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Altimeter read: is it correct?

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Is it correct that when the 767 is on the ground the altimeter reads 10 feet more than the airport altitude? I mean, when I am at LIRF, which should be 14 ft, the altimeter reads 24 ft. Shouldn't it read the exact airport altitude? Of course the exact qnh is set. Thanks,Luca

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I imagine the altimeter is referenced to the bottom of the jet when the wheels are up, not to the bottom of the wheels when they are down.

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Guest Ian_Riddell

An interesting (and technical) question, Luca....After recent discussions about a similar topic on the PPRuNe site, I believe the altimeter on a 767 should read the height of the airport plus the height of the airplane's Air Data Computer(s) above the ground. Since the Air Data Computers are located on one of the Main Equipment Centre racks, about 10 feet above the tarmac, then 24 feet should be correct.... er....unless, that is, the airport weather bureau measures the barometric pressure above ground level... i.e. if their instruments are in cabinets above the ground. If the baro reading is taken by an instrument, say, 3 feet above the ground, then the altitude readout on the airplane should be 24 feet minus 3 feet = 21 feet. However, I don't know if any compensation for cabinet height is made by the airport weather bureau.... and I know that the accuracy of these types of pressure-driven instruments may put the airplane's altimeter several feet above/below this value.Note also, that airports are not always flat. There could be a large difference in indicated altitude depending on where you are on the airfield.Hope this helps.Cheers.Ian.P.S. Note that on some aircraft (Later Airbuses and 777's), the pressure measurement is actually made at the height of the pitot-static probes, rather than at height of the Air Data Computer.As far as I know, there is no compensation for aircraft attitude, so you will most likely have a heavy landing if you rely on your barometric altimeter for flare height.

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