Unlocking The Mystery Of An Aircraft ADI

If you’ve ever seen the cockpit of an airplane, you’ve probably noticed the round ball that shows your attitude, and if you are like us, you’ve wondered exactly how the Attitude Direction Indicator (ADI) works. Well, [msylvain59] is tearing one apart in the video below, so you can satisfy your curiosity in less than 30 minutes.

Like most things on an airplane, it is built solidly and compactly. With the lid open, it reminded us of a tiny CRT oscilloscope, except the CRT is really the ball display. It also has gears, which is something we don’t expect to see in a scope.

Getting to the ball mechanism was fairly difficult. It is nearly the end of the video before the ball comes apart, revealing a pair of hefty but tiny autosyn units and a clockwork full of gears. Bendix equipment often used the autosyn to transmit positions over wire similar to a selsyn but using AC instead of DC.

Next time you peek into a cockpit, you’ll know what’s driving that eyeball or, at least, what might be driving it since not every one of these is identical, of course.

These cool devices show up in our feed every so often. If you can cram a CPU, a screen, and an accelerometer into a Lego, you could build one for your next block model.

13 thoughts on “Unlocking The Mystery Of An Aircraft ADI

  1. Nice description, these electro-mchanical devices are always fascinating (and with a great accent too ! ;)

    This reminds me of a French pilot joke about the ADI I heard when I was a kid.
    This pilot said, pointing to the ball: “Quand je pilote, je suis ma boule” – (“when I pilot, I follow my ball”).
    Which is in French a play on words to say: “I’m crazy”…

    Actually he was not, we landed safely.

    1. >”I follow my ball” which is in French a play on words to say: “I’m crazy”…

      It doesn’t quite translate like that though. “I am my ball” which is to say “this instrument represents me” – but then “maboul” means “crazy” – and that’s the pun.

  2. Pretty cool and ingenious device. Pretty useful too and increased aircraft safety enourmously under bad conditions. But no matter how cool or useful it’ll never replace the furry dice as vehicle option as that’s in a completely different ballpark regarding the cool factor.

    1. The turn-and-bank indicator is pretty much this.
      In general, aircraft have duplicate instruments, one relying directly on a physical input, and the other relying on a simulated version. The physical input one wobbles around a lot, and the simulated one slowly accumulates errors, so you spend some time mentally integrating the behavior of the physical input sensor and resetting the simulator to match it. Turn-and-bank-indicator/Artificial Horizon, compass/Directional Gyro, are what I’m thinking about here. At least with the modern version, pitot-airspeed/GPS airspeed, the GPS version will probably not drift.

      1. Incorrect. Both the Turn Coordinator and the ADI are vacuum-driven instruments. The only “ball in fluid” is the slip/skid indicator at the bottom of the Turn Coordinator

  3. If you want to see a student pilot sweat, say the magic words: “Partial panel”. Flight where this device has quit working.

    In the era before digital instrumentation made it all much easier there was at least one ferry pilot (pilot flying an empty airplane to a customer/repair facility) that would bring an electrically powered attitude indicator and a motorcycle battery in case the unfamiliar plane decided to misbehave. These days the digital equivalent is a common backup item, fitting where the mechanical clock used to go. they carry their own backup battery good for an hour or two of flight if the electrical system gives out and you’d be foolish to not have one.

    1. Illegal under 14 CFR. Even auto GPS and iPads with Garmin Spot pucks are not usable or reliable in the cockpit unless your operation has been approved to utilize them by the FAA. Situational awareness is one thing, but I’ve never known a freight dog to fly by the rules.

      1. >Illegal under 14 CFR.

        We’re talking about a certificated/legal but crotchety aircraft on a solo ferry/service flight that decides to fail something (the vacuum pump) mid-flight. Nothing to do with navigation other than keeping the tail skyward. These days the in-panel wonders are a real treat – AI + GPS + GS etc.

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