flyprivate
Posts: 96
Joined: Tue Aug 24, 2004 5:36 am

Aircraft Altitude Annunciator

Wed Jul 26, 2006 12:24 pm

I always wondered, when u watch cockpit videos, and you hear the annunciator call out the final altitude (e.g:500,200,100,50,40,10), is that altitude form the belly of the plane, or from the bottom of the landing gear. i would imagine it makes a big difference...if it from belly of the plan, how many feet do they add from large planes, like on the 747, when they are so high up?
 
MaxQ2351
Posts: 321
Joined: Sat Apr 01, 2006 6:41 am

RE: Aircraft Altitude Annunciator

Wed Jul 26, 2006 12:39 pm

Quoting FlyPrivate (Thread starter):
I always wondered, when u watch cockpit videos, and you hear the annunciator call out the final altitude (e.g:500,200,100,50,40,10)

Some of the called altitudes in the cockpit are automatic, and others are done by the pilot-not-flying, or PNF (at least in NWA terminology). At NWA, on the 747-400, under CAT-3 weather, the PNF will call (on final approach):

"200 to minimums"
"100 to minimums"
"Minimums"

And at that point, the pilot flying the aircraft needs to either:

A) See the runway, and land the aircraft
or
B) Execute go around, having not seen the runway at minimums

Quoting FlyPrivate (Thread starter):
is that altitude form the belly of the plane, or from the bottom of the landing gear.

The radar altimeter itself sits on the belly of the plane, and it does NOT correct for the gear.

Quoting FlyPrivate (Thread starter):
i would imagine it makes a big difference...

Not really. If you are making a visual approach, the plane will give you flare tones, plus you should know at what point you need to start your flare anyway. You won't be looking at your altimeter of all things, you will be looking outside!! If you are in CAT-3 weather conditions, the autoland knows when to flare based on the FMC, not the radar altimeter.

Now, as my little "asterix of liability", the above info is, as far as I know, specific only to the NWA 747-400's, the 747-451. Other airlines may use different procedures and equipment than what I described above...and I cannot speak for them!!

Hope that helped though.

-Max

[Edited 2006-07-26 05:39:39]
The 777-200LR......2 engines 4 longer haul
 
Goldenshield
Posts: 5008
Joined: Sun Jan 14, 2001 3:45 pm

RE: Aircraft Altitude Annunciator

Wed Jul 26, 2006 2:07 pm

Quoting MaxQ2351 (Reply 1):
The radar altimeter itself sits on the belly of the plane, and it does NOT correct for the gear.

That depend on the aircraft. If it were true for 100% of all aircraft, then the 10 foot mark would never sound for anything bigger than the A320.
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 
flyinTLow
Posts: 459
Joined: Tue Oct 12, 2004 10:40 pm

RE: Aircraft Altitude Annunciator

Wed Jul 26, 2006 2:41 pm

Quoting MaxQ2351 (Reply 1):
The radar altimeter itself sits on the belly of the plane, and it does NOT correct for the gear.

I am pretty sure that is wrong, at least for LH aircraft. And i doubt they customized them.

The altitude callouts come from the radio altimeter, as said before, and give There are 2 pairs of antennas a little further back behind the wing on the bottum of the fuselage which look like someone glued some plates to the plane (with the bottom side showing). The radio altimeter works during the entire flight, with an altitude envelope of I think 66000 to -20 ft AGL, but callouts and displays only work starting at 2500 ft. to AGL. And the -20ft. Both, call-outs and displays will give you the true height between your wheels and whatever surface may be beneith the antennas. It can actually even happen, that another aircraft crossing 2500 ft. or less beneith you will give you a callout.
The altitude call-outs are actually part of the GPWS System, which uses the radio altimeter for numerous warnings. And one is the call-outs before hitting the ground. Of course, if there is a cliff right before the runway, this system will not work 100%. Enhanced GPWS on the other hand has additional information in a database, which often fixes this problem.

This is what I know about the system, hope it helped you a little.

Cheers,

Thilo
- When dreams take flight, follow them -
 
PhilSquares
Posts: 3371
Joined: Sun Mar 28, 2004 6:06 pm

RE: Aircraft Altitude Annunciator

Wed Jul 26, 2006 2:50 pm

Every aircraft I have flown equipped with a radio altimeter was corrected for the gear height.
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