Partial Power Failures

The one time I suffered an engine failure, it was partial, not total. One cylinder’s intake valve spontaneously disassembled itself—a neat trick for a one-piece component—leaving the other five jugs to continue merrily along, albeit with some major vibration and a distinct power loss. I had plenty of altitude and enough engine power to reach a nearby 9000-foot-long runway after a couple of 360-degree turns to lose altitude. (Pro tip: Keep as much altitude as you can as long as you can until descending for the runway in such a situation.) The airplane was even able to taxi off the runway and to the ramp under its own power. Regardless, we were done flying that day. 

Of course, one pilot’s partial power failure is another’s partial failure. In this instance, pieces of the former valve spread throughout the intake system and into a balance tube, and acted as a flapper valve, interfering with the airflow to the cylinders on that side of the engine. It was kind of like the old joke about a car’s turn signal. “Yeah, it’s on; Wait, no it isn’t. Okay, now it’s back on again....”

The point is that engine failures, like many other airborne emergencies, are analog, not digital, as contributor J.R. Warmkessel once put it in these pages. They have nuance, are dynamic and can easily change their nature, rather than being on/off, true/false.

Unlike fine wine, engine failures rarely get better with time, however. Having some power available, but not all of it, can lure us into a false sense of security and may lead us to take risks we might not otherwise embrace if the power failure were total. This applies to both singles and twins with a sick engine. But just as our reactions to a total engine failure in a single will be different if we’re flying a twin, a partial engine failure also should be handled differently if we have another engine flying with us in close formation.

Figuring out if you have a partial power failure is a lot like Justice Potter Stewart’s definition of obscenity: you’ll know it when you see it. It may be the engine won’t develop more than some lower rpm and/or manifold pressure. It may be running rough and making power, but not enough to consider diverting. A turbocharged engine may only develop ambient manifold pressure, which is a sure sign the turbo has failed (hot exhaust gases may be blowing on combustible material) and means you probably want to get on the ground sooner instead of later. 

Your initial reaction to a partial engine failure aboard a single should be identical to a total power stoppage: troubleshoot the problem. It could be something simple, like carb ice, it could be a clogged fuel tank vent that switching tanks will fix. It could be the loose nut behind the yoke mismanaged the engine controls. If there was something you did a few seconds before the partial failure, undo it.

Your second reaction—which actually should be part of the initial one—should be to establish the best engine-out glide airspeed and ensure the airplane is aerodynamically clean: gear and flaps up, cowl flaps closed. If there is enough power remaining to produce a slight climb at best-glide, accept the opportunity to gain additional potential energy by adding a few feet to your altitude.

Once you’ve got the airplane stabilized but are unable to resolve the partial power loss, it’s time to start getting this thing on the ground. If you’re talking to ATC, you should go ahead and declare the emergency. The engine might be producing enough power to stay at altitude, or even climb, but there’s no way you can count on that continuing.

However you do it, you need a runway: You can query ATC and take vectors, you can hit the nearest button on your nav box or look out the window. Or all three. This is when I remind you to look out of both sides of the airplane, and even behind you—wasn’t there a runway you flew over a couple of minutes ago?

Keep in mind that your engine could easily get worse or fail completely while you limp to the nearest suitable runway. So maybe not choose an airport over the next hill, or the other side of a lake, okay?

What comes next as you near your divert airport depends on geography and a classic geometry exercise, plus some planning and luck. The last thing you want to do is make a normal descent and landing. Instead, fly to the airport at altitude before descending. From that perch, you can eyeball the airport and then execute a circling descent to both lose altitude and set up for the approach and landing.

There’s a longstanding admonition against turning or flying away from the divert airport with a sick/dead engine. But if you have enough altitude, you have the energy and flexibility to plan and execute a traffic pattern that puts you on final approach in a position from which to land. You’ll have to turn away from the runway—briefly—to get some distance and perspective for your approach. But you can always keep the runway in sight while you’re doing this, just as you would when shooting touch-and-goes.

My training for such landings called for establishing a “key point” roughly where a close-in base-leg turn would be made. I want to hit that point in space at about 1500 feet agl and immediately turn to the base leg, all the while keeping the runway in sight. Both flaps and gear stay retracted until the runway is made, and only then do you add drag and land.

I was reluctant to reduce power in my little episode. As a result, we floated about a third of the way down the 9000-foot runway before touching down. I had plenty of pavement that time, but next time I likely won’t be so lucky. Knowing when to reduce power in this situation is one of those pilot skills that can’t really be taught—you just feel it. It can be practiced, though.

Ironically, partial powerplant failure involving a multi-engine airplane can be much riskier than with a single. One reason is because the twin offers you more options, which in turn can get you into more trouble as you try to stretch the flight to the next big airport, and nurse along the ailing engine—hey, it’s putting out some power, right?

Unless you’re facing extreme circumstances and need that minimal extra power above and beyond what the good engine is providing, you might be better off to shut down the bad one and proceed on the good one. Why? One reason is you don’t know why the bad one is ailing. There could be a fuel leak needing just a little bit more encouragement to become a fire. Regardless, it easily could go ahead and fail at any minute. That could come as you turn final into the dead engine.

The temptation is to leave it running for the minimal power it’s putting out, in part because you may be having some difficulty accepting that you have a sick engine and making the mental leap from two-engine operation to only one.

Another reason to secure that bad engine is the topic of a recent NTSB Safety Alert, summarized in the sidebar below. Basically, if the engine is allowed below 1000 rpm or so, start locks may engage, preventing the propeller from feathering. And feathering the bad engine’s prop is a must to achieve the best single-engine rate of climb.

To close this out, keep in mind a lot of the foregoing presumes decent daytime VMC. If you’re in the clag on a dark and stormy night, there may be little you can do to reach a runway with a partial engine failure in a single. That’s a risk we all take when flying singles at night or in poor weather. The good news in this scenario is you at least have some power, which gives you time, which gives you options. To maximize both, stay as high as you can as long as you can.