Learning hub · Aircraft General Knowledge · Lesson 3
Fuel, oil, electrics and the propeller
An aeroplane engine needs three things from you before every flight: the right fuel with no water in it, enough oil, and a working electrical system. After that you need to understand what the propeller does with the power.
About 12 minutes, plus the quiz. Part of Aircraft General Knowledge. It builds on Piston engines, ignition and carburettor icing.
1. Fuel: grades and system
Most light piston aeroplanes burn AVGAS 100LL. It is dyed blue and has a distinct smell. Jet A-1 is a kerosene used by turbine engines. It is colourless or pale straw. Some diesel-engined light aeroplanes also use it, but only when their flight manual and fuel placard say so. A petrol engine given Jet A-1 can fail, and a car petrol must never be used unless the manual allows it. Always read the placard at the filler.
The system is simple. Fuel flows from the tanks through a selector valve, a filter and a pump, to the carburettor or the injection system. A high-wing aeroplane may feed by gravity, but most also have an engine-driven pump and an electric boost pump for start and for a failure of the main pump. The tanks have vents, so air can replace the fuel that leaves.
Fuel gauges are only indicators and may not be accurate. Always check the quantity by looking in the tanks (or with a dipstick, if your type has one) before flight, and keep a record of the time since the engine started.
2. Checking for contamination
Water is the main enemy. It gets in through condensation in part-empty tanks or through rain at a badly sealed cap. It is heavier than fuel, so it collects at the low points. A single drop in the wrong place can stop the engine.
At every pre-flight, drain a small sample from each sump and drain point into a clear tube or cup, and look at it. Water appears as a separate bubble or layer that does not mix, and it is clear against the blue of AVGAS. Look also for dirt, and for a wrong colour. If the fuel looks wrong, keep draining until it is clean, and ask for help if it is not. Check the vents, and that the caps are tight. After refuelling, let the fuel settle before you take a sample.
3. Oil
Oil lubricates moving parts, cools by carrying heat away, cleans by holding particles in suspension and seals the piston rings. In flight you watch two gauges: oil pressure and oil temperature.
On the ground check the level with the dipstick, with the engine stopped for a few minutes so the oil can drain back. The level should be between the marks, as the manual says. Too little oil can overheat the engine. Too much can be blown out through the breather. Use only the grade and type the manual lists.
4. The electrical system
Most light aeroplanes have a 12 or 24 volt direct-current system.
- The battery starts the engine and is a back-up if the generator fails.
- The alternator, driven by the engine, runs the system and charges the battery once the engine runs.
- A bus is a heavy conductor that supplies the individual circuits, such as lights, radios and pumps.
- Circuit breakers or fuses protect each circuit from overload by breaking the circuit. A tripped breaker is a sign of a fault, not something to push in again and again.
- The ammeter (or a load meter) shows whether the battery is charging or discharging.
The ignition magnetos work on their own and do not need the electrical system, so the engine keeps running if you lose electrical power. If the alternator fails, the battery supplies everything until it is flat, so switch off what you do not need and follow the flight manual.
5. The propeller
The propeller is a rotating wing: the blade makes a force by angle of attack. A fixed-pitch propeller has one blade angle, a compromise. A “climb” propeller has a finer angle and a “cruise” propeller a coarser one. The power you set is read on the RPM gauge (tachometer).
A constant-speed propeller has a governor that changes the blade angle to hold the RPM you choose. You have two levers. The throttle sets the manifold pressure (MP), which is the pressure in the inlet manifold and shows how much air the engine is taking in, and so how much power it can make. The propeller lever sets the RPM. In many aeroplanes, including in Europe, MP is shown in inches of mercury, so read the gauge.
The usual rule is to avoid high MP with low RPM, because the cylinder pressures are high. Follow the manual’s limits and power-setting tables, which override this general rule. Increase the RPM first, then the MP. Reduce MP first, then RPM.
Training material only. Fuel type, oil grade, limits and procedures differ for every type: use the flight manual and your school’s procedures. Treat every propeller as live: it can start if the ignition is on or a magneto is faulty.
6. Try it yourself
Work each one out on paper first, then open the answer to compare. The numbers are invented.
- You drain a sample from the sump of an aeroplane that uses AVGAS 100LL. You see a clear bubble at the bottom of a blue sample, and the bubble does not mix. What is it, and what do you do?
Show the answer
- Water is heavier than fuel and does not mix, so it sinks to the bottom as a clear bubble or layer.
- Keep draining the sump until the sample is clear blue and free of bubbles.
- Check other drain points and the cap seals. If you still find water, do not fly: ask an engineer.
- An invented aeroplane has 120 litres of usable fuel and burns 28 litres an hour. You want a 45-minute reserve on landing. How long can you plan to fly before you start the reserve?
Show the answer
- Total endurance is 120 ÷ 28 = about 4.29 hours, which is 4 h 17 min.
- Take off the reserve: 4 h 17 min − 45 min = 3 h 32 min.
- Plan to land with the reserve, so the flight, including taxi, should stay within about 3 h 30 min. Check your own reserve rules; 45 minutes is only the figure for this exercise.
- In cruise the ammeter shows a steady discharge and the low-voltage light comes on. What does that tell you, and what are the first actions?
Show the answer
- The alternator is no longer charging, so everything is running off the battery, which will go flat.
- Check the alternator breaker and switch, and follow the flight manual’s checklist.
- Switch off the equipment you do not need to save the battery, tell air traffic services if you are in contact, and plan to land at a suitable aerodrome. The magnetos keep the engine running.
7. Watch
A clear explanation of a constant-speed propeller. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.
Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.
More to watch
The parts of a light aeroplane’s electrical system. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.
Open on YouTube if the player does not load. Not made by VectorEFB.
8. Check yourself
Five original practice questions. Answer them, press Check answers and read each explanation.
Next: Airspeed markings, engine instruments and the compass. Back to the learning hub.