They say flying is hours and hours of boredom punctuated by a few fleeting moments of occasional terror. For the pilot flying a single, maybe it starts as a vibration you’re pretty sure you’ve never felt before, or as a slight pulse of the engine, a muffled thump, popping or a stumble. Maybe your airspeed has dropped off, and the gauges aren’t indicating what they should, or where you left them. The good news is engines rarely stop completely without warning. The bad news? Odds are, if it gets this far into the process of trying to get your attention about a fuel-related issue, things are poised to get more interesting rapidly.
There are many reasons an engine might act up in flight, but we’ll limit this discussion to what I sometimes fondly call liquefied dead dinosaurs—avgas—because it is that magic, expensive elixir that makes most piston-powered aircraft go. To “go” for a while, for which most light general aviation aircraft are designed, there has to be an adequate quantity of gas (a lack of this essential ingredient is, sadly yet predictably, the most common cause of fuel-related engine stoppages). If there is, then what? It’s all about scanning, planning and troubleshooting—in that order, please. In fact, the instrumentation in most modern cockpits offers pilots a plethora of tools with which they can diagnose a problem. Once the problem is determined, you can decide a course of action, avoiding many of the “gotcha’s” of in-flight emergencies.
Mixing It Up
That gas has got to flow from its stowage in a tank typically located safely away from the hot engine compartment. It’s fed to the engine through lines and a fuel selector valve to a carburetor or fuel distribution “spider” and finally to the intake manifold or the fuel injectors. Along the way, it’s atomized into a finely tuned spray or droplets that will, when ignited from a spark of the sparkplug, burn with the air in the cylinder (which has been compressed by the piston).
You may think it’s avgas that makes your light aircraft engine go, but really, it takes two to tango: avgas and air, to be precise. The perfect mixture is roughly 15 parts air to one part fuel by weight, give or take a little for temperature and humidity. That’s the peak ratio, though. This mix, sparked to burn at just the right point in the piston’s slide, pushes the piston downward in the cylinder, rotating the crankshaft via the connecting rod and the propeller along with it, pulling or pushing the airplane along. The whole mess does its thing at 2700 rpm at full power in my Lycoming-powered aircraft and many others. It never ceases to amaze me that it all happens so smoothly. Except for when it doesn’t.
High-winged aircraft often have simple, gravity-fed fuel systems, such as the one I’ve got in my Kitfox IV, to get the fuel from the tanks to the engine. Even with gravity’s assist, my Jabiru-powered machine has an engine-driven fuel pump to help keep positive pressure in the system at all times. Look around your bird. If it has a boost pump switch on the instrument panel, then it generally also has an engine-driven pump. The boost pump backs up the engine-driven pump in the case of a failure. Sure, in a gravity-feed system the fuel will probably keep feeding even if the engine-driven pump fails, but the electric pump is there “just in case” you need it. It can also come in handy for other problems.
If it’s a fuel issue causing an in-flight problem, your symptom could be a reduction of power or a stumble in an otherwise smooth-running engine. So, what causes that? We’ll start with the most common fuel-driven, in-flight issue: running one fuel tank dry. Shouldn’t have happened with good planning, but, say it does (perhaps you have a leaky gascolator valve on that tank and it has siphoned all your fuel out while you weren’t looking at the gauge).
Fly the airplane. Got an autopilot? Let that fly for a bit. Keep straight and level. Go to your emergency checklist. Do the red-box/boldface items: Switch tanks to the fuller tank quickly and use the boost pump to rush fuel back into the lines.
Maybe you’re feeding from both tanks and one of them suddenly stops feeding (perhaps debris from inside the tank blocks the feed port). You might see a decline in fuel pressure in the system and even a power reduction. If you can’t switch to another tank (my Kitfox and many other planes cannot), you could turn on the boost pump to bring the pressure in the system back up to normal. This is one reason many airplane checklists require the electric boost pump to be on for takeoff and landing (think go-around).
If the restored pressure brings your engine back up to snuff, you are in good shape again, as long as the boost pump continues running. But remember, the boost pump is backup equipment: If it fails, you’re back where you started. Meanwhile, if you can’t access that tank of fuel, you just lost approximately half of your endurance. Best you land sooner than you had originally planned.
