Originally Posted by
HoverFort
The first one is that a breach of the hydraulic chamber would cause pressure loss, which is can be seen in many accident reports.
The second failure type is jamming of the piston within the cylinder such that the servo actuation displacement is fixed. But this is confusing for me because there's very few documented accidents that are caused by such failure.
Have you seen any documented accident ?
The failure modes described appear to be for ballistic damage to a actuator hydraulic cylinder. Under normal operation boost actuators rarely fail structurally. If loss of pressure occurs it is almost always due to leakage in a hydraulic line, or failure of the hydraulic pump.
For helicopters rotor actuators there is no way to mitigate the failure of a jammed piston in a cylinder. For this reason extreme care is taken to ensure this doesn’t happen from normal operation. In fact for civil certified aircraft the probability of this occurring exceeds once in every 1 X E9 flight hours. Which explains why you will not find much evidence of the securing, short of battle damage.
Hydraulic boost actuators like the H-60 use a servovalve to control boost direction. Servo valves can sometimes jam from fluid contamination. When they jam they can result in a hard over command in either direction, or the equivalent of a stuck actuator piston. To appear to be a stuck actuator piston the servo valve needs to stick at the exact center position, which is very improbable. So you will find many accidents that describe the actuator going hard over, but next to none describing a jammed condition.
Because the jamming of a servo valve is a significantly probable event, modern large helicopters like the H-60 incorporate a bypass function in the actuator in this event occurs.