Ask the Captain: Fuel efficiency in flight

Question: On short flights where the plane is at cruising altitude for only a few minutes, why not just stop at, say, 15,000 feet? Isn't it a waste of energy to climb to 30,000 just to go right back down?

— submitted by reader Ryan Joyce, St. Louis

Answer: Jet performance is carefully calculated by flight-planning programs. Climbing to a higher altitude allows the jet to operate in thinner air, where fuel economy is better and drag is reduced for longer. Although it is counterintuitive, climbing to a higher altitude and then descending can save fuel.

Q: Is fuel consumption a function of cruising altitude? Are there assigned altitude levels to flight routes?

— David Gillespie, Bonita, Calif.

A: Altitudes are assigned by air traffic control to maintain separation from other airplanes. Flight plans are filed usually at the optimum altitudes for fuel economy (higher is usually more efficient). Air traffic control will assign the filed altitude unless there is conflicting traffic.

Yes, fuel consumption decreases with altitude.

Q: Does a plane become more fuel efficient on a trans-oceanic flight?

— Kevin, Raleigh N.C.

A: Yes, the fuel burn affects the weight of the airplane. As fuel is consumed, the weight decreases. This results in less lift being required to maintain altitude, consequently less fuel is needed to produce the necessary thrust.

Q: Is it true that a turboprop plane actually climbs more efficiently than a jet? If so, does it also climb faster, or simply more efficiently?

— Jack, Ohio

A: A turboprop is more efficient (pounds of fuel burned per nautical mile flown) than a jet at lower altitude and for shorter trips. Some of the modern turboprops approach jet speeds, making the time difference very little.

John Cox is a retired airline captain with US Airways and runs his own aviation safety consulting company, Safety Operating Systems.