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I came across a question in Quora asking the difference between altitude and flight level. One of the answers said that anything above the transition altitude is referred to as a flight level. So suppose an aircraft is flying at 35,000ft, it is flight level 350. Is this correct? Can we not refer to altitude as flight level below 18,000ft (the transition altitude)?

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    $\begingroup$ Also note: The transition altitude is not necessarily 18,000ft! That's just the US definition. $\endgroup$ – Bianfable Nov 18 '19 at 10:48
  • $\begingroup$ Yea, sorry my bad! $\endgroup$ – Johnson Nov 18 '19 at 11:15
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Yes, altitude is below transition level, flight level is above transition altitude.

Altitude is referencing to actual (local) air pressure, while flight level is against standard pressure 2992 / 1013.

The transition altitude is dependent on the location and are definded so both will not mix up.

When climbing through transition altitude, pilots set their QNH to 2992 - and so no longer need to set it to changes due to local weather.

When decending through transition level they set it back to the local QNH - as they are close enough to ground the real altitude (above ground...) is getting important.

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    $\begingroup$ Even where to set the QNH varies by region. The "through" statements apply to USA, not Europe, for example. $\endgroup$ – ymb1 Nov 18 '19 at 11:56
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    $\begingroup$ Setting your altimeter to QNH does not give you altitude above ground... For example at my home airport if the altimeter is set correctly it should show about 650 feet sitting in the ground. $\endgroup$ – Ron Beyer Nov 18 '19 at 13:00
  • $\begingroup$ Yes, but setting it to QNH shows the "true" altitude - and charts show the ground elevation or minimum altitude... So close to ground this can make a significant difference... $\endgroup$ – tsg Nov 18 '19 at 15:48
  • $\begingroup$ @tsg It does not even show you true altitude AMSL (above mean sea level), unless you are at airport elevation or the temperature happens to be at ISA. See e.g. this article. $\endgroup$ – Bianfable Nov 18 '19 at 19:19

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