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Aircraft Performance

Logging Rate of Consumption

Each year a number of aircraft accidents are related to starvation, exhaustion or contamination. There were numerous reasons for these avoidable accidents: ranging from inadequate fuel systems knowledge by the crew, preflight planning issues, takeoff and landing checks and failing to monitor consumption during flight. Or even failing to refuel the correct quantity before the flight.

During the flight fuel will be consumed from the tanks selected by the pilot. Keeping an eye on the rate of consumption, leaning the engine and changing tanks are the subject of this page.

Fuel log keeping

Maintaining an accurate log during any flight is mandatory. Monitoring the consumption rate in combination with the fuel gauge readings is an important task of the pilot in command. Making this a part of the cruise checklist is crucial, it will keep you from forgetting to change tanks when needed and thus creating an unbalance in the wings (in case of light aircraft with two or more wing tanks) or even trying to fly on an empty tank.

From the beginning of the flight you will have recorded the engine start time, aircraft takeoff time, selected the correct tank and determine a 'land by time'. After 30 minutes or so in the flight, change tanks (in case of two wing tanks) take note of the time and tank, keep on doing this every hour afterwards, this way fuel imbalance is no more than 30 minutes of flight.

Deduct the fuel used from this tank and note that on the log. This creates a running total (in minutes, liters or gallons) which should be kept for all tanks on board. This will give you a good idea of how much fuel is left in each tank at any time during the flight.

Fuel gauge readings should also be recorded on this log. Any problems with the fuel gauges can be noted as well. If for any reason your cockpit workload is too high (inexperienced pilots are easily overloaded in busy airspace and when weather plays a role), remember the overall fuel quantity and keep an eye on the 'land by time'. Just make sure that you are on the ground before this time passes.

Changing tanks

Whenever you are changing tanks in flight, the electric fuel pump should be switched on just before switching tanks and left on for a short period of time afterwards, 30 seconds should be enough. This is done so that any air in the fuel line from the other tank is purged.

Keep your hand near the fuel pump switch when switching it off. If the engine RPM drops for any reason drops you can switch the pump back on immediately. At this point you are quite certain that the engine mechanical pump has failed.
Never run a tank dry as that could introduce air into the fuel lines. Some engines have trouble restarting when air gets into the system. And never (only when you really need to) change a tank when over stretches of water, inhospitable terrain etc. Emergency landings could be really difficult.

Monitoring consumption

Aircraft Fuel Cap

An accurate fuel log should be kept by the pilot and checked after the first landing to verify the actual fuel level with the calculated consumption. This is even more important when hiring aircraft, as consumption can and will vary between the same type of aircraft and engines due to pilot technique.
Pilot who regularly fly their own aircraft have a will current history of the actual consumption of their aircraft, those who hire usually have not.

It must be remembered that a fuel log alone should not be relied upon. Variables as a loose fuel cap, leaking drains, higher consumption due to turbulence and not so perfect leaning techniques can result in higher fuel usage.

Checking total consumption

After completing the flight make it a habit of checking the fuel level in the tanks. This will give you a good indication of the actual consumption and will be a reference for the upcoming flights. At the same time you can verify the fuel gauges for accuracy.

Normally the aircraft should be topped off after the last flight of the day to minimize entry of water due to condensation in the fuel system. However, this may create a problem for the next pilot as this could mean a weight & balance problem and fuel needs to be drained from the aircraft. Try to determine if the aircraft needs full tanks for the next flight before topping off.

Leaving a topped off aircraft in the sun creates a temperature rise and a possible fuel spill from the tanks, fire could result.

Engine leaning

Lycoming Engine Leaning

Not leaning an engine during cruise could mean that your still air range is about 15-20% less with some engine types. Most aircraft engines can be leaned when power is set 75% or below and at any altitude. And not only when cruising higher than 3000 feet, which is teached at some flying schools.

By leaning, the engine runs more fuel efficient and with higher temperatures, the only requirement is that you keep an eye on the EGT and or fuel flow gauges. You do not want to burn out the exhaust valves prematurely. For aircraft with fixed pitch propellers and without EGT indicator engine leaning can be done by RPM only: just pull, gently, the mixture knob until you reach max RPM. If you pull to far the RPM will drop quite suddenly. Just put it back in and you are fine, no worries.

Written by EAI.

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