flexo
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Altitude Announcements Before Landing

Sat Jan 10, 2009 4:20 pm

Ok, didn't know how to phrase the title right, but we've all heard the voice in the cockpit before: "100" - "50" - "40" - "30" - "20" - "10"...

How is that altitude determined? I can't imagine barometric pressure being accurate enough, so I guess it is some other system that comes up with the figures.
Anyone know?

[Edited 2009-01-10 08:20:33]
 
EMBQA
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 4:24 pm

I'm pretty sure it come from the Radio Altimeter.

[Edited 2009-01-10 08:39:25]
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wilco737
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 4:33 pm



Quoting Flexo (Thread starter):
I'm pretty sure it come from the Radio Altimiter.

Yes, a radio altimeter measures the actual height above the ground and the computer calls out these heights.

wilco737
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flexo
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 5:09 pm



Quoting EMBQA (Reply 1):
I'm pretty sure it come from the Radio Altimeter.



Quoting WILCO737 (Reply 2):
s, a radio altimeter measures the actual height above the ground and the computer calls out these heights.

Thanks guys, is that something you have to specifically turn on before landing or is it always online? Is it also used during cruise?
 
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HAWK21M
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 5:22 pm



Quoting Flexo (Reply 3):
Thanks guys, is that something you have to specifically turn on before landing or is it always online? Is it also used during cruise?

The RA is very accurate.Its always functioning but on B757/737 measures 2500ft & below.Since the transmitted signal is reflected from the surface of the earth/water below & recieved,the time duration is calculated as the height.
regds
MEL
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David L
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 5:46 pm

Quoting Flexo (Reply 3):
is that something you have to specifically turn on before landing or is it always online?

Since I've only flown "toy" aircraft, don't take my word for it but I'd say it's always "on". How the FMS displays the redundant radio altimeter data at altitude, I can't remember. I think it just indicates "more than the maximum usable value".

Cue an expert...

Edit: My apologies. In your short post I managed to miss this:

Quoting HAWK21M (Reply 4):
Its always functioning but on B757/737 measures 2500ft & below

 embarrassed 

[Edited 2009-01-10 09:48:55]
 
AAR90
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 6:01 pm



Quoting Flexo (Reply 3):
...is that something you have to specifically turn on before landing or is it always online? Is it also used during cruise?

Always on-line and not used during cruise flight. For most airliners heights greater than 2500' are simply not displayed. The displayed height is "biased" to landing gear heights at "normal" landing attitude. During cruise, IF the radio altimeters get a good return on something below you, the display will show the height above that object and you'll sometimes get the aural altitude call-out.... but that doesn't happen very often.
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wilco737
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 6:27 pm



Quoting Flexo (Reply 3):

Thanks guys, is that something you have to specifically turn on before landing or is it always online? Is it also used during cruise?

It is always on, but not displaying anything higher than 2500'. Funny thing is, if another aircraft passes you 1000' below you, the Radio Alitmeter shows 1000' for a short time. There you can see it is on all the time, but just not displaying anything greater than 2500.

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mrskyguy
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 6:39 pm

Yep, that RDI can quite unnerving when you are descending at night over terrain on final approach down in South America.. if you hear "100!" when you aren't expecting it.. yeah, bring a change of underpants.
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113312
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RE: Altitude Announcements Before Landing

Sat Jan 10, 2009 8:43 pm

Yes, you get accurate readings over uneven terrain approaching a runway. Most airports have level ground in the immediate vicinity of the approach end of the runway. Some runways are located near ravines, sand dunes, or other uneven features and the radio altimeter will cause the talking GPWS to call it out just as it is. A pilot needs to be aware of this approaching to land at places such as San Diego, Seattle, and landing east at Los Angeles. Of course, there are many other locations but all of these places have higher minimums and require a visual reference to land.
 
FredT
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RE: Altitude Announcements Before Landing

Sun Jan 11, 2009 8:53 am



Quoting 113312 (Reply 9):
Some runways are located near ravines, sand dunes, or other uneven features and the radio altimeter will cause the talking GPWS to call it out just as it is.

