I've noticed that some planes cage their attitude indicators when Parked and uncaged them after start. As seen in this Lear 35 checklist. Why is this?
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2$\begingroup$ How do you know this attitude indicator is caged? I don't see a caging knob. Also, on what type of aircraft have you noticed the attitude indicator being caged while parked and uncaged after start? Thanks $\endgroup$– user22445Commented Nov 17, 2022 at 0:12
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3$\begingroup$ I don't see (and have never seen) a checklist item for caging after shutdown / when parking. Is this just an assumption on your part, based on the startup action? The startup action is just a check: obviously the AI won't work if caged, so it's important to check that it's not before flight. (On most [all?] AIs, a flag will be displayed when caged.) The presence of the check doesn't imply an expectation, in the same way "circuit breakers... checked" doesn't imply that the circuit breakers should be pulled after shutdown. $\endgroup$– TypeIACommented Nov 17, 2022 at 12:12
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1$\begingroup$ @Boeing787 OK, then you're correct, and that's new to me. I look forward to the answer to why. $\endgroup$– TypeIACommented Nov 17, 2022 at 13:35
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1$\begingroup$ Can you post the shutdown checklist that shows to cage the standby AI? Thanks $\endgroup$– user22445Commented Nov 17, 2022 at 14:17
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3$\begingroup$ If not caged the gyro can wobble and precess quite drastically while spinning down and could damage it. So: cage before letting it spin down and uncage only after it’s been spun up and erected. Same before doing extreme aerobatics in some planes. $\endgroup$– JimCommented Nov 18, 2022 at 6:57
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1 Answer
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The standby attitude indicator on the Lear 35 has its own power switch; it's not automatically energized when the battery masters come on. Therefore it doesn't spin up on its own when power is applied to the rest of the aircraft.
If it's uncaged before it spins up, it could be damaged, and unlike the other gyros, it doesn't begin the spin up process until you flip the switch. So you power it on and leave it caged until it's up to speed, then you uncage it. Similarly during shutdown, you cage it first and then remove power so that it can spin down safely.
A - Standby attitude indicator
B - Caging knob
C - Power switch
Original image by Don Popp, found here. Image is in the public domain.
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1$\begingroup$ Would this be the case for any plane? $\endgroup$ Commented Jan 9, 2023 at 19:40
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1$\begingroup$ @Boeing787 - I doubt it. I haven't seen this on other airplanes, so I assume whatever mechanism a normal gyro uses to protect itself while spinning down, the Lear 35 standby gyro doesn't have. $\endgroup$– Steve V.Commented Jan 10, 2023 at 18:46