Technical Articles - As published in The Blue

There are several areas where the composition and control of Mogas differs from Avgas and this can result in safety concerns with Mogas even if the rules and restrictions are observed.

The main areas of difference for Avgas and Mogas are:

• Less regulation of which constituents are allowed when making the fuel.
  - Mogas is allowed to contain a number of components which are known to be a problem with aircraft systems, or can even reduce power. One such item is alcohol which can be aggressive to seals, carburettor components, fuel tank linings etc. This can lead to leaks, engine failure due to poor mixture control and it has even been the documented cause of an in-flight fires. There is also no assurance that one supply source of fuel will have a consistent composition, so always buying from one forecourt is no guarantee that the fuel’s composition is always the same.

• Mogas routinely changes composition and properties between Winter and Summer
  - Winter grade Mogas is more volatile, which is intended to assist with engine starting in cold weather. This means that it has a lower initial boiling point and will form a vapour more easily; a problem with aircraft fuel systems as a bubble of vapour in the fuel line will prevent fuel from flowing and cause engine failure. Why is this a problem for aircraft and not cars? The significant difference here is that cars don’t get up to flight levels. The reduction in pressure with altitude causes vapours to form more easily even at moderate temperatures. This can be compounded by the fact that aircraft tend to be used less frequently meaning that there is a greater risk of winter grade Mogas still being in the tank on high temperature spring and summer days, making the problem of vapour lock more of a risk.

• Mogas is not designed to be stored for long periods.
  - Within the automotive world Mogas is generally burned within a few weeks of production so storage stability is not a concern. However, if kept for longer periods, Mogas can form sticky lacquers and gums that have the potential to result in fuel injector or carburettor malfunctions. The composition of Avgas is much more tightly controlled, allowing fuel to be kept for months without deterioration – significant in aviation as it is not uncommon for an aircraft to be in the hangar for several months with fuel remaining in the tank.

• Mogas does not have the same quality and handling restrictions.
  - Avgas quality is guaranteed by the use of dedicated manufacturing and storage vessels, road tankers that are only used for Avgas transportation, and dedicated airfield storage. Furthermore, water is removed and the fuel tested throughout the delivery system, almost totally eliminating the risk of contamination. Upon delivery to the aircraft Avgas is cleaned using filters so fine that they would be capable of separating bacteria from the fuel – the result is that you receive very clean, dry, on specification fuel.
  None of these quality restrictions are in place for the Mogas supply chain, in which there are numerous areas where there is the chance for cross-contamination. These range from the obvious, such as non-dedicated vehicles, to the less obvious, such as distributing Mogas large distances by pumping it down multi-product pipelines along which other products, ranging from diesel fuel to heating oil, are also being moved.
  Remember there is equivalent to the edge of the road at 3,000 feet.

There is one more fundamental difference where Avgas out-performs Mogas – that of Octane rating.

Octane is the measurement of a fuel’s detonation resistance. All fuels will automatically combust before the flame reaches it if the temperatures and pressures are high enough in the yet-unburned gas. This leads to explosive combustion of the remaining fuel in a phenomenon known as detonation.

Severe detonation leads to rapid and uncontained temperature and pressure increases in a combustion chamber that can destroy an engine a few seconds. Octane rating is a measure of how resistant a fuel is to detonation; the higher the octane rating, the more the fuel / air mixture can be compressed without detonation happening.

To make this clear, octane rating is not a measure of the amount energy in the fuel but is a measure of its resistance to detonation. The advantage or higher octane fuels is that a higher compression ratio or supercharging ratio can be used, which then leads to a higher volumetric efficiency within the engine, which in turn means more power output for a given fuel burn.

There are several things that determine the octane demand of an engine (in other words things that encourage the fuel to detonate in a given engine). These are numerous, but include: compression ratio, supercharger pressure, inlet temperature, cylinder wall temperature, ignition timing, load (or power setting), engine speed, cylinder bore etc.

In aviation, we generally have many factors that combine to result in high octane demand when compared to automotive engines: low engine speed, air cooled (high cylinder wall temperature), large cylinder bore, supercharged with no intercooler (high pressure and high temperature inlet air), fixed ignition timing with magnetos and relatively high average power settings.

