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 Savvy
Owner Notebook:
Troubleshooting From The Cockpit
Before an aircraft problem can be fixed, it has to be diagnosed … but a lot of problems are impossible to reproduce in the maintenance hangar. If you want to minimize expense, downtime and frustration, you need to learn to troubleshoot such problems from the cockpit.
by Mike Busch (mike.busch@savvyaviator.com)
 The ten-hour three-leg flight from Santa Maria, Calif., to New Haven, Conn., had been both delightful and uneventful. My Cessna T310R had just come out of a "heavy annual" during which both turbochargers were replaced, and the engines were running like fine Swiss watches. My good friend Chris -- a Bonanza owner and a superb instrument pilot -- was performing copilot duties. The flying weather was fine, the tailwinds were generous, and all seemed right with the world.
After a few days in New Haven visiting Yale University (where Chris's older son was a senior), we launched on the next short leg of the trip. We were headed for Hanscom Field near Boston, where I would be visiting my parents and sisters, and where Chris would be visiting his other son.
We had just leveled off and leaned the engines for cruise when suddenly the right engine stumbled abruptly for a moment. Instinctively, Chris and I looked at the right engine nacelle, then at each other, and then at the engine instruments. By that time, however, the engines were purring along as if nothing had happened, and the engine gauges showed normal readings.
During the remainder of the 40-minute flight, we both were alert for a recurrence, but both engines continued to run flawlessly. "Maybe there was just a little water in the fuel," I said hopefully to Chris, although I knew perfectly well that this explanation was unlikely since we'd encountered no precipitation at all during the trip.
After a few days in Boston visiting with our respective family members, Chris and I departed Hanscom for a 90-minute flight to Dutchess County Airport in New York's lower Hudson River Valley. Our preflight runup was normal, and the engines ran fine during takeoff and
climbout. But when we reached cruise altitude and adjusted the engine controls for 65% power and normal lean mixture, the right engine started running rough … REALLY rough. This time, it was no momentary hiccup, but sustained and alarming roughness.
What's wrong with this engine?
The right engine fuel flow and manifold pressure were oscillating in sync with the engine's surges. The right EGT needle was very erratic. (At that time, I had only the single-probe factory EGT gauge; I hadn't yet installed my probe-per-cylinder JPI EDM-760 engine monitor.) Cylinder head and oil temperatures looked normal. I didn't have a clue what was wrong or how serious the problem was. I briefly considered declaring an emergency and landing at the nearest airport, but since the engine was still putting out useful power I decided to try some troubleshooting first.
A piston engine needs three things to run: fuel, air, and spark. Air is usually a given, so fuel and spark are usually the principal suspects. It struck me that the roughness didn't start until I reduced power and leaned for cruise. I tried shoving the right mixture control to full-rich. The engine started running smoother. A lot smoother!
The fuel flow went through the roof, of course. But we were only a bit more than an hour's flying time from our planned destination and had lots of extra fuel on board. So I decided to continue the flight while continuing my troubleshooting efforts.
I tried re-leaning the right engine a bit to bring the fuel flow down to a more reasonable value. As soon as I started pulling on the red knob, the engine started running rough again and the EGT needle starting going beserk. I quickly returned to full-rich and things smoothed out.
I tried an in-flight mag check, and found that the engine ran smoothly on each magneto individually (so long as the mixture remained full-rich). This suggested to me that the problem was almost definitely fuel-related and not spark-related. Rough running is usually the result of one or two cylinders producing less power than the others (or perhaps no power). I suspected a clogged fuel injector.
"Boy, I sure wish I had a multi-probe engine analyzer," I remarked to Chris. "I bet we could tell precisely what's going on." Even without an analyzer, it occurred to me that I might be able to take my diagnosis one step further. When the engine ran rough, the right EGT needle was highly erratic. I knew the EGT probe was mounted in the exhaust cluster on the left side of the engine. I reasoned that if the problem was indeed a clogged injector, it probably was on one of the left-bank cylinders (#2, #4, and #6).
