Are airliners' cruising altitudes we know indicated or true altitudes?

Question

Are airliners' crusing altitudes, which commonly range from 25000 to 45000 feet depending on various conditions, indicated altitudes or true altitudes? If they are indicated altitudes, what are the range of cruising true altitudes?

Answer 1

The altitudes are "indicated." That is the altitude shown on the aircraft's altimeter.

In the U.S. at and above 18,000 feet (known as Flight Level [FL] 180) all aircraft altimeters are set to 29.92 in. Therefore the "indicated" altitude on the aircraft's altimeter (at and above FL180) is also "Pressure Altitude." In other parts of the world "Flight Levels" (where all altimeters are set at 29.92) varies.

"True Altitude" (actual altitude above mean sea level) is not, in an operational sense, used other than a reference for weather reports, terrain, obstacle clearance, etc. It is important, but when aircraft are flying (below FL 180) their altimeters are set to a "local" altimeter setting which adjusts the "indicated" altitude to correspond closely to "true altitude." This eliminates "true altitude" as being an operational consideration.

It sounds complicated, but it really is not. Just a bit difficult to explain.

The range of cruising altitudes varies considerably for air carrier aircraft. It would be fair to say that the range is generally between FL290 (about 29,000 ft. msl) to FL 400 (about 40,000 ft. msl). This varies due to ATC operational considerations, wind speed and direction aloft, and aircraft weight and performance capabilities. There are times when some aircraft will fly a bit lower or, if the capability exists, fly higher, say FL430.

Answer 2

The delta between "indicated altitude" and "true altitude" depends on how much the actual pressure differs from the set 29.92. If the actual pressure is 29.92, then indicated = true altitude.

For each 0.01 of altimeter setting, you change the altimeter reading by about 10 feet; for each 0.10 of altimeter setting, 100 feet, and for each 1.00, 1000 feet. So if your actual pressure is 30.92, then with 29.92 set in your altimeter, you'll be 1,000' higher than indicated, so at FL410 (read on your altimeter with 29.92 set), your "true altitude" would be about 42,000' MSL.

Anything approaching 31.00 as an altimeter setting is pretty rare, so I'd say that 42,000' is about the top end of the range for most commercial operations, although if you find a jet that can cruise above FL410 (at least some models of the 747 can, IIRC), then add 1000' to their max ceiling for your answer.

As far as the low end, anything below about 29.00 is a pretty rare altimeter setting, so subtract 1000' from whatever you want to use as your range for "indicated" altitude, and that will be the bottom of your corresponding range of "true" altitudes.

That all said, I've used altitudes below FL 180 for a cruise altitude in airline operations plenty of times, so there my indicated would essentially match my true altitude. Most often, this is on short flights where you can get more direct routing by staying low & out of more congested airspace up higher (U.S. east coast); sometimes the headwinds/tailwinds or the rides (turbulence) is enough better down low to make it worthwhile to fly below the flght levels.

So, if you want to consider what is the range of commercial operations, I'd say that FL410 is mostly a very firm upper limit, with very few airline flights above that, while the lower limit would be much less clearly demarcated. And converting to true altitude, add/subtract about 1,000' from your indicated (flight level) altitude to cover almost all of the likely variation in atmospheric pressure.