Old Times

My club washed the airplanes the other evening, and, especially when the weather is nice, airplane washing is inevitably followed by airplane drying, in other words, airplane flying. Since my medical is still in limbo I had another member ride around with me while I did some practice approaches. Even though I flew to ATP standards there was a lot of rust evident, but that's another story.

When flying an Instrument Landing System approach, it's nice to know your groundspeed so you can pick a proper descent rate. Since this was spur-of-the-moment, I did not have my GPS, and the DME readout on our Archer is a little bit, ah, flaky. How did I determine my groundspeed?

Take a look at this picture of my watch (Yes, I wear a Breitling, thanks to a generous wife!) The outer scale is a tachymeter, which is Latin for "speediness measurer thingy." You start the stopwatch and run it for one mile; at that point, the hand points to the speed made good. In the picture it's about 97 units, and in the airplane I know that the units are nautical miles. So my groundspeed was about 100.

(The principle here is the same as a ship's log: the number of seconds it takes the ship to travel its (known) length is translated into an actual speed. In ships they threw a log overboard at the bow and calibrated the knots on the rope so that each knot represented one nautical mile per hour. That's the basis of our modern terms "log" and "knot." I suppose you could do the same in an airplane by releasing a balloon and seeing how long it took to pass the balloon. This would give you a direct measure of true airspeed.)

This was the first time I had ever actually used the tachymeter in flight. In the early days of DME they were popular, because the early units only gave you distance, so it was nice to be able to determine a groundspeed quickly.

It's even nicer to be able to fly!