when to overhaul altimeter and encoder

jsnapp

jsnapp

Pre-Flight
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Jan 9, 2012
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Westminster, CO
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BJC-Cherokee
well my 2 year pitot static check is about to expire. I have recently noticed that encoder and alt are reporting about 120' different. it appears that when the altimeter hits 5680' the encoder will change from 5700' to 5800'. it appears that the altimeter will indicate 5500' when it should indicate 5550'.

what you do guys think. is it time to have my 40 year old altimeter overhauled. should I also get the encoder overhauled / calibrated prior to the pitot static check?
 
Is this with 29.92 set?
What are you using to determine the encoder is reporting 120ft higher and only at that altitude?

If you are reading the encoder alt as reported by the transponder, remember it reports based on 29.92 and your altimeter may be set to 30.00 as required by local reports. That's 80ft difference right there.

ATC radar displays correct reported altitude based on 29.92 to local alt setting for display.
 
So the altimeter is off by 50 feet based on sitting on the ground?

When they do the transponder check, they should be able to compare the altimiter and encoder to the gauge on the vacuum pump to determine which is off. At least some encoders can be adjusted to bring them back in line with the altimeter as part of the process. I see no value in running the whole process twice - once to calibrate the encoder and once to check the encoder along with the static system.

If the altimeter is consistantly off by 50 or so feet, the Kolsman setting can be adjusted in about 2 minutes.
 
I'd council having it chrecked out before spending any unneeded money.
 
When you get the altimeter/static and transponder checks done, the shop should give you any appropriate advice on whether it's time for any work on those systems based on how they respond in the tests.
 
jsnapp said:
well my 2 year pitot static check is about to expire. I have recently noticed that encoder and alt are reporting about 120' different. it appears that when the altimeter hits 5680' the encoder will change from 5700' to 5800'. it appears that the altimeter will indicate 5500' when it should indicate 5550'.

what you do guys think. is it time to have my 40 year old altimeter overhauled. should I also get the encoder overhauled / calibrated prior to the pitot static check?
Click to expand...
I think this is why we have the test requirements.

The inspection station will dial it in for you when they do the inspection.
 
Lots of good advice above. Just go get the IFR cert. During the cert they can adjust the baro screw of the altimeter to get it withing limits if they are an appropriately rated repair station. Additionally the encoder can be adjusted as well. Depending on the type encoder, most are not worth overhauling anyway. Both these 2 tasks are done frequently on IFR certs.

If it fails the IFR cert, then deal with it, but if its just off by what you say, it can usually be dialed back in.

Also, what someone above mentioned about reading your encoder at 29.92 is spot on. Alot of folks with the Garmins or newer Kings transponders that display altitude, frequently think they are off by a few hundred feet. Always match your altimeter up to 29.92 before checking the altitude readout.
 
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