Transport Canada has approved the use of automotive gasoline (mogas) in specific categories of aircraft, subject to certain limitations. Every pilot who contemplates the use of mogas should read Transport Canada’s Use of Automotive Gasoline (MOGAS) in Aviation.
There are three considerations regarding mogas: the use of mogas is not generally supported by engine manufacturers; mogas is not engineered for aviation purposes; and, in using mogas, the pilot assumes sole responsibility for quality (and therefore liability) associated with its use.
Transport Canada basically followed the process of formal approval established by the US FAA which issued Supplementary Type Certificates (STCs) for specific engines and aircraft; while these were recognized by Transport Canada, it imposed altitude and temperature restrictions in view of the unique characteristics of Canadian mogas—i.e., Canadian Aircraft were restricted to flight below 6000’, and at temperatures below 24°C. This restriction, however, has since been removed for certain category aircraft. As well, for aircraft categories, Transport Canada now provides blanket approval, meaning that STCs are not required for these aircraft.
- Material Compatibility. Transport Canada warns that mogas may be associated with the deterioration of rubbers and plastics in aircraft fuel systems.
- Alcohol. Fuels containing alcohol (methanol or ethanol) other than de-icing fluids are not permitted for aircraft use, owing to the manner in which alcohol attacks rubber and plastic components in the fuel systems. Importantly, fuel manufacturers need not indicate when alcohol is present in automobile fuel. Manual alcohol testing procedures undertaken by the pilot must therefore be used. As a means of monitoring elastomers (natural or synthetic rubbers or plastics), Transport Canada additionally recommends frequent inspection of the o-rings found in fuel sump drains—the pilot should look for o-ring blockage when the drains are open, and o-ring leakage when the drains are closed. As well, the fuel filter should be checked frequently for particulate originating from deteriorated elastomers.
- Carburettor Icing. Because mogas has higher volatility than aviation fuels, mogas absorbs more heat during air/fuel mixture process and is therefore subject to greater cooling during vaporization—the result being that ice accumulates at higher ambient temperatures, making the likelihood of carburettor icing higher while flying with mogas.
- Vapour Lock. Again, because of the increased volatility of mogas, there is increased probability of experiencing vapour lock whereby mogas vaporises in fuel lines. This is especially common in instances of shutting down aircraft immediately after running at full operating temperatures; the fuel in the lines adjacent to the engine become “heat soaked” and vaporization occurred. Full fuel flow should be verified prior to takeoff and effort to reduce engine temperature by reducing throttle should be used if vapour lock is encountered in flight. Automotive fuel varies in volatility owing to the four seasonal grades that are produced (Aviation fuel has only one volatility grade), and special consideration should be given to avoiding winter grade fuels which have increased volatility, and therefore increased risk of vapour lock, as well as carburettor icing.
- Filtering. Transport Canada recommends that all mogas be filtered using a 5-micron filter/separator, or finer; the filter should also have a “go/no-go” system, which responds to the presence of water contamination by shutting off. Makeshift filters, such as a chamois or felt material, should only be used in emergencies, owing to the possibility of fibres clogging the fuel system.
- Cross-contamination. Unlike aviation fuel, mogas is not transferred using dedicated fuel lines; there is therefore increased risk of fuel cross-contamination whereby two fuels are mixed. Transport Canada recommends that mogas vendors perform fuel-testing procedures referred to as the “Clear and Bright”, “Free Water,” and “Density” tests.
- Electrostatic Discharge. Transport Canada recommends that the fuel storage container should be bonded by wire to the tank being fuelled. It notes that the hazard of fire increases substantially with the use of plastic containers, which cannot be bonded to the aircraft. Transport Canada writes: “Many accident reports have revealed that an arc was created when the [plastic jerry] can was pulled away at the end of the pour following sufficient charge accumulation. By this time the tanks are likely full and the results can be lethal.”
- Of the aircraft receiving blanket mogas approval, the most common is the Cessna 150 using a Continental O-200 engine; aircraft not in this category (so called “Category 4”) require an engine and airframe STC.
- To qualify, the aircraft must be approved for operating 80/87 fuel.
- With all aircraft using mogas, the engine log must contain a complete record of all hours on mogas, including a record of fuel mixtures (avgas and mogas); the aircraft Journey Log shall be annotated “Mogas may be used based on the acceptance of TP 10737”. Placards should be installed at each fuel filler and in the cockpit; the flight envelope (temperature and altitude) should also be placarded.