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On July 13, 2007, at 1320 Pacific daylight time, a Eurocopter AS350 B3, N811HP, experienced an in-flight loss of control while performing a practice emergency maneuver with the hydraulic system off at the Paso Robles Municipal Airport, Paso Robles, 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), commercial pilot undergoing instruction (second pilot), and passenger were not injured; the helicopter sustained substantial damage. The local public-use instructional flight departed Paso Robles about 1300. Visual meteorological conditions prevailed, and a flight plan had not been filed.
The National Transportation Safety Board investigator-in-charge (IIC) conducted several interviews with the CFI and second pilot, both immediately after the accident, and in the weeks following. The purpose of the flight was for the CFI to give training to the second pilot, who was positioned in the right seat; another CHP employee occupied the aft left seat. The second pilot requested to perform practice hydraulic-off emergency procedures, as he had an upcoming check ride where he would be required to conduct such maneuvers. Prior to departure, the pilots determined that the helicopter was about 90 pounds under the published maximum gross weight. With the helicopter being heavy, the CFI opted to perform the first hydraulics-off maneuver and demonstrate the correct procedures.
After departure, the CFI adjoined the helicopter with the left downwind leg of the traffic pattern for runway 19, where he planned a touchdown on a dirt landing site adjacent to the runway surface. 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 cutoff switch on their respective collective]. While on the base leg, he noted that the controls felt as if they were "heavy" and he had to exert a stronger force to hold the collective up. As he maneuvered the helicopter onto final approach, the collective force required became neutral and the cyclic became stiff, with more force required to manipulate it. As the helicopter continued on final approach, the CFI noticed the control forces on the cyclic were becoming increasingly harder to overcome. He asked the second pilot to guard the collective while he repositioned his hand. He affirmed his grip on the cyclic and again took control of the collective.
As the helicopter slowed, the cyclic control continued to become extremely stiff and difficult to move, while the collective remained in the neutral position. The CFI was trying, using a great amount of pressure, to hold the cyclic in the forward-right position. The helicopter slowed out of effective translational lift (ETL), configured about 15 kts and 1 foot above ground level (agl). The helicopter began to drift to the left of the intended approach site and the CFI attempted to move the cyclic in response to the drift, but could not correct due to the stiffness of the control. He attempted to accelerate through ETL and touchdown further down. The helicopter continued to drift to the left [about 5 to 6 feet] and began climbing. The CFI then instructed the second pilot to "give me hydraulics back," knowing that only the collective for the right-seated pilot had the hydraulics switch. Seconds later the helicopter climbed to about 20 feet agl and the CFI had no control. The helicopter rolled to the left and landed hard on the left skid; it came to rest on its right side.
The CFI noted that the slowest airspeed the helicopter ever reached was a minimum of 8 kts. He added that he was not sure if the cyclic was immobile in any additional direction, aside from the forward-right position, as he did not move it into another direction to prevent possible further loss of control of the helicopter. The CFI estimated that he performs hydraulics-off simulated emergency procedures on a regular basis; he has never experienced any problems or difficulty controlling the helicopter.
The second pilot stated that he never turned the hydraulics back on because he recalled that doing so at such a low altitude could result in the pilot unintentionally over-controlling the cyclic and the helicopter crashing.
Certificated Flight Instructor [Left Seat]
A review of the Federal Aviation Administration (FAA) airman records revealed that the instructor held a commercial pilot certificate with ratings for rotorcraft helicopters, instrument helicopters, and private privileges in single engine land airplanes. He also held a flight instructor certificate for helicopters. The pilot's most recent second-class medical certificate was issued in January 2007, with a limitation that he must wear corrective lenses.
According to the pilot, at the time of the accident he had accumulated 10,400 hours of total flight time. He had amassed a total of 8,406 hours while employed with the CHP of which 2,887 hours were in the same make and model as the accident helicopter. In the previous year, the pilot had given about 50 hours of instruction. His last biannual flight review was completed on May 17, 2006, in a Eurocopter AS350 B3; he completed a semiannual evaluation with the operator on January 4, 2007.
