We were looking at the example where the radio altimeter reads down to 100… — Page 270, Lesson 319

Let me walk you through the final piece of the radio altimeter chapter, because this is where the installation details really matter for your understanding of what the instrument actually reads. We were looking at the example where the radio altimeter reads down to 100 feet. Now here's the key point: the aircraft installation delay is adjusted to compensate for two things — residual height and cable length. And notice the cable length is multiplied by two, because you have both the transmit antennas and the receive antennas running cables down to the avionics bay. So the signal travels down twice — once for the transmit path, once for the receive path — and that's why it's times two. The whole purpose of this adjustment is to ensure that at touchdown, with the main bogeys trailing, the radio altimeter reads zero. Let me unpack that. The main bogeys are the main landing gear wheel assemblies. When the aircraft touches down, the main wheels contact the runway first. The radio altimeter is calibrated so that at that exact moment — main wheels on the runway — it shows zero. That's the reference point the whole system is built around. Now here's something that surprises a lot of pilots the first time they see it. When the aircraft is on the ground, the radio altimeter may show a small negative altitude. Why? Because the equipment has been adjusted to indicate zero when the main wheels first touch the runway on landing. So when the aircraft is level on the ground — sitting flat, not in the landing attitude — the antenna is actually below its calibrated position relative to the aircraft's landing attitude. In other words, the antenna sits lower than where it was when the wheels first touched, so it reads slightly negative. This effect is particularly noticeable on aircraft like the Boeing 747, which actually indicates minus 8 feet. The reason is the multi-wheel assemblies. The 747 has multiple wheels on each main gear, and those assemblies are inclined at an upward angle when deployed in flight. That creates a larger difference between the antenna position and the wheels at the point of touchdown. So the geometry of the gear itself makes the negative reading more pronounced. Let me show you this with a diagram. This is Figure 20.7, "Undercarriage residual height." You can see the residual height marked, and then the reduced residual height. The residual height is the distance between the antenna and the ground when the wheels are about to touch. The adjustment we talked about — the installation delay — is what reduces that residual height effect so the reading comes to zero at touchdown. So to tie it all together: the radio altimeter isn't just measuring raw time-of-flight. The installation delay is deliberately set to account for cable runs and residual height, so that the instrument reads zero exactly when the main bogeys touch down. And because of that calibration, a level aircraft on the ground will show a small negative value — and on a 747, that's minus 8 feet. That's not a fault. That's the system working exactly as designed.