Improper and Proper D-Sub Connectors



The connector in the left photo was discovered on the back of an autopilot control head that was intermittent. D-subs require a backshell (right photo), which provides the mechanical connection from the connector to the device through the screws. Without the backshell, the connector will eventually become loose and perhaps even completely disconnect from the device.

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Vic is a commercial pilot, CFII with ASMEL/ASES ratings, an A&P/IA, DAR, and EAA technical advisor and flight counselor. Passionately involved in aviation for over 40 years, he has built 11 aircraft and logged over 10,000 hours in 72 different kinds of aircraft. Vic volunteers as a Young Eagle pilot and Angel Flight pilot. He chairs the EAA Homebuilt Council and is a member of EAA’s board of directors. He also has his own sport aviation business called Base Leg Aviation.


  1. Requires screws to hold it in place, not necessarily a back shell if the shrink tubing provides adequate wire relief.
    Why do you publish such misleading info?

  2. Yes, the screws are required to hold it in place. I don’t think it is misleading information. Heatshrink can and will come loose over time the way it is depicted in this picture. The wires should be mechanically supported so that they do not flex and work over time, causing breakage of the conductor. Perhaps in a non-aircraft, low vibration environment you might get away with it. But in ana aircraft, on a major system like an autopilot, I really think it is poor practice.


  3. You correctly identified missing screws as the reason the connectors came apart. A back shell, with its screws, conveniently provides wire support and ensures connectors do no come apart. But a back shell may not be the solution in some cases where there is no room for it. What is required: something to ensure connectors do not come apart (usually screws but zip ties cannot be ruled out) and something to ensure that wires are supported.
    My objection is to absolute statements like “D-subs require a backshell”.

  4. They make 90 degree backshells for areas where there is not room for a straight backshell. In all of the aircraft I have worked on, including military and commercial, I have not seen where there is not enough room for either a straight backshell or a 90 degree one. I know I haven’t seen everything in life yet, but we are trying to rpovide guidance to builders of homebuilt aircraft. Backshells should be used. Perhaps someone can come up with a valid example of where there is not room, but then I bet there is not enough room for the heat shrink to proved the necessary support either. I will also bet that any example with be a real outlier. I stand by my statement. You are welcome to your opinion. 🙂


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