Once we know the octane demand of the engine, how do we measure the performance of the fuel? The octane performance of a fuel is measured in the lab using single cylinder test engine, known as a CFR engine, in which the combustion chamber volume can be varied, therefore altering the compression ratio. However complication arises when it is realise that the way in which this engine is operated -with varying temperature, load, speed etc. – can result in different types of response and this leads to differing octane numbers for a given fuel.

Road fuels tend to be measured on a Research Octane Number (RON) scale, in which the test engine is run at low cylinder temperature and low load. This is intended to replicate a liquid-cooled car engine on the open road and so is commonly used as a reference point for Mogas fuels. Aviation fuels are measured using the more demanding Motor Octane (MON) method, which uses higher engine temperatures and high load conditions and higher speeds in the test.

As the MON test is more demanding, typical gasoline fuels have lower MON than RON values. Unleaded Mogas fuels tend to be controlled in the range 95 - 98 RON but can range anywhere from 82 - 90 MON. Avgas is measured on Lean Mixture (similar to MON). The Lean Mixture rating has a specification minimum of 99.5 octane (15 octane higher than the comparable 85 MON typical of unleaded Mogas), but in practice has actual production values around 104, so has considerably better detonation resistance than Mogas.

But for low octane demand engines, surely it is OK to use a lower octane fuel? This is the thought behind the Mogas STCs. For low-powered aircraft, Petersen Aviation Inc in the USA (and others, such as the EAA) have a test program agreed with the FAA that qualifies a given engine / aircraft combination to run on unleaded Mogas.

These approval methods have also been accepted by other aviation authorities, such as those in Europe and Australia and involve running the engine in the aircraft on the ground and looking for detonation problems and then flying the aircraft to altitude whilst burning a typical Mogas and monitoring for signs of vapour lock.

Significantly, these tests accept the engine conditions that are seen during the testing and do not look at the whole operating envelope of the engine. Now we come to the problem: engines don’t just operate at the normal condition of CHT and ground fuels changes seasonally. As part of their Research and Development programs, Shell Aviation has a low octane demand Lycoming engine - one apparently suitable for use with unleaded Mogas - on a test stand.

During some related combustion study work, Shell conducted some tests within the operating envelope of this engine, but with higher than normal cylinder head temperatures (CHTs). In spite of using unleaded fuels covered by the typical STCs, the engine suffered detonation when operating within the operating envelope of the engine, but at higher than normal CHTs. Reducing the CHTs to a more typical level allowed the fuel to work throughout the power curve, which is why many of the aircraft do not experience problems under the Mogas approval program, but it is proof that even high octane Mogas does not give an adequate detonation boundary throughout the operating envelope of the engine in all engines where Mogas is legally allowed.

It is not too hard to imagine a scenario: a hot day with a quick stop for fuel, a long hold on the ground, or a rapid turn-around all would allow the engine to heat up to higher than average CHTs. Shell’s engine bench testing work shows that having Mogas in the tanks at this point represents a much greater risk than choosing to use Avgas and adds weight to the position by the oil companies and engine manufacturers that aircraft engines should use aircraft fuel.

The additional problem of vapour-lock is also a risk with the higher volatility “winter grade” gasoline when used out of season or on a warm day. This is possible on a warm spring day or even if an aircraft has been stored for several months, having last been fueled during the supply window for winter grade Mogas.

If an aircraft is fuelled using a winter grade automotive gasoline, and is subsequently flown with this fuel on a warm day, then the higher ambient temperature combined with the reduction in pressure at altitude can easily cause vapour lock.
The situation can be even worse if the fuel system is of poor design. If a fuel delivery line passes close to some hot part of the engine then vapour is even more likely to form, especially if the fuel flow is low.

The scenario here is that the engine is at low power prior to take off and the vapour lock forms. The engine will keep on running until the fuel is used in the carburettor float bowl - probably enough to allow the aircraft to take off - and then the engine will fail. This has happened before with pilots using automotive gasoline and is a documented killer.

Several approval bodies, such as the UK CAA, put limits on the operational use of Mogas for these reason – in the UK case this is a fuel tank temperature of 20 deg C max and an altitude restriction of 6,000 ft.

However, even if it is observed, it is impractical for a user to control, or even know the tank temperature, which can be 15 degrees higher than the ambient air temperature on a sunny day: the pilot has no means of measuring the fuel temperature and is often tempted to take risks to get home in the face of no other practicable options.

