JET ENGINES

Comments, questions, stories

2011-10-31T18:13:00.002+08:00

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LAF and BAF Part 3.

2011-10-31T17:13:00.000+08:00

LAF and BAF Part 3.

And so we look at BAF.

BAF, or ‘Big Aeroplane Fuel’ as we have come to know it, is paraffin. Or, if you prefer, kerosene.

Why do we use it in big aeroplanes?

The flame rate is slower at only a few feet per second. It is harder to light and easier to extinguish. The energy content is acceptable at (Jet A-1) 43.15 MJ/kg (1 MJ/kg = 430 Btu/lb so about 18,500 Btu/lb.).

Aviation turbine fuel (Avtur) is commonly known in commercial circles as Jet A-1. There is also a Jet B. And a gasoline based jet fuel called Avtag.

Let’s look at some fuels that are kerosene jet fuels:

JET A A kerosene type fuel with a freezing point around -40°C. It is available only in the U.S.A.

SG Range = 0·775 to 0·83

JET A1 See below

JET B This is a wide range distillate known as a wide cut gasoline. Not in common use. It is a Naphtha based fuel used primarily for low temperature conditions.

SG Range = around 0·76.

JP 4 This is a wide range distillate known as a wide cut gasoline. When certain additives are present it may be known as AVTAG. For military use.

SG Range = around 0·76

JP 5 High flash point kerosene mainly for aircraft carrier use. May be known as AVCAT.

SG Range = around 0·83

PROPERTIES OF JET A1

FLASH POINT 38°C Minimum

SPECIFIC GRAVITY 0·81 at 15°C

CALORIFIC GRAVITY 18,560 BThU/lb OR 150,400 BThU/gallon

VISCOSITY- from 22 Centistokes at -60°C to 1·2 Centistokes at +43°C

FREEZING TEMPERATURE -40°C maximum.

Note: When kerosene freezes it forms a sort of sludge. Really, it is the water in it that is freezing and not the kerosene itself. Diesel does a similar thing that we call ‘waxing’.

Problems?

Well, OK.

Kerosene, in common with other hydrocarbon fuels, is hygroscopic. That means it loves water. It will soak up water until the water begins to coalesce. Major problem now. That water will form droplets in the tank that becomes larger pools. The water that goes down the feed line to the engine can cause flameout and the water that remains causes corrosion in two ways:

1. By direct contact with the metal walls of the tank and,

2. By promoting the growth of fungus.

Unlike gasolines, kerosenes all have microbial spores in them. If the temperature is satisfactory for it and if there is water present then these spores will hatch out into fungal growths.

Believe me, this fungus smells really, really bad. It also blocks filters, and pipelines as well as causing corrosion.

Another problem is that the fuel, in rolling around in the tank because of aircraft manoeuvres, will rub on itself creating friction that, in turn, creates static electricity. If there is one part of the fuel at a low level of electrical charge and another portion that is at a high state of charge then there will be lightning in the tank.

This is a bad thing.

To prevent this an additive is put in the fuel to make it electrically conductive. There are also other additives to prevent icing, to prevent fungus growing (biocidal additives) and lubricity additives to stop the pumps and things burning out.

These fuels are very well engineered, are they not?

Some general thoughts.

All fuels are toxic. If you are going into a tank that has had any fuel in it then you need to vent the tank thoroughly.

You also need a medical check before going in or there may be insurance problems in the event of a disaster.

Once the tank has vented remember that there will always be loose (wet) fuel somewhere that is sneaking up on you as vapour.

Measure, and monitor, the lower explosive limit all the time. If the tank is below 25% of the LEL you may go in with breathing apparatus. Below 5% LEL you can remove the breathing kit –but beware that it doesn’t sneak up while you are in there.

Note: ALWAYS HAVE A SAFETY PERSON

Make sure the safety person is strong enough to get you out if you become unconscious!

Make sure that you do all the talking so that the safety person knows you are all right!

If you get fuel on you—wash with cold water FIRST! Then wash with hot water.

Hot water opens the pores (little holes) in your skin and that lets the fuel into your body.

Now you know. BAF and LAF should never, ever be mixed up. Disasters have happened due to the refuelling of Big Aeroplanes with LAF and Little Aeroplanes with BAF.

Don’t let it happen to you!

BAF and LAF Part 2

2011-10-31T16:18:00.000+08:00

Let’s start with LAF.

No particular reason other than it’s my ‘Blog’ and I choose to start with LAF. OK?

LAF is short for ‘Little Aeroplane Fuel’ and is petrol. Also known in some, less developed, parts as ‘gasoline’. For this reason the petrol they put in aeroplanes is known as AvGas.

AvGas is ‘Aviation Gasoline’.

AvGas is very similar to the stuff you put in your car to make it go—unless your car runs on diesel or NGV in which case it is very different.

Putting petrol in aeroplanes gives several problems.

Let’s start with the idea that as you ascend up Mount Everest the boiling point of your water is going down. By the time you get to the summit your water will boil off by pouring some in your hand.

Similarly, the water in your car radiator will not boil until it gets to 110°C, or thereabouts, because the radiator and cooling system is pressurised.

This change in boiling point is all due to the different pressures on the surface of the water.

To say that water boils at 100°C is only true if we add “at ISA SL” atmospheric pressure.

Note: ISA SL = International Standard Atmosphere at Sea Level.

Caution: If you remove the cap from the radiator of a hot engine the pressure on the coolant will suddenly reduce—thus lowering the boiling point dramatically. Equally dramatically, every molecule of coolant will now wish to turn to vapour, which will cause it to occupy a much greater space. Since there is no more space in its immediate surroundings the only place it can move to is outside the system that contains it. It will do this by attempting to pass through the hole that you have just made.

Result? A jet of superheated steam in excess of 100°C will now come out of the radiator filling hole at huge velocities. Contact with your skin will scald the skin and the velocity of the escaping gas will rip it off.

Important to know this, isn’t it?

Note: The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid.

Now let’s think about petrol. Petrol is lighter than water. It evolves into vapour more readily than water. Its boiling point is lower than water (95°C at ISA SL). Like water, the boiling point will reduce with altitude.

Aeroplanes fly higher than Mount Everest but, in World War Two, the American high level bombers, flying lower than Mount Everest, were losing up to 10% (Ten Per Cent!!!) of their fuel load because the gasoline was boiling off—it was turning into vapour.

