Six events involving Eurocopter AS350 helicopters in the past six years, including two fatal crashes, have prompted the National Transportation Safety Board (NTSB) to issue recommendations to the Federal Aviation Administration (FAA) and European Aviation Safety Agency (EASA) that address concerns over the helicopter's hydraulic pump drive shafts. Those shafts are also used in the smaller EC130.

The Safety Board recommended that the FAA and EASA

• 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.
• 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.

"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 said.

"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," it added.

On March 8, 2007, a Eurocopter AS-35 (N354NT) reported a hydraulic failure and collided with terrain, resulting in four fatalities and three serious injuries. On-scene investigators found the left lateral main servo disconnected. Since the hydraulic pump and pulley assembly is part of the same hydraulic system as the servo, and given problems already being investigated in other Eurocopter AS-350 crashes, the hydraulic pump and pulley assembly was removed as a unit and sent to the Safety Board for examination.

Disassembly revealed that the drive shaft O-ring had not been installed. Although the drive shaft splines and the coupling sleeve splines were relatively intact, Safety Board hardness testing revealed that the measured hardness at the specified minimum depth for the drive shaft splines was significantly below the requirements specified in the engineering drawings.

Although the examination also revealed viable grease on the splines, the grease in the coupling sleeve cavity was dried and was no longer capable of lubricating the splines. Analysis revealed that the viable grease and the dried grease were similar but that neither was specified in the maintenance manual.

Total pump operating hours for this helicopter were not available, but a review of the servicing records revealed that a required 100-hour visual inspection and lubrication of the pump drive shaft splines had been performed 58.1 hours before the accident.