FDC
Measuring Aircraft Height
on November 7, 2012

So you’d like to know how high an aircraft flies. After all, this must be the most basic measurement we can have. In this blog we’ll look at how aircraft measure height, and then see where problems arise. In the next posting after this, I will explain how we make all the problems disappear – who said you can’t have suspense and excitement in a technical blog?

Wherever you are, the weight of all the air above you causes a pressure in the air around you. The higher you go, the less air there is above you and so the lower the pressure of the air. If you drive up a mountain you can feel your ears “pop” and this is because the pressure is decreasing. So all we need to do is measure the air pressure and we know how high we are.

The same works for aircraft. We measure height on a Barometric Altimeter and it detects the air pressure and displays the altitude. We normally use feet, although some countries use metres just to make life interesting!

If only life was that simple. The trouble is Blowing in the Wind.

OK, I’m not really a Bob Dylan fan, more JethroTull, but I couldn’t work “Don’t Want to be a Fat Man” into the blog. Must try harder.

So what’s wind got to do with it? Wind is caused by changes in air pressure across the world and this varies from one day to the next.

Imagine a pilot wants to go for a pleasure trip around his local airfield. His altimeter reads zero on the ground as he takes off. He flies around for an hour during which time the local pressure increases. When he returns to the airfield and flies down to zero feet the aircraft will still be in the air. The air pressure at the ground has increased and so he has to be higher to find air at the same pressure that he took off with.

What we need is a measure of the height above the ground, and for this we turn to radio waves. A radio altimeter on the aircraft sends a pulse of radio waves downwards and these bounce off the ground and return to the aircraft. By timing the journey to the ground and back we can work out how far away the ground is.

If only life was that simple. The radio waves get weaker with distance, and so we only use these radio altimeters for heights above the ground of 2,500 ft or less. More than that and the indications can become unreliable.

So we have a pressure altimeter that changes its readings from day to day, so can give misleading readings when we fly low, and a radio altimeter that doesn’t work when we are flying high. In the next blog I will explain how we merge these two to find the height of the aircraft…

TTFN
Dave