Things can go “bump” in the night. Daytime, too. Most of them either have been considered before or encountered by someone, resulting in a section of your AFM/POH labeled “Emergency Procedures.”
But not everything that can happen is covered there: Pilots are constantly inventing new ways to screw up, and the aircraft themselves can present something new and different, or at least something that appears new and different, especially as they age. The bad news is there’s no checklist to cover everything that can go wrong. The good news? Some portion or combination of checklists might get you through. The better news? A solid understanding of how the aircraft’s systems work and their various failure modes provides you an excellent chance to get home.
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Partial Engine Failure
Let’s start with engine failures of the partial kind, insofar as they involve a gasoline-fueled piston single (twin drivers have a bunch of other options). Your AFM/POH no doubt has a section covering complete engine failure, remedial actions and the procedures to follow to execute an emergency, off-field landing. But what if the engine is running, just not very well?
A partial engine failure can have many different causes. There are cylinder failures, induction-system blockages, fuel-flow problems and ignition-system faults. You have tools in the cockpit with which to tackle these issues by addressing the three things all engines need: fuel, air and spark. (Diesel aircraft engines don’t need the spark, but they may need some electrical power to run their electronic control system.) The only other kind of issue an engine is likely to suffer is mechanical, which rarely can be fixed from the cockpit. But if it involves fuel, air or spark, we might be able to fix it.
Electrical issues with the airplanes most of us fly begin and end with the magnetos, harness and spark plugs. A rough-running engine could be the result of fouled plugs, but this usually shows up in the pre-takeoff mag checks. Regardless, an in-flight mag check is appropriate if you suspect an ignition problem.
To perform one in cruise, leave the other engine controls alone, then do what you do during the pre-takeoff run-up, with one exception: If the engine dies on one mag, or you inadvertently find the “off” position, do not immediately switch back to both or to the good mag. However tempting/reflexive it may be, doing so risks a potentially damaging backfire from the accumulated air/fuel mixture in the intake system. Instead, reduce throttle to idle before switching the mags back to the loud position, then bring up the power.
During that mag check, and throughout any attempts to diagnose an engine problem, pay close attention to what your engine monitor is telling you (You do have one, right?). For instance, a well-running piston engine should exhibit an EGT rise on all cylinders after switching from both mags to only one. If one or more cylinders’ EGT drops out, there’s your problem. Proceed on the good mag. If nothing happens to the EGTs when performing this check, the switch may be defective.
Inadequate airflow to a rough-running engine also may be diagnosed from the cockpit. It can be caused by a blocked/collapsed intake or filter, airframe icing at the intake or good old-fashioned carb ice. Apply carburetor heat or alternate air, depending on the installation. If that doesn’t work, but you’re convinced the problem is insufficient airflow due to a blockage, our previous admonition about backfires no longer applies: Creating one as previously described may clear the blockage, but you’re risking damage.
Fuel-related issues probably are the most common source of partial engine failures, after mechanicals. Here, also, you have some tools. If you have it, monitor the fuel-flow/pressure instrumentation throughout your diagnostic efforts.
The first and most obvious action is to switch tanks, even if you’re running on “both.” Since the problem may involve one tank’s plumbing or venting system, see if the engine will run better on another tank, even if it’s not the fullest.
Engage the electric fuel pump, if you have one. Some electric pumps are single-speed (on/off); some are two-speed. Try the low setting first, then the high one if things don’t get better. Be aware that using an electric fuel pump can flood the engine with too much fuel, temporarily making the problem worse until you lean the mixture to compensate. If the problem resolves itself at the new mixture setting, you’ve likely experienced an engine-driven fuel pump failure.
Finally, don’t forget the engine controls: Pushing and/or pulling on the throttle, mixture and prop control may help you find a sweet spot that keeps the engine running, or fixes the problem, long enough to get to a runway. Which control to try first is a toss-up, though, perhaps depending on what the symptoms are. For example, a sudden vibration not reflected on the engine monitor could mean the prop is the problem. On the other hand, a gradual loss of power could result from either of the three control linkages having failed.
For a generic, not-enough-power kind of problem, we’d probably try the mixture first, enrichening it from leaned to ensure there’s enough fuel flowing. If it’s already full rich, lean it some to see what happens. The throttle would be next, running it up or down a bit to see what, if anything, changes. If none of that helps, use the prop control to find the best rpm setting, one that smooths out any roughness.
Other kinds of partial engine failures usually involve a mechanical issue, probably involving cylinders—a swallowed or stuck valve, for example. The one time we saw a swallowed valve up close and personal, it was an intake valve, the pieces of which bounced around in only one side of the induction system, creating all kinds of havoc. This was depicted on the engine monitor by wildly fluctuating EGTs for the affected three cylinders but normal indications for the other three. There was enough power remaining to get the airplane onto the nearest runway without additional damage.
Control System
Many pilots may find at least one control system-related failure checklist in their AFM/POH, perhaps involving a jammed pitch control. Few AFM/POHs, if any, get into what to do about a failure where the cables or pushrods have failed but the surface remains attached to the airframe. Presuming there’s no outright break in continuity from the cockpit to the control surface, something’s binding. If it comes on suddenly, as opposed to progressively stiffer ailerons, for example, there are a couple of likely causes.
