Wednesday, January 27, 2016


Air. Funny thing, air.
Once upon a time, during a practice oral examination for a gas turbine test, I asked a young lad what the diameter of a ball of air that weighed 10lb would be—roughly.
He looked at me like I had just fallen out of a tree and said, “Air doesn’t weigh anything.”
My turn to look askance.
“Suppose,” I asked him, “I drew a square on the sand down at the beach. A square with each side exactly one inch long. What would the air pressure be on that square?”
“15psi,” he said with supreme confidence.
“Correct,” I told him, “So that picture I have drawn is one square inch—what is the weight of air that is sitting on it?”
He was now bemused. He had no idea how to correlate between pressure and weight per area. Pressure is pressure. 15psi--pounds per square inch, at sea level (ISA SL = International Standard Atmosphere at Sea Level) is what you get; there is no way to rationalise between that and weight of air. Weight is a different thing entirely, it seems.

Another thing is humidity.
Humidity is a difficult thing to imagine. People often confuse ‘vapour’ with ‘steam’ but ‘vapour’ is gas whereas ‘steam’ is minute droplets of liquid.
Water vapour is gaseous water that merges with the atmosphere and can make the air ‘wet’. If the air is saturated with water vapour then, when you sweat (or ‘glow’, if you are one of our gentle female colleagues) it just runs off you and will not evaporate into the air around you.
Dry air is more comfortable for humans if you are from a dry part of the World.
To tell if your air is humid, or not, you will need a ‘wet and dry bulb’ thermometer. Then you look up the results on a ‘Relative Humidity Table’ (Appendix ‘C’, page 170, “A Simple Guide To Understanding Jet Engines”).
Mason's Psychrometer (Wet & Dry Bulb Thermometer)

RH (Relative Humidity) Table
Or a cold drink.
If you take a wonderfully cold drink of fresh, pure water that is slightly alkaline and, therefore, very good for you, pop a couple of ice cubes in it and sit the glass on the table. You will notice that there is condensation on the side of the glass. Or not.
In Yemen, where it is very, very humid, you will observe water running off the glass and then off the table rather in the manner that a tap will release water.
In Malaysia where it is less humid you will get a steady drip… drip… of water.
In West Africa where the air coming off the Sahara Desert is extremely dry there will be no condensation because the water is immediately evaporated off into the dry air. It will also be very dry in Antarctica but there the whole drink will turn to ice in a flash so it is hard to tell.
‘Do It Yourself’ humidity kit!

Another little thing is that irritating pressure problem we were talking about at the start.
The higher you go the less the air pressure around you is. Why? Because gravity is pulling the air down towards the surface of the Earth. The higher you go the less air is sitting above you to push down. Less weight above you—less pounds per square inch (in old money and I’m very old!).
Air Pressure (at International Standard Atmosphere) v. Altitude
Just recently I was flying back from Mauritania in West Africa to my home in Malaysia via Paris. At CDG (Charles de Gaulle) airport there was a change of aeroplane from the Airbus A330 of Air France to an Airbus A380 of SQ (Singapore Airlines).
The A380 cruised, eventually, at 40,000’ above sea level. That is high. Very high.
Just as we approached Singapore I put my plastic bottle of water on the armrest. The bottle was almost empty but I left a little to drink before disembarking.
When the aircraft sank to just below 10,000’ I took a photo of it.
Plastic Water Bottle After 30,000'+ Descent

Just goes to show what a difference 31,000’+ makes and remember—the pressure difference outside will be much greater, this is cabin pressure only.

Just a few things to think about when we say, “Just going out for a breath of fresh air…”

Saturday, January 9, 2016

Aerotoxic Syndrome

Firstly, my apologies for, perhaps, a lengthy ‘Blog’ this time. You will see why.

A question has winged its way over to me that set me back a little bit.
“Have you heard,” they asked, “about aerotoxic syndrome?”
The short answer to that is, “No. I have not.”
Sadly, easy questions tend to have difficult answers and so I need to go into a little detail here to sort this out for you.
The first thought was, ‘Is this another conspiracy theory that has been blown up out of all proportion?’
Some research seems in order here.

Sixteen years ago (1999) a group of chaps filed a report called ‘Aerotoxic Syndrome: Adverse health effects following exposure to jet oil mist during commercial flights’.
As a precursor to this, let us just review how the air gets into the cabin.
Ambient air—the air that surrounds the aeroplane in flight, is pushed into the engines because of forward speed. This is called ‘ram effect’. It is a good thing because the air is pre-pressurised before it gets to the compressor; it also makes the air ‘thicker’ for the fan to grip and push back.

