Tip What is the most fuel efficient altitude to fly at?

[OldHerkJock]

The general rule is, "as high as you can go."

I used to fly C-130's out of Yokota AB, Japan. We had a deployment to Clark AB in the Philippines. The normal cruising altitude for the C-130 was FL 240. Normal cruise speed was 280 knots. On this particular day, the headwinds were about 120 knots at FL 240. They were significantly lower at 14,000 feet.

I took off about a half an hour after the lead aircraft in the deployment. Out crew was already at the bar by the time they landed. I did not check how much fuel we burned when compared to the aircraft that flew higher, but the flight engineer was smiling when we landed.

Amount of Fuel you burn per unit if time = Ground Speed / Rate at which you burn the fuel, If the ground speed goes up faster than the rate at which you burn fuel goes up - you win.

Sometimes this means you push the throttle up - fly faster. You burn more fuel per hour but you spend less time burning it. Sometimes it means flying down where the headwinds are lower.

I was able to compute the best airspeed and cruising altitude based on calculus (I'm an engineer) and the performance manuals for the C-130.

If I have the right data, I can do the same for any aircraft. The slower the aircraft is, the more pronounced the savings become.

Of course, it works in reverse with a tailwind. There comes a point where max endurance airspeed it the winner.

There was one night when we were returning from Korea with a 150 knot tailwind. When we contacted the Japanese controller. He asked us to confirm our 430 knot ground speed. We did. Then he asked us to confirm type aircraft. I told him, "C-130." His response was "Are you sure?" My response was, "That's what it says on the yoke."

 

[Waldom]

Among other airplanes, I occasionally fly a PA-11 on long cross country flights. I can certainly concur that selecting the most favorable altitude for winds aloft is important with an airplane that cruises 82 knots. The Continental Motors operators manual can give you power setting vs. fuel burn vs. altitude numbers, but Piper Aircraft never published cruise performance data for the airplane. However, an app like ForeFlight is very useful in economical flight planning if you have entered accurate performance data based on refined experience with the airplane. Even with an average fuel burn of 4.8 gallons per hour, efficient flight planning can save enough money to pay for the expensive subscription price of the application.

It is true that turboprop and jet aircraft are more efficient the higher they fly if wind is not considered. For most normally aspirated reciprocating engine aircraft the best fuel burn vs. true airspeed pressure altitude is around 6500 feet.