Today I was asked what the difference is between the Distance Flown and the Distance Travelled. Like so many things in life it seems obvious if you know the answer, but confusing if not.
An Extreme Example
I will explain this by referring back to a time when I was working at Bristow Helicopters, who flew workers to the oil rigs on the North Sea.
When Airship Industries produced their Skyship 500 airship (hence the excuse for the photo above) we were approached with the view of using this, or the larger 600 model, to take men and equipment to the rigs. The new airships were filled with Helium, so the risk of fire was almost eliminated, and they were powered by two ducted fans at the rear of the gondola. These could be vectored to give excellent controllability.
As I was in the design department, some of my colleagues started a study of the practicality of using these craft. In particular their ability to take heavy loads was of interest.
The problem was one of speed. The maximum speed of the airship was 58 mph, but the cruise speed was only 35 mph. To take an example, the distance from Aberdeen to the Brent field is 250 miles. In a helicopter this takes about two hours but in this airship it would take seven hours on a still day. Today, as I am writing this blog, the wind in the Shetland islands is 15 mph. That headwind would make the flight last 12.5 hours and this is right up at the limits of endurance of the airship.
Now, in that 12.5 hours cruising at 35 mph the airship would have flown an air distance of 437.5 miles but only covered 250 miles to get to the rig.
So to answer the original question, the “Distance Flown” is the distance flown through the air, 437.5 miles in this example, and “Distance Travelled” is the distance travelled across the ground, 250 miles in this example.
Why does this matter? When we look at the approach to an airport, a standard descent profile is based upon a given glidepath angle which does not vary with the weather conditions. Therefore approaches are measured in terms of the Distance To Travel. However, if we want to know how much fuel the aircraft will have burned, it is the distance through the air which is of interest so for fuel consumption analyses we need to look at the Distance Flown.