Plain English Meaning
When a deice boot inflates, it pushes against the ice. If the ice hardens around the inflated shape, it can stay stuck in that shape after the boot deflates, leaving a layer of ice still on the wing that the boot can no longer reach.
Full Definition
A failure mode of pneumatic deice boots in which ice forms a shell that conforms to the boot's inflated shape, so when the boot deflates the ice does not break free but remains attached as a hollow dome standing off from the airfoil surface.
Derivation
From 'bridge' -- something that spans a gap. The ice spans across the now-deflated boot like a bridge over the airfoil surface, no longer in contact with it.
Why Pilots Care
Bridging reduces boot effectiveness, allowing ice to accumulate and degrade wing performance.
Analogy
It is like a thin crust forming over a dent in soft ground. If you push up from underneath but only fill the empty space, the crust may not break.
Intuition Check
Bridging does not mean connecting two places here. It means ice spanning over a moving boot like a small arch, so the boot may not press directly against the ice.
Example Sentence 1
The instructor explained that bridging was a real concern with older deice boots, which is why pilots were once trained to let ice build up before activating the system.
Example Sentence 2
In continuous icing, proper timing of boot activation helps prevent bridging on the wings.