Aviation numbers
Flight levels, and the pressure behind them.
Every altitude in aviation is a pressure reading in disguise. Once that is clear, flight levels, QNH and the old warning about high to low all fall into place.
- 1013.2 hPaThe standard setting. Above the transition altitude, everyone uses it.
- ≈27 ft per hPaWhat one hectopascal is worth in indicated height near sea level.
- 10,000–19,000 ftThe range of transition altitudes Türkiye publishes, aerodrome by aerodrome.
An altimeter does not measure height
It measures pressure. Air gets thinner with height in a way that is well described, so an instrument that reads pressure can print a height scale on its face and be right — provided it knows what the pressure is at the bottom. That reference is the setting, and it changes with the weather, from day to day and from place to place.
QNH — set this and the altimeter reads height above mean sea level.
QFE — set this and it reads height above a particular aerodrome, showing zero on touchdown there.
Standard, 1013.2 hPa — set this and it reads a pressure altitude, expressed as a flight level.
Why FL350 is not 35,000 feet
Above a published transition altitude, everyone stops using the local pressure and switches to the standard 1013.2 hPa. The reading is then no longer a true height; it is a pressure altitude, written as a flight level in hundreds of feet. FL350 means the altimeter shows 35,000 ft on the standard setting — the aircraft's real height above the sea may be several hundred feet either side of that.
That sounds like a defect and is in fact the point. Separation between aircraft is what matters in the cruise, and separation only needs everyone to be wrong in the same way. Two aircraft a thousand feet apart on the same datum stay a thousand feet apart whatever the weather below them is doing.
The rule of thumb, and the warning built on it
Near sea level a change of one hectopascal is worth roughly 27 feet of indicated height. Fly from an area of high pressure into one of low pressure without resetting, and the altimeter keeps reading the old, higher number while the aircraft quietly descends toward the terrain. Hence the line every student learns: from high to low, look out below.
The calculator applies that 27 ft per hPa figure. It is an approximation for understanding, not a flight-planning value.
In Türkiye
There is no single worldwide transition altitude — each state sets its own, and it can differ between aerodromes within one country. Türkiye publishes transition altitudes of 10,000 ft, 12,000 ft, 15,000 ft and 19,000 ft depending on the aerodrome, which is why a crew reads it off the plate rather than from memory.
Feet, exactly
The international foot has been exactly 0.3048 metres since the 1959 agreement, so every conversion on this page is a definition rather than a measurement. 35,000 ft is 10,668 m to the metre.
What this does not tell you
- The 27 ft per hectopascal figure is a rule of thumb for understanding, not a flight-planning value.
- There is no worldwide transition altitude. The one in force comes from the current AIP and the approach plate, never from a page like this one.
- A flight level is a pressure altitude. It is not the aircraft's true height above the sea, and it is not terrain clearance.
Standard atmosphere datum 1013.2 hPa; 1 ft = 0.3048 m (1959 international yard and pound agreement). Turkish transition altitudes: AIP Türkiye ENR 1.7, published by DHMİ. Altimetry here is explanatory — operational values come from the current AIP and the approach plate in force.