Yesterday I flew from Bedford, Massachusetts to Gaithersburg, Maryland. East Coast airspace was complex to begin with and has become further complicated by restrictions around the Washington, DC are. If you’re an instrument-rated pilot you can avoid all of this complexity by filing an ins trument flight plan and taking advantage of Air Traffic Control (ATC) services. The assigned route took me straight over JFK airport at 6000′ and then through central New Jersey before proceeding over Balitmore-Washington Intl. airport and into the Montgomery Country Airpark.
My Diamond Star (DA40) is a brand-new design but it uses an engine that hasn’t changed for 50 years. Old-style piston airplane engines require that the pilot constantly adjust the air-fuel mixture as the plane rises into thinner air or descends into denser air. When you’re done with your flight and parked at the airport, you pull the mixture control all the way back to “full lean” and the engine stops, starved of fuel.
Descending out of 6000′ over Baltimore I noticed that my exhaust gas temperatures were rising, despite the fact that I was enriching the mixture. Between talking to ATC and the other pilots at the busy non-towered Gaithersburg airport, I didn’t have much time to reflect on this odd behavior. After parking the airplane I pulled the mixture control back. The engine kept running. I shut the airplane down by shutting off the flow from the fuel tanks, then hopped out and unscrewed the cowling.
The mixture control itself is an L-shaped arm on the throttle body of the fuel injection system. It is attached to the mixture cable by a bolt. In case the mixture cable snaps, a spring is also attached to the arm to pull the mixture to “full rich” (engine runs but not necessarily efficiently). Sadly the engineers at Diamond decided that both the spring and the cable should be attached with the same bolt. The bolt was rattling around loose in the bottom of the cowl. The spring was hanging free. The end of the mixture cable was hanging free. My engine continued to run because (a) I had been conservative in running moderately rich at altitude, (b) the difference between 6000′ and sea level isn’t enormous, and (c) the L-shaped arm, free to rattle around a bit, hadn’t rattled its way to “full lean”. [This is more than a theoretical possibility; rumor has it that a plane similar to mine landed in a farmer’s field in the Midwest back in the Spring of 2002 after the mixture cable came loose. The incident led to a redesign, which was retrofitted to my airplane in June 2002.]
My mixture control was held together with a regular bolt and a locking nut (that apparently did not lock and is now on the ground somewhere between Long Island and Baltimore). Tull, one of the best mechanics at Gaithersburg, happened to be on the field at 6:00 pm on a Saturday and he reassembled the airplane, this time using a bolt with a little hole in the middle so that a safety cotter pin could be inserted to prevent future separations.
There are a bunch of ways to look at this incident. One is despair at the state of engineering in this world. Had an extra hole been drilled in the L-shaped arm, the spring could have been attached separately from the mixture cable. The engine would have gone to full rich after the mixture cable detached. Alternatively, Diamond could have used a bolt with a hole in the middle and a safety, like the one that the mechanic in Gaithersburg used. A few extra cents and the plane would have been spared the risk of an emergency landing.
Another way to look at this incident is to be ever-vigilant when flying a piston single-engine airplane: have an emergency landing spot in mind at all times. The #1 reason for engine stoppage is running out of gas but it apparently is not the only reason.
Project for today: make it to Williamsburg, Virginia with both the airplane and myself in one piece.Full post, including comments