CH-47D flight controls
Hello everyone,
Before I became a crew member on UH-60s I worked quite a bit on the CH-47D, I’ve done a bucket load of flight controls rigging on them and as SASless pointed out, the flight control system is not exactly simple.
Having said that, it’s not pure magic either. From memory it goes roughly like this: (anyone with more recent experience please correct me) the controls move through linkages horizontally from the cockpit to the companionway/flight control closet where th e pallets someone mentioned already are located.
The pallets have the force trim actuators(mag brakes) and position transducers etc. on them. Then the controls rods go vertical to the ILCAS(SAS Actuators) the on top of those the controls go horizontally to the first stage mixing assembly bell cranks, which transfers the control motion to the to the second stage mixing assembly where it goes horizontally across the top of the aircraft, then vertically to the swiveling and pivoting actuator pilot valve.
And here is where I suspect the failure may have happened. The previous incident involving the barrel roll of a CH-47D was attributed to a jammed pilot valve in a fwd pylon actuator, can’t remember which one, maybe swiveling, causing it to EXTEND fully. Anyway, This incident trigged a period of time where special recurring inspections were performed on the actuators where you ensured certain hardware was properly installed by pulling an inspection plate on the pilot valve. So, possible pilot valve jam? Or here’s another scenario.
While rigging flight controls we very often disconnected the input rod to the pilot valves to make adjustments, once you push/pull on the valve that actuator takes off. If the control rod anywhere in the system got severed/disconnected, the actuator would simply drive whatever direction had LEAST force applied to the pilot valve. In this case, If I remember right if you disconnect the control rod from the valve the actuator would RETRACT fully. There are a bunch of places where this disconnect could have occurred in such a complex system, I feel like it could have been anywhere after the second stage mixer, again just speculating. I lean towards the aft controls because of the tilt of the rotor system, also remember the lions share of the lift is on the aft system due to the amount of the weight contained in that section of the aircraft.
Anyway, no matter what the cause, such a sad situation. Prayers for the families/friends of all involved.
60Mech
Last edited by 60FltMech; 30th Aug 2022 at 19:41.