Second Pilot [Right Seat]
A review of the FAA airman records revealed that the second pilot held an airline transport pilot certificate with a rating for multiengine land airplanes. His certificate also was endorsed for commercial privileges in single engine land airplanes, rotorcraft helicopters, and instrument helicopters. He held a flight instructor certificate for airplane ratings of single engine land, multiengine land, and instruments. The pilot's most recent first-class medical certificate was issued in January 2007, with a limitation that he must wear corrective lenses.
According to the second pilot, at the time of the accident he had accumulated a total flight time of 9,056 hours. While employed with the CHP, he had amassed a total of 4,552 hours in both airplanes and helicopters. He had logged a total of 1,956 hours in the same make and model as the accident helicopter. His last biannual flight review was completed on May 18, 2006, in a Eurocopter AS350 B3.
The helicopter was a Eurocopter AS350B3, serial number 3404. It was manufactured in 2001 and had accrued a total time in service of 6,240 hours at the time of the accident. Review of the aircraft maintenance records disclosed that the last 100-hour inspection was completed 45 hours prior to the accident on May 30, 2007. The most recent annual inspection was completed 340 hours prior to the accident on February 7, 2007. The Turbomeca Arriel 2B engine, serial number 22179, had accumulated a total time in service of 5,758 hours. Both the annual and 100-hour inspections were accomplished on the dates noted for the airframe.
Prior Maintenance Issues
The second pilot stated that he had flown the accident helicopter the day prior to the accident. During the flight he noticed that the collective had a tendency to "creep up" and a lot of friction was needed to keep it in the correct position. No other maintenance anomalies were noted.
The Eurocopter AS350 B3 is equipped with a single hydraulic system, which provides hydraulic boost to the cyclic, collective, and tail rotor controls. The main rotor control system consists of a series of rigid rods interconnected by bell cranks and reversing levers. The respective control linkages interface with the swash plate through three hydraulic servo actuators, which are designed to exert the necessary control force. A mixing unit is located aft of the cockpit and utilized as an interface for the cyclic and collective pitch controls. The unit enables each control to operate independently without mutually coupling.
The helicopter is designed so that in the event of a hydraulic pressure failure, main rotor servo accumulators provide boost (over a duration of about 30 seconds) enabling the pilot to land if the helicopter is configured in a hover, or establish the recommended airspeed (40 to 60 kts) to lessen control forces in forward flight.
According to Eurocopter the helicopter can be flown without hydraulic pressure, which will require a lateral control force of about 9 pounds and a forward cyclic control force of about 11 pounds. If the pilot attempts to hover without hydraulic assistance, the control forces intensify and oscillate in direction. Lateral and longitudinal forces can be upward of 12 pounds which can change in direction. During a suggested run-on landing (performed at a minimum of 10 kts), a longitudinal force of up to 37 pounds may be required, with 33 pounds necessary for the right or left lateral control.
Hydraulic Test Pushbutton
The hydraulic test pushbutton (HYD TEST) is mounted on the center consol between the two forward seats and consists of two positions. The TEST position (button pushed in) initiates the test function and normal operation is restored when the button is released out. The primary function of the HYD TEST pushbutton is twofold: enable the pilot to verify the functionality of the servo accumulators prior to flight and simulate the onset of a hydraulic failure during training. Selecting the TEST position results in the solenoid valve opening on the regulator unit, immediately depressurizing the hydraulic system. It additionally opens the tail rotor servo solenoid, depressurizing the tail rotor accumulator and tail rotor load compensator, allowing the main rotor servos to be powered by their respective accumulators until the exhaustion of their stored energy.
Hydraulic System Failure Training
Hydraulic system failure is simulated by carrying out a specific sequence of switch selections and corresponding actions, which are documented in the helicopter's Flight Manual within Supplement 7. Practice 'hydraulics OFF' approaches are conducted in two phases: the transition to a recommended speed range and a transition to landing. The instructor is to depress the HYD TEST pushbutton to the TEST position and the student should respond by reducing the helicopter's airspeed to between 40 and 60 kts.
When in steady flight condition, the instructor is then required to reset the HYD TEST pushbutton, which restores the system pressure and recharges the main and tail rotor accumulators. The student then positions the collective hydraulic cut-off switch off, which within 2 seconds introduces the main rotor manual control loads. The tail rotor accumulator continues to assist the tail rotor servo and load compensator. This switch configuration ensures that if hydraulic power is required, turning the collective hydraulic cutoff switch on will immediately reinstate the powered controls.
