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By CloudHound
#1793345
I believe my altimeter is 3 millibar (100’ ish) wrong.

Comparing with others in the hangar, by setting each one to 0 feet, gives a different QFE on the sub scale. Mine is around 100’ low.

There’s a small screw next the the knurled knob which when removed reveals this.

So what next?
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By Flying_john
FLYER Club Member  FLYER Club Member
#1793350
You have undone the screw that allows the knob to be pushed in to decouple it from the Altimeters hands and just operate the subscale. Normally it just needs to be loosened, so I hope you can pick up the semi captive plate again when the time comes to relock the knob.

So you set the hands of the altimeter to zero, loosen the screw, push the knob, might need a wiggle too, until you are able to adjust the millibar scale without the hands moving, then set to your number of choice to match the altimeter you want to calibrate too, release the knob and replace the screw.

Probably need a couple of attempts before you get it right !

Then get the pitot static test set out and confirm all is well before entering the work up in your logs !
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By Flying_john
FLYER Club Member  FLYER Club Member
#1793390
A good printout from Misc - but be careful as there are a number of types. Some you just loosen the screw to enable the disengagement of the toothed wheel by depressing the adjustment knob (sometimes push sometimes pull) and on some (described in Misc's printout) you have to push the toothed wheel back with a small tool.
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By Miscellaneous
#1793398
Good point @Flying_john, I think the best option is to simply remove the screw and see if the adjustment can be made as described with the adjustment knob, without trying to poke around in the hole. :thumright:

Probably worth noting it is important to ensure the adjustment knob is locked on once the screw is relocated. IE the needles and the altimeter setting function as they should.
By MikeW
FLYER Club Member  FLYER Club Member
#1806301
Paul_Sengupta wrote:Any feedback on this?


Screw removed, internal locking bit moved with bit of wire, knob pulled and subscale adjusted 3mB (to match a brand new Avmap Ultra used as reference), knob jiggled back in (careful to match gear teeth), locking bit slid back, screw replaced. <5 mins.
By AlanC
#1806451
Using a GPS source as calibration brings in other errors, not least differences from the WGS84 geoid. If at a licensed airfield then current QFE would be the most accurate datum, while elsewhere the closest QNH factored back to 0 with ISA factors would make sense. EG at a strip with an elevation of 405' use the closest promulgated QNH, subtract 15hPa, then with that set the hands should indicate 0'.
By AlanC
#1806517
CloudHound wrote:Very interesting. Haven’t a clue what you mean :lol:


Hehe. Altimetry semantics often have that effect :D. Short version: using GPS as an altitude source to compare altimeters to introduces its own errors. Better to use a promulgated pressure setting from somewhere relevant.

Yes, I see a few altimeters cunningly set to 0 at the top of tower steps, the really sophisticated ones have a Sharpie mark on the edge based on exhaustive calibration and hanging weights out the window!
By MikeW
FLYER Club Member  FLYER Club Member
#1806537
AlanC wrote:Using a GPS source as calibration brings in other errors, not least differences from the WGS84 geoid. If at a licensed airfield then current QFE would be the most accurate datum, while elsewhere the closest QNH factored back to 0 with ISA factors would make sense. EG at a strip with an elevation of 405' use the closest promulgated QNH, subtract 15hPa, then with that set the hands should indicate 0'.


Totally irrelevant.
It's barometric. Pressure transducer.