Although the figures will vary considerably we could say that the RVP (Reid Vapour Pressure) for petrol is around 10-12psi.

So what is RVP?

Reid Vapour Pressure is defined as the absolute vapor pressure exerted by a liquid at 100 °F (37.8 °C) as determined by the test method ASTM-D-323.

What does this mean in practical terms?

There is a surface pressure at which hydrocarbon fuels will begin turning to vapour at a specific fuel temperature. Of course, gasoline and other fuels will vaporise over a period of time at any pressure and temperature but there comes a point at which vapour evolution is critical and measurable because of the reduction in boiling point.

The cure? Pressurise the fuel storage system (fuel tanks). This may create a heavier structure and a complex system to control it—also heavier.

What other problems are there?

In the old days there were lots of different petrols (Avgas) available. They were engineered to tolerate different compression ratios.

This leads us straight into a triumvirate of conditions.

Piston engines that use gasoline all operate as a ‘Modified Otto Cycle’ They are four stroke (for the most part) engines. The four strokes are:

1. Induction. The piston moves down to allow a fresh mixture (fuel and air) into the cylinder head.

2. Compression. The piston moves upwards to increase the pressure on the mixture.

3. Power. Just before the piston reaches the top of the compression stroke there will be a spark to ignite the mixture. The burning mixture now adds heat energy to the system at constant volume. The increase in pressure moves the piston down on the power stroke.

4. Exhaust. The piston moves back up to allow the burnt gases out of the cylinder head, thus making room for fresh mixture.

At 3,000 rpm each stroke takes ¹/₅₀^th of a second. Not much time, then.

Gasoline vapour burns at about 100’ (feet) per second. It takes time to burn and more time to transfer the heat to the air and then the air needs time to expand.

This is why the spark has to be before the piston gets to the top, to allow for a delay in the burning, transfer of heat, expansion in the cylinder head.

Another factor is mixture strength.

Complicated, isn’t it?

The stoichiometric ratio (sorry about that!) for gasoline is 14.6:1 (Natural Gas and Diesel are about 14.5:1; compare with Hydrogen at 34.3:1.).

Note: A stoichiometric ratio is the ratio at which all the fuel and all the available oxygen are completely burnt. A perfect mixture, if you like.

So you have 14.6 parts of oxygen and 1 part of fuel vapour. It will now burn nicely.

About 12:1 will also burn but that is called a ‘rich’ mixture. You are leaving a trail of black smoke (unburnt fuel turns to carbon as opposed to oil that burns blue-ish or white). A weak mixture extreme might be around 17:1. We like to burn a tiny bit weak because that saves us money on fuel but too weak creates a hole in the piston, valves and our bank account.

So what if we have a high compression ratio, a weak mixture and an advanced ignition?

Whoa? A what?

The faster that an engine is running the less time there is for the fuel to burn. For this reason the spark needs to be initiated even earlier. Then, as the rpm of the engine reduces, the spark can be moved closer to TDC (Top Dead Centre—the point where the piston changes direction at the top of the cylinder as opposed to BDC—Bottom Dead Centre which is the same thing at the other end of the piston’s travel).

Moving the spark away from TDC is advancing the ignition and moving it back towards TDC is retarding the ignition.

If we advance the spark too far the air will expand and push down on the piston too soon. We need this maximum pressure—called BMEP (Brake Mean Effective Pressure), around 22° after TDC for best results.

Too soon and the mixture in the cylinder head will cease to burn rapidly and will, instead, explode. This is called detonation or ‘pinking’ or ‘knocking’. It is very damaging to the engine.

Aero Piston engines needed increased compression ratios to develop more and more power. This increase in CR (Compression Ratio) creates a higher possibility of having detonation.

Note: Many piston engines have bimetal washers under the cylinder head wired to a gauge. This acts as a temperature sensor. A sudden increase in cylinder head temperature indicates that detonation is occurring.

Petrol is engineered to deal with detonation—up to a point.

Most petrols have an ‘octane’ rating. For your car it will be either 95 or 97.

For aeroplanes it is normally, now, 100LL.

The octane rating of petrol was increased by the addition of Tetra Ethyl Lead. This is a bad thing.

Now it is done with other additives and hence we have 100LL or 100 Low Lead.

An octane rating is found by increasing the compression ratio of an engine using the fuel at a specified mixture strength until detonation occurs and then comparing it with an iso-octane fuel. The 95 or 97 is a percentage of that figure—the iso-octane is always 100%.

You cannot get more than 100% so a higher number will be a ‘Performance Number’ and not an octane rating.

100/130 AvGas, now commonly called Avgas 100, is dyed green. 100LL has replaced 100/130 in most places, but AvGas 100/130 is still sold in Australia and New Zealand, I’m told.

In the past other grades were also available particularly for military use, such as AvGas 115/145 (dyed purple) and 91/96 (dyed brown).

Limited batches of 115/145, commonly called AvGas 115, are produced for special events such as unlimited air races; in the past 115/145 was used as the primary fuel for radial engines.

The second number is the Performance Number at ‘Rich Mixture’ conditions. On most of these engines the mixture strength can be adjusted for ‘warm up’, acceleration (Take off) and ‘Economical Cruise’.

So now you see that Little Aeroplane Fuel is quite complicated. You should also note that it catches fire more easily and is more difficult to extinguish than BAF. It is also more expensive.

We shall look at BAF in Part 3.

BAF & LAF Part 1.

2011-10-31T13:09:00.000+08:00

A short while ago I was asked, in passing, if the ‘petrol’ used by aeroplanes is all the same.

The answer to this is ‘no’. It is not.

And so we move on.

The subject this time is BAF and LAF.

You want to know, don’t you? I can tell.

OK.

BAF = Big Aeroplane Fuel

LAF = Little Aeroplane Fuel

Way, way back in the early days before even I was born—yes, yes, there was such a time, all aeroplanes used LAF.

Then, just after the Second World War, little aeroplanes used BAF until the de Havilland Comet was invented. Aaaah! De Havilland! A thing of grace and beauty that swung effortlessly into the air assisted by angels and four de Havilland Ghost engines on the 27^th July, 1949. BOAC started services with the Comet 1 in May, 1952.

Note: the Boeing 707 was not invented until 1955.