The recommended procedure for landing is to make a shallow final approach to a flat field in an effort to minimize operation of the collective lever. The manual states that the pilot should perform a no-hover, run-on landing with the helicopter configured at 10 kts and a headwind. Specifically, the helicopter should not be hovered or taxied without hydraulic pressure assistance.
The Eurocopter AS350 Instruction Manual states that the "helicopter can be controlled without servo actuators, but this requires the pilot to apply non-negligible forces that are difficult to gauge," and that the aforementioned control loads are "absorbed by hydraulic servo actuators so that the pilot can fly the helicopter PRECISELY and EFFORTLESSLY [Emphasis in original]."
TESTS AND RESEARCH
Following recovery, the Safety Board IIC examined the helicopter at the CHP facilities in Paso Robles, on July 20, 2007. Manufacturer's representatives from Eurocopter assisted with the examination, as well as maintenance personnel contracted by CHP and an FAA inspector.
The main and tail rotor control system was examined with no anomalies observed. Flight control continuity was established from the cockpit to the main rotor and tail rotor pitch change links. Several separated control linkages were noted with angular fracture surfaces, consistent with buckling at impact.
The hydraulic system was examined and no evidence of a catastrophic malfunction was visually apparent. Investigators activated that hydraulic test button and could hear the manifold solenoid activating.
The SAMM servo accumulators were pressure tested with the following results:
Right Lateral Servo Part# SC5083 Serial# 923 199 PSI
Longitudinal Servo Part# SC5084 Serial# 938 190 PSI
Left Lateral Servo Part# SC5083 Serial# 1820 190 PSI
The Eurocopter representative stated that the servo accumulators are required to be serviced with nitrogen and pressurized to 15 bars of pressure (220 PSI) [1 bar = 14.7 PSI] at 20 degrees centigrade. The representative noted that the pressures recorded from the accident accumulators were slightly low although the accumulators would be functional during a hydraulic failure. The hydraulic system drive belt was found intact. The chip detector on the hydraulic pump was removed and was found clean and free of debris.
Investigators removed the hydraulic pump from the mounting bracket. The splines were intact and undamaged; the o-ring was present and undamaged. No anomalies were noted with the hydraulic pump. The hydraulic fluid had leaked from the reservoir onto the helicopter from damage sustained during impact. A small sample of fluid remaining in the hydraulic pump's filter was obtained; no visible contamination was noted. The filter screen was inspected with only a nominal amount of debris found.
The servo accumulators were removed and investigators obtained the following rigging measurements (from lower rod-end attachment of piston) in inches:
Right Lateral Servo Upper: 1.272 Lower: 1.357
Longitudinal Servo Upper: 1.260 Lower: 1.382
Left Lateral Servo Upper: 1.357 Lower: 1.444
The limits given by Eurocopter are as follows: Upper: 1.260 Lower: 1.338. An engineer at Eurocopter stated that the out-of-limit left lateral servo (0.106 inches) would not have a noticeable affect in the capabilities or handling of the helicopter.
The servo accumulators were retained by the Safety Board IIC and taken to the facilities of Hawker Pacific Aerospace, Sun Valley, California. The investigation was completed on August 08, 2007, with Safety Board investigators and a representative from the CHP present. There were no anomalies found that could be definitively attributed to a preimpact condition. The complete Safety Board examination report is contained in the public docket for this accident.
Upon disassembly of the longitudinal servo, investigators observed that the liner exhibited a bulging area about 1 inch from the end, similar to mushroomed deformation. The piston was removed from the liner revealing that about 1/2 of the Teflon white piston pad was displaced from the piston seal groove. The surrounding piston rod area was free of score marks and appeared intact. The piston guides (bearings) and their respective pressure seals were removed from the liner housing. The cylinder end side piston guide had no damage to the o-ring seals. Removal of the rod end side piston guide revealed that the two end o-ring seals were free of gouges and nicks, with the most inner o-ring having 30 percent separated; the pieces were discovered within the cavity. It could not be determined if the anomaly was a result of impact damage.