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97 Octane Super Unleaded

Since the replacement of regular E5 Mogas at the pumps by E10, for the time being at least, Mogas users have an alternative option in the form of 97 RON Super Unleaded fuels. 97 RON Super Unleaded fuel supplied by garage forecourts is NOT necessarily ethanol-free, but its ethanol content will be capped at a maximum of 5%. 97 RON E5 Super Unleaded fuel can be used under the LAA process for E5 Mogas, as before. 


There’s currently no legal requirement for ethanol to be present in super unleaded petrol and in fact, quite a few members have reported that their tests have shown Super Unleaded fuel to be ethanol-free in their areas. Esso reports that “Synergy Supreme+ 99 is actually ethanol free (except, due to technical supply reasons, in Devon, Cornwall, North Wales, North England and Scotland). Legislation requires us to place E5 labels on pumps that dispense unleaded petrol with ‘up to 5% ethanol’, including those that contain no ethanol, which is why we display them on our Synergy Supreme+ 99 pumps”. Shell, BP, Tesco and other fuel suppliers don’t commit to such detail, merely stating their fuels “may contain up to 5% ethanol”. 


Of course, many LAA members have been using Mogas of one sort or another for many years. For many airstrip users, the prospect of having to go to an airfield to fill up with Avgas would seem at best very off-putting, and at worst totally impractical for their style of operation. Hopefully Super Unleaded will continue to be an option using the LAA’s E5 procedure. Where 97 octane E5 super unleaded can be found that’s ethanol-free, it can be used in the broader range of LAA aircraft with engines eligible for use with E0 unleaded fuel. But it is important that Mogas users take on board the fact that unlike with aviation fuels, automotive fuel specifications generally only describe the properties of the fuel blend, in terms of octane rating and other characteristics, they don’t specify the chemical make-up of the fuel. 


Apart from specifying broad maxima and minima for those components that the powers-that-be wish to either encourage or discourage, the specifications leave to the fuel supplier the choice of how to blend the fuel to achieve the described properties. In some cases it’s the relatively small amounts of additives that cause compatibility issues, rather than the main components of the fuel that cause the problems – and the mix of additives may vary from one batch to the next. 


With Mogas of any type, what you get from the pump one week may be different to what you get the week after, or got the week before. The fact that your fuel system pump diaphragms, seals, composite tanks and so on might have been unaffected by Mogas up till now does not guarantee that they will be OK with the blend that the next tanker-load brings to your garage forecourt. 


Unfortunately there is no simple answer to this, and the only way to mitigate this problem is by constant vigilance, and thoughtful adaption of your maintenance schedule to check for signs of problems developing before they create a safety issue – more frequent checking of filters, changing of fuel lines etc. Signs to watch out for are swelling of rubber components like diaphragms, fuel valve seals and O rings, fuel pipes hardening or developing surface cracks, fuel tank sealants wrinkling or detaching from the internal surfaces of fuel tanks. Varnishes on cork floats may go gummy, or plastic carburettor floats absorb fuel and lose buoyancy, causing a rich mixture and rough running. Watch out for discolouration of the fuel you take out as fuel samples, which may imply something dissolving somewhere in the system, and for corrosion in the bases of your carb float bowls and gascolator. 


Avoid letting Mogas go stale in your fuel system – drain it out before a long period of disuse. Don’t leave the tanks empty for a long period – better to fill them with Avgas which will reduce condensation, and also, particularly with plastic tanks, prevent the tank’s internals drying out which can cause problems with shrinkage, and in extreme cases, the tanks splitting open. O rings and other rubber parts are best kept submerged in fuel continuously. We’ve seen cases where composite aircraft skins have rippled apparently due to exposure to the vapours created by the break-down products in stale Mogas, after being left unattended over a single winter. Mogas is blended with the expectation that in a car it will be used within a few weeks of being supplied, so it doesn’t need to be as stable over a long period of time as an aviation fuel. 


If a composite or plastic fuel tank is built into your aircraft, consider carefully whether you want to run the risk of having to replace it should the tank deteriorate with Mogas exposure, with all that that implies. With an integral tank in a wet-wing Jabiru aircraft, the answer should be certainly not – but even with the embedded polyethylene fuel tank in a Europa’s fuselage, changing the fuel tank is not an operation for the faint-hearted, involving cutting out quite large parts of the fibreglass cockpit module to release the tank, and then scarfing them back again afterwards in-situ. The Europa kit was first produced in the era of four-star Mogas, a very different blend to what we find at the pumps today. 


Ethanol-proof rubber hoses are available. In particular SAE J30/R9 or the European equivalent DIN 7339 D3 are automotive hose standards that are widely available from auto factors, and should be used in preference to the older SAEJ30/R6 standard hose which is more permeable to fuel vapour and will more quickly harden and crack, particularly using fuel containing ethanol. The SAE spec J30/R14 is a lower pressure, more pliable version for carburettor systems. Watch out for cheap imitations – the real McCoy should come from a reputable manufacturer, be marked with the SAE number along its length and will likely cost at least £10 a metre. 


What can we do to help the situation ? It may be that off-the-shelf drop-in-the-tank additives can be used to mitigate some of the problems with mogas fuel, in particular, stabilisers and anti-corrosion products, just as they are routinely in the marine and race-car world. Not knowing what’s in them, we recommending steering clear of any additives that claim to boost the fuel’s octane. Definitely avoid additives that claim to actively enhance an engine’s power or fuel economy, other than just to restore proper performance by giving the carburettor jets and orifices an occasional clean-up. 


For most aircraft engines UL91 Avgas remains the ideal fuel, in that it is a tightly-controlled aviation-grade fuel of a guaranteed composition, blended for long-term stability and optimal volatility for aviation use. As it is in effect the familiar 100LL Avgas but supplied without its tetraethyl lead, any fuel system designed for 100LL Avgas will be unaffected by using UL91 fuel, or, if circumstances demand, by a mix of 100LL and UL91. Despite a pervasive mis-conception, UL91 fuel is NOT Mogas and suffers from none of the issues associated with Mogas use in aircraft. A list of engines suitable for use with UL91 fuel can be downloaded from the ‘operating and maintaining an LAA Aircraft’ page of the LAA’s new website. Moves are afoot to increase the number of airfields supplying UL91 fuel, with full LAA backing. 

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