Perhaps not as relevant as it may seem at first glance but on a local news group someone posted a link to this patent...
https://www.google.co.uk/patents/US8...ed=0CCEQ6AEwAA
"Locking and unlocking system for the cockpit door of an aircraft and door with such a system US 8505850 B2"
In the claims...
an accelerometer configured to detect a maintained constant acceleration exceeding a preselected acceleration threshold value that indicates that the aircraft is in a dangerous descent,