1
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ What do you mean by "indicated altitude"? We have "indicated airspeed" and "true airspeed". I believe the term you're looking for is "pressure altitude". $\endgroup$ – kevin Jul 22 '17 at 3:03
  • 1
    $\begingroup$ @kevin Although you're right in saying that the indicated altitude is the pressure altitude at crusing levels, there IS indicated altitude. It's a known concept. $\endgroup$ – lemonincider Jul 22 '17 at 4:26
  • $\begingroup$ Note that there are 3 kinds of altitude: geometric altitude (measured by GPS), indicated altitude (measured by barometric altimeter set to local reference datum) and pressure altitude (measured by barometric altimeter set to 1013 hPa (29.92 inHg)). The first two are different and the third is different unless local reference datum happens to be standard pressure. $\endgroup$ – Jan Hudec Jul 23 '17 at 19:50
1
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Thank you so much. May I ask you a follow-up question? When a jet is certified for cruising altitudes up to 45100 feet (B747), is it pressure altitude? $\endgroup$ – lemonincider Jul 22 '17 at 5:56
  • 1
    $\begingroup$ Always a Flight Level, as far as I've seen. For example, the 737 is good to FL410, which isn't changed no matter how high or low the.local pressure may be. $\endgroup$ – Ralph J Jul 22 '17 at 15:38
  • $\begingroup$ The difference between “indicated altitude” and “true altitude” actually depends on the actual temperature, humidity and bunch of other atmospheric properties that are never corrected for. The difference you describe is between “indicated altitude” (corrected for local pressure (but not density)) and “pressure altitude” (measured with standard setting of 1013 hPa (29.92 inHg)). “True altitude” is measured with GPS and only used for terrain proximity warning. $\endgroup$ – Jan Hudec Jul 23 '17 at 19:59
4
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ Thank you for your detailed reply. If the cruising altitudes (FL 290~400) are indicated altitudes, do you know what their true altitude range is? The numbers might be varied depending on the atmospheric conditions, but can you give me approximate numbers? $\endgroup$ – lemonincider Jul 22 '17 at 4:32
  • 1
    $\begingroup$ I can't give you an approximate range because the altimeters (above Fl180) are set to 29.92 (so all aircraft are being separated by ATC based on the same altimeter setting) and the atmospheric conditions (e.g. temp) could vary considerably. $\endgroup$ – 757toga Jul 22 '17 at 4:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.