Single Point Failure

April 1, 2009 by Bruce Landsberg

I was recently reminded of the vulnerabilities of our hardware. After a flight my passenger apparently twisted the mic jack when removing his headset such that it shorted out on something to totally foul up the system. On the next flight both transceivers were in transmit mode only i.e. hot mic. The avionics tech troubleshot the problem in 10 minutes, repositioned the jack and we were on our way. His parting comment, ” We see this a lot.”

That got me to thinking about other single point system failures beyond the engine itself. In IMC and at night the hardware becomes increasingly important. Dual radios incapacitated by a single jack, a transmit relay, or a sticky mic button are all examples just within the comm system. This is not a benign failure either. Not only does it booger up your aircraft (that’s a technical safety term) – a single stuck mic can mess up an entire ATC sector frequency.

As we go about improving the hardware, seems like a simple insulator around jacks would be an effective and cheap solution. Perhaps there are some avionics types reading who could shed some light on this. I wonder what other single point system failures are lurking. Long ago, I gave up on a single power source for primary flight instruments in IMC. A single dry vacuum pump to power some of the most critical equipment is not a recipe for success.

What single point failures have you had? Did you add redundancy or change your flight operation to avoid the problem in the future?

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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  • Wes

    On a clear night in a Piper Dakota I lost the alternator after two resets just after leaving Omaha NE. Luckily I was flying on an instrument flight plan even in clear wx and the intermittent radio was a big clue that I had problems.

    We (my wife and I) landed without incedent at an alternate field with no AC lights and only a pair of flashlights to check on the gages to verify settings by feel and sound.

    It turned out the next morning that a wire had come loose from the alternator and it was less than 10 minutes to fix on the ground.

    I now fly with a back-up hand held radio at night at all times and in addition to the required flashlight, cary a small clipon led light that I can clip to the dash and point at the gages if I loose electical power.

    I’m glad my instrument instructor had me practive flying by feel and sound during my training… I still practice from time-to-time just to keep in tune with my aircraft.

    Wes Waddell

  • Rick Tavan

    On my first flight with my wife after earning my private pilot’s cert, flying a rented 152, I had a total electrical failure resulting in a norad landing at my home, tower-controlled airport. Everything went by the book including some one-way comm, light signals and an emergency truck idling beside the runway. The next week I ordered a hand-held transceiver which remains in my flight bag.

  • Walt Roberts

    After losing an alternator en route, landing, having it repaired and a second failure due to a faulty regulator circuit while en route in IMC, about an hour after departure, I devised a new rule in the pre-GPS days.

    When asking for a pre-flight brief and there is high likelihood of requiring an instrument let down at my destination, I always ask the briefer where the closest VMC is along my planned route. If there is an electrical failure resulting in the potential for localizer/glideslope failure, I have the option of turning the aircraft in the direction of better weather and flying out of the weather to land in VMC.

    I have never had to use this option, but if there is no VMC within range of the aircraft on the flight, I consider the flight very carefully before I depart. Now, I also have a backup hand held WAAS gps and a spare set of batteries, as well as an ancient hand-held that amazingly, still works.

  • Vance Harral

    Here’s an interesting one: modern glass cockpit aircraft have redundant displays with reversionary modes, etc. But the few I’ve (briefly) fiddled with use a single knob to control the brightness of all the displays. Heard a tale of a bad dimmer rheostat causing all displays to go full dim (and therefore unreadable in daylight). No idea if the story was an urban legend, and perhaps all the systems have separate per-display dimmer controls somewhere… though kinda hard to set ’em if the display itself is unreadable. But something to think about.

  • Jeff Ward

    Two days after reading this blog post, I was flying VFR with advisories, and a prolonged silence (and no handoff) gave me a hint that something was amiss. My headset mike plug had been touchy earlier, and now appeared to have gone south, taking both TX and RX with it. Since I was barreling towards Class B, I slowed down, squawked 7600, and started a wide orbit while I troubleshot (try the copilot jacks – still bad, etc.) So, I reached into the back seat and grabbed my spare headset – the very same headset I had briefly considered leaving behind that day to shave a little weight out of my bag! The lesson I guess is whenever I can easily add redundancy to a critical system, I will! (Oh, and I learned that your blog is a jinx!) It occurred to me later that I could have tried the cabin speaker for RX-only, and I lke to think I would have gotten to that if I hadn’t cured the problem with the spare set.

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