The portion of a runway, beyond the threshold, where it is intended landing aeroplanes first contact the runway.
Source: ICAO Annex 14
The TDZ is placed after the runway threshold and runway designation markings. This is done to ensure that the aircraft lands on the runway and provide a safety margin in case of e.g. pilot error or sudden loss of height due to windshear or turbulence.
The TDZ is marked by pairs of stripes symmetrically placed on the two sides of the runway centreline. The number of pairs depends on the runway length (e.g. one pair for runways that are shorter than 900 m, 6 if the length is 2 400 m or more, etc.). The aiming point marking coincides with one of these pairs and is noticeably wider (see picture below).
Specific provisions about TDZ marking and aiming point marking, including requirements when to implement these, dimensions, distances, and spacing, are described in ICAO Annex 14, Chapter 5.
Aiming point and touchdown zone markings (illustrated for a runway with a length of 2 400 m or more)
Rubber desposits are a specific issue related to the TDZ. When aircraft touch down, their tyres are not spinning and therefore start dragging. Due to the pressure caused by the aircraft weight, some of the tyre rubber is deposited onto the runway. The process continues until the tyre rotation speed matches the aircraft ground speed. Rubber deposits can obscure runway markings, and, when wet, create a slippery area on the runway surface that can cause or contribute to runway excursions.
Rubber is removed on a regular basis (depending on the runway usage) by using chemical solvents, high pressure water blasting or hot compressed air.
- ICAO Annex 14 - Aerodromes, Chapter 5. Visual Aids for Navigation
- Chapter 8 - Removal of Rubber, part of ICAO Doc 9137 "Airport Services Manual Part 2"