Two tales, but both concerning SA/AS/EC type helicopters and servo/hydraulics:
1. SA-341 Gazelle (MIL version) suffered a fatal accident at Farnborough around 1973. The helicopter performed a low level, right-hand turn at high speed. The roll to the right continued unchecked and the helicopter crashed inverted. The conclusion was that by increasing the angle of attack on the advancing blade, which provides most of the lift, the forces on the blade at high forward speed were sufficient to overcome the hydraulic servo and jack stall occurred, and the pilot was unable to counter the roll.
This occurs mainly when turning to the right - French helicopter = advancing blade on the left side of the helicopter. When turning left the angle of attack on the advancing blade decreases, and the forces on the servo decrease accordingly.
Even though this was a known phenomenon, and demonstrated under training, it was the presumed cause of another Gazelle fatal accident in 1975, during a low level sortie over Dartmoor, killing the instructor and student.
2. AS-350B2 – When this type was introduced it had new spherical bearings, new single hydraulic system (both taken from the 355 dual hydraulic system), a new power plant, beefed up rotorhead, & etc. Better, more powerful version of the B & B1……?
The hydraulic pump is driven by a belt (“rubber band” - see previous postings on this). In cold weather (under - 25oC) the hydraulic fluid increases in viscosity and the belt starts to slip. As it cannot drive the pump at normal speeds, the belt deforms, due to the friction from the accessory drive, and eventually stops turning the hydraulic pump, or slips off the drive wheel. OK you think, small helicopter, no hydraulics, what’s the problem ?
Well it turns out that the new spherical bearings (“rubber balls” from the 355) freeze solid at temperatures under - 25oC, and without a hydraulic servo the whole control system locks up !
The pilot then becomes a passenger in the helicopter, and only by using extreme force can the controls be moved.
AS/Eurocopter say they have fixed this problem with new types of spherical bearings, but the fact remains that if you lose the hydraulic drive belt on a 350, it becomes extremely difficult to control – see the news gathering AS-350 that plunged to a rooftop in New York earlier this year