Let’s say you’ve done the red-box/bold items: enrichening the mixture (or leaning it if that didn’t work), turning on carburetor heat to eliminate carburetor ice, turning on the boost pump to increase or restore fuel pressure to the system, and changing to the fuller tank. You might even check the magnetos to see if your rough-running engine is caused by a magneto or ignition failure (even though technically not a fuel issue, you depend on this system to ensure fuel burns evenly and properly in the cylinders).
What other resources do you have? Most modern aircraft these days have engine analyzers and/or fuel totalizers. The engine analyzer is a special-purpose computer connected to temperature probes for all the engine’s cylinders and exhaust pipes, and usually also tracks oil temperature and pressure. It often also incorporates the fuel-totalizer function, which measures the engine’s fuel use via a transducer. Some installations may involve a separate fuel totalizer. When you add fuel, you tell the computer how much you put aboard and it calculates, real-time, your range and/or endurance. Neat stuff.
Once you document your cylinder head temperatures (CHT) and exhaust gas temperatures (EGT) in normal operating conditions, it is also quite easy for you to diagnose potential engine issues even before the engine begins to suffer damage and cough or sputter. If you’ve been operating an airplane without one, think about this.
It’s no exaggeration to say an engine analyzer has helped prevent me from having to declare an in-flight emergency about a dozen different times in 34 years of flying. And that’s in about a half-dozen different airplanes. Even my Kitfox has one. I do not like to fly aircraft without one. Seriously, just invest.
It will pay you back.
Now, oldtimers will tell you these new-fangled engine analyzers make pilots too jumpy. Heck, we got along just fine by leaning the mixture with our ears and eyes and feeling for the rumble of uneven fuel with our feet flat on the cabin floor, they’ll say. I’ll admit right out that fluctuations of five or even 10 degrees in CHT or EGT can mean nothing at all. But then again, an engine analyzer, properly interpreted, can tell me with a simple depiction of a jittery EGT that an exhaust valve could be going bad. A major split between CHTs and/or EGTs, or a sudden change from status quo means something’s happening in your engine, and it is time to figure out what.
If you take away anything from this article, don’t take away the impression that all you have to do is look at your engine analyzer to figure out what is going wrong with your aircraft engine. It’s just an indicator; you’ll have to do real troubleshooting to get to the core of most problems, and some of that troubleshooting is better done safely back on terra firma. Just remember that.
Here’s an example: I’m airborne halfway between Cuba and Grand Cayman, at 8000 feet msl, when my #5 cylinder’s EGT begins to drift away from the nice even zero line on the graphical display. That gets my attention. There are two possibilities: a clogged fuel injector or a failure in the ignition system somewhere.
How do I know? How do I tell the difference? With a clogged fuel injector, in a rich-of-peak operation, the EGT would climb away from the others, because as that cylinder was starved of fuel it would run leaner, approaching peak EGT (and then eventually drop off if the fuel restriction completely clogged the injector). The CHT on #5 will also rise. But here’s the kicker: in a lean-of-peak operation, the EGT and CHT would sink away from the others, because that cylinder had already peaked and moved lean of peak EGT (an argument for operating lean-of-peak, but we won’t get into that here).
By watching the EGT and CHT move and experimenting with mixture settings, I determine the #5 cylinder’s injector is clogged, saving me valuable dollars on a mechanic who works by the hour. The key to understanding what is wrong here is knowing what kind of power management I’m using for the phase of flight, and having a keen sense of what my normal EGTs and CHTs for this operation look like.
How do I fix the problem when over the ocean, as in the scenario above? Adjust the fuel flow with the mixture knob (and the electric boost pump, if I must) to bring the EGT/CHT of the offending cylinder back into line and land as soon as possible to have the injector cleared. If I’ve increased my cruise fuel flow considerably, I take note of my now-significantly reduced range and endurance. If I need to, I change my destination to make sure I’ve got adequate reserves on board at all times. I simply will not land with empty or near-empty tanks, and I plan even my longest leg flights considering I might have to divert. Never fly without an out.
