View Full Version : Radio altimeter
tvrao
21st May 2009, 12:01
For many radio altimeters they say the range is from -20ft to 2500ft.
The radio altimeter measures height above ground. The antennas which transmit and receive the radio signal, are fitted on the fuselage.What is the Zero reference for the radio altimeter? When will it read -20ft, that is negative radio altitude of 20ft?
When will it read -20ft, that is negative radio altitude of 20ft?
It obviously means you are flying 20ft underground !
No, its to do with instrument calibration delays. On a given installation it may read say +/- 4 ft (with a/c on the ground) the actual value displayed depends on the antenna slant distance (height) above ground. In a specific installation instrument setup software may allow you to calibrate out that delay to near zero to represent wheel height - which,then, if repeated on a shorter legged a/c it would read negative.
TyroPicard
21st May 2009, 13:47
When you land with your 20 ft. landing gear still up...
Mansfield
21st May 2009, 14:08
The radio altimeter is calibrated to read main wheel height above the ground while in the pitch attitude associated with a normal configuration and normal approach speed at the runway threshold. In most large transports, this means that the radio altimeter antenna, located forward of the main wheels on the fuselage, is quite a bit higher than the main wheels. When the main wheels touchdown, the radio altimeter should read zero, but the antenna is still several feet above the ground. As the airplane de-rotates and the nose touches down, the radio altimeter antenna continues to descend, and finally thinks it has gone below the ground level.
On the 767/757, the radio altimeter typically reads -8 feet on the ground, although the DH selector knob will rotate as far as -20 feet.
The flared attitude on landing and calibrating the RA to read zero when main wheels touch the runway certainly explains the reason for allowing a negative reading. What I am not sure about is the degree of 'zero' adjustment allowed by typical RA setup software.
Graybeard
21st May 2009, 15:45
There's no software involved. There is a selectable AID, Aircraft Installation Delay, that is a discrete. The 737, for example, uses a 57 foot AID. That's the distance from the transmit port of the transceiver, through the cable and antenna, to the ground at touchdown angle, and back through the receive antenna and cable to the receive port of the transceiver. Therefore, these are critical length cables, and any extra cable length needed to make the correct 0 feet is coiled in the belly of the plane.
When the radio altimeter starts reading -20 feet, it is because of signal leakage across the fuselage between antennas, or leakage at the mating connector to the transceiver. It can be a distance shorter than the indicated -20 feet, but that's the indicator limit.
The radio altimeter system is a good belly corrosion detector.
GB
ChristiaanJ
21st May 2009, 16:48
b377,
You are totally missing the point.
At the moment the main wheels touch (which is the moment you would like the radalt to read zero), the real signal path length between transmitter output and receiver input (in the case of the 737) is still 57 ft, made up out of the cables between the R/A unit and the antennas and the path from the antenna to the ground and back.
So there is no question of additional "transmission delay lines", analog or digital.
You simply offset the indicator, so it reads 0 ft, when the R/A measures 57 ft.
Since the exact AID varies between aircraft (which use the same R/A) that offset is made pin-selectable.
CJ
In that case it would be easier to adj with software as I said at first during setup...the red herring was input later.
ChristiaanJ
21st May 2009, 20:46
b377,
Thanks for your childish belief in "software".
Even today with the glass cockpit, where do you think the R/A unit software (or whatever calculates the offset) "finds" the right information, to know what aircraft it is in? Right. It reads a pin code.
We did it with a few resistors in the olden days, connected to those same pins.
Your red herring was the "digital delays....". No such animal in this case.
CJ
PIN code is one way, better still from memory with the exact offset set at initialization. Your point is valid though, if that is the way its generally done. Must find out how current Collins RAs do it.
ChristiaanJ
21st May 2009, 21:48
Read Graybeards comments above? His delays are wound cable.
Yes, I did ..... and no.
The nominal cable length (which is part of the AID) is always a bit more than is needed for installation, to allow for re-routing, different a/c variants, etc. The excess length is then coiled.
If a cable gets damaged (does happen) an exact equivalent length of cable is spliced or connected back in.
CJ
Answer to your edit...Graybeards a/c specific delay is 57m of coiled cable !Nope!
It's 57 ft, not 57 m.
And it's not the cable, it's the total signal path from transmitter output to receiver input, at the moment of touchdown of the main wheels.
So it's TX output > cable > aerial > ground > receiver aerial > cable > RX input.
Do the sums, but cable lengths are probably in the order of 15 + 15 ft .
fine i have no issues with that.
fine i have no issues with that.
What a relief.:rolleyes:
It would be interesting to know just what type of flying or aircraft/avionics license b377 holds?:hmm:
hey 411A
I think you can answer that yourself judging by all the questions you have answered so my rating can only improve with your replies.;)
This is what this forum is all about.