Get Primed and Ready

Aircraft Engine Priming

Engine priming

When summer weather arrives, modern automotive engine computers make adjustments based on temperature, air volume, and pressure sensors in the background to keep things running without thought or interruption, and give you slick starts and smooth revs even if you mistreat them.  Our piston AVGAS aircraft are another story, and this impact is the greatest and most notable when starting the engine.

Remember that aircraft checklists are for standard operations and cannot account for every variable.  Those adjustments for temperatures and pressures must be made by you, the pilot, when conditions change. 

The checklist recommended priming (number of strokes for carbureted, seconds of aux pump for fuel injected) work fine on a “cold” engine in mild to warm weather, but that same amount of fuel will be far too much for the engine to start easily when either the engine itself is warm, the ambient temperatures are warm, or both.  Most of our aircraft will start without priming if the plane has flown within the last four hours or so and the ambient temperatures are mild or warm.  If the plane has not flown that day but the ambient temperatures are hot, it is still likely to start with little or no prime.  In warm weather, if you omit priming and the engine doesn’t catch the first time, it’s easy to add a little fuel and try again (not the case in cold weather for a few important reasons, but that’s another story).

Why does it really matter if you’re able to start the plane eventually?  Primarily: Wear and tear on the aircraft.  An over-rich start can cause backfires, which is hard on engine parts and especially the aircraft exhaust because it is causing detonations of fuel in parts and at times not meant to happen.  Excessive fuel also washes oil off engine parts which can increase wear (another reason to lean as appropriate for all phases of operation). 

As with everything in life, there are exceptions, and it’s important to note that this information is most applicable to the aircraft we operate and their Lycoming engines.  There are piston engine aircraft that use slightly different fuel systems and require more fuel (six cylinder Continentals for instance) so as you fly other airframes you will need to adjust to their needs; but for 0-235 and I/O-360s and 320s, if you keep these things in mind, adjust your priming to what our Lycoming engines like and as the ambient conditions demand, you’ll experience more consistent starts and reduce required maintenance for those powerplants that keep you flying.

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