Why would an altimeter be incorrect?

dcat127

dcat127

Pre-takeoff checklist
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I recently bought an Saratoga, it is currently in the shop getting various items resolved that were discovered during the prebuy. During the test flight I noticed that while both altimeters matched on the ground, in flight they show about a 500ft discrepancy. My mechanic's comment was that it is likely a small leak in the air line going to the one altimeter. He is a good mechanic and I have no doubt he will figure out what the issue is. But I am puzzling, how would an air-leak cause such a large discrepancy in an altimeter? If the hose was completely disconnected, would it not simply show the cabin altitude, which would likely vary a bit, but not 500 feet.
 
Sounds more like an obstruction than a leak. I had a completely blocked static system once (this was five weeks after 9/11 when they finally let me get my plane out of Potomac Airfield). It was reading about 100-200 feet AGL even though I was much higher.
 
That would make more sense according to my understanding of how an altimeter works.
 
dcat127 said:
During the test flight I noticed that while both altimeters matched on the ground,
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Were both altimeters set to the same barometric setting, or just to airport altitude?

If they are connected to separate static sources, one of those sources might have something wrong with it. Or someone blew into the static port and ruined that altimeter.
 
Why two altimeters in a Saratoga ? Wouldn’t an avionics instrument shop be better?
 
Dan Thomas said:
Were both altimeters set to the same barometric setting, or just to airport altitude?

If they are connected to separate static sources, one of those sources might have something wrong with it. Or someone blew into the static port and ruined that altimeter.
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:yeahthat::yeahthat:
My 1st guess, one altimeter is not connected to the Static port and is just vented to the cabin.

Brian
 
dcat127 said:
But I am puzzling, how would an air-leak cause such a large discrepancy in an altimeter?
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Keep in mind the altimeter is off the static which is vacuum based. So yes a leak in the static system in flight can easily give you a discrepancy like that
 
Bell206 said:
Keep in mind the altimeter is off the static which is vacuum based. So yes a leak in the static system in flight can easily give you a discrepancy like that
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How is the static system vacuum based?
 
Dan Thomas said:
Were both altimeters set to the same barometric setting, or just to airport altitude?

If they are connected to separate static sources, one of those sources might have something wrong with it. Or someone blew into the static port and ruined that altimeter.
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Both were set to the same barometric setting. On the ground they both showed the same expected elevation.
 
dcat127 said:
How is the static system vacuum based?
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Only in the context of how the system works vs pressure system indicators. Its also the reason you pull a vacuum on a static system to leak check it.
 
dcat127 said:
I recently bought an Saratoga, it is currently in the shop getting various items resolved that were discovered during the prebuy. During the test flight I noticed that while both altimeters matched on the ground, in flight they show about a 500ft discrepancy. My mechanic's comment was that it is likely a small leak in the air line going to the one altimeter. He is a good mechanic and I have no doubt he will figure out what the issue is. But I am puzzling, how would an air-leak cause such a large discrepancy in an altimeter? If the hose was completely disconnected, would it not simply show the cabin altitude, which would likely vary a bit, but not 500 feet.
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Check static lines for leaks (will read cabin pressure), check if they share a static source, check exterior static ports for insect plugs
 
Which altimeter reads higher? One with a leak will read higher. Both altimeters are likely to be connected to the same static source, so there may be an issue in one of the indicators. You should have an avionics shop test the altimeters and static system.
 
Giving an update on what the problem actually was...turns out it was the altimeter itself. Upon closer examination the needle would jump to roughly the correct place when you tapped on the instrument.
 
dcat127 said:
Giving an update on what the problem actually was...turns out it was the altimeter itself. Upon closer examination the needle would jump to roughly the correct place when you tapped on the instrument.
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I flew "home" through Atlanta's airspace several years ago - dropped to 3500' to get under the shelf. It was like the airplane was on rails - didn't have to correct the altitude at any point on the ride. I had the airplane trimmed perfectly.

And when I dropped into the pattern at the home field? Still 3500'. And when I landed and rolled it into the hangar? 3500'.

I didn't get a phone call, so maybe I did hold altitude that well...

Turned out to be a failed/sticky altimeter.
 
dcat127 said:
Giving an update on what the problem actually was...turns out it was the altimeter itself. Upon closer examination the needle would jump to roughly the correct place when you tapped on the instrument.
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Second step in EVERY abnormal… tap the gauge. Young people keep telling me “you know there’s not really a needle there…”

I tell them to keep doing it until they quit messing up their abnormals!
 
Tools said:
Second step in EVERY abnormal… tap the gauge. Young people keep telling me “you know there’s not really a needle there…”
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Sometimes you gotta knock the electrons loose.

Naugga,
no harder than the bridge of his nose
 
The aneroid bellows (heart of a mechanical altimeter) can be affected by various things (fatigue, work hardening, shock), as well as the gear train between the bellow and the needles (worn, dirty, lacking lube, etc.)
 
40YearDream said:
The aneroid bellows (heart of a mechanical altimeter) can be affected by various things (fatigue, work hardening, shock), as well as the gear train between the bellow and the needles (worn, dirty, lacking lube, etc.)
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Which of those do you fix by tapping on it?
 
dcat127 said:
Which of those do you fix by tapping on it?
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You're basically overcoming the friction in the altimeter mechanism by tapping on it. There is a specific bench test to check this friction while under a vibration. The cause of this excess friction can be one of the items mentioned above.
 
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