Pilotage:

Hidden truths

March 1, 2010

I’m not much of an electronic gamer, probably because I’m decades removed from my prime eye-hand coordination years. The exception is when I’m flying. There’s no X-Box or Wii Nunchuck in my flight bag, however. My gaming hardware of choice is the database-rich, full-featured GPS navigation system in the panel’s center stack. The games I like to play on it are a clockwise turn of the big knob back from my default navigation page. Games with such titles as “Trip Planning,” “Fuel Planning,” and my current favorite, “Trip Statistics.”

The airplane I fly has a Garmin 430 that was installed some 16 years ago, which qualifies it for classic status. The “Trip Statistics” page provides three simple items of information: odometer reading (how many nautical miles the airplane has flown since the odometer was last reset), average ground- speed over the displayed odometer distance, and highest groundspeed over the displayed odometer distance.

When I first began flying the airplane I looked at the page and thought, Well, that’s nice-to-know stuff, especially the odometer distance, although I wasn’t really sure why. I reset the odometer to zero, then moved on to explore other trivia residing within the 430’s circuitry. Then I thought of a way to use some of it.

We have an annual one-day open house at our airport when the public is invited in free of charge to see a variety of general aviation aircraft on display, take cheap rides in fixed-wing airplanes and helicopters, peruse vendor exhibits, and chow down on boiled hot dogs and oversize hamburgers. I shine up the Aztec, park it on the display ramp, and prop up a sign extolling the virtues of general aviation flying, especially for business.

We had been operating the airplane for about five months when I first displayed it at the open house, and on my sign pitching general aviation to the public I noted that in a few short months we had flown to such places as Norfolk, Virginia, and back in a day; all over the state of Florida; and to several islands in the Bahamas—some 15,000 miles total. I got that figure off the 430’s odometer. Very nice to know, indeed.

As the Hobbs hours continued to build, I began to pay more attention to the other two fun facts on the Trip Stats page: average groundspeed and highest groundspeed. The latter is the gee-whiz number, because going fast is a kick no matter what you’re flying. Imagine a Cub driver boasting that he has clocked 120 knots over the ground, and calling up the Trips Stats page on his battery-operated GPS to prove it.

Eventually, however, I concluded that knowing the highest groundspeed the airplane has achieved over the odometer distance doesn’t mean much other than you got lucky one day and hitched a ride on a terrific tailwind. No, the revealing stat on the Trip Stats page is average groundspeed because it lays bare the hidden truth about your airplane’s day-in, day-out performance.

When I first started paying attention to this number it was about 150 knots. I was shocked. The Aztec is a 165-knot true airspeed airplane or better below 10,000 feet msl, which is where we cruise on at least 80 percent of our trips. How could it be that our average groundspeed over tens of thousands of miles is 10 percent slower than published cruise speeds?

The answer, of course, is the combination of relatively slow groundspeeds achieved in the climb (taxiing has no effect because the odometer doesn’t start until the GPS decides we’re doing at least 30 knots over the ground), the effect of headwinds in cruise (with the unfortunate result that we spend more time battling headwinds than enjoying tailwinds), and maneuvering in the terminal area on arrival.

Knowing the average groundspeed of the airplane over the 18 months we’ve been operating it is as sobering as knowing exactly how much it costs per hour to operate it. The first number is lower than expected, the second one higher.

But, like operating costs, average groundspeed is information worth having. It’s the figure we should be plugging into our flight planning to arrive at accurate estimates of block-to-block trip time. If you tell passengers it will be a two-hour flight based on average ground speed, and a welcome tailwind knocks 15 minutes off that estimate, then you’ve exceeded their expectations.

Checking the Trip Statistics page on the GPS Navigator has become something of an obsession. I seem to do it every flight, sometimes several times, and the reason is the number has slowly been on the rise. I’m not sure why. I’m not flying the airplane any differently, and we’re covering the same routes. But statistics don’t lie. At 39,250 nm on the odometer the average groundspeed was 150.8 knots. At 39,984 it had soared to 150.9 knots. Clearly, 151 knots was in sight.

Finally, at 41,272 on the odometer, the average ground speed ticked up a tenth to 151. I smiled. The glass is half full. Who knows, maybe the hourly cost of operation has dropped a nickel or two.

E-mail the author at mrtwombly@gmail.com.