Definition
A height above the touchdown zone, based on the characteristics of the aircraft and its fail-operational landing system, above which a CAT III approach must be discontinued and a missed approach executed if a failure occurs in one of the redundant systems required for the approach. Below the alert height, the approach and landing may be continued despite such a failure, because the remaining systems are sufficient to complete the landing safely.
Plain English
A specific height during a CAT III approach where the rule changes. Above this height, if something important fails in the airplane's automatic landing equipment, you must go around. Below this height, the system has enough backups that you can keep going and land.
Context Anchor
Seen in Category II and Category III approach procedures, especially for aircraft approved for automatic landings in very low visibility.
Derivation
The word 'alert' comes from the Italian 'all'erta,' meaning 'on the watch.' Here it marks the height at which the crew must be alert to a failure and respond by going around — above it, you abort; below it, you commit.
Why Pilots Care
It provides a clear, non-negotiable decision point that protects against attempting a landing without adequate visual cues, directly affecting safety in reduced-visibility conditions.
Grounding Statement
On final approach in very low visibility, Alert Height is the point where the failure-response rule changes.
Intuition Check
Alert Height is not just a reminder to be alert. It is a specific height used to decide whether to continue or go around after a required system failure.
Example Sentence 1
The crew briefed an alert height of 100 feet for the CAT III approach into Heathrow.
Example Sentence 2
The approach briefing included setting the alert height at 100 feet for the Category III landing.