B737NG radio altitude landing callout

[citizensun]

Hi all,
I got a question about the B737NG radio altitude callout when landing.
when the aircraft calls 50 40 30 20 10, and when it reaches 0, the lading gear touchs down.
But I know this altitude is not the distance from main landing gear to the ground, it`s the distance from the RA antenna to the ground.
I heard there is a program pin calibration in there between the RA and landing gear altitude. But I don`t quite know about it.
Can anybody tell me about this program pin? And from what altitude this calculation start?
Thanks a lot!

[BOAC]

But I know this altitude is not the distance from main landing gear to the ground - why, then, does the gear touch down at '0'?

My answer to the last question is RADIO ALTITUDE 2500'.

[Lord Spandex Masher]

- why, then, does the gear touch down at '0'?


...and the RA display a negative value when the nose wheel is on the deck...

[Skyjob]

The RA antennae are located on belly of plane.
The readings will be 0 for an inflight pitch attitude during flare, hence 0 ft is read at touchdown, negative values displayed when on ground.
At higher altitudes, due to the high variableness when flying over non flat terrain, such trivial differences in radio altitude can and thus are ignored.
But when over the landing strip, in LVO conditions, the aircraft must know its actual altitude above the ground and thus relies solely on the RA, as does the auto throttle FLARE command use its input and the trim change occurring prior to landing.
The slight difference in RA values displayed in the flight deck is due to the two antennae not being collocated nor shared.

[citizensun]

when passing the threshold, the typical altitude is 50ft, which is the GS height over the threshold. the aircraft GS antenna is locate near the nose, so when passing the threshold, the cockpit is around 50ft, and the GPWS anounces RA50, according to the FCTM, the landing gear is around 35ft height.
and when decend and flare, the RA and landing gear decrease syncronized to 0.
I heard that there is a program pin in there for calculation for the aircraft pitch, bank angle, antenna location etc. for landing.
does someone know this program?

[A37575]

I have often wondered if the RA count down from 50 feet to touch down in the 737 is used for anything particular. What I mean, does the pilot commence the flare manoeuvre at the first auto-callout or the second or third call-out. I would have thought the cockpit being relatively close to the ground compared to some wide-bodies, that the auto calls are superfluous as the pilot can easily judge his flare by simply looking through the window - unless Cat3 B or Cat 3C. I have been astonished to see some SOP where the PM actually calls out the RA heights from 50 feet in the absence of auto-call-outs and that is in fine weather. If the rate of sink is high he has to rattle off the readings so fast the aircraft is on the runway before he is past 30 ft. Bit of an overkill, as are so many SOP nowadays.

[aveng]

What you are referring to is called the " installation delay". Basically radalt is a timing signal from transmit to receive. Knowing the delay time due to antenna position and transmission cable type, a particular radalt transceiver is adjusted for the signal delay dependent on the aircraft it is fitted to. It is designed to give zero feet at touchdown with the aircraft in standard touchdown AOA. A330 has transceiver at rear of aircraft with the antenna aft of main gear. Boeing have the radalts and antenna fwd of main gear.