Up until the Comet and the 707 (720, according to United Airlines) all big aeroplanes used LAF. Most little aeroplanes were now using BAF

These little aeroplanes were Hawker Hunters, Supermarine Swifts, Fairey Gannets and the like.

Some mid-sized aeroplanes like Vickers Viscounts, F27 Friendships and Dart Heralds were also using BAF.

In the sixties and seventies a swarm of big aeroplanes took over commercial flying that were equipped with magical engines called ‘gas turbines’—jet engines, that were more powerful, more economical, more reliable, quieter (inside the aeroplane) and smoother than the old piston engines.

Suddenly the world had changed. Suddenly the world could afford to fly. The airways as the province of the wealthy and famous was gone.

There are still some hangovers from those days. To cross large bodies of water you still need to have more than two engines—that led to the development of a host of three-engined aircraft like the Lockheed Tristar, BAe Trident, Douglas DC10.

Big aeroplanes now use BAF and little aeroplanes use LAF except that there are still some preserved, old aeroplanes that are big and use LAF and, of course, there are a few little aeroplanes that use BAF.

There will always be delusions if grandeur amongst the smaller ones!

Now we shall look at what BAF and LAF is.

The Great Step Forward

2011-08-22T21:30:00.000+08:00

Going over some old thoughts the other day. Pondering, as it were, the way jet engines have changed over the years.

No change in the basic theory, of course, but a lot of refinements and improvements in materials.

Of course, the manufacturers have put in a lot of effort to fine tune engines and improve gas flows and aerodynamics—especially in the turbine but there has been one thing that, to my mind, was the real turning point. That moment in time when jet engines ‘grew up’.

Before I get into that, let’s just think about the ‘old days’ of gas turbines. Aero-gas turbines as opposed to hulking great industrial LP gas turbines with, or without, closed loop systems as made by the likes of Solar and ASEA Brown Boveri.

[NB: The Solar Saturn engine, first designed in 1950 for the US Navy and produced in 1960, went on to become the world's most widely used industrial gas turbine with some 4800 units in 80 countries. It remains in production today in two up-rated and enhanced configurations.]

Our good old ‘clunky’ engines were highly reliable and ruggedly built, for the most part. They were pretty heavy for their thrust output but the idea was to contain all that heat and energy.

Of course, many of the old engines were centrifugal flow. The wonderful sounding Derwent, Ghost, Goblin were all centrifugal flow as were the first turbopropellers—chief among them being the redoubtable Dart, still in use today. First produced in the late 1940s, it powered the first Vickers Viscount maiden flight in 1948, and was still in production until the last F27s and H.S 748s were produced in 1987.

These centrifugal flow engines were very robust and had very simple systems; the fuel system on the Dart is a model of simplicity.

“Whittle… stressed the great simplicity of his engine. Hives [Director of Rolls Royce] commented, ‘We’ll soon design the bloody simplicity out of it.’ ” [From Genesis of the Jet]

How successfully they have achieved that!

[NB: http://web.mit.edu/aeroastro/labs/gtl/early_GT_history.html

Is an interesting web page—especially the early history.]

Of course there were axial flow engines, too. The Avon, Sapphire, Viper were all axial flow as were many of the early Pratt & Whitney engines.

Some engines were compounded. The reverse flow Proteus turbopropeller engine had axial flow and a centrifugal last stage to ‘turn’ the air around the corner into the combustion section. This is still something that is widely used, not least by the wonderful PT6 engine.

All these engines had alloy front ends with aluminised mild steel rear compressor sections and outer combustion casings. The materials used were fairly standard in those days, the hot parts were various forms of nimonic—‘Hastalloy’, ‘Waspalloy’ (Registered Trade names of ‘Special Metals Group), which is a nickel alloy in various guises.

Materials have moved on. We still use nimonics in conjunction with crystals for turbine blades but there is increasing use of lighter and stronger metals like titanium. There is also more use being made of composites here and there on colder parts.

[NB: The ‘colder’ parts are not all ‘cold’. Remember that the temperature of the gas coming off the compressor of a twin spool engine like a ‘Spey’ is around 550°C which is the same temperature as the glowing end of a cigarette when you suck on the other end.]

So where is the ‘breakthrough? Is it the multi-spool engine like the JT9D or Spey that led to the Triple Spool RB211—a machine of great beauty and elegance?

Is it the development of high by-pass engines for the military airlift aeroplanes?

No, no. None of these.

Lets go back to the Viper.

No, really!

The Viper was developed from the Adder engine that was developed, in turn, from the Mamba that came from a Metrovick project. The twin Mamba was successfully installed in the Gannet and the Adder engine was the prototype Saab Draken engine.

‘Power by the Hour’ leasing was started in those days as the Viper had maintenance issues resulting from it being developed as a limited life engine (10 hours) for the Jindivik target drone. Operators would pay a fixed hourly rate to Bristol Siddeley for the continual maintenance of the engines.

There was one major step forward.

But the main one was that these engines had vaporising burners. Not a big issue. Vaporising, hockey stick, burners were very efficient but hard to start so atomising burners (4) were used as well to start the engine and get the vaporising burners going. The atomising burners would remain burning while the engine was running.

How is that a major leap forward?

Because they were in an annular combustion chamber.

Why is that so significant?

This engine, like the constant volume engine and the external combustion engine is a heat cycle machine. It relies for its effectiveness and operation on the addition of heat energy.

Irrespective of the temperature of the gas coming off the compressor, fuel is burnt in the combustion chamber in order to add (heat) energy to the working fluid (air).

Separate combustion cylinders and can-annular (cannular) systems are limited to the number of burners they can hold and control—in terms of flame shape; this includes multiple burner systems on early Pratt & Whitney engines.

But annular chambers can have as many burners as you can fit in them. This means you can add as much heat as the engine will take without a disproportionate increase in temperature.

Now, from that, we leap forward. Old engines ran at 10:1 compression ratios. It was thought that the maximum possible would be around 20:1 before all sorts of problems occurred.

Now we are running at over 40:1 compression ratio on a routine basis. We have high by-pass engines where 80% of the airflow is going down the by-pass duct and doing most of the work and that means that the fan is being driven by 20% of the airflow in a small core engine.

Huge amounts of power are generated in the core turbine to drive the big fans which means that the pressure ratios must be higher, the temperatures are higher, the stresses are higher.

