I am forever enthralled by the Magic of Flight.
Fairy Dust and Magic Juice and Blue Sky and Fluffy Clouds and Puffin Winds are the enablers of real practical aviation that I commit to earn a living.
Lets talk about one of the ingredients and look at it in a little bit of detail without destroying its magical qualities. Yes, you guessed it correctly. I am talking about the Magic Juice, also known as petrol to non-aviator, land-lubber, gentle folks.
This complex chemical liquid is made up of really nasty stuff that I imagine are the modern equivalents of “witches blood, sacrificial virgins’ fingernails, rotten sheep entrails” and other such mephitic stuff.
Magic Juice is naturally found deep inside the Earth’s Crust, where monsters reside. And no, it is not made up of old dinosaur blood and bones, but far deeper by natural chemical reactions of the components of the Earth’s crust. (“fossil fuel” a term encouraged by John Rockefeller, who made a vast fortune by charging a high price for a seemingly limited supply of easy to use fuel)
Naptha, Benzene, Toluene, Xylene, Ethylbenzene and more than 100 other such horrible things that come out of the ground are found in petroleum.
Petrol, or Rock Oil (Petra = Rock; Oleum = Oil) as the all knowing ancient Greeks called the liquid which came out of the rocks and could be set alight, resulting in oily, sooty smoke and light and heat is now our main source of energy all over the world.
Many thousands of Aircraft still use the Aviation Gasoline, laced with carcinogenic Lead particles, because the piston engines are of very old design. Our modern engines used on Microlight and Light Sport Aircraft use the “unleaded” type of Petrol.
Imagine the three Witches, cackling over their cauldron, bubbling and troubling, emitting noxious fumes, our modern Refineries all over the world do the same.
The boiling and evaporation of the base “Rock Oil” allows petrol to be extracted by a process called fractional distillation.. akin to some other liquids that are so much more pleasant to imbibe or used to set ablaze certain after dinner delicacies.
Now the crux of my articles after all the above vaporising is that Petrol, the unleaded kind that we use in our modern Rotax engines has a shelf life of about 15 to 30 days. Most car petrol tanks are well sealed, but our aircraft tanks are well vented to cope with rapid pressure changes due to rapid altitude and temperature changes occurring during every flight. It is these well vented tanks that allow petrol to evaporate more rapidly than in cars.
When aircraft are not flown weekly, due to weather related events, or other reasons, the various highly volatile components of the complex chemical liquid evaporate at different rates, leaving heavier components behind.
British Petroleum’s fact sheet states that petrol should be used within 30 days when filled in equipment petrol tanks, i.e. aircraft, boats, mowers, etc.
In 5 weeks, petrol will evaporate and lose 15% of its volume, increasing its density from .75 to .79 and increasing its Octane rating from RON 98 to RON 99.5.
At the end of 5 weeks of evaporation, petrol is 5% heavier, increasing the Stoichiometric Ratio from 13:1 to 12.3:1, thus making the engine run rich and fouling the plugs and increasing fuel consumption.
The different rates of evaporation allow the lighter components to evaporate first, leaving behind the green colour dye, naptha, benzene, gum, and other chemicals, which normally aid in better combustion in the engines.
The residue shows a change of colour of unleaded petrol from very light green to a darker shade which looks like a good French wine or bovine urine.
The evaporation of the lighter chemicals from petrol change the composition such that the chemicals will attack natural rubber parts in the fuel system and in the carburettor, causing leaks and mis-firing or rough running of the engine, fouling of the spark plugs, blockage of fuel filters, especially paper element filters. The residual chemicals may also allow some algae to flourish and block the very fine mesh metal or nylon filters!
The good pre flight practice of draining fuel, from the bottom of the fuel tank, in a suitable clear tube will give you an indication of how old the petrol may be. (I have written in the past how to detect whether there is ethanol present in the fuel when doing a fuel drain test.)
If the aircraft has not flown for more than 30 days; you must check the fuel quality and either top up with fresh fuel to improve the octane number or to replace the entire fuel in the system, while disposing off the old petrol in an eco-friendly and safe manner.
If your aircraft runs on a mix of petrol and two-stroke oil, evaporation of the petrol will cause the mixture of oil and petrol ratio will certainly change from 1:50 to 1:40 or worse.
See photos of 3 month old petrol and fresh petrol to compare……
11 March 2020.