Tapping on the altimeter

J

Jim Logajan

Administrator
Staff member
I started my flight training in gliders and one thing I picked up was it helped to tap on the altimeter. Otherwise the needle may sometimes move in jumps as friction seems to make movement a bit sticky.

Never did that in a powered airplane because the engine adds sufficient vibration to keep things moving. Until a few days ago while taking off in the club plane. The vertical speed indicator was showing about 400 fpm. My iPad GPS-fed vertical speed and altimeter showed similar vertical speed and steadily increasing altitude. But the panel's analog altimeter showed no change in altitude. Tapped on it and it shot up. Damn. After that the altitudes on the panel and iPad started to get close to each other and move roughly in sync. Maybe the engine needs to generate more vibration? ;)
 
Bob Noel said:
Maybe you could pull the sparkplug wire off one cylinder...


Just kidding.
Click to expand...
Or stick a wad of gum on the outside edge of a prop? Just kidding too!

Bob Noel said:
If it were my airplane, I'd be tempted to put in a new altimeter.
Click to expand...

I'm pretty sure I've seen hesitant altimeters before, mostly during climbs after takeoff when I'm looking at it more frequently to see when I've reached a specific altitude such as 300 ft below pattern altitude. Just did a quick search of "sticky altimeter" and came up with several hits, this one about a new altimeter sticking because it was tested outside an airplane:
https://vansairforce.net/community/showthread.php?t=1819

This FAQ suggests it's getting old:
https://tghaviation.com/aircraft-instrument-services/frequently-asked-altimeter-questions/

The pointers on my altimeter are very jumpy and stick sometimes.
What is wrong?

The altimeter is exposed to the outside atmosphere. This includes all of the dirt and dust present in that atmosphere. Dirt and dust will get into the gears and cause them to stick and bind, the vibration from the aircraft will help the gears to overcome this problem but they will be very jumpy and become worse as the unit gets older. This unit needs an overhaul.
 
Jim Logajan said:
this one about a new altimeter sticking because it was tested outside an airplane:
Click to expand...
FYI: 43 Appx E allows altimeters to be tested under vibration and most shops do. However, as I recall, the friction limits are only tested under decreasing altitudes and not increasing altitudes. Another common cause of altimeter indicating issues is a leaking static system and usually the first thing I would check.
 
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