Mogas, in or out?

• The Aircraft Owners and Pilots Association in Australia is funding a special research project by aviation expert Bill Abbott into the feasibility, safety and other aspects of using mogas in many light aircraft powerplants.

Bill has already researched the question in countries including the US and Great Britain, according to an AOPA spokesman. He has also prepared and presented papers on the question, in Australia.

• Aircraft insurance companies in Australia say unofficially that mogas IS being used by clients.

They have not yet been presented with claims by clients who have used mogas in their light aircraft.

• Transport Australia has not budged from its official policy stance in 1979, when it declared it could not approve the use of mogas, even on a restricted basis.

However, Transport Australia is understood to have expressed guarded interest in the AOPA’s special project.

• Meanwhile, in both Britain and the US there have been successful tests on light aircraft piston engines fed with mogas.

• And in America at least one leading supplier has already predicted that his company plans to make two kinds of fuel – one for cars, boats and small aircraft, the other for turbine-powered types.

Without wishing in any way to be mischievous, we present the Transport Australia statement (issued as an Airworthiness Advisory Circular in June 1979 – No. 110) and add some UK researchers’ findings, knowing that there are differences in production standards and climatic temperatures, etc.

The Transport Australia AAC goes to commendable lengths to explain the situation as the Department views it. This is the main content of that document:

. . . For safety reasons, the Department cannot approve the use of mogas in aircraft even on a restricted basis.

The manufacturers of aircraft piston engines are adamant that their warranties apply only when their engines are operated on avgas of at least the minimum knock rating (expressed as Octane or Performance Number) specified in the relevant type certificates and service documents.

Quite apart from its knock rating and vapour pressure, which will be discussed later, it is important to realise that mogas differ from avgas in being produced to much wider specifications allowing for considerable variability in chemical composition and physical properties.

It follows that the mogas marketed in Australia show significant variation in characteristics from refinery to refinery, from State to State and even from time to time at the same location.

In some States the maximum lead content of mogas is controlled by government regulation for environmental reasons; in others, it is not.

Available figures show that the actual lead content of standard (regular) grade motor spirit can vary from less than 0. 1 to as much as 0. 7 grams of lead per litre, and that of super (premium) grade motor spirit from about 0. 3 to more than 0. 8 gm/L, the highest levels generally being observed in South Australia, Queensland and Western Australia.

Comparison of these figures with the official specification maxima of 0. 14 gm/L for avgas 80/87 and 0. 84 gm/L for avgas 100/130shows Australian mogas is not, in general, a low-lead product as often assumed.

Publicized trials on selected light aircraft in the US are understood to involve unleaded mogas of a type not available in this country.

In avgas, the lead is added in the form of tetraethyl lead (TEL) with just the chemically correct proportion of ethylene dibromide (EDB) to act as a scavenging agent. Mogas may include tetramethyl lead (TML) in addition to TEL and has 50% more scavenging agent comprising a mixture of EDB and ethylene dichloride (EOG).

The excess of scavenging agents, especially in the chloride form, could well lead to accelerated corrosion of components of an aircraft engine not designed for such a fuel.

Depending on its source, mogas is likely to be less chemically stable and have a greater propensity for gum formation than avgas. It may also contain more sulphur (another possible cause of corrosion), and a higher proportion of aromatic hydrocarbon compounds to contribute to unwanted carbon formation and possible deleterious effects on rubber seals etc. in the engine and aircraft fuel system.

Mogas usually contain additives that are not approved for avgas and whose possible effects on aircraft engines are consequently unknown. It has also been postulated that the wider distillation range of mogas – e. g. final boiling point about 50°C higher than for avgas – could cause mixture distribution problems between the different cylinders of an aircraft engine.

Although it is possible that some of the potential problems arising from the above-mentioned characteristics of mogas could be monitored and controlled by special maintenance procedures in the form of regular, frequent “top end” inspections, more frequent oil changes etc. , it is a different matter in the case of the basic properties of knock rating and vapour pressure where any inability to match the requirements of the engine, or aircraft fuel system, could result in a sudden emergency situation.

It is important to note that avgas is given a dual knock rating – e. g. 80/87 or 100/130 – in which the first figure represents a lean mixture rating determined by what is commonly referred to as the “Motor” method, while the second figure represents a special rich mixture, high power rating established by the “Supercharge” method.

Mogas, on the other hand, is given only a single, lean mixture rating measured by the so-called “Research” method.

Although comparative testing of samples of mogas by the Motor method has shown that there is no direct, consistent correlation with results obtained by the Research method; it is clear that, as a rough rule of thumb, about 10 points should be subtracted from a high octane number.

Research rating in order to obtain a reasonable estimate of the equivalent lean mixture Motor rating used for avgas.

It will therefore be seen that even super grade mogas with their nominal Research rating of 98 octane would be quite inadequate for an aircraft engine rated on avgas 91/96 or 100/130, even for weak mixture cruise.

For take-off or high-power climb, one would certainly expect it to result in serious detonation, especially at high ambient air temperatures.

At the same time, super grade mogas could probably satisfy the anti-knock requirements of an aircraft engine rated on avgas 80/87, both in lean and rich mixture, though the other objections to the use of this fuel remain.

Standard grade mogas, with a nominal Research rating of 89 octane, could perhaps marginally meet the weak mixture requirement for avgas 80/87, but would almost certainly fail the rich mixture Supercharge test for that fuel.

Indeed, this would be consistent with certain recent overseas accident reports that attribute partial power failure during take-off to the unauthorized use of mogas in various light single-engined aircraft types.

