HAWK21M From India, joined Jan 2001, 31457 posts, RR: 57 Reply 4, posted (4 years 11 months 5 days 9 hours ago) and read 5627 times:
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.
David L From United Kingdom, joined May 1999, 9295 posts, RR: 42 Reply 5, posted (4 years 11 months 5 days 8 hours ago) and read 5618 times:
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
AAR90 From United States of America, joined Jan 2000, 3434 posts, RR: 49 Reply 6, posted (4 years 11 months 5 days 8 hours ago) and read 5600 times:
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.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
WILCO737 From Greenland, joined Jun 2004, 8763 posts, RR: 77 Reply 7, posted (4 years 11 months 5 days 8 hours ago) and read 5587 times:
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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.
MrSkyGuy From United States of America, joined Aug 2008, 1210 posts, RR: 3 Reply 8, posted (4 years 11 months 5 days 7 hours ago) and read 5580 times:
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.
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air
113312 From United States of America, joined Apr 2005, 553 posts, RR: 1 Reply 9, posted (4 years 11 months 5 days 5 hours ago) and read 5530 times:
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 From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 10, posted (4 years 11 months 4 days 17 hours ago) and read 5399 times:
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.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Woodreau From United States of America, joined Sep 2001, 935 posts, RR: 7 Reply 11, posted (4 years 11 months 4 days 13 hours ago) and read 5341 times:
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.
Good judgement comes from experience. Experience comes from surviving bad judgement.
BuckFifty From Canada, joined Oct 2001, 1314 posts, RR: 20 Reply 12, posted (4 years 11 months 4 days 10 hours ago) and read 5278 times:
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'.
HAWK21M From India, joined Jan 2001, 31457 posts, RR: 57 Reply 13, posted (4 years 11 months 3 days 19 hours ago) and read 5121 times:
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.
TimT From United States of America, joined Jun 2001, 168 posts, RR: 0 Reply 14, posted (4 years 11 months 3 days 12 hours ago) and read 5013 times:
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.
Heinzmahrer From Switzerland, joined Feb 2008, 6 posts, RR: 0 Reply 16, posted (4 years 11 months 1 day 16 hours ago) and read 4668 times:
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 From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 19, posted (4 years 11 months 1 day 9 hours ago) and read 4577 times:
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?
I thought I was doing good trying to avoid those airport hotels... and look at me now.
ROSWELL41 From United States of America, joined Aug 2001, 635 posts, RR: 1 Reply 21, posted (4 years 11 months 1 hour ago) and read 4324 times:
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.
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.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
AlexEU From Serbia, joined Oct 2007, 1793 posts, RR: 2 Reply 24, posted (4 years 10 months 3 weeks 6 days 11 hours ago) and read 4083 times:
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.
25 David L: 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 move
26 Zkpilot: Besides the 100,50,40,30,20,10 calls there is also the actual "Radio Altimeter" call that comes up first.