All this is possible because the amount of heat that can be added is exponentially increased by the use of annular combustion chambers

And that, dear friends, is all thanks to Bristol Siddeley and the humble, disposable, Viper engine.

2011-08-18T13:00:00.000+08:00

Recently I have been speaking about safety. I have said, time and again, to the point, no doubt, of boredom, that there is only one way to work on aeroplanes.

The right way.

Is 'The Book' always right? How often have you read an entry in the Aircraft Maintenance Manual (AMM) and wondered if there could be a 'better way'?

Rarely, one hopes. If you analyse 'your way' you may, very often, find that there is a flaw in your analysis or sequence of events for a particular task.

Sometimes events occur that highlight a flaw in the system. A system that we have, hitherto, considered flawless.

The NTSB (National Transportation Safety Board in the United States) has written a letter to the FAA (Federal Aviation Administration) pointing out something that should have been obvious from the very start of commercial aviation and yet...

http://1.usa.gov/pLRZmM

This is very well worth reading, it will take a few minutes of your valuable time but make the effort.

We are not only concerned with what it is saying about this specific point but also we should consider the ramifications on everything else that we do.

Next time you work on an aeroplane - and by 'work' I am not just referring to maintenance people (wonderful souls though we undeniably are) but to flight crews, cabin crews, baggage handlers, catering delivery people, fuel tanker drivers, etc., and say to yourselves, "Am I missing something obvious? Is what I am doing safe? Is it safe for me, for the aeroplane, for other people?"

"IS IT SAFE?"

Flying is, inherently, safe. Modern aircraft design and materials make it so. Humans working on it reduce that level of security.

Working to 'The Book' should prevent that reduction.

Check.

Does it?

Addendum to "Think Safe"

2011-08-09T21:54:00.000+08:00

In the last 'Blog' I have mentioned that there is a rule that every one of my students hears. This is,

“Rule 1: Look after yourself”.

This is number 1 of “The Three Leyman Laws of Aviation Maintenance”

Let’s have a look at them now while they are fresh in the mind and applicable to the previous ‘Blog’.

Rule 1: Look after yourself. Only you know what you are doing and why you are doing it. You are the person best placed to observe and assess the risks and to take the appropriate action to avoid harm to yourself.

If you do not look after yourself then you are not going to be able to obey rule no.2.

Rule 2: Look after your colleague. It is eminently possible that the person, or persons, that you work with are not your friends. It would be nice if this were always so but life being what it is you will find that, sometimes, you will work with someone you dislike.

Fact, you still have to work with them.

Fact, you are still in a team with them.

Fact, they are your colleagues.

Look after them because there may come a time when you need them to look after you.

Rule 3: Look after the equipment. Much of the equipment used on modern aeroplanes to maintain them is extremely expensive. Some of it is fairly delicate (ask your avionics people about the TDR – Time Domain Reflectometer!). Certainly a lot of the equipment on the aeroplane is delicate and expensive – need I mention inertial navigational boxes?

If the equipment becomes defective it will require repair or replacing. All airlines are pricing to the bone to be competitive, additional repair costs for equipment, especially Ground Servicing Equipment (GSE) could be the straw that broke the camel’s back.

At best you could be looking at a zero pay rise or, at worse, looking for a new job.

Possibly, defective or ill-maintained GSE could cause you injury (have you seen a toe that had a tow-bar - no pun intended, dropped on it?).

Either way, not looking after the equipment contravenes Rule 1 (see above).

Don’t say you weren’t warned!

Think 'SAFE'!

2011-08-09T17:24:00.000+08:00

At great risk of sounding boring or repetitive, I should like to say, again, that when one is involved in aviation maintenance there is only one way to do things and that is the right way.

There are no ‘ifs and buts’, there is no negotiation or discussion necessary and there is no such thing as ‘it will do’!

Quite apart from the number of injuries and fatalities to passengers and crews due to aircraft making big black smoking holes in the ground, there are also risks involved with maintaining aeroplanes.

Time and time again I have said to students taking part in all sorts of courses, “Rule 1: Look after yourself.”

Sadly, this advice tends to be disregarded. Primarily, I suspect, because most people believe that accidents happen to other people, they forget that, to everyone else you are the other person!

On my ‘Facebook’ page [http://on.fb.me/nDwHMO] I have given an example of what happens when the appropriate safety precautions are not taken. There is also a mention there that two people a year, on average (I am told) are sucked into jet engine intakes.

The number of people who are struck and killed by propellers and tail rotors is significantly higher.

How does this happen? Why does it happen?

For many years the makers of propellers and various Aviation Authorities – the CAA (UK) not least among them, have attempted to discover the magic paintwork that will show a propeller up even when it is spinning.

They have failed. Without exception.

The fact is the propellers become invisible when they are rotating at any rpm that becomes inimical to human health (Translates: ‘lethal’).

I spend much time emphasizing to students that if they cannot see a propeller it is very dangerous. Very. This was even stated clearly in “A Simple Guide to Understanding Jet Engines” on Page 59 with a sad example given.

The air rushing into a gas turbine intake is also invisible. There is a famous incident on a US aircraft carrier (USS Theodore Roosevelt, I do believe) where an airman was sucked into an A-6 Intruder intake and survived. He survived because his helmet came off and smashed the compressor blades before his head arrived on the scene.

He was lucky. Extremely lucky. He was also foolhardy.

Propellers and air rushing into intakes are both invisible. The movement of air from one place to another is unstoppable unless you are aerodynamically shaped to permit the flow of air to slip past.

Most people, however, tend to wear clothes when working.

“A Simple Guide to Understanding Jet Engines” Page 131, qv.

The primary cause of these accidents is somewhat deeper. It is invisibility and the inability of the human person to fight the movement of air which creates the accident but there is something much more sinister that creates the situation!

Mindset.

We are, all of us, victims of compulsion and impulsion. Every one of us has pressure applied to do the job quickly. We understand that aeroplanes only make money when they are flying – on the ground they soak up money like a dry sponge.

There is constant pressure on us all to get the aeroplane back up in the air as quickly as possible, that there are schedules that must be kept.

Combine that with the idea that “familiarity breeds contempt”.

We adopt a laissez faire attitude, a feeling that ‘nothing ever happens’ or that the risk has been overstated (by people like me!).

