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I was up flying today – VFR – and the attitude indicator tumbled. It's a 6-year-old RC Allen 22-7, without the ability to cage, so I just wound up letting it be all off-kilter until things stabilized... which took almost an hour. The vacuum was in the green the entire flight, and at no point was I doing any wild maneuvering.

Is there anything I could have done to accelerate the process? Could I have caused damage by not landing and shutting down?

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  • $\begingroup$ If it took an hour to stabilize, I'd certainly talk to maintenance about it. The chance of survival in an AI failure is 1 in 20... $\endgroup$ – kevin Apr 19 '15 at 6:14
  • $\begingroup$ That's definitely happening - but I realized I didn't know the process (or expectations) for older model AIs! Maybe the process is to replace it with a newer one. $\endgroup$ – egid Apr 19 '15 at 6:30
  • $\begingroup$ @kevin I'm not doubting you, but do you have a reference for the odds, or is it an educated guess? $\endgroup$ – CGCampbell Apr 19 '15 at 11:30
  • $\begingroup$ @CGCampbell It's a stat mentioned in one of the AOPA seminars. They mentioned several emergencies : VFR into IMC, AI failure, engine failure after takeoff, smoke in the cockpit. The speaker is a guy who has 6000 hours in a 172. I don't remember which one, but it's on YouTube. $\endgroup$ – kevin Apr 19 '15 at 12:43
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    $\begingroup$ @kevin No problem in VFR though. Odds of survival, 100% $\endgroup$ – Simon Apr 19 '15 at 19:07
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Only a mechanical inspection is likely to give you a definitive answer for why the AI tumbled, but typically it'll happen for one of two reasons: either the vacuum pump has decided to take a break, in which case the AI gyros slow down and will eventually no longer hold the instrument up, or you've exceeded the instrument's pitch and/or bank limits and caused it to tumble.

In the case of the former, there's not much you can do in a stock -172 about that. Section 3 in the PIM just tells to verify that the vacuum pump is operating with normal parameters (green arc is 4.5-5.5 in.hg). I've seen PA-28Rs with electric vacuum pumps that served as standbys to the engine-driven ones, but never seen one in a -172, not even as an STC mod. You could always try increasing the throttle; pumps in older aircraft will sometimes fail to perform as expected at lower throttle settings, but there's no guarantee there.

The latter case can happen during maneuvering flight if you exceed +/-30 degrees of pitch and 60 degrees of bank on a non-acrobatic AI. This can inadvertently happen when certain training maneuvers such as power-on or cross-controlled stalls are performed... enthusiastically. I've also seen tumbling occur during spin training. And then there was the Husky we used for banner-towing and which was pitched to 45 degrees on a regular basis during banner picks (waivers are wonderful things). The AI tumbled so regularly, the owner gave up on having mechanics reset it.

But yea, should this happen to you again, run any checklists that might exist on the topic (either directly concerned with the AI or with the vacuum pump system), and switch to partial panel ops if necessary.

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    $\begingroup$ The pump was in the green (and is plumbed off the back of the AI, which is actually a 6-year-old RC Allen 22-7), and there was no unusual maneuvering, which is partly why I was surprised. It's been ok on a subsequent flight so who knows. $\endgroup$ – egid Apr 21 '15 at 21:46
  • $\begingroup$ That is unusual, but probably it's not something would warrant a more detailed inspection, especially if it recovered okay and worked subsequently, so yea. $\endgroup$ – habu Apr 21 '15 at 21:57
  • $\begingroup$ The (now defunct) club I used to fly at had 5 C172P aircraft, and as I recall all of them had electric standby vacuum pumps. $\endgroup$ – Fred Larson Apr 22 '15 at 16:43
  • $\begingroup$ @FredLarson do you remember if this was a stock configuration on the P model or an after-market mod? $\endgroup$ – habu Apr 22 '15 at 19:00
  • $\begingroup$ @habu: I'm not sure I ever knew which it was. Might have been an STC add-on. But I have a C172P pilot manual at home. I'll see if it mentions anything about it. $\endgroup$ – Fred Larson Apr 22 '15 at 19:06
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You indeed have some kind of failure. Except for performing aerobatics, there is nothing a pilot can do to harm the gyro.

The AI itself is probably failing. But even though vacuum shows green, there's an unlikely chance the instrument itself is not getting adequate suction. The vacuum lines should be checked at least visually before sending the unit in for repair.

Often times when a gyro starts to fail, you will notice it slow to stabilize when first starting the engine. A healthy attitude indicator will become erect almost immediately. If it wobbles for more than a short moment, be suspicious. Also become suspicious when a gyro become loud or whines. You should be able to hear the gyro if you get close, but modern healthy gyros are pretty darned quiet.

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