# Why might a passenger jet fly at altitudes in a strong headwind when the winds are more favorable at a different altitude?

I was a passenger on an east to west flight recently. During the flight I was playing with Garmin Pilot's new "Flight profile" feature that nicely shows the wind speeds at various altitudes, and I noticed that we were flying against an 80+ knot headwind at FL320, but if we'd dropped to FL220 we'd shave about 40 knots of headwind. That seems pretty significant. A 1,000 NM flight with 355 knot ground speed (which we were cruising at) would save 17 minutes of cruise time with 40 knots less headwind. That's a 10% increase in efficiency.

So the question is, did we stay (or even plan in the first place) to fly at FL 320 the entire cruise portion of the flight simply due to ATC needing to keep us there for traffic routing purposes, Is there an even larger increase in fuel efficiency between FL320 and FL220 than 10%, or is the difference negligible enough that it's not worth asking for a different altitude from ATC, or some other factor not considered here?

• winds are quite volatile so you can't rely on them staying the same for several hours – ratchet freak Apr 30 '16 at 13:22
• Pretty good answers below working through the math, but it must be stressed that the airlines optimize for minimum fuel cost much more strongly than for minimum flight time. While there are some other costs that go up with flight time (crew pay, aircraft maintenance), fuel is a big, big component and they fly to make money. I also find it useful to keep in mind the very approximate rule of thumb that true airspeed increases around 2% per 1,000 feet of altitude. (The real equation is far from linear so this a crude estimate, but useful.) – RobP May 7 '16 at 23:46

You assessment is flawed in that it does not take different air density in account. You state the following conditions:

• FL320
• 355 kt groundspeed
• 1000 nm distance

and you are proposing descending to the following conditions:

• FL220
At 32,000 ft, standard conditions are around 275 hPa, 225 K and density of 0.43 kg m$^{-3}$. If your 80 kt wind is a pure headwind, a 355 kt groundspeed translates into a 435 kt TAS. If we correct this TAS for the ambient conditions we get an indicated airspeed of about 255 KIAS. This is the airspeed that really matters because it is what the airplane experiences as it interacts with the atmosphere.
If we descend to FL220, where standard conditions are 430 hPa, 244 K and 0.61 m$^{-3}$, an airspeed of 255 KIAS yields a TAS of 363 kt TAS. If we have a headwind of 40 its, this yields a groundspeed of 323 kt. This is about 30 kt slower than you were flying at FL320. A flight of 1000 nm will take about 15 minutes longer to complete.