Accuracy of altimeter QNH settings

[F Smallwood]

I find when I set QNH before take-off to altitude of airfield I need a higher QNH setting than other pilots. In consequence I have stopped quoting my QNH setting when reporting position on radio as I feel it may distort information to other pilots. It is not due to meteorological conditions as the altitude is correct when I return to base. I typically have to set the QNH 2 to 3 hPa higher than other aircraft are broadcasting to get airfield altitude. This could obviously be an issue when flying into controlled airspace if the tower tell me to set QNH to 1021 and mine is reading 1024. Should I worry about this? How/is it possible to "calibrate" QNH on a Skydat system? If so can an AP do this. I am scared to fly for example pinedene route until it is sorted out. if my calcs are correct 1hPa is equivalent to 30ft so a couple of hPa would make a significant difference.

Any advice/flaming will be apreciated.

Cheers Frank

[WhiteEagle]

Very good question F Smallwood. I know what the altitude is at our manned earodrome. When ATC gives QNH it is about 120ft (also 4hPa) below what it should be (who am I to differ). Flying low level "above the surf" , I am actully submarining 60ft below according to altitude reading. Also SKYDAT system.This has bothered me for a while now as well. Will try and remember next time when me and my mates fly to go to the same reading level and see what happenes. Would also like to hear from anyone else, Thanks.

[MAGNIficent]

The unit must be RE-calibrated.

Agent for your SKYDAT, surely must be able to give you information.

AMPtronic
email: amptro@iafrica.com
Tel/Fax : +27 (0)11 679 27 54
Mobile: +27 (0)73 31 55 757

[nickjaxe]

Deleted due not reading the original post correctly.

[Bundy]

Interesting thread Frank, I also seem to have about a 2 bar error on our skydat. I called the above no and chatted to the guy (I think it was Serge?) and he said they can recalibrate it. Unfortunately you may have to wait till Jan as they are about to close.

My problem is that not only is the QNH out...my ASI is underreading by 8-10mph! He told me that this is commonly caused by a small crack/puncture in the pipe leading to the pressure sensor...I'll have to open it up and see.