Altimeter malefunction. What's wrong?

chrisbre

chrisbre

Filing Flight Plan
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Jul 16, 2014
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Hamburg/Germany
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chrisbrenner
My aircraft sat on the ground for 15 weeks without being moved in a quite humid hangar (no idea if this was a factor). When I flew her home, I noticed the altimeter showing 500ft wrong. So I re-adjusted it by loosen the screw next to adjustment knob. For the next flight, the indicator just worked fine (could crosscheck with 3 different altitude sources). However, a couple of days later, the altimeter again showed a "too high"-error when adjusted to the correct altimeter setting (been at a controlled airport with ATIS). The error now increases from day to day. Apart from that the altimeter still works "fine". I know the instrument has to be send to an avionics shop for overhaul, but I'm really curious what technical issue causes that error. The indicator has been installed only four years ago.
Any ideas?
 
Slight static blockage (could be water, could be something else). However, I'm real concerned. There's no pilot legal adjustment on the altimeter (Other than the altimeter setting knob). Have a proper facility check out the static system. You'll need to be recerted (if you fly IFR) anyhow, you've already made your plane unairworthy.
 
No reason to worry. Sure I'm aware of the regs. Of course I have an A&P installed a new indicator last week. Got a fresh pitot static check as well. The new one works perfectly, so water in the static system obviously is not a factor. I will let the shop overhaul the broken indicator and keep it as a spare part. But still I'm curious what's the technical reason for the malfunction.
 
While your bird is in the shop, get one of them AOAs installed. It'll ...well you know. ;)
 
Altimeters can get "sticky". Been there, done that, paid the $250 or whatever to have one repaired.
 
Check pitot for block (spider web, dead insect, crud). Static pressure sensor for block (crud). Water in pitot system freezing and thawing.
 
chrisbre said:
So I re-adjusted it by loosen the screw next to adjustment knob.
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I've never seen an altimeter adjusted that easily. I'm guessing that your fiddling removed some mechanical error, which is why many altimeters have internal vibrators. And you must be half crazy to attempt that adjustment without qualification and not have a pitot static check afterwards.
 
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I'm curious how you knew for sure it was off and exactly how far in the first place.
 
Salty said:
I'm curious how you knew for sure it was off and exactly how far in the first place.
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That wasn't too hard... Set altimeter according to ATIS, landed at my home airport and checked indication for field elevation after landing.
 
chrisbre said:
That wasn't too hard... Set altimeter according to ATIS, landed at my home airport and checked indication for field elevation after landing.
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And you are certain atis does not have an issue? How old was the atis info? Even just an hour lag in reading could be 100 feet off easily. I've heard atis that we're lagging much more than an hour.
 
GlennAB1 said:
He said the new altimeter has fixed the problem.
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The old one that wasn't screwed with might also be "fixed" when the atis information is up to date and accurate.

It seems like a real bad idea to screw with your altimeter without the proper equipment to calibrate it. JMO
 
ATIS required at least once hourly. More often with significant weather changes. Bring it to someone's attention if that's not occurring, at least once an hour when the new weather come out at the top of the hour.
 
mscard88 said:
ATIS required at least once hourly. More often with significant weather changes. Bring it to someone's attention if that's not occurring, at least once an hour when the new weather come out at the top of the hour.
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I'm not saying ATIS had anything to do with it. I'm saying it could explain why the altimeter was off, and then after adjustment it was off again. Personally, I wouldn't use my local airports ATIS to calibrate my altimeter. There is equipment specifically designed to do this task properly.
 
Salty said:
I'm not saying ATIS had anything to do with it. I'm saying it could explain why the altimeter was off, and then after adjustment it was off again. Personally, I wouldn't use my local airports ATIS to calibrate my altimeter. There is equipment specifically designed to do this task properly.
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Salty said:
And you are certain atis does not have an issue? How old was the atis info? Even just an hour lag in reading could be 100 feet off easily. I've heard atis that we're lagging much more than an hour.
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I was addressing your comment above if the ATIS was more than an hour old. No worries.
 
Salty said:
The old one that wasn't screwed with might also be "fixed" when the atis information is up to date and accurate.

It seems like a real bad idea to screw with your altimeter without the proper equipment to calibrate it. JMO
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Of course. But "on wing", it would only be adjusting, then checking, the altimeter to make sure it was within calibration tolerances. Calibration is shop/bench level maintenance, altimeters are hermetically sealed.
 
mscard88 said:
I was addressing your comment above if the ATIS was more than an hour old. No worries.
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Understand. I wouldn't expect an ATIS to be more than an hour old, but it happens.
 
Pretty sure your AOA is fowled
 
I had a blockage in my static system and had a mechanic look at it. I got the log books back and it said he removed a "doober" from the static line. I was beginning to wonder who had been getting high in my pitot-static system when Margy pointed out he probably meant dauber (as in a wasp).
 
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