CEN14FA193

The pilot's loss of yaw control during takeoff due to the absence of hydraulic boost to the tail rotor pedals for reasons that could not be determined based on the available information. A finding in the accident was the lack of a caution indicator to alert the pilot of the lower hydraulic system configuration.

The commercial rated pilot planned to depart on a repositioning flight from a medical helipad located on a hospital rooftop with two medical technicians on board. The pilot reported that he completed all of the pretakeoff hydraulic checks and did not note any abnormities with the pedal movement. As the helicopter lifted from the helipad, the pilot expected a slight left turn; however, the helicopter kept turning. The pilot tried to stop the turn without success, and the helicopter then entered a left spin. The pilot reported that the (antitorque) pedals felt jammed or locked in the neutral position. The pilot added that, during the spin, he looked for a light but that he did not recall seeing any (warning) lights. Video footage from a security camera captured the helicopter completing several rotations before it impacted the rooftop and then came to rest adjacent to the helipad.  The helicopter was equipped with a dual (upper and lower) hydraulic system, and the lower system was used to power the single-servo tail rotor servo control and the yaw load compensator. Testing and examination of the lower hydraulic system did not reveal any abnormalities. Data from the helicopter's quick access recorder (QAR) and nonvolatile memory (NVM) from the engine controls were also downloaded; no abnormities were noted.  An examination of the cockpit found the yaw servo hydraulic switch on the collective in the "on" (flight) position, the correct position for the flight. The "ACCU TEST" switch, which controls the accumulator for the tail rotor, was also found in the normal (flight) position. The NVM does not record the positioning of the switches, and analysis of the recorded data provided no indication that the switches were activated during flight.  The investigation tried to determine a reason for the development of the helicopter's spin. Given the pilot's statement that the wind was "relatively calm," which was corroborated by the security camera video footage that showed the wind effect on the nearby smoke and water, a loss of tail rotor effectiveness likely did not occur. Drive continuity of the tail rotor and control continuity from the pedals to the tail rotor were established. No evidence of foreign object debris (FOD), including any witness marks that could be associated with the presence of FOD, was observed in the pedal control system, and there was no evidence indicating that a pedal had jammed.  During takeoff, it is likely that there was an absence of hydraulic boost to the tail rotor pedals, either from a misconfiguration of the yaw hydraulic isolation switch or a failure in the lower hydraulic system that was not evident during postaccident testing. Although the specific cause of the absence of hydraulic boost to the pedals could not be identified, there was no evidence of either abnormal functionality of the lower hydraulic system or a tail rotor hydraulic circuit misconfiguration. Additionally, by design, the helicopter's caution panel does not provide a warning indication when the yaw hydraulic isolation switch is activated.  The manufacturer had originally equipped the helicopter with a cockpit imaging system; however, the operator had removed the system. The removal of this system precluded a determination of the configuration of the hydraulic control switches before takeoff. Further, due to the lack of available cockpit images, the investigation was unable to verify the pilot's actions before takeoff, including whether he moved the hydraulic isolation to "off" before the loss of control.

