No smoking gun

The Epsilon had flown twice that warm spring day with no issues. I fueled it up, then hopped in the backseat to give an Epsilon checkout and spin instruction to a newly minted CFI.

We cranked normally, then started a long taxi. We were number seven for takeoff at Frederick (FDK). We leaned the Epsilon during ground operations and occasionally bumped power and tickled the boost pump to keep the engine idling on a hot, stagnant ramp.

We turned the boost pump on—normal Epsilon takeoff procedure—and the front-seater went full throttle for a takeoff with normal engine readings. He cleaned up gear and flaps, turned the boost pump off at about 1,000 feet, and began a climbing turn for departure. All seemed nomal, albeit with warm cylinder head temperatures in the 400- to 418-degree range. A CHT of 420 degrees is higher than preferred for cruise, but not unheard of for big Lycomings on a warm-day climb, so we retarded power to slightly below normal climb setting, checked the mixture rich, and continued a 120-knot cruise climb on our way to 6,000 feet.

A few minutes later, still climbing, the front-seater reported engine oil temperature rising rapidly and approaching redline. I smelled burning oil. I took the airplane, lowered pitch, powered back, and the engine stumbled—hard. I powered back up, but the engine kept stumbling, so I turned on the boost pump and checked mixture, still full rich. The engine responded better, but still wouldn’t produce full power. Oil temperature hovered just below the redline. We powered back, turned back to the field, and began pitch to best glide. The engine still wouldn’t make enough power for level flight. In the descent with power back, the oil temperature began dropping. One problem neutralized.

We switched back to tower, declared an emergency, and reported our position, eight miles to the northeast. If the engine held, we seemed to have enough altitude and power to make it back to Frederick. We scouted farm fields and a couple of grass strips along our path, good options for a potential off-airport landing if the engine degraded further.

What could be the problem? We tried isolating magnetos to no effect and switched fuel tanks, also to no effect. We turned the boost pump off and the engine lost more power and seemed likely to quit. OK, boost pump needed to keep the engine running. Even then it only resulted in rough partial power. We flew a high straight in, pointing a third of the way down the runway and delayed gear extension until just before the threshold, then dropped in a steep descent to land a couple thousand feet long. On the ground, no difference from the air, the engine would quit without the boost pump.

We shut down and an A&P/IA investigated. He checked pressure from the engine-driven fuel pump; flow through the fuel lines; and fuel sump samples. No smoking gun. Later he cranked the engine, idled it, ran it up, then powered back to idle, but could not duplicate the problem. The dreaded CND: could not duplicate—annoying to maintenance, hated by pilots.

Another inspection and analysis of engine data downloaded from the Garmin G3X clearly indicated where the problem had occurred in the flight profile. Several phone calls with Epsilon club members and engine experts left the general consensus that it must have been an airborne vapor lock, a burglar that will steal thrust and leave no trace. A similar event happened another time, in similar conditions with an Epsilon. They never found evidence of a problem, and it’s never occurred again. Other airborne vapor lock stories began to emerge from interested maintenance professionals now engaged in analyzing my CND event.

The clearing procedure for airborne vapor lock is boost pump on, full rich, full power, pitch down if able to help cooling. But in our case, that didn’t work. At least I don’t think it did. I started second-guessing my throttle position after I powered back to control the oil temperature. I was worried about protecting the engine, so maybe I didn’t ever go back to full throttle once I knew partial power would make the field. I can’t be certain.

Several of us with a lot of GA time learned of a new bogey that day: airborne vapor locks. Fortunately they’re rare and isolated to hot days where the engine gets warm on ground operations and continues warmer than average operations airborne.

Go fly. Think hard about engine temperature management if you find you need higher power and the boost pump to keep your engine idling on the ground.

Your engine might be warning you of a stealthy burglar casing the joint.

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