Is My AI About To Bite The Dust?

Stingray Don

Stingray Don

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Stingray Don
I pulled out the ol' 172 the other day and had an issue with the AI. As the gyro began to spool up, the AI went inverted and stayed there. Now I was always told to trust the instruments no matter what it feels like in the plane, so I must have been inverted on the ramp. I checked the POH and couldn't find a required runway length for an inverted takeoff so I figured I would just shut it down. As the gyro began to spool down, it began spinning around like a clothes dryer. Once it stopped, I started the engine again and it showed about a 30 degree left bank. After several minutes it ever so slowly began to right itself. Now it seems to have a slight case of the leans with maybe a couple of degrees left bank even when sitting on the ramp. The vacuum system was operating properly and the gauge was in the green, so I don't think there is an issue with the vacuum system.

So is this a symptom of an AI about to fail? How much does it cost to overhaul on of these bad boys?
 
Are you sure you weren't inverted? Did you cross check with your turn indicator? :D
 
Stingray Don said:
I pulled out the ol' 172 the other day and had an issue with the AI. As the gyro began to spool up, the AI went inverted and stayed there. Now I was always told to trust the instruments no matter what it feels like in the plane, so I must have been inverted on the ramp. I checked the POH and couldn't find a required runway length for an inverted takeoff so I figured I would just shut it down. As the gyro began to spool down, it began spinning around like a clothes dryer. Once it stopped, I started the engine again and it showed about a 30 degree left bank. After several minutes it ever so slowly began to right itself. Now it seems to have a slight case of the leans with maybe a couple of degrees left bank even when sitting on the ramp. The vacuum system was operating properly and the gauge was in the green, so I don't think there is an issue with the vacuum system.

So is this a symptom of an AI about to fail? How much does it cost to overhaul on of these bad boys?
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Sounds like the perfect time to upgrade to a dynon.
 
Don't be too quick to overhaul or replace the AI, your problem could still be in the vacuum system depending on how your gauge is plumbed. If it's connected to measure the pressure delta across the AI and there's no possibility of a collapsed hose between the low pressure input to the AI and gauge then your diagnosis is very likely correct. But many airplanes have the suction gauge connected between the regulated vacuum source and the cabin. With that setup any filter or other obstruction between the AI and ambient air would have a normal reading on the gauge but limited airflow through the AI. And even if the suction gauge is connected to both ports on the AI, any hose on the suction side that's only feeding the AI could collapse and block airflow through the AI with a normal reading on the gauge.

As to the slow eventual erection that occurred after a restart, that could be "normal" behavior. It's common to see that same behavior when you shut down the engine just long enough for the gyro rotor speed to decay to the point where it starts to roll over and then restore the vacuum source. This happens because the erection force from the "pendulous vanes" has to overcome significantly higher rotary inertia at that point than the same force does when the rotor is barely spinning.
 
:ohsnap: He typed erection!

Since you're seeing symptoms in both the AI and the DG, start by check those vacuum lines and filters. Then buy the Garmin G5.
 
rolivi said:
:ohsnap: He typed erection!

Since you're seeing symptoms in both the AI and the DG, start by check those vacuum lines and filters. Then buy the Garmin G5.
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And "Pendulous" which at least sounds perverted. Reminds me of the time in 4th grade when I got razzed for mispronouncing sheet of paper while attempting a southern accent. :p
 
It has failed,time for an overhaul,or replacement.
 
gismo said:
And "Pendulous" which at least sounds perverted. Reminds me of the time in 4th grade when I got razzed for mispronouncing sheet of paper while attempting a southern accent. :p
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Southern accents are best left to the professionals.

<------ former Southern accent professional.
 
Do you have a polaroid to prove you were inverted? #becauseiwasinverted
 
Joshuajayg said:
Do you have a polaroid to prove you were inverted? #becauseiwasinverted
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No pic but this does demonstrate how powerful spatial disorientation can be. I would have sworn I was sitting on the ramp with wings level and right side up. However, we all know we must always trust our instruments no matter what our body is telling us! :D
 
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Stingray Don said:
No pic but this does demonstrate how power spatial disorientation can be. I would have sworn I was sitting on the ramp with wings level and right side up. However, we all know we must always trust our instruments no matter what our body is telling us! :D
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There's something in that instrument flying handbook about identifying failed instruments I think. Either way, if there's no Polaroid, it didn't happen, Maverick.

 
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