Understanding Transition Altitude, Transition Layer and Transition Level

Introduction

In aviation, proper understanding of the transition altitude, transition level, and transition layer is essential for safe vertical navigation. These terms relate to the changeover between altitudes (based on local pressure – QNH) and flight levels (based on standard pressure – 1013.25 hPa).

This post will explain the differences between these concepts and how they are used in airspace management, along with helpful diagrams.

1. Transition Altitude (TA)

• Definition: The altitude at or below which the vertical position of an aircraft is controlled using altitude (QNH).
• Set by: Local authorities depending on terrain and airspace.
• Example: In many countries, TA is 18,000 feet, but it can be lower in others.
• Use: Aircraft climbing switch from altitude to flight level upon reaching the TA.

2. Transition Level (TL)

• Definition: The lowest flight level available for use above the transition altitude.
• Determined by: Actual atmospheric pressure — it varies.
• Use: Aircraft descending switch from flight level to altitude when passing the TL.

3. Transition Layer

• Definition: The airspace between the transition altitude and transition level.
• Purpose: Provides a buffer zone to prevent collision between climbing and descending traffic.
• Size: Varies depending on QNH — typically 1000 ft or more.

Diagram: Explaining TA, TL, and Transition Layer

       +----------------------------+
       |      FL 70 (Transition Level)     | ← Start using Flight Levels (1013.25 hPa)
       +----------------------------+
       |       Transition Layer        | ← Buffer between TA and TL
       +----------------------------+
       |    6000 ft (Transition Altitude)  | ← Start using Altitudes (QNH)
       +----------------------------+

• Aircraft climbing: Change from altitude (QNH) to flight level (standard pressure) at or above TA.
• Aircraft descending: Change from flight level to altitude at or below TL.

Why It Matters

• Ensures consistent separation during climb and descent.
• Critical for safe and standardized vertical navigation.
• Helps manage transition between local and en-route airspace systems.

Practical Example

If TA is 6,000 ft and current QNH makes TL FL70:

• Aircraft climbing from 5,000 ft → use altitude until passing 6,000 ft, then switch to FL70.
• Aircraft descending from FL90 → continue on FLs until FL70, then switch to altitude below 6,000 ft.