A quick, cheap fix
An hour later, we landed at Dutchess County Airport and taxied to the sole FBO, Richmoor Aviation. I went to the maintenance hangar and found one A&P mechanic at work. I explained that we had a Cessna T310R with a rough-running right engine, and that I suspected a clogged injector on the left bank of cylinders. The mechanic suggested we have the line crew tow the airplane into the hangar and have a look. Thirty minutes later, I had uncowled the engine and held the three suspect injectors in my hand. The mechanic and I inspected each one under a bright light. Sure enough, the #4 injector was clogged with what looked like a small glob of grease!
We cleaned all three injectors in MEK, reinstalled them on the engine, and replaced the cowling. We towed the airplane out of the hangar so I could take the airplane up for a quick test flight. Problem solved! Total time in the maintenance hangar: 60 minutes. Total involvement by Richmoor's A&P: 10 minutes. Total maintenance cost: "That's okay, don't worry about it."
Now, here's something to think about: Suppose I hadn't made the effort to troubleshoot the problem in-flight, or didn't know my airplane well enough to do so. Suppose I'd simply brought the plane to the mechanic and told him that the right engine was running rough in cruise. What do you suppose the A&P would have done? How long do you think the airplane would have been down? How much do you think his bill would have been?
Another trip, another glitch
A few years later, I was making another transcontinental trip in the T310R. (I typically make three or four such trips a year.) This time, I was alone in the airplane, cruising along at Flight Level 190 with an oxygen cannula up my nose to take advantage of an unusually nice wintertime tailwind. The OAT was -25°C but the cabin was toasty warm thanks to the avgas-fired Southwind combustion heater mounted in the nose of the airplane. My TAS was a respectable 210 knots, and my groundspeed was a blistering 290 knots. Life was good.
About two hours into the flight, crossing the highest part of the Rocky Mountains, I started to have that awful feeling that something was wrong. But what? I scanned the gauges and everything looked fine. Then I realized the source of my malaise: my feet were cold. I reached my hand down to the heater outlet and felt a stream of frigid air coming out. The cabin heater had indeed flamed out. On a winter day at FL190 crossing the Front Range, this is not a good thing!
I was pretty familiar with the heater system in my airplane, because I'd removed the system from the airplane and sent it out for major overhaul a few years before. What I remembered best was that the heater cost between $1,000 and $3,000 to overhaul (depending on whether the combustion tube was repairable). The heater has a TBO of 1,000 heater hours (mandated by AD), and mine had less than 200 hours
SMOH.
Like an engine, a combustion heater needs three things to function: fuel, air and spark. But unlike an engine, the heater has no cockpit instrumentation to tell the pilot what's happening, nor controls that can be manipulated by the pilot. The only cockpit controls and indicators are an on/off switch, a thermostat knob, and a heater-overheat warning light. The overheat light was not illuminated. I tried turning the switch off and on a few times, and turning the thermostat knob to a higher setting, but nothing helped. The heater remained non-functional and my feet were getting numb.
What's wrong with this heater?
At first, I couldn't think of anything else to try that would help me troubleshoot the heater problem, much less resolve it. I concluded that I'd have little choice but to ask ATC for a descent into warmer air. Worse yet, this probably meant that I would have to do without any cabin heat for the remainder of my multi-week wintertime transcontinental trip. At best, I would be confined to low altitudes; at worst, I'd freeze my buns off.
On further reflection, however, I thought of one more thing I could try. I remembered from my Cessna 310 systems groundschool at FlightSafety that the combustion heater obtained its fuel supply from the left crossfeed fuel line. After passing through a heater fuel filter, that fuel went through the heater's fuel pump and ultimately into the combustion heater itself, where it mixed with air from a blower, was ignited by a spark plug, and heated the combustion tube (like a blowtorch inside a can).
