Definition
An emergency procedure used when a pitot-static instrument (typically the vertical speed indicator) becomes unusable due to a blockage in the static system. The pilot deliberately breaks the glass face of the VSI to vent the static line to cabin pressure, restoring usable static air to the altimeter and airspeed indicator. It is a last-resort action used only when no alternate static source is available.
Plain English
If the static port gets blocked and there is no backup, the pilot can smash the face of the vertical speed indicator. This lets cabin air into the static system so the altimeter and airspeed indicator can work again, even if not perfectly.
Context Anchor
Seen in pitot/static system failure procedures, especially when discussing a blocked static source and no available alternate static source.
Derivation
The phrase comes from the everyday idea of "break glass in case of emergency" — the literal act of smashing a protective cover to access something only used in emergencies. Here, the glass face of the VSI is the barrier being broken.
Why Pilots Care
Restores usable altitude and vertical speed information when primary static sources are lost, allowing continued safe instrument flight instead of immediate loss of attitude reference.
Grounding Statement
Picture the normal outside-pressure opening blocked; breaking one instrument face creates another way for air pressure to reach the instruments that need it.
Intuition Check
Do not read this as a figure of speech like “break glass in case of emergency.” In this context, it means physically breaking an instrument face as a last-resort pressure source.
Example Sentence 1
With both static ports iced over and no alternate static source fitted, the pilot considered breaking the glass on the VSI to restore the altimeter and airspeed indicator.
Example Sentence 2
The emergency checklist listed breaking the glass only after confirming the alternate static valve was inoperative.