With regards to BUCS on the WAH-64
'Unlike other fielded Army helicopters, the AH-64 Apache has an emergency back
up, electro-hydraulic, fly-by-wire system available to the crew in the event of a jammed
or severed flight control. This back up control system (BUCS) allows the crew to
bypass damaged mechanical flight controls and safely land the aircraft. The BUCS can
be found on both A- and D-model Apaches.
In the AH-64A normal flight control inputs from the pilot or copilot/gunner (CPG)
are relayed to the hydraulic servo-actuators, which control the flight surfaces, using
mechanical linkages (push-pull tubes, bellcranks, etc.). If this mechanical system is
jammed or severed by combat damage or maintenance problems, the BUCS will
recognize the problem and enable fly-by-wire control of the affected axis.
The BUCS uses linear variable differential transducers (LVDTs) to signal flightcontrol
position, and shear-pin-actuated decouplers (SPADs) to separate flight controls
from the mechanical linkages. Eight LVDTs are located in the cockpit to sense flightcontrol
positions from the pilot and the CPG. Other LVDTs transmit the servo-actuator
positions to the Digital Automatic Stabilization Equipment Computer (DASEC). Among
its other functions, the DASEC recognizes problems with the mechanical control system
and enables the BUCS. SPADs are located at the base of each control axis (cyclic
longitudinal, cyclic lateral, collective, and pedals) for each crew station. There are eight
SPADs in all.
When a jam occurs, either crewmember can decouple, or "break out," of the
jammed axis by pushing hard on the affected flight control and breaking the SPAD on
that axis. As soon as the SPAD is broken the BUCS is enabled. All other undamaged
axes will continue to function normally using mechanical linkages. The crew can safely
land the helicopter.
In the event of a severed control linkage, the DASEC recognizes the mistrack
between the flight-control position and the position of the hydraulic servo-actuator. With
sufficient mistrack (17.5 percent, or approximately two inches of control movement), the
DASEC automatically enables the BUCS for the defective axis. All other undamaged
axes will continue to function normally using mechanical linkages. The crew can safely
land the helicopter.'