But accidents happen.

“An Air New Zealand spokeswoman said the engine (RR-Allison T56) was sitting on a stand without propellors (sic) attached and was not affixed to a plane (C-130) at the time of the accident.”

A 51 year-old mechanic was sucked into the intake and died.

It is possible that he did not get ‘chopped up’ but his body, in blocking the intake, created a huge depression that damaged his lungs. This must have been, necessarily, at low power because the propeller was not installed.

Low power - and yet...

This is a turbopropeller engine that develops around 6000HP. Low power? Still significantly higher than your Ford Focus engine!

It has something in common with a Ford Focus - it is NOT a toy!

Careless? Yes. Should he have known better? Yes.

Ask yourself ‘how did it happen’? Try to go through it in your mind. Ghastly, isn’t it?

Most people believe that accidents happen to other people, they forget that, to everyone else you are the other person!

Wildlife Pt 2

2011-05-02T16:20:00.001+08:00

http://bit.ly/jHckWV

Just in case the article is taken off the 'net before you read it, here is what it says:

Deep sea search parties have found one of two recorders from an Air France flight that crashed off the coast of Brazil in 2009, according to investigators.

The news on Sunday revived hopes of understanding what caused the crash.

The Airbus 330-203 jet fell into the Atlantic Ocean off the northeast coast of Brazil en route to Paris from Rio de Janeiro in June 2009, killing all 228 passengers and crew on board after the flight hit stormy weather.

The flight data recorder (FDR), which stores technical data, could give vital clues about the flight's final moments.

French investigators said in a statement the FDR, which along with the cockpit voice recorder is called a "black box", had been hauled up to the deck of a search boat.

Pictures published on the website of France's BEA air accident inquiry office before the instrument was pulled to the surface show an orange cylindrical object half-buried in sand.

A BEA statement said the "memory unit" was "in good physical condition" after it was "raised and lifted by the Remora 6000 ROV [robot submarine] on board the ship Ile de Sein at 16h40 UTC".

'Good physical condition'

The discovery comes after years of start-and-stop search efforts on a 10,000sq km area of sea floor to locate the aircraft's two recorders, which investigators hope will settle a dispute over the cause of the crash.

Jean-Paul Troadec, the BEA's director, said: "At this stage, the box seems to be in good physical condition. Our experts will tell us if there's hope to read the data.

"If the data can be used it will allow the enquiry to make headway because the FDR records the altitude, speed and the various positions of the rudder."

The FDR was expected to arrive at BEA offices within eight to 10 days, to allow for the search of the cockpit voice recorder, so the two can be taken back to France.

Speculation about what caused the accident has focused on the possible icing up of the aircraft's speed sensors, which seemed to give inconsistent readings before communication was lost.

Investigators announced on Wednesday that search teams had retrieved part of a "black box" from the aircraft but not the part containing the key data.

BEA said the chassis that held one of the recorders was found a day after a salvage ship began working to retrieve bodies and recently discovered wreckage using the Remora submarines.

Previous searches had recovered a limited amount of wreckage and about 50 bodies.

Air France and Airbus - which are being investigated for alleged manslaughter in connection with the crash, the deadliest in the carrier's history - are paying the estimated $12.7 million cost of the search.

What is NOT Said!

2011-04-21T21:37:00.002+08:00

The TV was on this morning, coffee time. The programme was ‘Air Accident Investigation’. Seemed good enough to spend an hour watching. Us aviation type people will watch anything that’s got aeroplanes in it even if it is a little traumatic.

This was a lot traumatic. This was the story of how two pilots fought a B747 for over thirty minutes, not knowing their efforts were useless. It was Japan Air Lines (JAL) Flight Number 123.

Dreadful accident. Just dreadful.

Miraculously there were survivors. A few. There might have been more but rescue was late and the mountaintop was cold.

The programme detailed the events that led up to the accident. It was both enthralling and horrific.

At the end they reported on the cause. In some detail. They stopped there.

They did say that there was to have been a prosecution against Boeing but it never came about. They did say that there were lots of conspiracy-mongers who said that the blame lay with JAL and not Boeing, that JAL was being ‘protected’ by the Japanese Government and Boeing.

Still, JAL very nearly went under because of it.

JAL was entirely innocent. As innocent as all those victims.

The cause? A repair to the rear pressure bulkhead had blown apart and removed, in so doing, the fin of the aircraft. Thus the aircraft had become uncontrollable with the inevitable consequences.

I remember, many years ago, that there was talk about the engineer who had come out to Japan from Boeing to effect this repair. That it was his fault. The finger of blame descended upon him and his team.

But let’s just think about that for a moment.

This is a skilled engineer and his colleagues. They are all experienced and know the B747 well. They know their job well. These are good people at the top of their profession or they would not have been sent out to Japan to do this work.

Shall we go back to the beginning? This B747 made an appalling landing, for reasons best known at the time to the pilot alone. He managed to skid the empennage along the ground causing damage to the under-skin and the pressure bulkhead. Structural checks, carried out at the time by JAL engineers, revealed no problems anywhere else (top skin, wing retaining bolts, engine pylons, etc.).

What is the next step?

Telephone Boeing. “I’m sorry, Mr. Boeing-san, but we feel that this repair is beyond our expertise. Please help.”

Boeing looked at the damage and produced a ‘Repair Plan’.

Got that? A ‘Repair Plan’. This is a normal procedure for manufacturers of all aircraft. Your aircraft is broken and there is nothing in the Structural Repair Manual (SRM) to cover it? Call the manufacturers and get a ‘Repair Plan’ from them.

Boeing produced the plan and sent engineers, at the request of JAL, out to do the job.

A note about engineers. And American practices.

Thirty years ago we worked with the United States Air Forces in Europe (USAFE). They were good and conscientious lads—and girls, too. If they received an amendment to one of their manuals (TO—Technical Orders, they called them) they would not just insert the amendment as we did. No, no! They checked EVERY page in EVERY book! Very, very time consuming and a task that would bore any living engineer or mechanic to tears.

Similarly, when they went out to do a maintenance check on an aeroplane, they would follow the book precisely:

Page 1, Paragraph 1—Check ‘item’ in left undercarriage bay. (Tick—done.)

Page 1, Paragraph 2—Check ‘item’ in right undercarriage bay. (Tick—done.)

