So it’s really easy to measure heading. You just get out a compass and it will point North. Easy, eh?
But, you’ve guessed it, life is never that easy.
Firstly, the compass in an aircraft has to compensate for the motion of the aircraft. Even from the earliest days of aviation a compass was combined with a gyroscope so that the gyro pointed in the same direction as the aircraft moved around the sky, and in the long term the gyroscope direction was aligned with the magnetic compass.
Secondly, the earth’s magnetic field is very uneven and the difference between the magnetic North (where the compass points) and true North (where Santa lives) can change from a few degrees to perhaps twenty degrees or more depending on the route flown.
From an analysis point of view, we can play a trick which allows us to compensate for errors in the aircraft measurement system and for the earth’s magnetic variation, both at the same time. As the aircraft travels down a runway, its direction is very close to the direction of the runway centerline. We know the true heading of the runway, and we have the recorded compass reading. The difference is a combination of magnetic variation and compass errors.
Frankly, we don’t bother separating the errors – we can convert magnetic compass recordings to local true headings and that is good enough for our analysis.
With altitude, airspeed and heading we have now dealt with the three key measurements for flight data analysis and in a future blog I will explain how we can use these three parameters to check the other recorded parameters.