Aircraft maintenance: Listen to your airplane

January 22, 2014

Jeff SimonAircraft do not heal themselves. That sounds obvious, perhaps, but if you sincerely want to protect yourself, your family and your aircraft, read that sentence again and take it to heart.

As aircraft owners and/or pilots, we develop an acute ability to be “in touch” with the aircraft we fly. Sometimes it is conscious, such as when we hear a new noise or feel a vibration. Other times it can be almost subconscious, such as when we have a nagging feeling that something just isn’t right about how the aircraft is operating. Either way, it is critical that we address the situation head-on and take it upon ourselves (or our mechanic) to resolve the issue.

This can be especially challenging in cases of transient symptoms. The temptation to dismiss a symptom that has not re-occurred is firmly rooted in normal human psyche. However, there are cases where we only have one chance to recognize that a serious situation is developing.  The consequences of wishful thinking in these situations can be disastrous.

Case in point: About a year ago, I was returning from a routine flight in my Bonanza. During the roll out from touchdown, I detected a faint burning smell, possibly electrical in nature. It persisted for only a few seconds and was gone by the time I turned off the runway. After shutdown, I put my nose to work trying to track down the smell. I tried on top of the panel, below the panel, in the back of the aircraft, and in the engine compartment. No smell. I flipped some switches, tested avionics, etc. to no avail. Everything appeared to be working fine and no matter what I tried, I couldn’t reproduce the smell.  Thinking it might have been in my head, or caused by something external (flying over someone burning leaves, perhaps), I tied the airplane down and went home.

A few days later, I went back to the airport. The plan was to go flying, but I decided to take some of the advice I dish out on a regular basis and challenged myself to find the source of that one-time, faint smell before flying again. I ran the same tests to no avail. Then I decided to dive in head-first (literally), grabbing a flashlight and mirror and assuming the back-breaking contortions required only of circus performers and avionics technicians. 

With my head down around the rudder pedals, I began inspecting behind the panel while simultaneously flipping switches on the front of the panel. Still no smell; but then my fingers hit something hot enough to get my attention and the attention of anyone listening nearby. It was the main bus-bar for the lights, pitot heat, etc. It was practically red-hot. After shutting everything down and letting it cool, I found the culprit. The nut attaching the main power cable to the bus-bar was very loose. The screw connecting that thick cable and the copper bus bar had been arching, widening the attachment hole and working its way loose, causing intense heat.

The smell, it turns out, was caused by the overheating of the nylon lock-nut on the screw. It heated up until the nylon inside burned and dripped out all at once. That moment that the nylon burned out of the nut was the single opportunity to notice a problem from the pilot’s seat. Once the nylon was gone, so was the smell. The other consequence of the lost nylon was the loss of the “locking” part of the locknut. Upon inspection, the nut was already halfway down the remaining threads of the screw.

The consequences of ignoring this one-time, subtle signal could have been a complete electrical failure in-flight or much worse. The repair was simple. The theory about how the situation arose is that the cable was loosened intentionally during the replacement of all of the breaker-switches, as required by an AD, but never retightened properly. Another case of maintenance-induced failure, but I’ll leave that to a future article.

Until then, listen closely to what your aircraft is telling you…and happy flying!

Social FlightJeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series, The Educated Owner. Simon is also the creator of SocialFlight, the free mobile app and website that maps more than 5,000 aviation events. Free apps are available for iPhone, iPad, and Android, and on the Web at www.SocialFlight.com.