John R81
30th Jun 2017, 06:51
Did a quick search and couldn't find reference to this. G-KAXT Westland Wasp (https://assets.publishing.service.gov.uk/media/591c30aaed915d20f800001c/Westland_Wasp_HAS1_G-KAXT_06-17.pdf) Loss of collective pitch control resulting in a forced landing, Bishopstone, near Salisbury, Wiltshire, 23 September 2016.
With the helicopter straight and level at approximately 1,150 ft, the pilot felt vibration through the collective lever. The vibration ceased after two or three seconds. Approaching higher ground, the pilot pulled up on the collective lever but the helicopter did not respond. He lowered the lever and again, there was no response. Faced with a loss of collective pitch control, the pilot made a precautionary landing.
The collective pitch control rod in the main rotor gearbox had broken. The control rod failure was secondary to a universal joint failure that had occurred in the cyclic control circuit due to lack of lubrication and a build-up of corrosion deposits. That was a result of the design of the lubrication system; fill a reservoir during overhaul of the gearbox when changing the universal joint. The overhaul in this case was 600 hours, and with anticipated military use of 200 hrs per year that would be 3 years. After retirement, the machine had low hrs and so over 20 years the 600 hrs had not been achieved - the universal joint lubricating oil was exhausted from the reservoir and the joint ran "dry" until it failed.
Just goes to show; you have to understand exactly how the thing is built and what it means to depart from the "expected use" which the maintenance schedule is designed around.
With the helicopter straight and level at approximately 1,150 ft, the pilot felt vibration through the collective lever. The vibration ceased after two or three seconds. Approaching higher ground, the pilot pulled up on the collective lever but the helicopter did not respond. He lowered the lever and again, there was no response. Faced with a loss of collective pitch control, the pilot made a precautionary landing.
The collective pitch control rod in the main rotor gearbox had broken. The control rod failure was secondary to a universal joint failure that had occurred in the cyclic control circuit due to lack of lubrication and a build-up of corrosion deposits. That was a result of the design of the lubrication system; fill a reservoir during overhaul of the gearbox when changing the universal joint. The overhaul in this case was 600 hours, and with anticipated military use of 200 hrs per year that would be 3 years. After retirement, the machine had low hrs and so over 20 years the 600 hrs had not been achieved - the universal joint lubricating oil was exhausted from the reservoir and the joint ran "dry" until it failed.
Just goes to show; you have to understand exactly how the thing is built and what it means to depart from the "expected use" which the maintenance schedule is designed around.