FDC
Looking Closely at Radio Altimeters
Dave Jesse on November 13, 2013
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After last week’s hard sums, this week is much easier. We are going to look at the signals provided by Radio Altimeters when the aircraft is very close to, or on, the ground. There only be one easy sum and you can skip this if you prefer.

At the end of the day you will be able to answer the question:

“Why do some aircraft take off underground”?

Radio Altimeter Principles of Operation

The Radio Altimeter (or Radar Altimeter) measures the height of the aircraft by sending a radio signal to the ground and measuring how long it takes to make the journey to the ground and back.

It’s unlike the pressure altimeter that I looked at ages ago, because the pressure altimeter relies on the air pressure and that changes with the weather from day to day. So a pressure altimeter cannot tell you anything about where the ground is, whereas the Radio Altimeter ONLY tells you how far you are above the ground.

Radio Altimeters have a limited range of operation, and the display in the cockpit normally stops at a minimum of 0ft and a maximum of 2,500ft. The actual signal from the sensor has a greater range, including a small negative range and values up to, perhaps, 5,000ft. Above that the signal is too weak and unreliable to be used.

An aircraft may have multiple sensors, for example to provide the Captain and First Officer with independent sources of information the aircraft will need two systems while for a full autoland system three separate sensors will be required. Each radio altimeter has two separate antennae fitted to the belly of the aircraft, one to transmit the radio signal and one to receive. Here is a photograph which shows them.

Boeing 757 Radio Antennae

You can readily see four small square antennae in a square pattern on the belly of the aircraft midway between the wing fairing and the nosewheel bay doors. Look more closely and you can see another pair of antennae mounted flush to the aircraft skin on the centreline.

The signals are used by many systems on the aircraft, so you will hear them referred to when we look at Terrain Awareness Warning Systems (TAWS) and autoland, but that’s for another day.

Where To Put the Antennae?

Let’s think carefully about where the antennae are located, especially in terms of their use at takeoff and landing. Just before the aircraft takes off, the wheels are still in contact with the ground. We can reasonably say that the aircraft altitude is zero, but the nose of the aircraft has gone up and the tail has gone down.

Here’s a reminder for you…

Aircraft Rotating on Takeoff

After the Sums

So if the radio altimeter antennae was mounted in the nose of the aircraft it would read almost 9ft before the aircraft had left the ground ! Equally, the tail will be about 9ft lower. If we started from 0ft with the aircraft “standing still” it would read -9ft at takeoff.

The pilots really want to know the height of the mainwheels, so we could put the antennae between the wheels and that way they not change as the aircraft rotates.

There is a problem with this.

747 Undercarriage

The wheels get in the way. Both because this is where the undercarriage bays are, and because the radio antennae may “see” the wheels and so give false measurements, especially while the gear is being extended or retracted.

So we have to put the antennae in front of, or behind, the main undercarriage. The effect is to change the height reading as the aircraft rotates. Now, the primary use of the radio altimeter on an airliner is to measure the height of the main wheels prior to touchdown. The system is therefore calibrated for the pitch attitude at landing, so that the height indications and automatic callouts in the cockpit will allow the pilot to judge the final stages of the flare and landing.

Measuring the Effect of Antennae Location

To measure the effect of antennae location on a given aircraft we first plot the graph of altimeter reading against pitch attitude during the aircraft rotation but BEFORE the main wheels have left the ground. This is an example:

Radio Altimeter Height (ft) plotted against Pitch Attitude (deg)

On the left of the graph, the close blue dots at -4ft radio altimeter height and around -1 deg pitch attitude represent the aircraft condition as it accelerates down the runway. At some point the pilot initiates the rotation and as the pitch attitude increases through zero to +6 degrees, the radio altimeter reading increases towards zero. At the point of takeoff the altimeter reads zero and the aircraft is in a similar attitude to that at landing. From this dotted blue line we can plot a green “best fit” straight line and measure the change in recorded height for changes in attitude and the offset (the radio altitude at zero degrees pitch).

The graph above is for a 737, and has a slope of 0.44 ft/deg, while the one below is for a 747 with slope 1.04 ft/deg because the aircraft is longer and the antennae are further away from the main wheels.

Boeing 747 Radio Altimeter vs Pitch Attitude Plot

These slopes are close to those computed for the known antennae locations, but there is a fair amount of variation from flight to flight. I guess this depends upon aircraft loading and speed of rotation but at least the principle is right.

…and Finally

And then there was the Bombardier CRJ 900 aircraft. It has a chart like this…

CRJ 900 Radio Altimeter vs Pitch Attitude Plot

Here the antenna is located behind the main wheels and the altimeter reads greater than zero while taxiing, only to reduce as the aircraft rotates. Now because aircraft takes off at a more nose-up attitude than landing, the altimeter reading passes through zero until it actually reads negative values at the point of takeoff.

CRJ 900

So now you know the answer to the quiz question at the beginning. Aircraft with radio altimeter antennae mounted behind the main wheels will take off with the altimeter showing them under ground.

Endpiece

Can I offer one piece of advice? Please do not go into your local pub and open a conversation with “Did you know something interesting about Radio Altimeter antennae location?” You are unlikely to make friends that way.

TTFN

Dave