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Aircraft Altitude Annunciator  
User currently offlineFlyPrivate From United States of America, joined Aug 2004, 105 posts, RR: 0
Posted (8 years 4 months 4 weeks 2 hours ago) and read 1691 times:

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?

4 replies: All unread, jump to last
 
User currently offlineMaxQ2351 From , joined Dec 1969, posts, RR:
Reply 1, posted (8 years 4 months 4 weeks 2 hours ago) and read 1674 times:

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]

User currently offlineGoldenshield From United States of America, joined Jan 2001, 6117 posts, RR: 14
Reply 2, posted (8 years 4 months 4 weeks ago) and read 1639 times:

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.
User currently offlineFlyinTLow From Germany, joined Oct 2004, 525 posts, RR: 0
Reply 3, posted (8 years 4 months 4 weeks ago) and read 1617 times:

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 -
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 4, posted (8 years 4 months 3 weeks 6 days 23 hours ago) and read 1611 times:

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

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