The radio altimeter antennae should be preferably placed in a position where the changes in aircraft pitch attitude least affects the transmission and reception of the radio altimeter signal. As the aircraft pitching moment occurs through its center of gravity (CG) point, this makes it the perfect position to place the radio altimeter antennae. However, this is not very practical as in most aircraft this is also the position where the main landing gear is placed. If the radio altimeter is placed near the main landing gear, the signals can be blocked and interrupted by the gear, which can cause inaccuracies in the radio altimeter readout.
So, most manufacturers either place the radio altimeter between the main and nose landing gear or place it near the tail. However, when the antennae are placed in these positions, the pitch changes during take offs and landings can make the radio altimeter give out false altitudes. This issue is mitigated by the fact that the pilots use radio height mainly during the landing. Thus, the radio altimeters in aircraft are calibrated to read zero, when the main landing gear first hits the runway, with the aircraft in the landing pitch attitude (i.e. nose up). It does not really matter where the antennae is placed. But still they are placed as close to the CG as practical.
In Boeing aircraft, the radio altimeter antennae is placed on the belly between main and nose landing gear. When placed in this position, the aircraft radio altitude can read negative numbers when parked at the gate. The reason for this inconsistency is because, when the aircraft is in landing attitude, the nose is pointed up. And at this attitude, the radio height is calibrated to zero. When the nose finally comes down during the landing roll out, the radio altimeter goes below its calibrated height and it ‘thinks’ that the aircraft is experiencing a negative altitude. This is most significant in the Boeing 747, where the outboard landing gear (first gear that hits the ground when landing) is tilted to such an extent that when they come down (the vertical tilt zeroing out), the radio height becomes negative. A 747 typically reads a radio height of -8 ft when parked on the ground.
The radio altimeter on the 747, in this attitude with the outboard landing gear in contact with the ground will read zero. As the gear tilts on the ground and as the nose settles the radio altimeter senses a false negative height.
A 747 primary flight display on the ground. The yellow circle points to the radio height, which reads -8 ft.
In the Airbus side of things, the radio altimeter antennae are placed in the tail belly area. As with Boeing design, it is calibrated to read zero when the aircraft is in the landing attitude. As the tail moves a lot less significantly than the nose during the landings and even in take offs, the radio height is less affected by aircraft pitch moments when antennae is placed in the tail. So, on ground, Airbus aircraft tend to read a zero radio height. However, I have seen Airbus aircraft registering a negative radio height during landing. This happens when a pilot flares the aircraft a little more than normal. When the aircraft is flared more, the tail comes down more and the radio altimeter antennae are exposed to a higher landing attitude than to which they were calibrated for. This causes a momentary negative height readout. But as the nose comes down, the radio height goes back to zero.
During take offs, the tail is pushed down a lot more than during the landing as the take off pitch attitude is generally higher than the landing pitch attitude. This makes Airbus aircraft on the take off rotation to register a negative radio height. This happens for a brief moment and the radio altimeter goes back to measuring the correct radio height.
An Airbus A320 on take off rotation. the blue circle points to a -1ft radio height as the aircraft tail initially comes down for the take off.
Author – Anas Maaz
