Altimeter-Setting Procedures: Transition Altitude, Transition Level, and QNH/FL References

Procedures to ensure vertical separation and terrain clearance using standard barometric references.

Basic Principles

• Transition Altitude (TA): The altitude at or below which the vertical position of an aircraft is expressed in terms of altitude (QNH).
  • Usually specified by each State for each aerodrome.
  • Rounded up to the next 1000 ft (300 m).
  • Published in charts and AIP.
• Transition Level (TL): The lowest Flight Level (FL) available for use above the transition altitude.
  • Usually reported by ATC (in approach clearances or ATIS).
• Transition Layer: The airspace between the TA and the TL.
  • Climbing: Change to FL when passing TA.
  • Descending: Change to QNH when passing TL.

Vertical References

1. At or below Transition Altitude:
   • Reference: Altitude (QNH).
   • Used for takeoff, initial climb, final approach, and landing.
2. At or above Transition Level:
   • Reference: Flight Levels (Standard 1013.2 hPa).
   • Used for cruise and much of the climb/descent.
3. Flight Level System:
   • Separated by pressure intervals corresponding to at least 500 ft (in standard atmosphere).
   • FL0 is located at the 1013.2 hPa isobar.

Operational Procedures

• Takeoff and Climb:
  • Before takeoff: One altimeter set to the latest QNH.
  • During climb: Reference QNH until TA. Upon passing TA, change to 1013.2 hPa.
• Approach and Landing:
  • Before starting initial approach: Obtain the transition level.
  • Before descending below the TL: Obtain the latest QNH.
  • Upon descending through the TL: Change reference to QNH.
  • Exception: If the aircraft descends continuously from high levels for landing, QNH may be used above the TL if authorized by ATC, provided level-off is not made above the TA.
• Separation and Terrain: The goal is to provide adequate separation between aircraft and terrain clearance in all phases.