And yet other aircraft will (or at least would) change parameters in the A/P approach glide path tracking loop with decreasing RA returns, in order to account for the increased sensitivity of the GP... which for airports with non-flat terrain under the approach could give interesting results.
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Woodreau
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RE: Altitude Announcements Before Landing

Sun Jan 11, 2009 12:38 pm

The altitude callout is part of the GPWS/EGPWS system, what altitudes it calls out depends on the airline operator

If you taxi over snow/ice covered ramps and taxiways sometimes the radar altimeter gets confused and you'll get "minimums, 10, 30, 10, 50, don't sink, 20, 100, 10" call outs during taxi - it's quite annoying sometimes.
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buckfifty
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RE: Altitude Announcements Before Landing

Sun Jan 11, 2009 4:27 pm

With navigation so accurate these days, quite often the radalt will be active when another aircraft pass 1000' below us. For the uninitiated, it will seem a bit disconcerting when the final items/checklist pop up on the ECAM display, as it does so automatically when the radalt measures the height below 2000'.
 
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HAWK21M
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RE: Altitude Announcements Before Landing

Mon Jan 12, 2009 7:16 am

The RA jump is quite significant on table top runways like Mangalore.Where on approach the RA suddenly drops from 900 to approx 300 ft during landing.
On an anolog gage thats quite interesting to view.

Talking about RA system....The B757s have three Transmit & three recieve antennas located side by side under the aircraft forward fuselage.due to the surface mounting,the side antennas have supports to keep then flat & parallell.
Will try to post a pic.

regds
MEL
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TimT
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RE: Altitude Announcements Before Landing

Mon Jan 12, 2009 2:19 pm

The Radio Altimeter is active below 2500'. The callouts are selected by the operator. EGPWS systems have other inputs from landing gear position, flap position and others that will enable/disable some of the callouts.
 
CosmicCruiser
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RE: Altitude Announcements Before Landing

Tue Jan 13, 2009 1:07 am



Quoting Woodreau (Reply 11):
you taxi over snow/ice covered ramps and taxiways sometimes the radar altimeter gets confused

I've never seen that and the RA should be calibrated to the main ldg gear. If it wasn't at 10' the gear may be smacking the pavement.

Quoting BuckFifty (Reply 12):
the radalt will be active when another aircraft pass 1000' below us.

That I've seen a time or two.
 
heinzmahrer
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RE: Altitude Announcements Before Landing

Wed Jan 14, 2009 10:22 am

Yes, the altitude call-outs (100, 50, 30, 20, 10, 5) come from the radar altimeter but primarely the radar altitude is displayed on the instruments. The call-outs are there so that in the landing flare the pilot can look outside and still get the hight above the surface.
But the radar altimeter has other functions:
a) ILS localizer gain programming (adjusting the sensitivity of the localizer at radalts below 1000ft)
b) it provides vital information to the EGPWS (Ground Proximity Warning)
c) it provides vital information to the TCAS (Traffic Collision and Avoidance System: you don't want a collision avoidance command to descend if you are already at 1000ft or below!)
d) it provides information to the Auto-Throttle System (Idle Command in Auto-Land Mode, minimum engage height)
e) it provides information to the HGS (Head-up Guidance System: Flare Command, Approach Warnings)
f) it provides information to the Auto Pilot (Flare Command in Auto Land Mode, minimum engage height)
g) it provides information to the fly-by.wire system (if the aircraft is so equipped): Flare and Landing Modes, limitations, warnings
h) it provides information to the FGS (Flight Guidance System, CAT 2 or 3 validity)
i) it provides information to the aircraft warning system: Landing gear and flaps warnings, configuration warnings (i.e. aircraft not in landing configuratin at a certain hight)
These are some the uses of the radar altimeter, depending on the aircraft and type of operation this list may be different.

So yes, the radar altimeter is always "ON", as a matter of fact it does nor even have a switch to turn it off. If it fails some systems described above will be no longer be available or degraded in their function and some flightoperations are no longer possible (CAT 3 Auto-Land, and that could mean diversion to another airport). Of course there are 2 radar altimeters installed but regulations dictate that for certain situations (again Auto Land, CAT 3 operations) both be available and functioning.
 
