From 1st September 2021, the government mandated that all regular grade unleaded petrol must contain up to 10% ethanol, rather than up to 5% as previously. In the autumn of 2021, most filling stations around the UK re-marked their E5 petrol pumps to show that they now supply E10 specification fuel, and started receiving tanker-loads blended with up to 10 percent ethanol content.
For the automotive world this means reduced carbon dioxide emissions, which is better for the environment, and, arguably in terms of eco-desirability, a small further reduction in the reliance on fossil fuel. The automotive industry had been preparing for this change, and people with road vehicles registered after 2002 should be able to switch to E10 without noticing any difference.
Unfortunately, the additional ethanol content is bad news for aviation users if we were to ignore the change and start putting E10 in our aircraft regardless. Ethanol is a powerful chemical solvent which can attack components including rubberised gaskets and fuel pipes, older lacquered carburettor floats and composite or plastic components in some newer fuel systems. The doubling of the concentration of ethanol in E10 compared to E5 makes it much more likely that problems will be experienced if these parts are not designed to be ethanol-proof.
Ethanol also has an affinity for absorbing water, which over time can then become acidic, attacking metal components in the fuel system and engine. Even if there was a practical way to do so in the volumes of fuel we use in our aircraft, we cannot safely remove the ethanol content from the fuel because the ethanol acts as an octane enhancer, so the washed-out fuel would be left with a reduced octane value, likely to cause detonation problems. Another issue is that ethanol-blended fuel has a lower energy density than petrol, so to develop full power from our engines using an E10 fuel, we may need to richen the fuel/air mixture, for example using a bigger carburettor jet size.
Owners of Rotax and Jabiru engines are probably already aware that many of these engines are supplied as being able to use E10 fuel – though with Jabiru engines, in particular, it’s far from straightforward because some of the earlier engines require cylinder head modifications and reduced compression ratio if they are to avoid detonation issues when using any form of Mogas. Jabiru also say that Mogas of any type should not be used in commercial flight schools and only at the owners own risk.
Even with a supposedly E10-compatable engine, the chemical compatibility problems with fuel system components are such that the LAA does not at present approve the use of E10 Mogas in any LAA amateur-built or vintage aircraft. For factory-built microlights and factory-built gyroplanes, where LAA is not the approving authority but only renews the CAA’s Permits to Fly, owners need to refer to the TADS for the types for details of the approved fuel types, and monitor the service bulletins from the approved manufacturers for news of any updates.
Where we need to be particularly careful in reading-across from (hopefully) a trouble-free transition to E10 in automotive use is the big difference between our petrol cars and the way our aircraft engines are configured. Our cars generally have submerged fuel pumps in their petrol tanks and a sealed fuel system. In our aircraft we have an open-vented tank and usually a fuel pump several feet away, often mounted in a hot area of the engine bay near to the engine exhaust, dragging the fuel through a fairly convoluted pipework system, a filter and fuel selector – all features encouraging a vapour lock - and then to make matters worse we want to climb up to altitude and operate in reduced atmospheric pressure.
Modern petrol cars have a fuel injection systems rather than carburettors, a circulating fuel system designed to purge any vapour forming in the fuel line, and an ECU that monitors the engine’s parameters constantly and adjusts the fuel mixture strength and ignition timing to prevent damage to the engine – and if all else fails and the engine should ‘pink’, we can hear it from the driver’s seat and drop a gear to lighten the load. Because of the much higher background noise level in our aircraft, detonation cannot be heard and the pilot’s first indication of a problem may be when the first piston crown disintegrates, or a valve head departs its stem.
In conclusion - E10 is presently not approved for use in any LAA aircraft. ‘Hoping for the best’ and using E10 fuel in your aircraft regardless could risk ruining the fuel system components, fibreglass tanks falling to bits, engine failure through contamination of the fuel or ruptured fuel pump diaphragms etc, or more serious engine damage.
For most of our engines, UL91 Avgas is the best choice, but 97 octane E5 Super Unleaded remains an alternative to the now-obsolete E5 spec Mogas.