Fresno, CA, USA
N414HP
Eurocopter France AS350 B3
The certified flight instructor (CFI) configured the helicopter for a practice emergency maneuver with the hydraulic system off. Upon disengaging the hydraulics, he immediately noticed that an abnormal force was required on the cyclic control to prevent the helicopter's nose from pitching up and to the left. He elected to continue the landing with the hydraulics off and began to slow the airspeed as the helicopter adjoined final approach. He was convinced that he would not be able to land the helicopter without it incurring damage due to the severe control restriction. He managed to complete a run-on landing without mishap by maintaining an airspeed of about 10 kts. When the helicopter came to rest, the pressure was released on the cyclic and the second pilot restored the hydraulics via the collective switch. Immediately thereafter, the cyclic began a hard over and displaced to the left against the CFI's leg. He attempted to center the cyclic with both hands, but he was unable to move the control. After 40 seconds the pressure released and the second pilot centered the cyclic with ease. The hydraulic servos were tested and a complete teardown of each servo was performed. The pressure testing of one of the servos revealed that the unlocking pressure was out of tolerance (too high). Upon disassembly of this servo, wear was observed on the conical surface of the locking finger and on the diameter in contact with the bearing. According to the servo manufacturer, the wear resulted in excessive friction in the bearing, which led to the high unlocking pressure.
HISTORY OF FLIGHT On December 14, 2007, about 1620 Pacific standard time, a Eurocopter AS350 B3, N414HP, experienced an in-flight control restriction while performing a practice emergency maneuver with the hydraulic system off at the Fresno Yosemite International Airport, Fresno, California. California Highway Patrol (CHP) Air Operations was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 91. The certificated flight instructor (CFI) and commercial pilot undergoing instruction (second pilot) were not injured; the helicopter was not damaged. The local public-use instructional flight departed Fresno about 1600. Visual meteorological conditions prevailed, and a flight plan had not been filed. In a written statement, the CFI reported that the purpose of the flight was for the CFI to give training to the second pilot, who was positioned in the right seat. Prior to departure, the CFI discussed the proper procedures for performing practice hydraulic-off emergency procedures, as the second pilot was completing pilot phase-training, which required the achievement of such a maneuver. The CFI further stated that he opted to perform the first hydraulics-off maneuver and demonstrate the correct procedures. After departure, he adjoined the helicopter with the right downwind leg of a traffic pattern for taxiway "C", where he planned a final touchdown at area 5. With the helicopter at 800 feet mean sea level (msl) and 90 knots (kts), the CFI initiated the maneuver by activating the hydraulic test push button (HYD TEST). After the illumination of the hydraulic pressure light, he configured the helicopter to an airspeed of 60 knots (kts) and instructed the second pilot to turn the hydraulics off [the right-seated pilot has the hydraulic cut-off switch on their respective collective]. The CFI immediately noticed that an abnormal force was required on the cyclic control to prevent the helicopter's nose from pitching up and to the left. The CFI elected to continue the landing with the hydraulics off and began to slow the airspeed as the helicopter adjoined final approach. He was concerned that he would not be able to land the helicopter without it incurring damage due to the severe control restriction. He managed to complete a run-on landing without mishap by maintaining an airspeed of about 10 kts. When the helicopter came to rest, the pressure was released on the cyclic and the second pilot restored the hydraulics via the collective switch. Immediately thereafter, the cyclic began a hard over and displaced to the left against the CFI's leg. He attempted to center the cyclic with both hands, but he was unable to move the control. After 40 seconds the pressure released and the second pilot centered the cyclic with ease. AIRCRAFT INFORMATION The helicopter was a Eurocopter AS350B3, serial number 3378. It was manufactured in 2000, and had accrued a total time in service of 9,019 hours at the time of the last 100-hour inspection, which was completed November 12, 2007. The Turbomeca Arriel 2B engine, serial number 22436, had accumulated a total time in service of 4,790 hours. TESTS AND RESEARCH Following the incident, the three control servos were removed and sent to France, where a complete teardown inspection was conducted at the Goodrich Actuation Systems facilities under the auspice of a Bureau d'Enquêtes et d'Analyses (BEA) investigator. The following SAMM (Goodrich) servo accumulators were tested as per the Acceptance Test Procedure (ATP): Part# SC5083 Serial# 724 Part# SC5083 Serial# 1228 Part# SC5084 Serial# 491 The pressure testing of servo 724 revealed that the unlocking pressure was out of tolerance (too high); the unit was noted to be dirty. Upon disassembly, wear was observed on the conical surface of the locking finger and on the diameter in contact with the bearing. According to the servo manufacturer, the wear resulted in excessive friction in the bearing, which led to the high unlocking pressure. Servo 724 had accumulated 4,681 total flight hours. It flew 2,199 hours on another AS350 B3 helicopter between September 1998 and January 2006; then it was installed on the accident helicopter and accumulated 2,482 hours between March 20, 2006, and the time of the accident. The servo manufacturer’s report concluded that the wear pattern on the locking finger was unusual and most likely "related to abnormal external pollution associated with intensive use of the locking device." The complete test examination report is contained in the public docket for this accident.
Malfunction of a hydraulic servo while on final approach due to excessive wear of the locking finger.
Source: NTSB Aviation Accident Database
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