Alcohol
Interestingly the STCs to use Mogas are an approval only by the licensing authorities (Civil or Federal Aviation Authorities); the oil companies, engine manufacturers and airframe manufacturers generally do not approve or endorse its use. Shell has never supported of the use of Mogas in aviation applications and our concerns essentially centre on the fact that the product is poorly defined and controlled for the demands of aviation.

One aspect of this, namely the ability for Mogas to contain alcohol, is even recognised as a problem within the licensing authorities and Mogas supplies containing alcohol are specifically excluded from the approval of the STCs and therefore must not be used.

Why would this be the case? On the face of it alcohols might be seen as being a beneficial component; after all it is a bio-fuel derived from plant material and so should make the fuel have a lower carbon footprint, making our flying more ‘green’. Surely this is a good thing?

In fact, in a parallel process, work continues on a number of different technologies as a potential replacement for leaded Avgas. One of the possible alternatives that the Aviation Authorities are considering is an aviation grade ethanol, with the ultimate intention being to define a specification that would allow Type Certification of new aircraft to operate on such a fuel.  However, this is distinct and separate from the STC process for approving Mogas in Aviation and, for ethanol-based fuels, there are still many technical barriers yet to be overcome.

One of the primary concerns associated with alcohol in aircraft fuel is that it can be aggressive to the elastomers, seals and diaphragms used in the fuel systems on aircraft, causing them to fail; of course the fuel systems, from tank to engine, have been originally designed to use Avgas. The presence of alcohol does not only adversely affect fuel tank linings, but also components within the carburettor or fuel injection system, potentially causing them to fail.

When using alcohol as a blending component in unleaded Mogas, one of the other significant issues centres on the fact that, as we all know from diluting whisky with water at the bar, alcohol and water mix. Alcohol is used not just to increase the amount of bio-component in gasoline, it also contributes to the octane performance of the finished product.

Combining water with fuels that contain alcohol will tend to remove the alcohol into the water phase from where it will separate and be drained from the fuel. The removal of the alcohol in this way decreases the octane availability in the remaining fuel, and potentially takes it below the octane requirement of the engine. As we have covered in past issues of Technical Talk, having a fuel that has too low an octane rating can lead to catastrophic engine failure and remember Aviation engines generally have a much higher octane demand than an equivalent automotive engine, due to their design.

So why would water cause a problem in aviation fuel tanks and not with ground fuel use? In aviation we fly at altitude where the air pressure and temperature is relatively low, which does two things: first of all the airframe, and fuel tank become cold and secondly, as the aircraft descends, the increasing pressure forces warmer, moist air into the fuel tank where the water vapour condenses onto the fuel tank and results in water in the fuel.

Of course there are also problems of utilisation – it is not infrequent for aircraft to be left unused for weeks or even months, during which time condensation, and even rain water, can accumulate in aircraft fuel tanks. These mechanisms contribute to the reason why we conduct daily water drains from fuel tanks in aviation whilst it is seldom a problem in automotive use.

The material compatibility and potential for water to remove a high-octane component from the fuel are not the only concerns. There are several other issues with alcohol-containing fuels when used in aviation applications such as their tendency to promote carburettor icing due to the high latent heat of evaporation of alcohol and as a result of all of these factors it has been decided that alcohol-containing, unleaded gasoline fuels are unsuitable for aviation use.

For these reasons pilots who have authority to use Mogas are obliged, as part of the STC approval, to test each Mogas fuelling for the presence of alcohol prior to use. I should point out that most Mogas specs, such as EN228 used in Europe, already allow the use of alcohol in the formulation without needing to declare it to the customer.

This means that the fuel supplier can change the components in the fuel without notice and the only way a pilot can be aware whether or not a fuel contains alcohol is to test it. A simple way for determining the presence of alcohol in fuel is to pour approximately 10% water, followed by 90% fuel into a clear test cylinder. At this point the meniscus between the two products should be marked. The mixture should then be shaken thoroughly and allowed to settle. If any apparent increase in the water volume is observed, then it is an indication that the fuel contains alcohol and should not be used in your aircraft.

