Flight Levels

Definition

A Flight Level (FL) is a standardized way of expressing altitude based on standard atmospheric pressure (1013.25 hPa or 29.92 inHg) rather than local barometric pressure. Flight levels are used above the transition altitude to ensure all aircraft in higher airspace operate using a common pressure reference, enabling consistent vertical separation.

How Flight Levels Work

A flight level is an altitude expressed in hundreds of feet
Example: FL100 = 10,000 ft on standard pressure
Used by both IFR and VFR aircraft in controlled airspace above a specific transition level
Enables accurate aircraft separation regardless of varying surface pressure

Transition Altitude vs. Transition Level

Term	Definition	Used When
Transition Altitude (TA)	The altitude at which aircraft switch from QNH to STD (1013)	During climb
Transition Level (TL)	The lowest available FL when descending on STD pressure	During descent

The space between TA and TL is the transition layer, which ensures vertical separation between climbing and descending aircraft.

Typical Phraseology

“Climb to flight level 90”
“Descend to flight level 70”
“Report reaching flight level 85”

Flight Level Format

Always spoken as individual digits, not whole numbers
FL80 → “Flight level eight zero”
FL105 → “Flight level one zero five”

Example Conversion (assuming 1013 hPa = actual QNH)

FL	Altitude (ft)
FL50	5,000 ft
FL70	7,000 ft
FL100	10,000 ft
FL195	19,500 ft

If QNH is lower than standard, FL100 will correspond to an actual altitude below 10,000 ft, and vice versa.

VFR and Flight Levels

In some countries (e.g., Austria, Germany), VFR aircraft must fly odd or even flight levels based on their magnetic track:

Track	Altitude/FL (VFR)
000–179°	FL55, FL75, FL95 (odd + 500 ft)
180–359°	FL65, FL85, FL105 (even + 500 ft)

This is part of the semi-circular rule (or hemispheric rule), designed to maintain vertical separation between opposite-direction traffic.

Flight Levels vs. Altitudes

FL (Standard Pressure)	QNH Altitude
FL50	~5,000 ft (if QNH = 1013 hPa)
FL50	~4,700 ft (if QNH = 980 hPa)
FL50	~5,300 ft (if QNH = 1040 hPa)

This variation is why all aircraft above the transition altitude must use FLs, not local altitudes.

Important Notes

✅ Always verify the transition altitude for the FIR you're operating in
✅ Switch from QNH to STD at the transition altitude, and back to QNH at the transition level
✅ In VFR flight, ensure that the selected FL complies with visibility, airspace class, and hemispheric rules