Hydraulic Failures Can Lead To Controllability Issues

Citing six events involving Eurocopter AS350 helicopters in the past six years -- including two fatal crashes -- on Tuesday the National Transportation Safety Administration issued recommendations to the Federal Aviation Administration and European Aviation Safety Agency to address concerns over the helicopter's hydraulic pump drive shafts.

Those shafts are also used in smaller EC130, the NTSB noted.

The Board recommended the FAA:

• Require Eurocopter to identify AS-350 and EC-130 helicopter hydraulic pump drive shafts and coupling sleeves with splines that do not meet design specifications and take appropriate action to ensure that these parts (that is, replacement parts and parts to be installed in new helicopters) are expeditiously removed from the supply chain. (A-08-75)
• Require operators of Eurocopter AS-350 and EC-130 helicopters to perform a wear check, visual inspection, and lubrication of the hydraulic power assembly splines and coupling sleeve splines in accordance with the latest version of the maintenance manual at the earliest opportunity, and, thereafter, require operators to repeat the wear check, visual inspection, and lubrication of the splined connection at 100-hour intervals, and remove unairworthy parts from service. (A-08-76)

The same recommendations were also made to EASA, under A-08-77 and A-08-78, respectively.

"The Safety Board Materials Laboratory examined parts of the helicopters involved in these events, specifically the hydraulic pump assembly and pulley assembly that provide all the hydraulic power for the main rotor controls and the tail rotor controls," the NTSB notes. "Staff found that the splined connection that couples the hydraulic pump to its pulley assembly displayed excessive wear that in some cases caused it to fail with subsequent loss of hydraulic power. Loss of hydraulic power hinders helicopter control and increases the possibility of a serious accident...

"The helicopter can be flown without hydraulic power (that is, in manual mode) but doing so increases the physical demands on the pilot and can cause a serious accident if the pilot has not maintained familiarity with operation of the helicopter in manual mode or if an uncommanded reversion to manual mode occurs suddenly, especially during a critical maneuver."

FMI: Read The NTSB Recommendations Here And Here (.pdf)