here is what I (think I) know: an airplane uses the air pressure to measure the altitude - above 10000feet the crew sets the altimeter to a default pressure (QNH) (1013hPa in europe I think) - below 10000feet the altimeter should be set to the local air pressure (information being received from ATC during approach). could be that the crew set their altimeter incorrectly - causing the aircraft to display a wrong altitude (ref. sea level).
however, if the airplane (at least airbus do that) gets below something like 2000feet - you also get the distance between ground and plane measured by a radar instrument. that one only displays wrong information if the system itself is broken...
if the aircraft gets below something like 100feet - there is either a hearble countdown (100fett / 50feet / 20feet / 10feet) or a "terrain" warning (depending on whether the airplance is "supposed" to be landing or not)
it's still strange three pilots in the cockpit did not notice the wrong altitude despite all the warnings, etc...
(if I just wrote a total incorrect nonsense, please correct my statements...)
I believe it's above 18,000 ft that they set to the standard altimeter setter(in the U.S. its 29.92), and below that it is set to local reading. I don't think an incorrectly set altimeter could cause this. Variations in altimeter setting would only cause the sea level altimeter to be off by a few hundred feet. I've heard on some of the approach feeds something like "So-And So 1234, you a 500 below/above(what ever the case my be) your assigned altitude, current altimeter 30.12." (or what ever current reading is).
If that is incorrect, please correct me!
You are correct. Aircraft altimeters are set to 29.92 at 18,000 ft and above. Below 18,000 ft, they are set to local barometric readings.
An aircraft's mode C readout (altitude), as monitored at controller displays, must be within 300 ft of assigned altitude. That is, less than 300 ft, for controllers to be able to use that information for separation purposes (terminal environment). A controller must verify an a/c's mode c readout periodically. If pilot states level at 9000, but his mode C, as depicted on the controllers display, is reading 8500, the controller will say something like, "N1234, verify at 9000, altimeter 3002" If pilot states level at 9000, even after resetting to correct altimeter, and the a/c's mode C is still reading 8500, than the pilot should be told to, "stop altitude squawk, altitude differs by 500 ft."
Hi all, the barometric altitude has no bearing on this subject.
The aircraft was on the final approach so APP was the active FD and autoplilot mode.
In this mode, the Flight Directors gives the steering commands for the autopilot to steer the aircraft to the glidepath.
Barometric altitude is not used for landing, nor does a QNH setting or QFE setting have any influence on ATC altitude.
The Flight directors in this situation only look for the G/S and LOC signals until the radio altitude drops down.
The altitude call outs come from radio altimeter #1.
Offcourse the flare command also comes from radio altimeter.
If the FCC sees the GS and LOC signals as 'on course' it will then wait for a decreasing radio altitude and at roughly 50 feet or so go into FLARE.
This is the way the system works and is cause for concern as this defect seems to happen quite a lot.