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Transponder Pressure Altitude vs Altimeter QNH Corrected Altitude


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When on area frequency, I often hear ATC report position and altitude of an unknown aircraft to a known aircraft. The source of the unknown aircraft's altitude will be the transponder pressure altitude which is referenced to 1013 hPa (not compensated for QNH). If the QNH is high at say 1031 hPa, then the height difference between the transponder's pressure altitude and an aircraft altimeter altitude set to QNH would be (1031 - 1013) x 30 = 540 feet. So what altitude is the ATC reporting? Is it the transponder pressure altitude or a QNH corrected altitude to reflect what an aircraft altimeter would be reading? I am hoping it would be the latter. Eric

 

 

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The Transponder reports Pressure Altitude, and the ATC software system uses the local QNH to see the aircraft altitude.

 

Prevents the dumb pilot reporting wrong altitudes cause he has the wrong QNH, thus assisting noise minimisation.

 

 

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The Transponder reports Pressure Altitude, and the ATC software system uses the local QNH to see the aircraft altitude.

Just to clarify,.... Local QNH as in relevant to the aircraft location, not the " local QNH" you might have set before departing Bob's farm

 

 

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Just to clarify,.... Local QNH as in relevant to the aircraft location, not the " local QNH" you might have set before departing Bob's farm

They are pretty damned good, though.

 

I was lazy once and rounded of my altimeter reading of 2700 to 2500' when asked and was told by Melbourne Radar to get my transponder checked!

 

Saturday, Centre on 122.4 called an un-identified aircraft 2 NM east of YEUA at 2500 after the meat bomber asked for a traffic advisory as it departed. It was me and he had me nailed with both location and altitude (there is no established drop zone there and I actually saw the jump aircraft start its take off roll).

 

Kaz

 

 

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Also depends on how well your pitot systems are working. The ATC asked us one day if we were really at 22,000 ft over the Hawkesbury Bridge. When we said no, 2,000 ft they suggested brusquely that we turn the TXPNDR off Alt and get it checked. That was how we discovered that the ALT encoder had had a nervous breakdown. Replaced the encoder NAND now check he TXPNDR against the Altimeter routinely.

 

 

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I often wonder how many RAA registered aircraft are tested in compliance with 4.2.4 - 6 of the tech manual? From my "observations" an audit would produce some interesting results!

I wonder that too. Just did 4.2.4 instrument checks on a Tecnam at Coldstream that we maintain this morning + we keep the Rad47 up to date, however I suspect many RAAUS aircraft have not had these checks done when required.

 

 

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If I'm tooling about the Yarra valley and Melbourne radar seem quiet I'll often ask for a TXP check, they always ask what my alt is before giving me what they read, I've learnt to be exact in my answer cause they're exact in the reply, usually within a few feet!

 

 

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If I'm tooling about the Yarra valley and Melbourne radar seem quiet I'll often ask for a TXP check, they always ask what my alt is before giving me what they read, I've learnt to be exact in my answer cause they're exact in the reply, usually within a few feet!

Glad to hear your txpr is behaving :-)

 

 

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I often wonder how many RAA registered aircraft are tested in compliance with 4.2.4 - 6 of the tech manual? From my "observations" an audit would produce some interesting results!

I know RA-Aus has similar concerns and are working on a positive solution - make it easier for members to have the check done and then remind the masses who have never heard of it that it must be done.

 

 

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