Transition Altitude

Transition Altitude (TA) is the altitude at or below which the vertical position of an aircraft is expressed as altitude (above mean sea level) using the local QNH altimeter setting. Above the transition altitude, vertical position is expressed as flight levels (FL) using the standard pressure setting of 1013.25 hPa (29.92 inHg).

Key related concepts:

• Transition Altitude: The altitude above which pilots switch from QNH to standard pressure (climbing)
• Transition Level: The lowest flight level available above the transition altitude (descending)
• Transition Layer: The airspace between the transition altitude and transition level — aircraft should not level off here

The transition altitude varies by country and even by airport. For example:

• United States: 18,000 ft (FL180) nationwide
• Europe: Varies (3,000–10,000 ft), with harmonization to 18,000 ft under discussion
• Japan: 14,000 ft

Correct use of transition altitude is critical for maintaining vertical separation between aircraft operating on QNH versus standard pressure.