Plane crash detection : quadplane config

Crash detection in Plane only looks at the x-axis accel. With a quad-plane config we recently experienced a hard landing in VTOL that neared 10g's in the Z-axis, but it failed to trip the crash detection as x-axis accel was minimal. Suggest a second CRASH_ACC_THRESH specific to the z-axis

Version
AP3.8.5beta1

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[X] Plane
[ ] Rover
[ ] Submarine

Airframe type
OctaQuadplane X8

Hardware type
PH2.1

Start by locating Plane's crash-detection logic and references to CRASH_ACC_THRESH, then trace how acceleration axes are evaluated for quadplane VTOL landings. Done means a z-axis threshold can detect the described hard landing without disrupting existing x-axis detection; verify the behavior with the relevant crash-detection tests or simulation if available.

Written by the indexing model from the issue text.