Mastery of Flight: When Would You…?

Flying a twin, you have a lot of capabilities that your friends in single-engine airplanes do not have. You also have a lot more choices to make. Most of the additional options are a direct result of having that second engine. Let’s take a look at some of the unique options presented to the multiengine pilot and when you might actually use them.

Precautionary shutdown
One option available to the pilot of a twin that is not usually considered an alternative for the single-engine flyer is a precautionary shutdown. A precautionary shutdown means just that—shutting one of the engines down before it might fail. You might do this for one of two reasons:

1. To prevent or minimize damage to an engine as a result of indications that suggest additional damage may occur, and/or
2. To make the engine failure happen on your terms instead of just as you’re turning inbound on the approach or entering the traffic pattern.

Personally, I think reason #2 is at least as important as the first reason, at least from a safety standpoint.

So, when would you perform a precautionary shutdown? The most likely reason is a loss of oil pressure combined with an increase in oil temperature (which confirms the oil loss), especially if there is a visible oil leak on the cowling of the engine in question. Whereas in a single-engine airplane, similar indications would suggest reducing power, staying high (for greater glide radius if the engine seizes) and aiming for the nearest suitable airport; in the twin, you can shut down the ailing engine, transition to single-engine flight, then fly an almost-normal descent, approach and landing on one engine at the nearest good option…including flight to an airport with better weather within the one-engine range of the fuel left on board if conditions at closer airports aren’t good.

Other possibilities include a surging propeller, a propeller overspeed, or a severe vibration that suggests a propeller problem that could quickly lead to blade separation. Vibration could also be the result of a mechanical failure of the engine itself or an engine mount issue, with the possibility the vibration could tear the engine off its mounts. A strong vibration might also break hoses or fuel lines and lead to a fire. So, any wild vibration of a propeller or engine is grounds for a precautionary shutdown. You certainly would not want to land trying to manage this vibration when it could impair control or suddenly develop into a catastrophic condition close to the ground.

The condition need not be dramatic for you to at least consider a precautionary shutdown. If something just isn’t working right with an engine or propeller, you might decide to shut it down and feather the propeller to prevent damage or at least additional damage. This is less obvious than the other scenarios and may open you up to criticism if it turns out the condition wasn’t as bad as you thought. But don’t listen to the nay-sayers. It may be hard to determine exactly how bad something is in the air. It’s always better to troubleshoot on the ground, so a precautionary engine shutdown is an option.

So, oil loss, overspeed, vibration, or something else prompts you to perform a precautionary shutdown. Just perform the Precautionary Engine Shutdown checklist, right? Except many Airplane Flight Manuals or Pilot’s Operating Handbooks (AFMs/POHs) do not provide one. We can, however, co-opt the Engine Fire in Flight checklist for this purpose. The Beech Baron 58 Engine Fire in Flight checklist, for example, reads:

In Flight
Shut down the affected engine according to the following procedure and land immediately. Follow the applicable single-engine procedures in this section.

1. Fuel Selector Valve – OFF
2. Mixture Control – IDLE CUTOFF
3. Propeller – FEATHER
4. Fuel boost pump – OFF
5. Magneto/Start Switch – OFF
6. Alternator Switch – OFF

If all you do from memory is to shut off the fuel selector, pull the mixture control and feather the propeller (being very careful to pull the correct prop handle), you can transition to and trim for single-engine flight, then reference the checklist to “clean up” by performing the remaining checklist steps.

Air Start
On the other end of the engine spectrum, from an inflight precautionary shutdown, is an air start. That is, taking an engine you shut down for some reason (or that shut down on its own) and firing it back up. For that, most AFMs/POHs do have a checklist in the Emergency Procedures section. For our example Baron 58, this checklist directs:

That’s a fairly complex procedure. The good news is that none of it must be done from memory. If you’re performing an air start, you’re already under control in single-engine flight. When you’re ready, pull out the checklist, review it before actually performing checklist steps, and then read a step and do a step until the engine has restarted.

So when would you perform an air start? The key is in that caution at the top of the checklist: only if you know why the engine was shut down in the first place. More importantly, only if you know the engine should start back up and run properly.

This means you would only perform an air start if you had shut the engine down yourself for something other than a known or suspected mechanical reason. In
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 engine 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 during 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 procedure (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 limitation (in Section II of the AFM/POH) tells us something to this effect: “The fuel crossfeed system is to be used during 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.

For our example Baron 58, the ONE ENGINE INOPERATIVE OPERATION ON CROSSFEED checklist starts with a note repeating that limitation, then says:

There is a similar checklist for when the right engine is inoperative.

Often, the AFM/POH gives us a checklist for getting the engine into crossfeed, but it does not provide guidance for getting out of it. I teach the exit by starting at the bottom of the checklist for the inoperative engine and working your way back up in reverse order, i.e., starting at step 4, then steps 3, 2, and 1, turning the boost pump on LOW at the beginning and OFF at the end. Talk to type-experienced instructors, or look at the AFM/POH for the twin you fly and see what works best in that type. The good news is that, whether entering or exiting crossfeed, none of this is so time-critical that you must do anything from memory. Pull out the checklist, review what you’re going to do before you do it, then perform the procedure step by step.

So when would you use crossfeed? For almost all of us, almost never. If you lose an engine anywhere in the continental United States, there will almost always be a suitable airport within one-engine range using that engine’s normal main fuel tank. Pilots flying in the Australian Outback or someone ferrying a twin across the ocean or remote parts of Africa or Asia, Canada, Alaska, or South America may have different circumstances and a need to extend the airplane’s range or balance the fuel load before landing. It’s possible even in the mainland U.S., you were at just a bit more than an hour of fuel remaining (total for both sides) and just about to land when an engine quit, and to get to better weather, you decide to divert to a nearby airport. In this case, you might choose to operate in crossfeed for a short while to avoid a very low fuel level on the main tank for descent and landing. But that seems much less likely, at least to me.

If you do use crossfeed, use it in level flight. Before you begin descent, exit crossfeed and use the main tank on the same side as the running engine to avoid unporting and interruption of fuel flow to the one engine that is still earning its keep. All this, of course, unless otherwise directed in the AFM/POH for the airplane you fly.

Flying a multiengine airplane gives you options unavailable to the single-engine pilot. It also requires you to consider whether, when and how to execute those options.