You Must be Choking!

Sorry about that.  Some things are just naturally irresistible!

The point of it is that I want to talk about ‘stall’ and ‘surge’.

Why?

Because there seems to be some confusion in the ranks. There have been questions posed that would lead me to believe that these are two aspects of a compressor that are not fully understood.

Firstly, are we talking about axial flow compressors or centrifugal flow compressors?

Both, really, but we will come to the centrifugal flow compressors later since they are somewhat different in their ‘cause and effect’.

Shall we take out an axial flow compressor blade and take a peek at it?

Hold it so that you are looking down from the tip to the root.

What do you see?

Yes, several things:

1.     It has an aerofoil shape, the same as a wing.

2.     It is ‘twisted’.

If we take out two blades and put them together, by holding the base of each blade firmly abutting (touching) each other, then we see something else.

That the gap between the blades, in the direction of airflow, is divergent – it gets bigger as the gas leaves it.

These designs in the blades are there to do one thing – to ‘do work’ on the air.

The turbine supplies energy to the compressor and the compressor uses that energy to bunch up the air.

Now we have to be very careful.

The flow of air through the compressor has to be (almost) uniform.

Not in terms of velocity but in terms of mass flow.

If we have 500 kgs per second going in the front of the compressor then we absolutely must have 500 kgs per second coming out of the back into the combustion section.

That is vital.

The velocity of the gas is, more or less, irrelevant and will depend on the rpm of the compressor and not (as many seem to believe) the speed of the aeroplane through the air.

Note that I have just suggested that the rpm of the engine is, for most engines, a variable.

Note that the cross-section of the blade is an aerofoil.

Aerofoils do well at an optimum angle of attack of four degrees (4°). This will only happen at one rpm.

That means that there is a compromise somewhere.

Listen to a jet engine starting up.  At some point in the start sequence you will, very likely, hear a rasping noise from deep within it.

This is a ‘stall’ occurring. It will always do this. You will never get a nice airflow over all the blades all the time to create that lovely angle of attack.

There are going to be times when the airflow through the engine does not match the speed of the blades (rpm). Start-up is one of those times. Every time.

If the angle of attack of the air on a blade is bad – naughty air! It may produce turbulence down the back of the blade.

Turbulence means that the blade is no longer ‘doing work’ on the air. But the turbine is still sending energy down the shaft to the compressor (remember that!).

It is unlikely that only one blade will stall.

It is more likely that a whole stage (row) of blades will stall. It may be the ones in the middle, the little ones at the back or, even, those hulking great big ones at the front.

If the conditions that created that breakdown of airflow over that stage are restored to normal, then the blades will continue to work.

If conditions do not return to normal there is a danger that other stages will also stall.

Why?

Because the stage that has stalled is doing no work on the air, which means that the air being received by the next stage is not at the best condition for it.

Similarly, the air leaving the stage in front of the stalled row is not being accepted by the stalled stage – slowing it down.

Air that changes speed is not at the same angle of attack that it was previously. This is why we cannot suddenly change the rpm of the engine because it will rapidly change the angle of attack on the blades.

We now have an engine that is in a state of stall.

The compressor is still receiving lots of energy to do the work with but it is doing less work. More stages stalling will reduce the workload even more.

How do we know that this stall is occurring?

Because the exhaust temperature will be going up, the engine is likely to be vibrating (because the gas is turbulent) and it is very likely that you will hear it.

What to do?

Close the throttle and watch the instruments. If the engine slows to idle rpm and the temperature comes down then run the engine for three minutes at cooling rpm (not necessarily idle – the RR Viper engine cools at a slightly higher rpm than ground idle), and shut it down to investigate the cause.

It may be that the temperature continues to climb (burning fuel with less load on the compressor and reduced air) and the rpm stays where it is.  Now all you can do is shut-off the fuel and hope that the bearings don’t suffer too much.

This last is what is called a ‘locked in’ stall.

But what if all the compressor stages stall?

Then the compressor is doing no work on the air at all. The high pressure air at the back can now come to the low pressure at the front of the compressor. It will do this suddenly and, almost, explosively.

You will hear a loud bang. If you are outside the aeroplane it will feel as if you have been punched all over by a giant fist. If you are very close to the engine it may knock you over.

If you are inside the aeroplane at the controls, you may see the exhaust temperature and rpm begin to rise. The rpm will only go up for a moment but the temperature will keep going up. This is a really good time to shut the fuel supply off. Of course, it may just be a ‘pop’ surge where engine ‘clears its throat’, as it were. Things may go back to normal immediately but it is still worth investigation.