Once the air gets into the compressor the air pressure is raised some more to make it suitable for combustion where we shall add energy to the air by burning fuel in it.
Now back up just a little bit.
When the air is passing through the compressor there are off takes of air called ‘parasitic bleeds’ that are used for several functions on the engine and the flight computer and also a large amount goes off to the air conditioning unit(s) on the aeroplane.
This is the bit the air comes from

The amount of air passing from the air conditioning machine into the cabin is controlled. Too much bleed from the engine and fuel burn increases because we should need more ‘throttle’ to increase the airflow into the engine.
Remember that the primary need for air into the engine is for burning (for thrust) and for cooling the engine. Providing air for the passengers is, in reality, a necessary evil as far as the engines are concerned—remember the term ‘parasitic air bleeds’?
For this reason most of the air in the passenger cabin is re-circulated with only a percentage (about 50%) being replaced on a continuous basis.
How is it replaced? By making sure that the flow into the cabin is greater than the flow being released from the cabin. This will, in turn, raise the pressure of the air in the cabin to an acceptable level for human survival.
Sorry again

Survival? At 36,000’ there is not really sufficient air for a human being to survive. You would pass out in seconds and die in minutes from lack of air and cold.

So that is how the air gets there.
What about this ‘oil mist’ situation?
The oil that is used in jet engines is not dug out of the ground. It is synthetic. It is born in a factory. This is because mineral oil—oil that is dug out of the ground, is incapable of withstanding the temperature differences and pressures that exist in a turbine (jet) engine.
Note: When you see advertisements for engine oils for your car it will show you how great it is, in a CGI perhaps, at protecting the engine with the pistons going up and down and flames bursting around the cylinder.
What is really needed is for the oil to protect your engine when it is not running; which is most of the time in your car. If the coating of oil runs off your car engine parts will rust. This is a bad thing.

Synthetic oil is thin to start with. When it gets hot it becomes even less viscous (it gets thinner). Keeping it where it is supposed to be is difficult.
Fortunately, those clever people at Rolls-Royce, for example, are very good at designing seals and sealing systems that will keep the oil where it is supposed to be.
But engines wear. Not much but over a period of time it can become significant.
To have an engine overhauled is expensive. Very expensive. We are talking in terms of millions of US dollars per engine.
Clearly the airlines will want to maximise the use of every engine before it gets sent back to the manufacturers, or an overhaul facility, for maintenance and servicing.

Because the air is tapped off from the compressor, the only sealing part of the engine that we need to worry about for the passengers is the bit at the front. This is called the ‘cold end’ of the engine.
The oil is re-circulated around the engine all the time while it is running. There are places where it gets really hot and the oil will partially break down. This is called a ‘pyrolised’ condition—a chemical alteration in the oil. The toxicity appears to be confined to the additives in the oil and the decomposition of the oil itself.
One of the primary checks the maintenance staff carry out every time the aeroplane is on the ground is to make sure there is no oil leak from the front of the engine—the nose cone and the fan cowl.
If oil is seen here then the engine comes off. No discussion necessary.

What is the likelihood of oil coming from the engine into the cabin? About 1% of the time. The possibility of the oil becoming hazardous is considerably less because stringent checks are carried out between each flight to prevent this from happening.
How do we know if there is a fume hazard in the cabin? Only the crew’s capacity to detect it is available at present. There are no detectors installed that might spot it—or other contaminants in the air.

Perhaps there is another cause for this aerotoxic condition that has not been looked at?
When the aircraft is at 36,000’ the air, in theory, should be relatively clear although certain gases are found at high altitude as a result of pollution. It is likely that these gases are a very small percentage of the total and so it is better to look at the flight envelope through the lower altitudes where industrial pollution is to be much higher.

Climbing out of, or entering, the Beijing area, for example, is likely to introduce toxic chemicals into the cabin because the incidence of industrial pollutants is much higher than in many other places. Beijing is not alone in having a pollutant rich air zone.

What are the symptoms?
            Fatigue – feeling exhausted, even after sleep
            Blurred or tunnel vision
            Shaking and tremors
            Loss of balance and vertigo
            Loss of consciousness
            Memory impairment
            Light-headedness, dizziness
            Confusion / cognitive problems
            Feeling intoxicated
            Breathing difficulties (shortness of breath)
            Tightness in chest
            Respiratory failure requiring oxygen
            Increased heart rate and palpitations
    Irritation of eyes, nose and upper airways.
Can we cure it? Given rest and recuperation time your body should recover but repeated flights under these conditions can lead to considerable neurological stress that may become chronic.

Are all aircraft susceptible to engine oil bleeding into the compressor? Early BAe 146’s and Boeing 757’s are poor but the new Boeing 787 is exempt because the cabin air systems do not get their air from the engines. It has a separate electrically driven system for air conditioning and pressurisation.

“Should I worry?” you might ask.
According to the CAA (Civil Aviation Authority) in the UK there are about 30 pilots grounded by the CAA’s Medical Department because of a ‘suggested association between illnesses and the cabin environment’.
But remember that these are people who are sitting in this environment all their professional lives.
Passengers will consume far less time in the air and thus the risk is proportionately smaller.
There is no point in wearing a mask, it will not help. Even the ‘drop-down masks’ on the aeroplane will not help. The only answer is to get off.

At 36,000’ that could be a problem!