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

> Learn altimeter-setting procedures for safe vertical separation, including transition altitude, transition level, transition layer, and standard pressure use.

- **Subject:** 010 — Air Law
- **Section:** 010-06 — Aircraft Operations
- **Topic code:** 010-06-06
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/air-law/aircraft-operations/altimeter-setting-procedures-icao-doc-8168/

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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.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.