View Full Version : between RW surface and landing gear or antenna?
wingtip777
21st May 2002, 22:34
hi guys!
as we know , many of jets have height callout when aproaching RW, like "50" " 30" etc. so is the distance called out between RW surface and landing gear or antenna? could anyone spread lights on this? thanks!:D
Draculas Teabag
22nd May 2002, 04:16
I can speak for the B744.
The "voice callouts" are based on a radar altimeter, the antenna of which are somewhere near the main gear. There are two separate systems, one for the F/O and one for the grumpy old fart. Both are required for Cat III low vis ops (maybe Cat II, but can't remember). The actual distance is displayed on both pilots PFDs, but only when below 2500AGL.
I believe the heights that are called out are a customer option, our airline has 100, 50 and 30.
On the ground the reading usually shows as -8 feet. Dunno why.
The radar altimeter also provides inputs to the GPWS (ground prox warning system) for voice warnings such as "too low, flap", "too low, gear" and "terrain, terrain".
:)
expedite_climb
22nd May 2002, 06:54
The readout is normally 'Gear Height' with the shocks un-compressed.
The reason it reads a negative value is because the rad - alt's themselves are forward of the gear on the body - and so a figure is applied to the height calculated to allow for height above gear at normal body angle on landing.
After landing this figure becomes negative because the figure is still applied. You occaisonally see it change on the ground as someone / something goes underneath it.
TRUE FAITH
22nd May 2002, 14:55
Radio Altimeter antenna cables are of a critical length. On the
747-400 the Rad Alt's are pin programmed to calibrate the system to an AID (Aircraft Installation Delay) of 57 ft. This AID basically compensates for antenna cable length, fuselage to ground distance and flare angle so the pilot will see 0 ft displayed at touchdown.The 57 ft is not the actual antenna cable length it is a bias signal.The RA's normally display - 8 ft when all gears are on the ground. The 3 Rad Alt systems have different modulation rates for system isolation.