Aititude Indicator shutdown procedure

[Bennett]

It is true. You can learn something new every time you fly. I recently had a mild "bobbing" in pitch when in altitude hold (KAP150). A bit annoying, but not a real problem. I took the airplane to LAC Avionics at San Jose to have repairs made. They tested the AI, KAP 150, and the pitch servo. The AI was failing, running too slowly, and the pitch output voltage was erratic. They suggested a rebuilt unit by Mid-Continent, and I concurred. The "new" AI was calibrated for the KAP 150, and I picked up the airplane and flew her home - no more "bobbing". The next time I prepared to fly, I noticed that the AI was not erected. It was tumbled about 90 degrees, and would not come back to normal after engine start. I have a 2" vacuum gauge, and it showed 5"(right in the middle of the normal range). I turned on my electric back-up vacuum pump which raised the vacuum to 5.2", but did not cause the AI to erect. Dang! (and other such words). I obviously surmised that the newly rebuilt AI was defective, and LAC agreed to have Mid-Continent ship out a replacement. They did so, and while I don't know if it really was initially defective, (nor did the shop), they did tell me about shutdown situations that could cause an AI to tumble this way. In all my years of flying, and in all sorts of aircraft, I had never seen this. There are a few discussions available on the web that address this: "When the engines and the vacuum stop, and when the electricity is turned off, the erection system no longer functions. Normal drifting factors together with the dissipation of braking rotor energy around the axis of rotation necessarily results in precessional force and a slow toppling action. The gyro will slowly drift off vertical both in pitch and roll. It will take 5 minutes to stop when freshly overhauled. It will take as much as 20 minutes at the end of its service life to come to a stop. IT IS IMPORTANT THE AIRCRAFT (OR UNIT) NOT BE MOVED DURING THIS SPOOL-DOWN PERIOD. Such a turning or bumping can precipitate a violent toppling and/or motoring of the whole mechanism. At the least the centrifugal forces of this will change the calibration. At the worst the force is quite capable of breaking off parts. The Honeywell-King Service Memo 377 on the KI256 stresses the importance of not moving the aircraft for 15 minutes after the engine is powered down. Jet Service Letter SL-35A recommends 20 minutes. Most vertical gyros have solenoid brakes or thermal devices to prevent this destruction. In any event without electric power the flight director will not show the vertical gyro's behavior. But it is still good practice to wait till gyros stop." The link: http://1journey.net/1candle/av/FL-107/FL-107.htm Worth reading. When I returned to San Carlos right after the new AI was installed, I stopped perpendicular to my hangar (tight restraints between tie downs and my hangar row), shut down the engine normally, opened the hangar doors, and used my RoboTow to turn the airplane ninety degrees, and pushed it into my hangar. Apparently this change in direction, with a "new" AI, that takes a different time to spool down, caused this problem, as parts can break within the AI in such a situation. No matter what, LAC and Mid-Continent stood up, and there was no charge for the replacement unit, of for the time it took to re-calibrate the newest AI. I thought I would pass this along to the forum. I supect that some knew this already, but this information does not appear to be in wide distribution.

[Steve Dawson]

Thanks for the info Bennett. Definately going to change my shut down procedures.

[duke]

I too did not know that--thanks for the info

[N33GG]

And while you are at it, don't push or pull on the prop. Bah humbug!

[Bennett]

According to the avionics techies I have been working with, it takes a very narrow set of circumstances - AI spinning down, and at a critical RPM, a sharp turn, and the AI WILL tumble, and damage often occurs. The window is small - see article I posted above, but it does happen, a little caution makes sense. An overhaul and re calibration is not cheap. They tell me that they get calls from time to time from pilots who have shut down their engines "normally" and the AI or DG has tumbled and won't erect. All to often, they say, the pilot is at a small field, away from their home base, and they want instructions as to how to get the AI to erect. Those of us who have an auxiliary electric vacuum pump, and patience, have an advantage in this sort if situation. If the AI will not erect when you start the engine, shut it down, and turn on the electrical pump - leave it on, might take several minutes. Or even one or two tries, but the AI should start to level off. At that point start the engine, and the AI should work normally. The LAC folks showed me a disassembled AI and you can see that the air vanes are out of the air stream when the gyro is at 90 degrees.

[N33GG]

I am very familiar with the design and manufacture of gyros. I have seen many of them assembled new and torn down after use. I fear no tumble.

[AmigOne]

Very interesting, thank you.   Amazing and scary how much I didn't know about this instrument.   Perhaps because in all  my years of flying I only had one AI failure, and it was after coming out of more than 2 hrs of solid IMC!! It was certainly news to me that the AI can be damaged by just moving the airplane after shutdown and I was also surprised by the amount of time required for the unit to work properly on very cold days.  I'm based on a non-towered field and have launched into intruments certainly in much less than the 40' which the article states it can take for the AI to work properly.  I will be using a borrowed Dynon D1 on my flight to Sun&Fun next week and I think I'll get one.

[Bennett]

AmigOne, Over the years I have always trusted my AIs more than vacuum pumps, hence the back-up electrical auxiliary vacuum pump. The article I posted above was startling to me, and I might have dismissed it, had I not had the experience. My conversations with the LAC techies pointed out that this tumbling only can happen in a specific set of circumstance - RPM and sharp turning, but it does happen. I understand that some AIs have a "quick erect" button (no Viagra jokes, please) to deal with this situation.

My experience with the Dynon D-1 is generally good. I do not use their external antenna, but the windscreen mount gives reasonable sky coverage. In the panel mount option, I would use the external antenna and cig lighter power. As much as I like the Dynon, if you can push the budget, one of the electrical back up AIs - Mid-continent, Castleberry,RCA, to name a few, would be a better idea, in my opinion.

[bd32322]

Bennett - can the damage show up in the middle of the next flight after the botched parking job with spinning gyros? I am not very familiar with internal gyro ops except for the baiscs in pilot training.

If the damage is a tumbled gyro and clearly shows up before the next flight - then I can rest easy. If it can show up as fine in the preflight but fail mid-flight, that would worry me more...

[Bennett]

I'm not the guru here. All I know comes from a few conversations with avionics techies, and the article I posted above. My personal experience with an AI. failure while flying (in IMC) was the result of a vacuum pump failure that sent filter debris into the AI itself. The use of the backup electric vacuum pump was thwarted, and I was forced (and lucky) to descend to VFR conditions. This is a very interesting topic for all of us who depend on our AIs I hope that forum members with technical expertise would join the conversation.

[jetdriven]

A Clearview CV1J4 filter will keep pump debris out of the gyros.  This is an M20J, mount it to the firewall with a Clickbond stud and a MS21919-DG50 clamp.