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’.
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!