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practical terms, the air start is only really done after a practice engine shutdown for training. You might come up with some unusual precautionary shutdown or en- gine failure in flight scenario that can be rectified in flight, such as:
• You ran an auxiliary tank dry in an airplane with independently selectable aux tanks, and when the engine quit, you pressed rapidly through the engine failure checklist through shutdown and feathering— then remembered there was fuel in that wing’s main tank. So, you may try to restart the engine with that tank selected.
• You encounter airframe ice, and an engine’s induction system is blocked so that even the alternate induction air source is unable to run the engine. After entering warmer air that melts the ice, you restart the engine.
Even in these scenarios, you might think twice before attempting a restart because there is a hazard. To prevent propeller feathering when you shut down at the end of a flight, most propellers have a set of anti-feather locking pins. These engage and prevent propeller feathering dur- ing a normal shutdown with no air load on the propellers. For the pins to disengage, the propeller must usually be spinning more than about 600-800 rpm. If you try an air start but the attempt is unsuccessful, the prop may come out of feather but not spin fast enough to allow you to
re-feather the propeller. You’d be in a far worse situation than if you had kept the prop feathered and used your training and proficiency to land on one engine.
About the only time you’d perform the Air Start proce- dure (in my opinion) is after shutting down a perfectly good engine for training. Even then there’s the chance it might not restart, which is why I suggest doing training shutdowns at a good altitude close to a runway suitable for a single-engine landing.
Crossfeed
Most multiengine airplanes have fuel crossfeed lines, so the engine on the left wing may burn fuel from the right fuel tanks and vice versa. Crossfeed cannot transfer fuel from one wing’s tanks to the other’s; it can only direct fuel across the centerline to the engine on the other side. The purpose of crossfeeding is to extend the airplane’s range on one engine and to balance the airplane laterally when only one powerplant is burning off fuel.
In every twin AFM/POH I’ve seen, an airframe limita- tion (in Section II of the AFM/POH) tells us something to this effect: “The fuel crossfeed system is to be used dur- ing emergency conditions in level flight only.” The level flight stipulation is the important part; fuel unporting may occur in some flight attitudes while in crossfeed, and you wouldn’t want one or both engines to quit when you still have fuel available.
22 • TWIN & TURBINE / February 2024