What is the TDZ beam?
How a touchdown zone light aims its white beam back toward the approach so pilots can judge where to touch down.
The TDZ beam is the beam pattern of the touchdown zone lights, the pairs of short white bars (barrettes) set in the runway on either side of the centreline after the threshold. Pilots see them on the final approach and in the flare, so each light is aimed back toward arriving aircraft, at low vertical angles, and the many bars together help the pilot judge the touchdown zone.
The lights are inset, fixed, white and unidirectional: a barrette serves aircraft landing in one direction only, so a runway used both ways has a separate set for each. The pattern extends along the runway for up to about 900 m from the threshold, shortened on short runways so it does not pass the midpoint. ICAO requires touchdown zone lights on Category II and III precision approach runways; elsewhere the authority decides.
This follows the earlier beam entries: the beam direction (Entry 26) points toward the approach, the spread (Entry 27) must cover the pilot's view, and the complete distribution (Entry 28) is what the isocandela diagram for touchdown zone lights describes. Compared with the runway centreline beam (Entry 29), the TDZ beam is aimed one way, and compared with the threshold-light beam (Entry 37) it is white and inset. Because tyres deposit rubber here, dirty lenses and misalignment matter: a contaminated inset fixture shows reduced light to the pilot it is meant to guide.
Frequently Asked Questions
Q1What is the TDZ beam?
The beam pattern of a touchdown zone light: how it is aimed and shaped so that pilots on the final approach and in the flare can see the white barrettes and judge the touchdown zone.
Q2Are touchdown zone lights unidirectional?
Yes. They are fixed white lights that face the approaching aircraft only, so each barrette is seen from the landing direction it serves and not from the opposite one.
Q3How does the TDZ beam differ from the runway centreline beam?
The centreline beam is normally bidirectional and concentrated near the axis. The TDZ beam is aimed one way, toward the approach, and the lights sit in pairs of barrettes on either side of the centreline, so each pilot sees several bars at once. The exact shapes are set by the standard.
Q4What can change the beam of a TDZ light in service?
Rubber deposits from landing tyres, which are heaviest in the touchdown zone, dirt on the lens, misalignment after fixture replacement or pavement works, and degraded light sources. This is why inset lights here are cleaned and checked photometrically as well as visually.
Q5Where is the TDZ beam defined?
In the isocandela diagram of ICAO Annex 14, Appendix 2 for touchdown zone lights, and in FAA AC 150/5345-46 and the fixture manufacturer's photometric data. Always confirm against the current edition.
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