[zettabomb]

A proper deadman's switch can't just be a button - you could weight it down, or just fall asleep on top of it. Trains commonly use a pedal you have to hold down halfway. Not sure this is really the best idea though, as pilots typically need both hands (and usually feet) free to actually, you know, control the plane.

[ceejayoz]

A button press could be required, though.

[lupire]

If the pilots are controlling the plane, they are doing detectable actions and pre-empting the dead person switch.