Meanwhile, a failed spark plug causes incomplete ignition of the fuel/air mixture, which is going to mimic an enriched mixture condition, and produce less power from that cylinder. So, if I’m lean-of-peak EGT, I may see the EGT and CHT rise, as less fuel is burned, and rich of peak you’ll definitely see the EGTs and CHTs cool off in that cylinder. Diagnose the problem by doing an in-flight mag check. The dead spark plug will show up when I select the mag opposite the mag that powers it, because suddenly that cylinder will have no spark at all and the EGT will disappear completely. The engine likely will run rough, also. Sure, the issue could be with the ignition harness, an insulator or contact spring or even contamination in the magneto’s distributor cap—but odds are, it is the workhorse: the spark plug. I adjust my fuel flow with the mixture to keep the EGT / CHT in line, and land as soon as possible to fix the problem.
And what about a problem that shows up at full power, such as on a go-around or a takeoff? Can an engine analyzer warn you early enough to save the engine from these more catastrophic events, such as thermal runaways? You bet.
If you are in an over-lean condition with high power on a hot day, the temperatures rise fast; sometimes so fast that the only way to stop the actual melting and warping of a cylinder from thermal runaway is by simultaneously reducing power, increasing airflow over the cylinder and—if at all possible—increase cooling fuel flow to the engine. Oh yeah, and get the airplane safely back on the ground where you can check compression on the cylinder and figure out what went wrong, too. This is no condition to troubleshoot in flight.
How can this be prevented? One way is by ensuring the engine’s takeoff fuel flow is set high enough. Generally, manufacturers specify a a takeoff fuel flow value many think is too low. While the manufacturer-recommended fuel flow value is guaranteed to extract maximum power from minimal fuel, one trade-off is high CHTs. Ensuring the engine’s sea-level, full-rich takeoff fuel flow is adjusted higher—to roughly 10 percent of the engine’s maximum-rated horsepower—provides extra fuel with which to help cool the cylinders while not materially impacting power output. For my normally aspirated 265-hp Lycoming, that’s about 26.5 gph at full sea-level power. Remember, though, a clogged injector could cut some of that critical flow to a cylinder, so be conservative when setting the device’s warning bells and lights to give you time to catch a problem in the seconds before the runaway starts. Better yet, watch the analyzer carefully in the run-up for anomalies and fix them before hitting the power on the go.
Analyzers may have been developed for the high performance engines, but carburetor-equipped engines such as in my Kitfox or in a simple legacy Cessna 182 can benefit from their information, too. Creeping high, then suddenly low EGTs indicate the onset of carburetor icing before the engine coughs, giving the alert pilot the opportunity to pull the carb heat knob full open and defrost the venturi. Better yet, he can even re-lean the engine using the analyzer to re-establish the best possible power once the heat is in full force, increasing the airplane’s performance. If you’ve ever encountered carb ice on climbout in clouds, you’ll know how reassuring it is to have good power restored quickly.
I was even able to diagnose a breach in the flange holding the Bing carburetors to the engine in my Kitfox’s Rotax 582 engine using the EGT/CHT gauges (they began bouncing all over on taxi, despite a smooth-running engine). I shut down before the rubber flange failed completely, saving the engine from damage.
Most engine issues that come up surrounding fuel in flight can be dealt with safely in the air in such a manner as to avoid damage first to the engine, and second to the airframe and its occupants (by virtue of the pilot executing a timely and safe landing, preferably on an airport).
The simple trick to this desirable outcome is for the pilot to fly the airplane first, and to be scientific and methodical in approaching the problem. The diagnosis includes identifying the problem, verifying the problem and then isolating the problem. At that point, attempt to fix the problem with what you have (mixture, fuel selector valve, fuel pump, throttle and propeller levers, and the ability to increase the cooling air flow through the engine).
Beyond that? Land and let a mechanic finish the diagnosis and fix based on your scientific findings. It’ll save you money because you’ve done some of the hard work for him. Better yet, it could save your engine, and even your life.