Measuring Aircraft Height (continued)
Dave Jesse on November 15, 2012

Last time, I spoke about the ways of measuring the altitude of an aircraft, as the most important thing we need to know is the altitude above the local airfield or Altitude Above Airfield Level (AAL), as it is here that you will have to transition from aviation into the world of the programmer.

The first and last things we know are that we start and stop on the ground. For fixed wing aircraft, this is simple – if we are moving too slowly we must be relying on the undercarriage to support the weight of the aircraft. For rotary wing aircraft, the equivalent is that if the rotor speed is too slow we aren’t flying.

If we are moving fast enough, we can measure the pressure altitude and compare this to the pressure altimeter at the takeoff, to find how high we are above the takeoff airfield. Similarly, by subtracting the altitude at touchdown from the altitude on the approach, we can compute the height above the landing airfield.

The two height measurements will, at some point, have to join up. We just have to make a decision about the point of joining. It could be at the transition level, where air traffic control change from altitudes relative to the airfield, to flight levels relative to standard pressure. This, however, is not globally consistent and is difficult to implement.

Alternatively, we could use the highest point in the flight, or consider using the midpoint between the top of climb and top of descent. It is this latter option that FDS have selected, ensuring that we have a changeover point at the highest point of the cruise.

The next thing that we have to understand is the airflow around the aircraft. At the point of takeoff (or landing), there is a change in the pressure around the plane, which is partly due to the air being squeezed between the wing and the ground. We call this the ’ground effect’ as it gives us the lift we need to become airborne, while increasing in the pressure at the altimeter.

This causes the altimeter to display a lower altitude, indicating that the aircraft is “flying” below the runway, for an instant – and it is for this reason that we must use the radio altimeter to measure height at takeoff and landing.

At some point, we need to transition from radio altimeter readings to pressure altimeter readings – and this needs to be done before the aircraft leaves the flat surface of the airfield, but at a time when the aircraft is out of ‘ground effect’.

If only life was that simple: Sometimes an aircraft will descend without landing – for instance, when the conditions at the airfield are not suitable for landing causing the pilot to divert.

We really want to display the altitude above the local airfield – and the best way to do this, is to measure the lowest radio altitude reading, then compute an altitude AAL using this datum for the local heights.

It gets worse. Sometimes the dip will be low enough to be significant, but the radio altimeter will not be low enough to provide a reliable indication, and sometimes, the aircraft doesn’t even have a recorded radio altimeter signal.

As well as computing an altitude above the airfield, we can work out the altitude above mean sea level (altitude QNH) by adding the airfield elevations at takeoff, landing and any intermediate approaches. This is used to drive Google Earth displays as the Google terrain map is in metres above mean sea level.

At last! This is the end of this two-part mammoth blog about height. You should now reward yourself with a cup of tea/coffee/beer to celebrate.
I promise the next blog won’t be so complicated!