Standard mogas would clearly have the sufficient anti-knock capability for the smaller, older aircraft engines originally certificated on avgas 73, which has no separate rich mixture rating, but here again the other objections still apply.

The Reid vapour pressure (RVP) of avgas is required to lie in the range of 5. 5. to 7. 0 pounds per square inch (38 – 48 kPa) and engine and aircraft fuel systems are designed, tested and certificated on that basis. It is understood that the RVP of locally-produced avgas is normally around 6 psi.

The RVP of Australia mogas ranges from approximately 8 to 12 psi (55 – 83 kPa), being held to a maximum of about 10 psi throughout the year in the northern areas of the continent and during the summer months in the south.

The difference in volatility and vapour pressure between avgas and mogas can be highly significant in relation to the risk of vapour lock.

Aircraft fuel systems are not designed to cope with large volumes of vapour and may be especially susceptible to this problem when climbing to altitude with warm mogas in the fuel tanks.

Although simple gravity systems fed by high-wing tanks may be somewhat less sensitive in this regard, there would be quite serious risks for fuel systems using pumps drawing from low-mounted fuel tanks, even at low altitudes and during ground operations, and particularly under hot weather conditions.

It is also thought that the light hydrocarbon fractions present in mogas would cause an increased tendency for carburettor icing in any aircraft engine at all susceptible to that problem.

Carburettor icing is by no means unknown in motor vehicles, especially on a cool morning with any visible moisture present in the atmosphere.

The standard and purity of avgas is closely controlled by official specifications, supported by the fuel companies’ aviation quality control and distribution systems, providing reliable assurance that the product is fully satisfactory at the point of delivery to the customer.

Mogas, of course, is not subject to nearly the same degree of control in manufacturing and subsequent handling, and anyone intending to use such fuel in aircraft would accordingly have to accept full responsibility for verifying its identity, condition, cleanliness and freedom from extraneous contamination.

The Departmental AAC ends by stating that: . . .

It will be appreciated that the Department is in no position to approve the use of mogas in aircraft, even to help ease the difficult situation created by the current shortage of avgas.

In the UK, flight tests to FL140 using an Airedale spanning more than 200 hours running on what they call four-star over there, has not only passed airworthiness standards but encountered far less lead fouling.

The British CAA (Civil Aviation Authority) has reportedly seen for itself the results of bench-tests on a Lycoming 0-320 160 hp motor. The tests were entirely satisfactory and further tests are being made using a Tomahawk’s 115 hp Lycoming 0-235.

There HAVE been some snags, of course. Hot starting after long heat soak periods (hot to UK standards, about 20°C, and a mite higher). Carby icing has on occasions shown itself, but as the UK testers pointed out this can be controlled by a light aircraft’s engine’s current standard hot-air systems.

For the quality control pundits, the UK pro-mogas lobby has a ready answer. Most motorists have driven half a million miles by the time they reach the mid-50s, and most of them have never encountered contaminated fuel. And anyway, most light aircraft have fuel drain checks. . .

A significant number of British insurance underwriters have permitted some light aircraft to fly on mogas under their existing cover, adds the UK report.

And about 100 glider tugs operated by the British Gliding Association gulp mogas. British operators have bench-tested a 260 hp Lycoming 0-540.

One of the surprising revelations in the British report was the claim that the vital Reid vapour pressure requirement of 7 psi was added to specifications for aero-engines by mistake – when the researchers thought carby icing onset was fuel vapour lock trouble!

The UK pro-mogas viewpoint is that the use of premium (super) motor spirit could save about $8 to $12 an hour in direct costs to operators of average two-four seat light aircraft.

Using ballpark figures, AUSTRALIAN AVIATION has arrived at a slightly less optimistic Australian figure: around $4/hr taking the difference between 100 octane available at Bankstown and super coming out of an average service station tank.

The annual saving would be around $2000 with a conservative 10 hrs/ week utilisation – a very low figure.

We hope to continue this article in our next issue. AUSTRALIAN AVIATION intends to contact industry suppliers, aeronautical researchers and, where possible and keeping anonymity, actual users of mogas, in a sincere effort to arrive at some concrete and worthwhile conclusion.

It would appear at this stage that a tradition born 50 years ago when aircraft used high-grade motor spirit is being kept alive with insufficient justification.

After all, as one British source points out, today’s light aircraft piston powerplants are relatively unsophisticated compared to some car motors; a standard Jaguar V12 has been developed to put out 71 bhp per litre, a Lycoming 0-360 produces 31 bph/litre – and although it might not be truly relevant, an old de Havilland Gypsy produces 21 bhp/litre!

Makes one think. . .

Next Issue …

Just some of the exciting feature articles in our next edition include –

• Airtests of two great twins, Gulfstream rapid 1000 a n d Aerospatiale’s new AS.355F twin Squirrel helicopter.

• The first Battle of Britain was fought a quarter century before the one that is commonly remembered. Read how the Germans successfully employed mammoth Zeppelins and large multiengine bombers against England in the First World War. This story WILL amaze ·you!

• Roy Braybrook visits Fokker to discover what makes one of the world’s oldest aircraft manufacturers the success it is today.

• Part II of our Radio Band Directory shows you how to monitor international aircraft VHF frequencies. If the thought of tuning into VHF has ever fascinated ‘you then this is the series you have been waiting for.

• John Vogel shares a day in the life of a typical DC-9 operation. See what makes this fascinating aeroplane tick as Ansett farewells one of its most reliable aircraft types.

• Not many readers remember when East-West used Lockheed Hudsons on their rural NSW network, read all about this fascinating aircraft in our multi-part series, ‘In Australian Service’. These are just a few of the exciting features in the JUNE edition of Australian Aviation don’t miss it!