Radalt readout.

[Noah Zark.]

Please excuse a question, hopefully in the correct forum, from an enthusiast.
In any amount of video/film footage taken on an airliner flightdeck during touchdown, an automated voice can be heard calling out the remaining height to touchdown, i.e. '50 feet, 40 feet, 30 feet, etc.
My question is, where is the height mentioned referring to? Is it to the main undercarriage wheels, the belly of the aircraft, or w.h.y.?
Thank you.

[dixi188]

The Rad Alt call out is the height the wheels are from touchdown. The antennas are on the lower surface of the fuselage aft of the landing gear. They are calibrated to give Zero when the wheels touch down.

Back in the day, before the automated call out, it was one of my jobs as Flight Engineer to make the call out from the Rad Alt indicator.

[MarkerInbound]

On the 744 the radio altimeter antennas are forward of the main landing gear. The instruments are calibrated to read "0" when the main gear touches down during a normal landing. As the nose is lowered the antennas are lowered further so the radio altimeter reads -8 feet as you taxi around the airport.

[Noah Zark.]

Thanks for the replies and the info, guys.