NYC06IA176

While en route to the destination airport, the helicopter's hydraulic system caution light momentarily illuminated and the associated aural warning sounded. The pilot elected to continue to the intended destination, and nearing the runway, the helicopter lost hydraulic boost. The pilot subsequently performed a run-on landing to a runway. Following the incident, a company mechanic removed the hydraulic pump and pulley, and found that the hydraulic pump drive shaft splines and the coupling sleeve exhibited "excessive wear." A new hydraulic pump was installed, the helicopter was test flown, and was then returned to service. About 3 weeks later, while flying the same helicopter, the pilot experienced a similar series of events, resulting in another run-on landing and no damage to the helicopter. Following that incident, the hydraulic system test switch was identified as faulty, and was subsequently replaced. After a check by a mechanic and another company pilot, the helicopter was test flown with no discrepancies noted and returned to service. Examination of the hydraulic pump drive shaft and coupling sleeve revealed that while the mating splines of both components were severely worn, there was enough spline material remaining to enable the coupling sleeve to drive the hydraulic pump. Examination and functional testing of the hydraulic switch removed from the helicopter revealed no anomalies.

On July 17, 2006, at 1200 eastern daylight time, a Eurocopter AS-350 BA, N606TV, was not damaged during an emergency landing at Northeast Philadelphia Airport (PNE), Philadelphia, Pennsylvania, following a loss of hydraulic boost. The certificated commercial pilot and passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the flight, which departed a private helipad about 1155. The positioning flight was conducted under 14 CFR Part 91.

According to the pilot, while en route to the airport, the hydraulic system caution light momentarily illuminated and the associated aural warning sounded. The pilot elected to continue to the airport, and informed air traffic control of the situation. He was cleared to land, and about 1 mile from the runway, the hydraulic system caution light illuminated steadily. Shortly thereafter, hydraulic boost was lost, and the pilot subsequently isolated the hydraulic system using the toggle switch on the collective pitch control. The pilot then completed a successful run-on landing.

Following the incident, a company mechanic removed the hydraulic pump and pulley. According to the mechanic, the hydraulic pump shaft splines and the coupling sleeve exhibited "excessive wear." The pump and coupling sleeve were forwarded to the Safety Board Materials Laboratory for further examination. A new hydraulic pump was installed, and the helicopter was test flown, then returned to service.

On August 3, 2006, about 0800, the pilot was on a routine flight near the airport when he again observed the hydraulic caution light momentarily illuminate, and heard the associated aural warning. Believing that a hydraulic failure was imminent, the pilot made a shallow turn back toward the airport, which was then about 5 miles away. As the helicopter continued toward the airport, hydraulic boost was initially retained, with the hydraulic caution light and aural warning intermittently activating in 5- to 10-second intervals. About 2 miles from the airport, the hydraulic caution light activated steadily, as did the aural warning, and 1 minute later, the helicopter experienced another loss of hydraulic boost. The pilot subsequently completed another run-on landing.

Following the second incident, the hydraulic system test switch was identified as faulty, and replaced. While being removed from the helicopter, an identifying error occurred between that switch and another switch that had been removed from the instrument panel. Both switches were also forwarded to the Safety Board Materials Laboratory for further examination.

After a "thorough" check by a mechanic and another company pilot, the helicopter was test flown with no discrepancies noted. The helicopter was then returned to service.

According to the Materials Laboratory Factual Report, an examination of the splined shaft on the hydraulic pump, which was mated with a similar spline in a coupling sleeve, revealed that the splines of both components were worn. The required grease-retaining o-ring was installed, but the grease-retaining coupling plug was not.

Closer examination of the splined portion of the hydraulic pump drive shaft revealed that the normally flat crowns of individual splines were rounded, and that the space between the splines was occupied with the appropriate lubricating grease. The driven flank of each spline was worn appreciably more than the non-driven flank. The amount of wear on the splines prevented any meaningful dimensional comparison with the requirements of the spline specification. Only the outside diameter of 0.388 inch was recorded for comparison with the specified outside diameter of 0.3877 to 0.3897 inch.

Specifications from the hydraulic pump manufacturer stated that the steels used to fabricate the drive shaft would be heat treated and case hardened to produce a surface layer of 61 to 65 Hardness, Rockwell, C (HRC), and a core hardness of not less than 39 HRC. Hardness testing performed at various depths from the surface of one spline root, between 0.004 and 0.040 inch, revealed hardness ranging from 50 to 69 HRC, with a core of 47 HRC.

Examination of the coupling interior revealed streaks of grease on the inner surface, oriented in a spiral direction, consistent with being almost thrown out of the coupling during rotation. The grease occupied the space normally occupied by the plug. The splines displayed a triangular profile, and not the trapezium profile normally associated with the spline. The splines were uniformly worn across their entire length. Specifications from the helicopter manufacturer stated that the splines were nitrited to achieve a minimum hardness of 64 HRC to a depth of 0.004 to 0.008 inch. Hardness testing performed at various depths from the surface of the coupling interior to between 0.002 and 0.020 inch, revealed hardness ranging from 31 to 42 HRC, with a core of 31 HRC.

Both of the switches provided by the operator, and a third exemplar switch provided by the manufacturer, were x-rayed. Examination of the x-ray images did not reveal any differences between the switches. Repeated functional tests of all three switches revealed that they operated consistently and with similarity.

The pilot held a commercial pilot certificate with numerous ratings, including rotorcraft helicopter. He had accumulated 3,200 total hours of flight experience, 2,600 hours of which were in rotorcraft, and 1,000 hours of which were in the accident helicopter make and model.