Page 2, Paragraph 3—Check ‘item’ in left undercarriage bay. (Tick—done.)

Page 2, Paragraph 4—Check ‘item’ in right undercarriage bay. (Tick—done.)

....and so on.

We did a circular route around the aircraft rather than hop from one side to the other and back again. But their rules said ‘follow the book’.

But we still ‘follow the book’ when it comes to what to check, when to check it and how to check it.

Back to Japan.

The engineer from Boeing has arrived in Japan with his favourite tools and gets out the ‘Repair Plan’.

He follows the ‘Repair Plan’ because he knows that if he gets creative with it there could be very nasty repercussions. The words ‘Big Black Smoking Hole in the Ground’ spring very readily to mind at such times.

He does as he is told. The supervisor checks ‘The Plan’ and the repair. The Quality Assurance manager checks ‘The Plan’ and the repair.

Everything matches. All is well.

But it wasn’t.

The person, or team, that drew up the ‘Repair Plan’ got it horribly wrong. The Manager who approved the ‘Repair Plan’ got it horribly wrong.

And then....

The engineer, his supervisor and the QAM should have thought “Hello! Something odd about this.” They should have queried Headquarters.

These are experienced people. They didn’t ask.

Never be afraid to ask.

Never.

We are so drilled into ‘following the book’ that we forget that, sometimes, it is right to ask. Sometimes your duty is to ask.

It can save lives.

Geared TurboFans (GTF)

2011-03-29T15:31:00.000+08:00

This concept is intriguing. Any observations or comments about it would be most welcome.

Why?

Because the idea of gearing the fan came up many years ago in a seminar held by some people from Rolls-Royce plc. The question arose from the floor and so the answer given, it was stressed, was not R-R official policy but more the opinion of the engineer representing Rolls-Royce.

The seminar was not on the subject of single stage fans - nor of high by-pass fans in general, but it was something that the person posing the question, on behalf of an operator, clearly thought strongly about.

The pros and cons stated were as follows:

Pros were that the current situation where the turbine and fan have to be a compromise in design, in order to maximise the power produced by the turbine and the power consumed effectively by the fan with minimum losses, could be avoided by having both the fan and the turbine rotating at their individual optimum rate. This is bound to give an increase, even if only a small one, in the overall efficiency of the engine. This leads to a decrease in overall sfc that is going to please the operator.

The cons are that the introduction of a gearbox does two things:
1. It increases the overall weight of the engine unless savings can be made elsewhere in the construction. On a twin spool (co-axial) engine, or a triple spool, the length of the LP spool must be increased to accommodate the gearbox. This increase needs a corresponding increase in the case, carcass and fairing (cowl) which will add to the overall weight and this does not yet include the weight of the gearbox itself. The alternative might be to snug up the gearbox into the nose cowl which will prevent the additional weight of the external parts but will not avoid the mass generated by the gearbox itself.

2. No matter how this is done, and, one suspects, a single stage or two stage spur epicyclic will be used, the gearbox will need to be rotated. Rotating any mass, however small, will result in power being used to do it. Is it possible that the optimisation of the turbine and fan speed will produce sufficient benefit to both to increase the efficiency to the point that the residue from driving the gearbox will still give increased performance and improved sfc?

Intriguing. We await figures from Pratt & Whitney and your opinions.

Wildlife!

2011-03-28T21:17:00.000+08:00

Aircraft on the, almost treeless, Shetlands only need about twenty minutes, or so, to build nests in the rudders. Birds don't see aeroplanes - they see trees or nest-building opportunities. Snakes and other creatures see caves for warmth and sanctuary. Rodents on aircraft - as well as bugs (see 'British Airways' bed-bug problem out of Thailand and Berjaya Air's German Cockroach infestation) are an ongoing and insidious problem. Rodents are especially problematical because they are inclined to chew through wires and cables.
Now:

"Airbus charged over Rio tragedy

March 18, 2011

JET builder Airbus has been accused of involuntary manslaughter over the deaths of 228 people in the Rio-Paris crash in 2009.

Preliminary charges have been laid by a judge to start a formal investigation into the crash of the Air France flight.

Airbus said there was an “absence of facts supporting the charge” and chief executive Thomas Enders said it was premature. He added it would be better to focus on finding the cause of the crash and making sure it never happened again.

Investigators found automatic messages from the Airbus A330-200’s flight computers indicating an electrical fault. The pilots may have been receiving false speed readings from sensors.

Air safety authorities ordered the pitot tube sensors to be replaced on other aircraft after the disaster, although the problem had been known about since 2002.

Franco-German company Airbus says that only finding the black box flight data recorders will give answers to what happened when Flight 447 crashed into the Atlantic during a storm.

A fourth phase of searching for the black box is due to begin this weekend, with Air France and Airbus paying the seven million euro cost. It will use a mini-submarine searching in the south Atlantic crevices, which can be as deep as 13,000ft.

Just three per cent of the plane has been recovered, including a large part of the tailfin. Fifty bodies have been found."

What does this have to do with wildlife?

Well:

The pitot tube problem cropped up much earlier in Birgenair Flight 301, in 1996. The reason for the faulty pitot tubes. Investigators suspected that some kind of insect could have created a nest inside the pitot tube. The prime suspect is a species called the Black and yellow mud dauber wasp, well-known by pilots flying in the Dominican Republic. The aircraft had not flown in 25 days during which time the pitot tubes were not covered, giving the wasps an opportunity to build nests in the tubes.

(NB: Final comment courtesy of Charles Thomas)

ICING - Part 2

2011-03-28T21:15:00.002+08:00

A Word (or two) About Icing. Part 2

We need to get rid of ice.

Ideally we get rid of ice on the ground before the aeroplane leaps into the air. Sometimes the ice will form after the aeroplane leaps into the air and that presents us with a whole new set of problems.

There are lots of different forms of icing but we will just think:

‘ice’ = ‘hard water’ on ‘aeroplane’.

Very often ground crews will de-ice aeroplanes on the ground before take-off. They will do this so that the aeroplane is not damaged by ice (or snow) building up on the aeroplane and weighing it down.

The ice or snow can be removed by sweeping or spraying with a de-icing fluid.

Aeroplanes with a small wheel at the front (a nosewheel known as a ‘tricycle undercarriage) will be de-iced starting at the back because de-icing the front first can cause the aeroplane to tip onto its tail!

