Now here’s the subtle consequence — Page 270, Lesson 319

Let’s finish the radio altimeter story properly, because the last piece is the part that catches people out on the ground. We ended with the aircraft installation delay being adjusted to compensate for residual height and cable length — and I said the cable length is counted twice, because you have transmit antennas and receive antennas running down to the avionics bay. That adjustment is made for one specific reason: so that at touchdown, with the main bogeys trailing, the radio altimeter reads zero. Now here’s the subtle consequence. Because the equipment is calibrated to read zero at the instant the main wheels first touch the runway, when the aircraft is sitting level on the ground the antenna is actually below its calibrated position relative to the landing attitude. So the radio altimeter can show a small negative altitude on the ground. That’s not a fault — it’s the direct result of the zero calibration point being the touchdown moment, not the parked attitude. The effect is especially noticeable on aircraft like the Boeing 747, which actually indicates minus 8 feet. Why? Because the B747 has multi-wheel assemblies that are inclined at an upward angle when deployed in flight. That inclination creates a larger difference between the antenna position and the wheels at the point of touchdown, so the residual height — the vertical gap between the antenna and the ground at the moment the wheels touch — is bigger, and the negative reading on the ground is bigger too. So the key idea is residual height: the physical distance between the radio altimeter antenna and the runway surface at touchdown, before the gear compresses and the aircraft settles. The installation delay is tuned to cancel that residual height and the cable run, so the zero point is the main-wheel touchdown. And the price you pay is that small negative indication when parked — minus 8 feet on the 747. That figure shows the undercarriage residual height — the gap between the antenna and the ground with the gear deployed, which is exactly what the installation delay is compensating for. So to tie it together: the radio altimeter is calibrated for the touchdown condition, not the parked condition. That’s why a negative reading on the ground is normal, and it’s more pronounced on aircraft with inclined multi-wheel gear like the 747. That closes out the radio altimeter chapter. Next we move into the Flight Management System — the FMS — starting with its principle of operation and the Control and Display Unit, the CDU.