I wondered what would happen if I pressurized the fuel in the left crossfeed fuel line by turning on the electric aux pump? If the heater problem was due to insufficient fuel, I reasoned that would help. If it was due to insufficient air or spark, it wouldn't help. I flipped the lefthand aux pump switch to "low." This seemed like a long shot, but what did I have to lose by trying it?
Two minutes later, my feet told me that warm air was coming out of the heater! I couldn't believe my good fortune. This meant I could continue my flight at FL190 as planned. More importantly, I'd proved that the heater flame-out had been caused by inadequate fuel pressure.
For the next hour, I flew along in comfort and smugness, patting myself on the back for my cleverness in working around the heater problem. Then my self-congratulatory mood was shattered by a familiar sensation: cold feet. The damn heater had flamed out again!
Hmmm. If pressurizing the left crossfeed line was good, I thought, maybe pressurizing it more will be better. I flipped the left aux pump switch to the "high" position, while simultaneously pulling back on the left engine mixture control to prevent the engine from being flooded by the high fuel pressure.
Two minutes later, my feet were warming up again. The heater was heating! Yesss!!!
For the remainder of my transcontinental trip, I was able to keep the cabin heater functioning using this "high aux pump" trick. When I finally returned home to California, it was time to complete the diagnosis and fix the heater system.
I'd already established that the heater problem was due to inadequate fuel pressure. It seemed to me that there were only two likely possibilities: either the heater fuel filter was clogged or the heater fuel pump was bad. I checked the fuel filter and it was okay. That left the heater fuel pump as the prime suspect.
It took me an hour to open up the nose and gain access to the heater fuel pump. Upon close inspection of the pump, the problem became apparent: the fuel pump's drive coupling was sheared. I telephoned Aircraft Heating and Electrical in Redding, Calif., and ordered an overhauled/exchange heater fuel pump for $121.50. A few days later, the new pump was installed and the heater worked fine again (without the aux pump).
Again, think about what would have happened if I'd simply put the airplane in the shop and told my mechanic that the heater had failed. Wanna bet that the entire heater system would have been removed and sent out for overhaul? Wanna bet that the tab would have been something north of $1,000?
Troubleshooting from the cockpit
I've been long convinced that one of the most important things an aircraft owner can do to minimize maintenance expense, downtime and frustration is to make a real effort to troubleshoot every problem as far as possible before putting the aircraft in the shop. Many owners believe that troubleshooting is the mechanic's job, but I disagree. Many aircraft problems occur only in-flight and are difficult or impossible to reproduce in the maintenance hangar. If a mechanic can't reproduce a problem, he can't troubleshoot it. If he can't troubleshoot the problem, he has little choice but to "shotgun" the problem -- replacing various components until he gets lucky and finds the culprit -- or to send the entire system out for overhaul. Either way, you're not going to like his invoice.
Furthermore, many owners actually understand the systems in their aircraft better than their A&Ps do. After all, you only have one aircraft to learn, so you have the luxury of becoming an expert on that aircraft. Your mechanic probably works on dozens or hundreds of different aircraft makes and models, so he can only learn enough about each make and model to be dangerous. Your mechanic may be the best equipped to fix a problem, but very likely you're the best equipped to diagnose it. When
I do my weekend Savvy
Owner Seminars I spend several hours teaching the aircraft
owners who attend how to do a better job of troubleshooting problems
before putting their aircraft in the shop. I review common problems
with engines and electrical systems and talk about how to
troubleshoot them. I give special attention to the most difficult
kind of problems to troubleshoot -- intermittent ones -- and discuss
how to deal with those.
Even if you can't completely diagnose a problem, take the troubleshooting as far as you can. There's a big difference between putting the aircraft in the shop because "the cabin heater doesn't work" and doing so because "the cabin heater isn't getting enough fuel pressure, probably due to a clogged heater fuel filter or a bad heater fuel pump." Probably a $1,000 difference.
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