Plane & Pilot
Tuesday, June 4, 2013

Hot Starts

Fuel injection is a wonderful feature—most of the time. In hot-weather restarts, however…

For that very reason, make certain the engine cowling is opened up to the max before you leave the airplane. Assure that the cowl flaps are full down, and open any oil access or turbo inspection doors that are available. (The LoPresti cowling on my Mooney has two large doors on top of the cowl, one for oil and the second primarily for inspection and ground cooling on the opposite side. Together, they do an excellent job of removing hot air from beneath the cowl.)

If your cowling has "wing" doors that uncover the whole top left or right side of the engine (a la Bonanza), consider opening at least one of them, though both would be ideal. The problem may be in securing the doors, so they don't thrash the airplane if the wind comes up. Bungee cords often can handle the latter problem.
Fuel injection results in more even fuel delivery between cylinders, but it also can play havoc with hot starts, specifically because the pilot usually has full manual control of the start process.
Ambient temperature starts are usually no problem, even in the dog days of summer, but hot starts after shutdown can be challenging. They're most difficult for engines that have been cooking for periods from a half hour to three hours. After three hours, no matter what the outside air temp, most engines will have cooled enough to allow normal start procedures.

As every pilot who has read his pilot's handbook knows, the problem with fuel-injected engines in extreme high temps is that warm air rises, and the fuel delivery lines of many injected powerplants lie in the worst possible place, on top of the engine, often directly above the super-heated cylinder fins. This may be great for maintenance, but it only makes it easier for heat to boil the fuel in those lines as it settles at the top of the cowling. The temperature under a typically tight cowl may soar to 170 degrees F or higher, when the engine is shut down.

The characteristics of 100LL only exacerbate the problem. Avgas boils at a low 135 degrees F, and the very qualities that prevent vapor lock at high altitude tend to make fuel reluctant to vaporize easily at startup. When avgas turns to vapor in the fuel lines in high temperatures, fuel will begin to bubble and leave nothing but hot air in its place.

Once the engine is vapor locked, a pilot must wait until the fuel lines cool sufficiently to bring the gaseous avgas back into solution, or find another way to reintroduce raw fuel to the cylinders.

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