FredT
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RE: Altitude Announcements Before Landing

Wed Jan 14, 2009 5:20 pm

I've only worked on one aircraft which had a radar altimeter. It had a pointy nose and an off switch for said device, to avoid cooking eye- and other balls of ground personnel.

Radio altimeters will not exhibit this rather unpleasant behaviour towards human tissue and thus generally do not require an off switch.
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saab2000
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RE: Altitude Announcements Before Landing

Wed Jan 14, 2009 5:21 pm

Herr Heinz Mahrer is correct. And very thorough, as I would expect a Swiss aviator to be.  Big grin
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FredT
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RE: Altitude Announcements Before Landing

Wed Jan 14, 2009 5:30 pm

If you insist, I insist on including the specification FMCW. It is a very different beast indeed from what people will assume it to be if you don't, and confusion is (for obvious reasons, including those displayed in this very thread) not good.

Obviously four letter acronyms always create a fair bit of confusion on their own, so why make it harder than it has to be?  Smile
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ALexeu
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RE: Altitude Announcements Before Landing

Thu Jan 15, 2009 11:56 am

What would happen if you just flew low flight? e.g. airshows with Boeing and Airbus a/c ?
And why does only Airbus has ''retard''?
 
ROSWELL41
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RE: Altitude Announcements Before Landing

Fri Jan 16, 2009 1:30 am

On the CRJ's at my operator, the radio altimeter gives callouts at 500', 100', 50', 40', 30', 20', 10'. Its very comforting to hear these callouts on approach even though most times you don't actually need them for anything. I recall flying an aircraft recently delivered from another carrier. The RAlt callouts were different and had not yet been adjusted to our carrier's specifications. Something that insignificant actually proved to be distracting during the approach. You really get used to hearing certain sounds at certain times.
 
KDTWflyer
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RE: Altitude Announcements Before Landing

Sat Jan 17, 2009 7:46 pm

I always like the airbus aural altitude callout at the very end .... retard.....retard....retard lol.
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Jetlagged
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RE: Altitude Announcements Before Landing

Sat Jan 17, 2009 10:47 pm



Quoting AlexEU (Reply 20):
What would happen if you just flew low flight? e.g. airshows with Boeing and Airbus a/c ?

You'd get a lot of callouts! There's no way to disable them on the flightdeck except by pulling the EGPWS circuit breaker.

Quoting AlexEU (Reply 20):
And why does only Airbus has ''retard''?

If you didn't retard the thrust levers in an Airbus after touchdown the engines would spool up to climb power as soon as the autothrust disengages. Not something you want during rollout. Hence the aural reminder in case reverse thrust is not being used.
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ALexeu
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RE: Altitude Announcements Before Landing

Sun Jan 18, 2009 3:11 pm



Quoting Jetlagged (Reply 23):
If you didn't retard the thrust levers in an Airbus after touchdown the engines would spool up to climb power as soon as the autothrust disengages. Not something you want during rollout. Hence the aural reminder in case reverse thrust is not being used.

Thanks, I didn't know that. What's the purpose of that Airbus design? Isn't the idle always supposed to be used on touchdown. What if you can't use it? Will the aircraft automatically put full thrust? Pardon me, for off-topic.
 
David L
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RE: Altitude Announcements Before Landing

Sun Jan 18, 2009 4:14 pm



Quoting AlexEU (Reply 24):
What's the purpose of that Airbus design? Isn't the idle always supposed to be used on touchdown.

Yes but the FBW Airbus thrust levers don't move on their own so the crew has to pull them back to Idle on landing. On other types the levers are moved automatically to match the auto-throttle setting.

In their simplest use, the FBW Airbus thrust levers are moved back from the take-off position (whichever that might be) to the "Climb" position shortly after take-off and they stay there until the "Retard" call on landing. They can be moved for "manual" thrust control (sort of) but they don't have to be. As Jetlagged says, you're asking for trouble if you leave them above Idle when landing if you're intending to stop.
 
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Zkpilot
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RE: Altitude Announcements Before Landing

Mon Jan 19, 2009 2:08 am

Besides the 100,50,40,30,20,10 calls there is also the actual "Radio Altimeter" call that comes up first.
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