In this series of articles, we have been reviewing component failures and safety issues caused by well-intended maintenance tasks. Our mission is to explore a variety of cases in order to learn from the experience of others and to understand why it is so important for maintenance providers and owners alike to look at inspection and preventive maintenance situations with a more critical eye.
This case comes to us from one of our readers, David Fisher, who flies a Mooney M20M. The aircraft had been running a bit rough at idle (a frequent issue for this aircraft/engine combination), so the aircraft was brought into the shop for an injector cleaning. This wasn’t a bad call on the owner’s part, because rough idle is often caused by dirty injectors and cleaning them is an economical first step in diagnosing the issue.
Following the routine preventive maintenance, the shop’s ferry pilot flew the aircraft back to the owner (and the owner returned the favor with a round trip to return the ferry pilot back to the shop). This trip put about an hour of time on the engine, post-maintenance. The owner selected a relatively short runway for the final leg of the trip to bring the airplane home. Having already put an hour on the engine after the injector cleaning, his confidence was high that the aircraft was out of the “post-maintenance woods.”
Unfortunately, that wasn’t the case. Just after rotation on takeoff, the engine started running very rough, back fired, lost power, and trailed black smoke. It was too late to abort the landing, as the pilot was staring straight ahead at a line of hangars. Fortunately, he managed the emergency skillfully, finding a path between two hangars (in case the aircraft stopped climbing altogether) while looking for a suitable field in case of full power loss. The tower cleared him to land any runway, which they could have done with hand signals since the aircraft was about eye level with the tower while slowly turning downwind. He pulled the power back, the engine began running smoothly again, and the landing was smooth and uneventful.
During the post-incident investigation, the mechanic found pieces of rubber fuel line clogging the injectors on three cylinders. The injectors have a small metal “barb” on which the short section of rubber hose slides over and is secured by clamps. It is believed that, during the process of removing the hose from the barbed connectors, cleaning the injectors and reconnecting the hose, the mechanic scraped off pieces of the inside of the hose that found their way into the injectors. They had been floating in the fuel system, slowly working their way into the injectors during the first legs of the post-maintenance flight.
The backfire and thick trail of black smoke was caused by the three non-clogged cylinders running excessively rich while the other three were starved for fuel. Pulling the power solved the over-rich problem, restoring the engine to partial power.
The owner reports that the airplane was only 5 years old and the hoses were still soft and compliant at the time of maintenance. Ensuring that all of the flexible hoses on the aircraft are inspected and replaced on both condition and age is critical, but that doesn’t cover every failure case. It’s also critical to inspect hoses after removal and ensure that they are free of contaminants before putting everything back together.
Fisher’s own words convey some of the lessons to be learned: “Fortunately training, calmness and my long time flying mentors voice in the back of my head came through loud and clear, ‘Just fly the plane, just fly the plane, just fly the plane.’ A simple mistake turned into a life-long flying experience. No maintenance is routine. I won’t take any passengers and always plan a few hours solo in good weather after any maintenance.”
As with the other cases in this series of articles, when we perform preventive or repair maintenance on an aircraft, we run the risk of introducing a maintenance-induced failure. This is only one more case, but it helps us understand why we need to be extremely careful and systemic with all aircraft maintenance. Attention to detail matters and we should always carefully weigh the risks of the task with the benefits.
As we continue this series, we will review more interesting cases of maintenance-induced failures. If you have your own story of “well-intentioned maintenance gone wrong,” please send me an email. Until then, happy flying!
Interested in aircraft maintenance? View the archives of Jeff Simon’s Aircraft Maintenance series.
Jeff 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. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 20,000 aviation events, $100 hamburgers, and educational aviation videos. Free apps available for iPhone, iPad and Android, and on the Web at www.SocialFlight.com.