Aircraft restraint systems with unfixed default mode

A modal restraint system for an occupant of a seat of an aircraft having a mechanical failure sensor includes a strap feeder unit and a strap selectively retractable into and extendable from the strap feeder unit. The strap is positionable adjacent to the occupant. The strap feeder unit is adapted to switch between a plurality of modes including an unfixed mode and a fixed mode. The strap is extendable from and retractable into the strap feeder unit in the unfixed mode. The strap is substantially unextendable from the strap feeder unit in the fixed mode. The strap feeder unit is in the unfixed mode by default. The strap feeder unit is operable to switch to the fixed mode in response to a precautionary event including detection of an aircraft mechanical failure by the mechanical failure sensor, thereby reducing freedom of movement of the occupant relative to the seat.