Apart from discovering the source of the problem, what are we looking for after a surge?

Check the oil (SOAP) and MCD’s (Magnetic Chip Detectors) as well as the oil pressure filter and scavenge strainers in case the bearings have been damaged.

(The SOAP sample results will have to be compared with the previous SOAP results to see if there is a sharp up swing in the contaminant level.

NB: SOAP = Spectrometric Oil Analysis Programme. This is done by a lab unless you work for a very big major airline that has it’s own kit!)

Do a borescope check – especially on the rear stages where possible.

Check the turbine for ‘spatter’ – it will look like the sky at night with little ‘stars’ of melted metal all over the turbine blades and, often, the exhaust.

Naturally, look down the intake. Several times we needed to go no further than a quick peek past the first stage on co-axial (twin spool or dual-axial) engines.

So what about centrifugal compressors?  Do they stall? Yes. They do.

Try and stay with this because it gets a bit tricky.

Between the rotating part of a centrifugal compressor and the static part there is a gap.  Not a big gap but it is there.

The reason for the gap is that the air coming off the impeller vanes is quite thick, it has been compressed, it is hot and it is moving quite fast.

As each impeller vane on the rotating bit goes past a diffuser vane on the static bit there will be a ‘thud’ as a lump of compressed gas gets caught between the two. The air behind the impeller vane is not quite so ‘thick’ so the sudden increase in viscosity is really noticed by the diffuser vanes.

(Listen, if you get the opportunity, to a RR Derwent engine on, say, a Gloster Meteor going overhead at low level; it sounds like a V16 piston engine because of all the ‘thuds’ as each impeller vane’s air hits each of the diffuser vanes.)

The gap has to be wide enough – remember the gap? For the air to ‘spread out’ a bit before going into the diffuser to lessen this ‘thud’ because too much ‘thud’ creates too much drag – wasted energy. Too small a gap = too big in the ‘thud’ department.

But!

As the aeroplane goes up into the clouds, and beyond, the air gets thinner.

At some point the air will be so thin that, even with the impeller working very hard to compress it, it will slip around the gap. If air is going around in circles it will block off the air trying to get out of the impeller.

The compressor will now stall.

It is unlikely to surge (but it can happen) but the conditions of the stall are exactly the same as for an axial flow compressor.

So what causes a centrifugal compressor to stall?

Low air density - altitude.

There you have it.

Surge and stall made easy. I hope.

Now my brain has stalled, my temperature is going up and work is ceasing.  Time for bed after a quick refuel!

Heat

Before I write anything else about anything else, there needs to be a clearing up of confusion.

Just recently there have been a few misconceptions posted here and there. 

For instance, there is a person on ‘E News’ who has taken to using the expression ‘Ell-Beez’. This is usually in conjunction with the revelation that a certain celebrity (who is, invariably, completely unknown to me!) has gained, or lost, a few ‘Ell-Beez’.

Let us all, with one accord as distinguished Jet Engine Professionals, put these people into the light of knowledge.

The plural of ‘lb’, the accepted abbreviation for ‘pound weight’, is ‘lb’

It comes from the Latin ‘Libra’ meaning ‘pound’; the plural of this is ‘Librae’. Observe – no ‘s’.

Ergo, said ‘celebrity’ has gained, or lost, a few ‘Ell-Bee’. Not, perhaps, quite so catchy but much more accurate. It also avoids pitying looks from the intelligentsia who watch the show.

Now heat.

Heat and temperature are not the same thing.

Heat is a form of energy. We get ours from the Sun. Exclusively. Try very hard not to tell me that we can get heat from burning coal or wood. The energy that gave us both these fuels came from the Sun so they are ‘second-hand’ solar heat.

Cold is not energy. Cold is an absence of heat in the same way as there is no such thing as ‘dark’. Dark is an absence of light (which is another form of heat energy.

Temperature is a measurement of how much heat is present in any particular volume of… well… anything.

Let’s take an example.

10 lb of dry air at ISA (International Standard Atmosphere – 14.7 psi at 15°C) takes up the space of a sphere approximately 6.28’ in diameter.

If we pass this volume of air through the compressor of a jet engine, the compressor will make it smaller. We shall say that the compressor gives a CR (Compression Ratio) of 10:1.

Ten to one? Time for lunch – see you later.

Ah! Good food.

Where were we? Oh, yes. We are going to reduce the volume of air by a factor of ten.

It will now be 0.628’ in diameter or, if we convert that, 7.536”. Sit in the hand just nicely; just under an octave

Of course, you would not want it sitting in your hand because we have also compressed the heat in it by a factor of ten.

