747 hydraulic failure
According to the QRH, if both Systems 1 and 4 are Inoperative, the speedbrake lever does not extend past the flight detent until the nose gear is on the runway and thrust reversers are only available after the nose gear is on the runway. But there is nothing in the systems description that I saw that explains why.
The Air/Ground Sensing System is based on main gear tilt while the nosegear extension sensing provides a signal to relays controlling functions in the stall warning and nose gear steering systems.
Any explanation.