The New Mexico Heliport (NM11) is a 28 foot by 28 ft raised platform located on the rooftop of a six-floor hospital building at the University of New Mexico, Albuquerque, New Mexico. It is a private-use helipad used for EMS operations. The helicopter previously landed at the helipad to deliver a transfer patient and remained on the helipad for about 20 minutes before taking off for the accident flight. The helicopter was removed from the hospital's rooftop and transported to Air Salvage of Dallas, Texas, (ASOD) for further examination. The examination of the helicopter was conducted by representatives from the National Transportation Safety Board, Federal Aviation Administration (FAA), Bureau d'Enquêtes et d'Analyses (BEA), the operator, and technical advisors from the engine and airframe manufacturers. Investigators tried to determine if the pilot had departed with the yaw hydraulic isolation switch in the "off" position, if a malfunction occurred in the tail rotor drive system, or if a malfunction in the tail rotor controls allowed the helicopter to spin. A visual examination of the hydraulic system was conducted followed by an examination of the nitrogen charge on the yaw load compensator accumulator with a pressure gauge. A hydraulic pump rig was used to drive the helicopter's lower hydraulic system. Several test scenarios were evaluated, including using the ACCU TEST and yaw hydraulic isolation switches. Additionally, return hydraulic fluid was captured to determine if there were any contaminants in the fluid. No abnormalities were noted during the tests. The tail rotor servo, yaw load compensator/accumulator, isolation valve, engine data recorder (EDR), digital engine electronic control unit (DECU), and the vehicle and engine multi-function display (VEMD) were removed from the helicopter and shipped to the airframe manufacturer in France. Under the supervision of the NTSB and BEA, the tail rotor servo was bench tested and then partially disassembled; no evidence of abnormities was found. The yaw load compensator/accumulator and isolation valve were also bench tested with no abnormities noted. Under BEA supervision, data from the EDR and DECU were downloaded; no abnormalities were noted with the units; and failure flags in the units were attributed to the helicopter's impact with the rooftop. Data from the QAR were also downloaded, and no abnormalities were noted. The nonvolatile memory (NVM) does not record the position of the yaw servo hydraulic isolation switch or the "ACCU Test" switch. The examination of the tail rotor drive system and lower hydraulic system did not reveal any abnormalities in the components that would have resulted in a loss of tail rotor thrust or pitch control. The pitch on the tail rotor blades is not a recorded parameter on the QAR. However, the QAR does record data from various sources, including the DECU. When an additional flight load is added by the main and/or the tail rotor system, the engine's DECU would sense the additional power requirement and respond with additional fuel to the engine. A potentiometer located in the tailboom is used by the engine control to signal when additional power is required from the engine as a result of increased pitch on the tail rotor blades. The potentiometer parameter (XPA) is recorded by both the QAR and DECU. A limitation of the XPA value is that it is an indication of power requirement, but is not directly an indication of pitch on the tail rotor blades. Additionally, the value for XPA needs to exceed 70 percent before additional power is required from the engine; thus, changes in engine power (and corresponding tail rotor pitch changes) below 70 percent would not have been recorded. In reviewing the data, the XPA value did not exceed 70 percent until the end of the data, consistent with the tail rotor striking the rooftop during the accident. Additionally, the data were absent a "normal" XPA spike during the takeoff, which was expected to be present due to the additional power requirement at takeoff. The investigation was unable to verify the pilot's pre-take off actions due to the lack of a cockpit recording system with the ability to capture image data. During the course of the investigation, investigators learned of two events in which a pilot initiated the takeoff with the yaw hydraulic isolation switch in the "off" (cutoff) position. The events did not result in an accident and were not reported (nor was a report required). The actual number of similar occurrences is unknown because pilots/operators are not required to report such events. Airbus Helicopters issued Safety Information Notice No. 2776-S-29, "Hydraulic Power – Information about the Dual Hydraulic System," dated August 21, 2014, to operators. The air safety notice, noted that if the pilot fails to move the yaw servo hydraulic switch back to the "on" position before takeoff, there will be a complete lack of hydraulic boost to the tail rotor system. Before switch activation, the yaw load compensator would have been discharged to verify proper operation of the hydraulic accumulator test switch and valve. This situation could be perceived by the pilot as a tail rotor control failure due to the increased load required to move the control pedals. If not quickly identified and corrected, this situation could lead to a loss of helicopter control. The notice also identified two modifications that would help prevent departures in the helicopter without the hydraulic systems properly configured: (1) new production helicopters have been fitted with a flashing "HYDR 2" light on the caution panel, which will illuminate when the yaw hydraulic isolation switch is in the "off" position, (2) a future modification will also change the "ACCU TEST" button from an "on-off" push-button to a momentary push-button switch. On February 25, 2015, Airbus Helicopters issued Service Bulletin (SB) No. AS350-67.00.64, "Rotor Flight Controls – Double Hydraulic Servo Controls." The SB recommends modifying the hydraulic indications to the pilot, on helicopters equipped with the dual hydraulic system. The recommended modifications would provide an indicator light on the caution and warning panel, when the yaw hydraulic isolation switch is activated and a second light to indicate the status of the two systems. Rotorcraft Flying Handbook, FAA-H-8083-21 Pinnacle and Ridgeline Operations FAA-H-8083-21 states the following: A pinnacle is an area from which the surface drops away steeply on all sides. A ridgeline is a long area from which the surface drops away steeply on one or two sides, such as a bluff or precipice. The absence of obstacles does not necessarily decrease the difficulty of pinnacle or ridgeline operations. Updrafts, downdrafts, and turbulence, together with unsuitable terrain in which to make a forced landing, may still present extreme hazards. Vertical Takeoff to a Hover FAA-H-8083-21 also states the following: A vertical takeoff to a hover involves flying the helicopter from the ground vertically to a skid height of two to three feet, while maintaining a constant heading. Once the desired skid height is achieved, the helicopter should remain nearly motionless over a reference point at a constant altitude and on a constant heading. Further, Helicopter Flying Handbook, FAA-H-8083-21A states, ..... Very slowly increase the collective until the helicopter becomes light on the skids or wheels. At the same time apply pressure and counter pressure on the pedals to ensure the heading remains constant. Continue to apply pedals as necessary to maintain heading and coordinate the cyclic for a vertical ascent. As the helicopter slowly leaves the ground, check for proper attitude control response and helicopter center of gravity. A slow ascent will allow stopping if responses are outside the normal parameters indicating hung or entangled landing gear, center of gravity problems, or