Making Fuel

Buffalo Airways C-46 Commando.

While watching an “Ice Pilots” episode on the Weather Channel I heard a pilot of a Curtiss C-46 Commando talking to his inexperienced copilot during a flight. At one point he said they were “making fuel.”

I have enough common sense and experience as a pilot to know that could not be literally true. But I had no idea what the Ice Pilot’s comment  really meant until recently returning home during a non-stop flight from Dallas, Texas to the Florida Panhandle.

I had purposefully climbed to 11,000 feet to catch good tailwinds heading east. The winds were even stronger at higher altitudes, but if I’d climbed to the next allowed altitude, 13,000 feet, my passenger and I would have needed to wear an oxygen mask. And I’d left the oxygen system at home.

During flight planning before departure, it looked as if going high would give us enough of a tailwind that we would be able to make the trip without a time consuming fuel stop.

Modern aircraft often have fuel computers communicating with the aircraft GPS navigation system. Fuel computers track every ounce of fuel burned during taxi and flight. The pilot programs the total fuel available and then the fuel computer checks with the GPS to see how many miles remain to the destination, and the ground speed. Every few seconds the pilot sees an update of the fuel burned, gallons remaining, predicted flight time available, the fuel required to reach the destination, and the bottom line, the predicted fuel reserve at the destination.

Typically, I want to land with no less than 10 usable gallons remaining, which is enough to remain aloft for an additional hour at the normal fuel consumption rate. If the weather is bad at the destination, then the required fuel reserve is considerably larger.

On the first phase of my flight to Dallas, once I had reached cruising altitude the fuel computer calculated that if the current ground speed and fuel burn were to continue to the end of the trip, I would have five gallons of fuel left at the destination. That is not enough for safe flight, so a refueling stop was looking inevitable. As the flight continued, the estimation of reserve fuel barely budged from its first estimate. In other words, nothing was changing, and the decision to refuel was firmly made.

East Texas from 11,000 ft. Click to enlarge.

On the return flight, however, flying relatively high where the prevailing westerlies were strong,  the computed reserves (RES) were changing. They were growing. As the flight progressed I watched the estimated fuel reserve rise slowly from 8 gallons to 9, then 10, and finally 11.4 gallons. By the time we landed we had 12 gallons of fuel remaining in the two fuel tanks.

It truly looked like we were making fuel.

We weren’t, of course. The reality of it was that the tailwind was increasing in our favor for the east-bound trip. But the fuel computer gave every impression that for every gallon of fuel we burned, we were getting a little bit back.

I finally understand what the Ice Pilot meant; I think. If I ever meet him, I’ll ask.