Challenger Ultralight Aircraft Maintenance

Tip #4 – EGT Drop Out

There I was flying along minding my own business when, out of the corner of my eye, I noticed one of the needles on my dual exhaust gas temperature gauge drop right off to nothing. The other needle still read about 1000 degrees F. My heart leaped and a knot formed in my stomach as I glanced at the fields around me for a suitable landing spot. I always have one picked out as a “just in case” place but when the brown stuff hits the rotating air moving device one likes to make sure there isn’t a better alternate landing field around. Then I realized that the engine sound had not changed and the CHT’s were still fine. The aircraft was flying the same and the engine still producing power. It was a gauge problem, not an engine issue. Phew! I continued on with my flight with the right hand EGT needle at zero.

But now came the challenge. Was it the gauge or the sensor or the wires or the electrical connectors. All 4 components could be the culprit. It would involve some keen investigative work to narrow down to the truly guilty party and fix the problem.

First I decided to check the gauge. I switched the two pairs of EGT leads coming from the temperature probes where they connected to the back of the dual EGT gauge. If the zero reading changed sides on the gauge, that would mean that the gauge was not at fault. It would mean that the instrument was not receiving a signal through that pair of wires. That is exactly what happened. When I ran the engine next, the left hand needle was at zero, while the right hand needle was indicating normal temperature. The gauge was fine.

Next I decided to switch the positions of the EGT probes just to make sure that it was not an engine or exhaust system issue. I was pretty sure it wasn’t, but this would confirm it for sure. I switched the EGT probe positions and made sure that they were at the same depth into the exhaust manifold and tightly connected. This had no effect on the readings at the panel. The engine was running fine but the left hand gauge needle still read zero.

This meant that the problem had to be between the temperature probes and the panel. Perhaps a wire had parted or the insulation had rubbed off and shorted the signal to ground. I crawled through my Challenger and checked the wires as they passed through the gussets and other places where a rabbit couldn’t go. It involved gymnastics and acrobatics that a contortionist would have admired. I found no problems. All the wires passed through rubber grommets to prevent chafing. There were no breaks or worn insulation.

That left only one set of components that could be the problem – the electrical connectors that mate the temperature probes to the wires running to the front of the aircraft. This involved removing the heater and fabric cover under the engine and pulling out the wiring harness that was affixed to the triangular plate in fuselage below the engine compartment. At this point I was wishing that the gauge had simply expired. It would have been a lot simpler to replace.

When I finally had the wiring connectors in my hand i carefully examined them but all seemed fine. However, when I went to disconnect the two mating halves they remained united while the wire easily pulled away from the crimped end of one fitting. The crimp had been poorly done by the avionics technician (me) and no doubt resulted in a weak physical connection and a bad electrical connection.

A better connection would be accomplished with soldering rather than crimping. I went to Radio Shack and purchased a battery powered cordless soldering iron at the suggestion from the store clerk. It was on sale for 1/2 price too! It worked remarkably well. I soldered all four connectors and stuffed them back under the engine compartment. I reinstalled the heater and fabric cover. problem solved.

Lesson – Solder electrical connections where possible especially if they are buried deep in an inaccessible part of the aircraft. Otherwise ensure that the connectors have a solid fit and are tightly crimped on the wires.