Small propellers can be ‘greased’ with a de-icing compound that prevents ice from sticking to the blades.

Once an aeroplane has been de-iced it must be got off the ground fairly quickly or the process will have to be repeated to prevent further build up of ice or snow.

The problem now is the build up of ice on the aeroplane, and in the jet engines, in flight.

Why does ice form on the aeroplane?

Because the way an aeroplane gets ‘lift’ from the wings is to create a low pressure on top of the wings and a higher pressure under the wings. The aeroplane is ‘sucked’ into the air! Reduce the pressure of gas and the temperature drops, if it drops sufficiently any water in the air will freeze and stick to the wing.

The engine air intake also has a low pressure in it when the engine is on the ground and the aeroplane is not moving forward. Ice can (and does) form inside the intake ‘lip’.

Propellers act very much like an aerofoil (wing) and so ice can form on the propeller blades, too.

How to get rid of ice in the air?

Two methods:

1. Anti-icing.

2. De-icing.

Anti-icing prevents ice from forming on a surface and de-icing allows the ice to form and then gets rid of it – usually by using it’s own weight to help.

Anti-icing and de-icing use several ways of working. Sometimes heat is used. Heat is obtained from two main sources:

1. Hot air from the engine compressor (High pressure air is hot)

2. Electrical heater pads

It is also possible to use low pressure air from the compressor to inflate rubber balloons - called ‘boots’, that will cause ice to crack up and flake off from wings, tailplanes, fins and engine intakes. This pressure is usually very low and rarely goes much above 20 psi. The boot is then ‘sucked shut’ to conform to the shape of the surface it is protecting. The valve that controls the inflation and closing of the boot needs to be heated so that it will not freeze and cease to work.

Electrical de-icing heater pads use a quick burst of electricity to heat up. The heat needs to be applied at high temperature very quickly so that the minimum amount of ice is melted. The ice will then blow off or be flung off, on a propeller blade, and the airflow will then cool the heater. Not too much ice should be melted because water will flow back from the heater element and cause ‘run-back’ icing that cannot now be removed! These heater elements should not be used on the ground when there is no cooling airflow over them or they may well burn out.

Hot air from the compressor is applied to surfaces to keep them free of ice. A small bleed of air at low pressure - possibly around 30 psi, is allowed to flow out from small holes around the area that needs to be kept free from ice. Sometimes it is discharged gently from a pipe that has many tiny holes in it called a ‘piccolo tube’, the air sprays onto the inside of a metal skin to keep it just above freezing point so that no ice forms. This is commonly used on engine air intakes and spinner fairings in the middle of the intake.

The other area that needs de-icing is the windshield. This is usually electrically heated.

The contents of windscreen washer bottles can be kept liquid by electrical heater elements or by using waste air from fans cooling the electronic equipment.

Electrical elements are rarely used for anti-icing because the load on the electrical system would be too high. Much better to use short bursts of electricity every so often.

ICING - Part 1

2011-03-28T21:14:00.002+08:00

Given the Weather in Europe recently -

A Word (or two) About Icing. Part 1

Icing is a terrible thing to happen to an aeroplane. It is also a terrible thing to happen to a jet engine and the propellers that some of them drive.

Let’s consider what happens.

Firstly, ice is water. Water is heavy. Hard water is also heavy and it sticks to things very well indeed.

Lift a bucket of water, feel the weight of it and now imagine that bucket of water smeared thinly all over an aeroplane. Not a very thick layer, is it? That amount of ice will not do terrible things to an aeroplane.

Now imagine a layer of ice two inches (five centimetres) thick. All over the aeroplane. How many buckets do you think that will fill up? You are right - lots. Now we have a lot of very heavy hard water.

The weight of the ice on the aeroplane can cause damage to the structure of the aircraft by pushing down on it.

Can you imagine how hard it is to get an aeroplane off the ground when it is covered in ice?

The four forces on an aeroplane are:

Drag. This pushes against an aeroplane in flight trying to slow it down. The faster you go the more drag you get.

Thrust. The engines push the aeroplane forward against the drag of the air flowing over it.

Gravity. The aeroplane is being pulled down towards the ground by gravity. The heavier the aeroplane is the harder gravity will pull at it.

Lift. The wings generate lift. Lift works against gravity. To get lift you need the aircraft to be going forward fast enough for the wings to give lift.

Ice works with gravity to stop the aeroplane going upwards. Weight is the enemy of aeroplanes. More weight? You need more lift. To get more lift you need to go faster. To go faster you need more power (thrust) from the engines. To get more power from the engines you need to burn more fuel. Burning more fuel makes the engines hotter.

Ice wears out the engines faster.

But.

That was only ‘Firstly’!

Secondly, the ice breaks up the airflow over the aeroplane. Instead of a nice, smooth surface there is now a rough surface that is not the right shape to make a smooth airflow.

Turbulent airflow makes for a lack of lift that no amount of thrust from the engines will overcome.

Ice forming in the intakes and around the engine is not only heavy but also affects the airflow going into the engine in two ways:

1. It creates turbulence so that the smooth flow of air into the engine is now rough. The engine doesn’t like this and may well ‘cough’ causing it to break. The ‘cough’ is what we call a ‘surge’; this is when the airflow in the engine decides to suddenly (very suddenly) change direction and go from back to front.

2. The diameter of the intake may be reduced by the thickness of the ice. This reduces the amount of air going into the engine. The engine needs air to burn the fuel, less air going in means that there must be less fuel being burnt. Less fuel? Less thrust. This is at a time when the aeroplane really, really needs more thrust - not less!!

We need to get rid of that terrible ice.

How?

Watch out for the next episode.

Power v. Thrust

2011-03-28T21:13:00.002+08:00

There was a bold statement that said "Piston engines develop power but jet engines (gas turbines) develop thrust"

This is, broadly true.

Why?

Power, however it is measured, is the RATE at which WORK is done. Work can be described as torque.

Torque is a twisting moment about a shaft. If someone grabs your hand and rotates it there is a twisting moment in your forearm. Work is being done on your arm.

Now imagine a mangle. Oh. You're not old enough to know what a mangle is! Hmm. Two sets of rollers between which you pass clothes and sheets, etc, to squeeze most of the water out. The rollers are operated by a handle that cranks them around.

