Definition
The maximum allowable vertical position error for a WAAS-based approach before the system must warn the pilot that the guidance can no longer be trusted. If the estimated vertical error exceeds this limit, the avionics flag the approach and remove the glidepath indication.
Plain English
It is the largest vertical error the system will tolerate before it tells you the glidepath is no longer reliable. Cross that line and the approach is no longer usable.
Context Anchor
Seen in WAAS and LPV approach discussions, especially when explaining how the system decides whether precise vertical guidance is available.
Derivation
Alarm limit' is borrowed from satellite navigation engineering, where it describes the threshold at which the system must raise an alarm to the user. 'Vertical' simply restricts that threshold to up-and-down position error, as opposed to horizontal.
Why Pilots Care
Exceeding the limit removes vertical guidance, forcing a missed approach and protecting against unsafe descent.
Grounding Statement
The vertical alarm limit is the line the system must stay inside before it can be trusted for vertical guidance.
Intuition Check
Do not read “alarm” as only a sound or cockpit noise here. In this term, it means the point where the system must warn you or stop providing approved vertical guidance.
Example Sentence 1
When the WAAS vertical alarm limit was exceeded just inside the final approach fix, the glidepath flagged and the crew flew the missed approach.
Example Sentence 2
If the aircraft's vertical error approaches the vertical alarm limit, the system removes the glidepath guidance.