My friend landed at Timaru Airport and when he put the aircraft into the hangar the altimeter reading was approximately 100ft [airport elevation is 89ft]. When he got back to go flying two weeks later the altimeter reading was over 4000ft.

He replaced the altimeter with a new one and now the same thing has happened again. Has anybody got any idea about what has happened?

  • $\begingroup$ Welcome to the site! Can you add any information about the type of aircraft? It might be relevant. And did he really just replace the instrument, or did an avionics mechanic examine it first? $\endgroup$
    – Pondlife
    Commented Aug 31, 2015 at 0:57
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    $\begingroup$ Try switching to alt static source and see what happens. $\endgroup$
    – ryan1618
    Commented Aug 31, 2015 at 2:42
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    $\begingroup$ This could become a safety issue. I think he better get to the root cause whatever it is. $\endgroup$ Commented Aug 31, 2015 at 4:35
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    $\begingroup$ Sound like a static tube blockage. But to rise to 4000ft the pressure in the static tube must decrease significantly. Is running close to the exhaust / engine? I'd thoroughly check the whole static system. Ryan Brunette gives good advice to start with. $\endgroup$
    – DeltaLima
    Commented Aug 31, 2015 at 7:39
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    $\begingroup$ Was the setting still showing the previous QNH or some nonsensical value? Was the previous QNH significantly different from current one? $\endgroup$
    – Jan Hudec
    Commented Sep 2, 2015 at 5:17

1 Answer 1


This is how it works and I am surprised you switched the altimeter, unless I am missing something in your question. The altimeter reads the barometric pressure and should be set before you go flying. The airport will report the elevation and the knob on the altimeter can be turned till the altimeter is set to the correct altitude. You can confirm there is no blockage and the altimeter works correctly by getting the barometric pressure for the airport from the control tower or weather station, and it should match the barometric pressure numbers in the corner of the altimeter. Pressure changes on a daily basis and must be set each time. When we do a B check before flight, this is the exact procedure we do.

There is more information needed to properly diagnose this problem. With the lack of information I will continue on a couple of assumptions.

1) The altimeter has been bench tested and is serviceable 2) The altimeter system when installed in the plane does not match the local barometric pressure with the corresponding elevation

It was stated that the altimeter was replaced with a known good one. When this occurs the whole system must be pressure checked with a pitot-static test kit. This is a requirement since the system was opened up by replacing the altimeter. If this is not done the system is considered unserviceable. It appears this was not done.

To be complete I would check out the lowest point in the system for moisture and drain it if present.

Generally the procedures including connecting the tester to the pitot static system and taping (covering) all ports not connected. Then pressure is applied to the system, simulating different elevations. The airplane altimeter should match the testers simulated elevation within a certain + or -, for all specified elevations. Additionally, the rate of change of elevation is monitored for any abnormalities that may indicate a problem with any internal mechanisms.

Using the pitot-static tester is an absolute requirement in this situation.

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    $\begingroup$ An error from 100' to 4000' is roughly 125 mbar. That would be a surface pressure of 875 mbar and the only thing that is going to produce that pressure deficit at the surface is a tornado (the record for a hurricane is 882 mbar). That kind of error isn't just day-to-day pressure fluctuations and you probably cannot even correct for that much error with the kollsman knob if you were trying to just set field elevation. Perhaps he mistakenly connected his altimeter to a vacuum line :) $\endgroup$
    – casey
    Commented Sep 2, 2015 at 2:50
  • $\begingroup$ I agree with you it is a lot. But we are lacking more info that is why I said it is important to get the local pressure. It was stated that the altimeter was replaced. If that is the case then it is required that the whole system be pressure tested or it would be considered unserviceable. Anytime a component is removed the system has become unsealed and requires a full test with a pitot static tester. To be complete I would check out the lowest point in the system for moisture and drain it if present. My concern is why wasn't the system pressure tested after the replacement. $\endgroup$
    – Mark
    Commented Sep 2, 2015 at 21:39
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    $\begingroup$ I can edit my answer to include this? $\endgroup$
    – Mark
    Commented Sep 2, 2015 at 21:40
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    $\begingroup$ Yea, feel free to include in your post. $\endgroup$
    – casey
    Commented Sep 2, 2015 at 21:55
  • $\begingroup$ The altimeter is in a Rans S7. Ultralight, so they are Non-TSO’d. Falcon Wultrad. I have both altimeter sitting on my desk and if I set them to the current QNH they wonder off after about an hour even if the QNH remains the same. $\endgroup$
    – Art
    Commented Sep 15, 2015 at 22:11

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