Why would an altimeter be incorrect?

[dcat127]

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.

 

[flyingron]

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.

 

[dcat127]

That would make more sense according to my understanding of how an altimeter works.

 

[MauleSkinner]

Is the tubing for one of them near the heat vent?

 

[Dan Thomas]

During the test flight I noticed that while both altimeters matched on the ground,

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.

 

[bonefishpete]

Why two altimeters in a Saratoga ? Wouldn’t an avionics instrument shop be better?

 

[brcase]

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.

My 1st guess, one altimeter is not connected to the Static port and is just vented to the cabin.

Brian

 

[Bell206]

But I am puzzling, how would an air-leak cause such a large discrepancy in an altimeter?

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

 

[dcat127]

Why two altimeters in a Saratoga ? Wouldn’t an avionics instrument shop be better?

It has a gi275, along with the original altimeter.

 

[dcat127]

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

How is the static system vacuum based?

 

[dcat127]

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.

Both were set to the same barometric setting. On the ground they both showed the same expected elevation.

 

[Checkout_my_Six]

So....maybe they both don't share the same static source.....and one of them is seeing cockpit pressure.

 

[Greg Bockelman]

500 feet?!? I would expect maybe 150 feet but not 500.

 

[Bell206]

How is the static system vacuum based?

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.

 

[sansoneservices]

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.

Check static lines for leaks (will read cabin pressure), check if they share a static source, check exterior static ports for insect plugs

 

[John Collins]

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.