The touchdown zone (TDZ) is the section of runway where an aircraft is designed to first contact the ground during a normal landing. For instrument approaches, RVR (Runway Visual Range, a precise measure of forward visibility) recorded at the TDZ is the most critical visibility value for determining whether a landing can legally proceed.
How It Works#
The TDZ begins at the runway threshold and extends 3,000 feet down the runway, or half the runway length, whichever is less. It is marked by pairs of white rectangular bars painted symmetrically on both sides of the centerline. These markings help pilots judge their position during the final flare and touchdown.
Runway Visual Range sensors sit close to the TDZ to capture the most relevant visibility reading for a landing aircraft. The sensor measures how far down the runway a pilot could see from cockpit height. This value, called TDZ RVR, feeds directly into go/no-go decisions on low-visibility approaches.
On approaches with multiple RVR reporting points (TDZ, midpoint, and rollout), the TDZ reading carries the most weight. A pilot may be able to complete a landing even with degraded midpoint or rollout RVR, but if TDZ RVR falls below the approach minimum, the approach is not authorized.
Example in Aviation#
An airline crew is flying an ILS (Instrument Landing System) Category I approach with a published visibility minimum of 1,800 RVR. As they reach the final approach fix, the ATIS (Automatic Terminal Information Service) reports TDZ RVR at 1,600. That reading is below the published minimum. The crew must either hold and wait for conditions to improve or divert to their alternate. The midpoint and rollout RVR values are irrelevant to that decision.
Why It Matters#
Pilots must understand the TDZ because it anchors two separate but related concepts: physical aircraft placement and legal visibility minima. A landing that touches down well beyond the TDZ uses up stopping distance and increases the risk of a runway excursion, especially on wet or contaminated surfaces.
On low-visibility operations, TDZ RVR is not just a number to cross-check. It is often the single value that determines whether an approach can continue. Misreading or ignoring it can place a crew outside the safety envelope the approach procedure was designed around.
Key Takeaways#
- The TDZ spans the first 3,000 feet of runway from the threshold, or half the runway length if shorter.
- White rectangular bar markings on the runway identify the TDZ visually.
- TDZ RVR is the primary visibility value for low-visibility approach authorization.
- Touching down beyond the TDZ reduces available stopping distance.
- If TDZ RVR is below the published minimum, the approach is not authorized.