Of course many pilots who choose to use Mogas increase their risk by not even considering whether or not the fuel they are using might contain alcohol. Of course they should be aware of this fact at all times as it is one of the specific operating constraints of the approval to use Mogas; without testing, pilots are not only potentially operating outside of their approval, but also exposing themselves to uncontrolled flight safety risks.

So why have I decided to highlight the particular issue of alcohol in Mogas now?

No doubt you will be aware that there is a lot of pressure on reducing greenhouse gas emissions throughout all areas of the energy sector. For this reason, many governments have issued either legislation or targets to include bio-components into ground fuels thereby reducing the percentage of fossil fuel content and reducing the net contribution to global warming.

This means that in many regions, including Australia, USA and Europe, there will be an increasing bio fuel component in ground fuels. For diesel fuel, this generally means the inclusion of Fatty Acid Methyl Esters (FAMEs), but in gasoline it means an increasing use of the alcohol ethanol, generally derived from either sugar cane or corn.

In some countries, such as Sweden, the standard Mogas supplies already routinely contain alcohol and in other countries it may be that all premium grades currently contain alcohol; however, you should be aware that, in most developed economies, the conversion is underway with different suppliers moving towards the inclusion of bio-components at different rates.

This is a gradual, but increasingly common practice, normally introduced to a given market through the use of limited scale local trials, but the inclusion of bio-components will escalate resulting in the fact that in the next few years most of the Mogas supplies available in Europe, USA and Australia will contain alcohol.

This rate of change and amount of alcohol is different within different countries, but you can be sure of one thing – it is coming. Even in the USA, the 2007 Federal State of the Union Address committed the USA to a 20% reduction of greenhouse gas emissions from the use of ground transportation over the next 10 years, the decrease coming predominantly coming from the increased use of ethanol as a blending component in gasoline.

This has predictions for an almost 10 fold increase in alcohol use in Mogas over the next 10 years in the USA – to a predicted 135 billion litres of ethanol by 2017.

In Europe and Australia, the targets are focussed on much shorter timeframes: in the next 3 years the target for Europe is to have 5.75% of bio components in ground fuels and in parts of Australia the mandate is for 10% by 2011.

Of course this is positive news from an environmental viewpoint, but it will mean that the sources of Mogas approved by the current aviation STCs will become increasingly scarce and what is the norm at the moment - of being able to find alcohol-free Mogas at most forecourt filling stations - will become a rarity.

This might be a good time for users to re-evaluate the balance of risks of using Mogas. Even if accepting of the increase in risk with using Mogas the introduction of bio-fuels also means that this is a time to be aware of the increasing importance and imperative to be continually testing for the presence of alcohol for Mogas users, even if only buying from a single forecourt source.

Of course, although generally more expensive, the alternative is to use Avgas; Avgas is not permitted, by specification, to contain any alcohol and furthermore it is formulated, stored, handled and subjected to rigorous quality assurance procedures that is have been developed purely with the safety of aviation in mind.

Many users accuse the oil companies of protectionism and profiteering by not supporting the use of Mogas, but in fact the concerns are driven purely by safety; Mogas may work in some engines much of the time, but it does not mean that the product will not change without notice and neither does it mean that it is suitable throughout the operating envelope of either the engine or aircraft.

The truth is that Avgas is expensive simply because it is expensive to manufacture; the fact is that a refinery would typically make more money by utilising the facilities to produce the larger volume demand unleaded motor gasoline rather than a niche product that ties up facilities that cannot be used for unleaded grades and for a grade that is typically less than 0.25% of the Mogas volume in a given market. This is why most refineries don’t make Avgas: it simply makes no economic sense.

As for margin, in fact the vast majority of the retail margin is taken by the airfield reseller and it typically forms the major part of their income, in many cases it being the difference between an general aviation airfield existing or not.

In summary, the pilot may see the advantage to Mogas use as being the price, but the true cost is the risk involved in using an unsuitable and uncontrolled product in an unforgiving environment.

The aircraft that I am referred to in this article are principally the common Category A light aircraft – which includes Pipers, Cessnas etc. – rather than aircraft in the microlight and ULM categories. Certain small microlight / ULM engines, such as Rotax and their like, seem to operate well on a diet of unleaded fuel but, whilst this prevents the problem of lead fouling, some of the compositional concerns outlined in this article still apply, and should be considered if deciding to use Mogas.