15°C now becomes 150°C – considerably more than the boiling point of water (at ISA).

It is exactly the same amount of HEAT but it is now occupying a smaller VOLUME of, in this case, AIR.

Of course, this is ignoring the work done on it to compress it. Since we are less than efficient, the work done to do the compressing will be mostly wasted in heat energy transferred into our bundle of gas and so the heat within our sphere will be much greater than it was at the start.

In other words, the temperature will be much higher. Much.

An axial flow compressor giving a CR of about 11:1 will give us an exit gas temperature of around 550°C. If you yearn to know how that feels then go to a friend who longs to kill him/herself with noxious gases. Ask them to light the cigarette and request that they suck long and hard on it.

When the tip glows brightly that will equate to around 550°C. Try to avoid the temptation to put your finger on it.

The HEAT in the cigarette end will be quite small but the TEMPERATURE will be very high.

ICE

Ice

A small, simple word. How devastating it can be.

We have already discussed the difference between anti-icing and de-icing on these pages. Now let’s have a tiny peek at the effect of ice.

There are 6.229 Imperial Gallons (7.48 US Gallons) contained in one (1) cubic foot of water. Get your rulers out and construct a cube with each side measuring one foot (1’).

Not very big, is it?

That much water, irrespective of whether it is the tiny US Gallon or the proper sized Imperial Gallon, weighs 62.288 lb (27.76 kgs). Heavy, yes?

One cubic foot is 1,728 cubic inches so a one-inch thick layer will cover 144 square feet (12’ x 12’)

(Sorry about all of this math – just trying to make a point here!)

The surface area of a small jet airliner – like a Boeing 737-200 is 6,500 square feet (compare with your home). This is equivalent, using the above data, to 281.17 Imperial Gallons of water at one inch thick.

That is 2811.7 lb (1253 kgs) of water. One and a quarter tonnes.

Want to consider the top of the fuselage and the tailplane? Hmm. Thought not.

Well, we could more than double that weight of water if we look at those other areas.

Is a one-inch thick layer of ice outlandish – an exaggeration? Not really. This is a perfectly feasible thickness. But, even a quarter inch thick layer of ice over the whole aircraft will come to nearly a tonne Actually, around 785 kgs). This is, you will recall, only a small aeroplane we are considering. We could, if you have time, do the calculations on something a shade bigger – like a B747?

Very well. We’ve made the point.

Ice is very heavy.

The enemy of aircraft is weight. The greater the mass you have the more power/thrust you need to lift it into the air. If the total mass of aeroplane, fuel, passengers and luggage plus the ice is too great then the aircraft will not fly. The limit is in the books as the Maximum Permissible Take-Off Weight (MTOW). Of course, we cannot measure the weight of the ice which is why it is vitally important to get rid of it before take-off; it is also vitally important to get rid of it as close to take-off as possible to prevent a further build up if it is still snowing or there is freezing rain.

Then there are aerodynamics.

This hits the aeroplane in two distinct ways.

Firstly, the ice changes the shape of the wing and also it can create rough surfaces on the wing.

This is critical because the wing is designed specifically to produce sufficient lift at a certain forward speed to get 20,000lb of B737 (in our example) off the ground safely and smoothly.

Note: ‘safely’.

Changing the shape of the wing changes the lift characteristics of the aerofoil (cross-sectional shape of the wing) to the point where, in extreme cases, all lift is lost.

Certainly there is a major move of the airflow towards stalling (the point where the work done to lift the aircraft into the air reduces to the point where no work at all is being done.

While the stalling point is being approached the centre of lift – this is the point under the wing where the air is pushing the aeroplane up, is on the move. It is moving forwards.

Eventually it moves so far forward that the nose of the aircraft pitches up and airspeed reduces dramatically reducing lift even more and the aircraft now possesses the flight characteristics of, say, a cat.

The second consideration is that an ice build-up in the engines, especially, but not solely, in the intake, increases the mass of the aircraft and disrupts the flow of air into the engines.

Jet engines really, really, like a smooth flow of air going in. Anything that ripples or is in any way turbulent tends to disagree with the first bit of the engine which will now stall.

This is a bad thing.

The engine now produces less thrust at a time when the main thing that the aeroplane really needs is? Thrust!

The result of not de-icing a ‘plane before take-off and not switching on the anti-icing and de-icing systems is that the ‘Air Florida’ B737 lands in the Potomac River after crushing vehicles on the 14^th Street Bridge.

Ice.

A small word but a big force.

Comments, questions, stories

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LAF and BAF Part 3.

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

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.

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.