By turning the handle you are putting torque on the rollers. If you apply sufficient torque to overcome the resistance of the rollers - and the wet clothes between the rollers, then they will begin to rotate. The speed at which they rotate will depend on the effort you wish to put into the job.

The torque that you apply is the work done. The rotation is the rate at which the work is done.

You are applying Horse Power to the mangle.

A piston engine applies torque to the wheels of a car or a propeller on an aeroplane. In both cases the resistance to motion has to be overcome by the torque to get movement, after that the speed of rotation (rpm = revolutions per minute) is the rate at which the work is done.

Power can be high rpm and low torque or, for the same power, low rpm and high torque.

Power = RPM x WORK.

Jet engines apply a reaction force to the engine by the hot gases rushing out of the back of the engine. This is Newton's Third Law - to every action there is an equal and opposite reaction.

The reaction felt by the engine is an applied force just like someone pushing you in the back but doing it continuously. This force is described in pounds (lbf) or in kN (kiloNewtons).

1000 lbf of thrust = 4.45 kN (Actually: 4.4482216 kN)

1 kN = 224.81 lbf (Actually: 224.8089431 lbf)

On a jet engine - or a rocket, there is only the 'push', there is no 'rate' at which this is done so a jet engine does not develop power.

Rolls-Royce RB211 Trent 900 Series

2011-03-28T21:12:00.000+08:00

Following the failure of an engine on a QANTAS Flight out of Changi, Singapore, which caused airframe (wing) damage and the aircraft to turn back, Rolls-Royce have really pulled out all the stops and got to the bottom of it very quickly to the extent that there has been a EASA EAD (Emergency Airworthiness Directive) - courtesy of RR and the UK CAA, issued already.

EASA EAD 2010-0236-E: RB211 Trent 900 series engines: Engine - High Pressure / Intermediate Pressure (HP/IP) Structure – Inspections

Details of the EAD have been copied onto the following site:

http://www.facebook.com/pages/A-Simple-Guide-to-Understanding-Jet-Engines-become-a-fan/396570288622

for those who wish to see the details of the action required. It is in a picture album titled "RB211 Trent 900 Series".

Rolls-Royce are to be highly commended on their swift and positive response to this situation.

If there is more news on this we will let you know.

Why does air get bigger when it gets hotter?

2011-03-28T21:10:00.002+08:00

Why does air get bigger when it gets hotter?

This happens anywhere and not just in a jet engine but it concerns us because we need to add energy to the engine to get thrust (energy), or power (energy) out.

(Note that piston engines develop power but jet engines develop thrust. We shall look at that later.)

When energy is added to air, or anything else, the energy (normally 'heat') spreads through the air causing all the molecules in the air to get excited. Watch a football crowd, when they get excited they start to jump around and wave their arms. They are, individually, taking up more space. Molecules do that. Their mass (weight) remains the same but they take up more room - they expand. They become less dense (less 'crowded together'). If they are not allowed to expand they will press against one another, they will have increased pressure.

In the jet engine we allow them, the molecules of air, to have more space by keeping the pressure (nearly) constant. They will spread out and become bigger; to escape they will need to accelerate.

Why?

The engine is a tube.

Air goes into the front and comes out the back.

Let's forget 'velocity' for a moment and think in terms of the weight of air.

Suppose that, over a one second period of time, one pound of air goes into the front of the engine.

We now have a "Mass Flow" (Wf) of 1 lb/sec. We should reasonably expect that we should get 1 lb/sec of air out of the back of the jet engine. If we only get half a pound/sec where has the other half a pound gone? If we get 2 lb/sec then we have miraculously created 1 lb of air, every second, within the engine. Wow! That would be something wonderful!!

No. what goes in the front comes out the back. But the air coming out of the back is a different size. It is bigger - much bigger. To maintain a mass flow of 1 lb/sec it has to come out MUCH faster.

Momentum = Mass x Velocity

Same mass going in as coming out but the velocity has changed because we have added energy to it.

More velocity? More momentum.

More momentum? More reaction - more thrust.

See how simple Jet Engines are?

Jet Engines

2011-03-28T21:04:00.000+08:00

Let's start from the very beginning:

There are two types of heat engines - External Combustion Engines and Internal Combustion Engines.

An external combustion engine is a steam engine like they have on trains and ships (paddle steamers out on the Mississippi of the Old West; these are cool.).

An internal combustion engine comes in two basic forms:
The Piston Engine - or reciprocating engine
The Jet Engine - or Gas Turbine Engine.

The similarities are that they both burn a fuel that transfers the heat energy to air. The effect on the air is what drives the engine.
This means that, in both cases, AIR is the working fluid.

The differences are that the piston engine has lots of moving parts which means lots of bits to break and lots of friction and lots of power needed to drive each part - power that does not get to propel whatever it is that you want to push long.
Jet engines have one moving part (for a 'basic model') and thus it becomes more efficient because less energy is wasted in driving different bits.

The MAIN Difference is that the Piston Engine is an (Modified) 'Otto' Cycle machine and a Gas Turbine is a 'Brayton' Cycle.
What does that mean?
An 'Otto' Cycle is a constant VOLUME engine.
A 'Brayton' Cycle is a constant PRESSURE engine
AT THE POINT WHERE HEAT IS ADDED TO THE SYSTEM.

Now you are going to say that the piston in a piston engine is going up and down so the volume in the cylinder constantly changes.
That's true.
But.
Imagine any very small moment; the piston will appear, in that very small moment, to be in one place. The volume will appear to be fixed. The air is getting hooter - it is expanding with nowhere to go. What happens? The pressure builds up and pushes the piston down to a new "fixed" volume.
So it is the increase in pressure at a fixed volume that drives the piston down (or across if you are in a Beetle!).

In a jet engine the pressure is held constant (it doesn't work in practice - we shall look at that later, 'almost constant' is good). The volume increases and, in a confined and fixed space, the only way for it to get out of the machine is to increase in velocity. The increase in velocity on a mass of air means that the momentum is increased.
If you run faster your momentum increase - try walking slowly into a wall and then running as fast as you can into that same wall. See the difference?
Isaac Newton said (Third Law, you know): To every action there is an equal and opposite reaction.

Increase the momentum of the escaping air and the reaction against the engine increases.

You now have a 'Jet Engine'