One potential cause is frozen water impinging on the linkage (cable/pushrod) or the surface itself. Obviously, any water present in the system can’t freeze if it’s too warm outside, so descend to warmer air and wait a few minutes, then try the control again. Use reduced power to descend if the stuck surface is your pitch control. Another potential cause can be a cable that’s jumped off its pulley. That usually doesn’t happen unless the pulley mount itself fails or the cables have much more slack in them than specified.
A third cause can be a nearby wiring bundle or other obstruction interfering with full and free control movement. This usually happens behind the instrument panel and, if that area can be accessed in flight and it’s safe to do so, you might give it a shot. Another cause can involve something—or someone—interfering with the controls. Is your right-seat passenger resting his/her feet on them? Is there an obstruction underneath one of the pedals? Is a passenger interfering with the yoke’s full movement?
A final control-system malfunction involves a split-flap condition: One of them is up; the other is down, creating a rolling moment you may or may not be able to counter with aileron alone, especially at slow speeds. Hopefully, the fix for this is simple: retract the flaps, raising the one that extended and leave them alone until engine shutdown. If one won’t retract and the other one will, extend the working flap to balance the rolling moment. Split-flap conditions are rare, but do happen. Preventing them from happening at the worst possible time is a good reason not to extend the flaps while in a bank.
Partial Gear Extension
In many popular airplanes, it may not be possible to determine from the cockpit alone if all three landing gear legs are extended. Some models—early Bonanzas and Barons, for example, may have only a single light to indicate the gear is down, with a mechanical indicator linked to the nosewheel leg. With these aircraft, all the single light really signifies is the gear motor has run to its set limit; whether the gear extended and is in the down-and-locked position may depend on whether the linkage is intact. It’s basically the same for the nosewheel indicator.
Other aircraft have three-position gear indicator lights illuminating when the respective gear-leg switches are activated as the gear is extended. You can reasonably be assured all three legs are down and locked if you see the three lights. But what if one of them isn’t illuminated?
First, check the light itself for proper seating in its socket. Also check the panel lighting controls to ensure the light can be seen in bright sunlight. Some older designs mount their bulbs in a dimmable socket: twist it to open or close an internal shade. You won’t be the first to discover the indication resulted from a dimmed bulb and everything is fine. Of course, the bulb itself can fail. That fix is easy: Exchange the suspect bulb with a known good one; most types allow this. If not, you do carry spare bulbs, right?
Finally comes the Big Kahuna of potential gear problems: What to do if one leg remains completely or partially retracted and the other two are down? Well, at some point soon, you’re going to land. The airplane will be damaged during the landing. You need to accept both realities. Your job is to minimize that damage so you and your passengers can get a safe distance away before calling your insurance company. How you do that depends on a number of variables.
Since you’ve got a bona fide emergency, you probably should tell someone at ATC. They likely can’t do a thing to help you except make sure the rescue equipment is standing by. The corollary? You may want to choose a towered airport equipped with its own crash/fire/rescue squad, trained to handle aircraft accidents. You also can do a tower fly-by to verify the gear’s condition if it makes you feel better. Land on pavement, not grass or other soft terrain. (For reasons why, check the reader e-mail on page 32, then “The Art Of Crashing” in our September issue.)
If it’s a main-gear leg that’s hung up, you may have the choice of landing with the other gear legs down or retracted. We’d probably opt for retracted: There will be airframe damage regardless, and if the hung gear leg retracts itself upon contact, you’ve eliminated the biggest risk of such a landing. If that’s not an option, touch down on the extended gear leg and use aileron to keep the opposite wing off the pavement as long as possible. Before aileron authority fades altogether, gently lower the wing to the pavement; don’t allow the aileron to lose all authority, slamming the wing down. If the nosegear won’t extend, land on the mains and hold the nose off as long as possible.
A final note: Some pilots may advocate shutting down the engine and slowing down enough to stop the prop, potentially saving it from damage. Doing so likely will cost you altitude as you slow below best-glide speed. It’s not worth the risk, in our view.
Putting it all together
Intimate knowledge of your aircraft and its systems can get you through a problem not covered in the AFM/POH. Careful study of the control and landing-gear systems can highlight weak points worthy of extra attention during inspections and regular maintenance, and help with diagnosing the problem from the cockpit. Meanwhile, flying an airplane into the freezing flight levels after it sat outside in the rain might not be a good idea, nor is allowing loose items to roll around in the cockpit, or under the instrument panel.
In any event, encountering a problem such as we’ve described likely is a clear sign your flying is done for the day. Even if you can resolve the issue while airborne, it’s probably a good idea to land at the nearest suitable facility to regroup. A “suitable” facility can be the nearest runway if you’re concerned the problem isn’t resolved and could worsen, or if you’re too shook up to continue. Presuming none of that applies, the nearest airport with full maintenance facilities, an open FBO and rental cars, plus a nearby adult drinking establishment, would be our first choice.
