I remember this one. Max speed with the ladder extended is 400 Kts!!! Not bad.
You would only know it wasn't locked in the up position it you checked the inticator on the left console/front cockpit, your No2 told you, you felt the vibrations (although you'd be unlikely to know what what was causing it) or when it bloke off and went down the left engine (which would only have been very slightly above 400 kts if it were fully extended).
I have come back from a sortie with a boarding ladder that had become partially extended (similar to the photo). I don't know at what point in the sortie it had become unlocked, but we had certainly been well over 400kts - because we could!
