4

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?

enter image description here

enter image description here

Boeing787
  • 6,454
  • 3
  • 33
  • 82
  • 2
    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 –  Nov 17 '22 at 00:12
  • @757toga I've seen it on fighter jets and older model learjets as well. I put this photo just to give people an idea. – Boeing787 Nov 17 '22 at 00:19
  • @757toga I've updated the picture if it makes more sense. – Boeing787 Nov 17 '22 at 04:42
  • 3
    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. – TypeIA Nov 17 '22 at 12:12
  • @TypeIA it says to make sure standby attitude indicator is off and caged at shutdown https://www.google.com/url?sa=i&source=web&cd=&ved=0CAMQw7AJahcKEwig99v9pbX7AhUAAAAAHQAAAAAQAg&url=https%3A%2F%2Ftomgorski.com%2Fasr%2FLear%2FLR35_Checklist_ATI.PDF&psig=AOvVaw0bGavFEmCJXiWR2w1jegto&ust=1668777277355921 – Boeing787 Nov 17 '22 at 13:15
  • 1
    @Boeing787 OK, then you're correct, and that's new to me. I look forward to the answer to why. – TypeIA Nov 17 '22 at 13:35
  • 1
    Can you post the shutdown checklist that shows to cage the standby AI? Thanks –  Nov 17 '22 at 14:17
  • @757toga I uploaded it – Boeing787 Nov 17 '22 at 15:58
  • 1
    Good enough. I will be interested if someone can explain why the caging is done/necessary upon shutdown. Thanks. Upvoted...good question. –  Nov 17 '22 at 15:59
  • 1
    I thought caging after parking protects the AI from movement/jarring while being towed. – Juan Jimenez Nov 18 '22 at 01:04
  • 3
    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. – Jim Nov 18 '22 at 06:57
  • @Jim this looks like an answer! – jcaron Jan 08 '23 at 17:38

1 Answers1

6

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

Image of Lear 35 cockpit with standby attitude indicator, caging knob, and power switch marked

Original image by Don Popp, found here. Image is in the public domain.

Steve V.
  • 23,246
  • 14
  • 99
  • 162
  • 1
    Would this be the case for any plane? – Boeing787 Jan 09 '23 at 19:40
  • 1
    @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. – Steve V. Jan 10 '23 at 18:46