Relative to reference frame of the airplane he's sitting in:
-- Gravitational potential energy = (mass) (gravity) (height)
His height above the airplane is zero, so his GPE is zero.
-- Kinetic energy = (1/2) (mass) (speed)²
His speed relative to the airplane is zero, so his KE is zero.
Mechanical energy = (potential energy) + (kinetic energy)
Mechanical energy = ( 0 ) + ( 0 )
The poor fellow's total mechanical energy is zero .
