Plane & Pilot
Thursday, September 1, 2005

The Go/No-Go Decision

It’s better to be safe than sorry

Every pilot has his own tolerance for risk, but most of us who fly ferry across the oceans on a semi-regular basis have developed our own set of guidelines for when we will or won’t fly. We like to hope that those guidelines make perfect sense, but they often don’t. They’re just our way of doing things, they work, and that’s all that matters as long as they keep us alive.

This comes to mind because of a recent experience with a Grand Commander 680FL from Van Nuys, Calif., to Melbourne, Australia. The airplane was old, but in generally good condition. It came fitted with a standard 156-gallon center tank that feeds both engines, plus a pair of 33.5-gallon wing auxiliary tanks dedicated to their respective engine. That’s 223 gallons of internal fuel.

The ferry system included a pair of interconnected tanks in the aft cabin with 330 gallons of total capacity and a 137-gallon tank at the center cabin. That means a total capacity of 690 gallons. If that sounds like a lot, consider that the airplane was fitted with a pair of eight-cylinder TIO-720 Lycoming engines, which were rated for 400 hp per side.

If the little O-320 and IO-360 Lycoming engines are among general aviation’s most efficient powerplants, the big Lycomings have always been among the least efficient, and the TIO-720 is about as big as they come. Specific fuel consumption is roughly 0.45 pounds per hp per hour on the largest Lycoming. Do the math, and the total burn at 75% power works out to 22.5 gph per engine or 45 gph total. Of course, that’s using best power rather than best economy, and you could certainly expect to do better running slightly leaner, probably 21 gph or so. You obviously could save yourself some fuel by throttling back to 65% power.

In seeming contradiction to the math, fuel flows on this airplane were reading unusually low, 16 gph on the left and 18 gph on the right, and I was fairly certain that wasn’t correct either. To get a better handle on fuel burn, I climbed out of Long Beach, Calif., on the short positioning flight to Oakland, Calif., using the center tank, leaned both engines to the proper level, switched to the auxiliary tanks and ran for exactly one hour, then switched back to the mains. I was hoping for a burn of 20 gallons per engine per hour, so I might have expected to see the gauges for the 33-gallon auxiliary tanks reading about 13 gallons on landing.

Instead, they read slightly below 10 gallons. When I refueled in Oakland, the auxiliary tanks took 23 gallons each, suggesting that burn was much higher than normal, rather than lower as indicated by the fuel flows. If the burn really was 15 to 16 gph, my 690-gallon capacity meant that I had about 15 hours of fuel to exhaustion.


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