WPR17LA159

An improper repair of the right magneto's tachometer and governor signal wire, which failed during cruise flight and initiated a simultaneous malfunction of both the engine tachometer and governor control system, which resulted in erroneous and contradictory information between the engine tachometer and main rotor tachometer that led the pilot to perform a forced landing. Contributing to the accident was the lack of guidance on how to address the simultaneous failure or malfunction of both the engine tachometer and governor control system.

**This report was modified on July 26, 2022. Please see the public docket for this accident to view the original report.** The commercial pilot reported that, during a local sightseeing flight over a densely populated area, he heard an unusual sound and saw that the engine tachometer was indicating a higher-than-normal engine rpm. He reduced engine power and attempted to manipulate the helicopter controls to align the engine and rotor speeds to no avail. Subsequently, the rotor rpm began decaying, and the pilot assumed the helicopter had experienced a catastrophic drive failure and chose to conduct an autorotation, during which the helicopter landed hard. The airframe sustained substantial damage. Postaccident examination of the helicopter and engine did not reveal evidence of any mechanical malfunctions or failures that would have precluded normal operation, and witness signatures indicated that the engine was producing power at the time of ground impact. However, a wire within the right magneto, which provided a signal to both the engine governor and engine tachometer on the instrument panel, was found detached at a solder joint where it had been soldered to the connector rather than crimped. Further, neither the wire nor its connector was aviation-grade. The investigation could not determine when the modification to the wire was made. Review of onboard video and a spectrographic analysis of the recorded audio revealed that the engine rpm initially decreased but then appeared to be restored a few seconds later while the helicopter slowed. For about the next 40 seconds, the engine continued to operate at the same rpm and the helicopter continued to slow and gradually descend. The engine rpm then quickly decreased, and the helicopter rapidly descended in a manner consistent with the pilot initiating an autorotation. The detached wire would have resulted in both the tachometer and governor not receiving a correct engine rpm signal. As a result, the engine governor would not have been able to make appropriate adjustments to engine power as demand changed, and the pilot would not have had a way to gauge engine power and make corrective manual throttle inputs. The pilot's statement that the tachometer rpm increased does not match a scenario where the wire completely detached. However, it is possible that the event was initiated when the wire remained partially connected and thus created a spurious increase in signal frequency to the tachometer and governor due to vibrations. Thus, in this scenario, the tachometer would read higher than normal and the engine rpm would decrease as the governor tried to compensate for the high engine rpm signal. The Helicopter's Operating Handbook provided separate procedures for the failure of the engine governor and tachometer but not for the failure of both. Because the engine governor control system and engine tachometer both share a common speed signal source and thereby a common failure point, this signal wire failure left the pilot with a confusing set of cues and no definitive procedures for corrective action. In addition, the helicopter's low rotor rpm warning horn was not working, but the low rotor lamp was working. This failure would have resulted in another set of contradictory cues for the pilot to process, further compounding the confusion he was likely experiencing during the high-stress event.

HISTORY OF FLIGHT On July 21, 2017, at 1211 Pacific daylight time, a Robinson R44 II helicopter, N3254E, landed hard during an autorotation near Van Nuys Airport (VNY), Van Nuys, California. The commercial pilot sustained minor injuries, the three passengers sustained serious injuries, and the helicopter was substantially damaged during the landing. The helicopter was registered to and operated by National Helicopter Service and Engineering Company, as a Title 14 Code of Federal Regulations Part 91 revenue sightseeing flight. Visual meteorological conditions prevailed, and no flight plan had been filed. The local flight departed Van Nuys about 1112. The three passengers and pilot boarded the helicopter at 1100 for what was to be a 1-hour tour around the Los Angeles area. The pilot reported that the tour was uneventful, and as they approached the airport for landing, he heard an unusual sound. He glanced down at the engine tachometer, and it was higher than normal, indicating above 110%. The pilot began manipulating the engine throttle to reduce power and bring the engine and rotor speeds into alignment, but the rotor RPM began to decay. He was concerned that the engine might overspeed if he adjusted the throttle higher, and presumed that there was likely an unrecoverable failure in the transmission system, so he initiated an autorotation. He stated that trees and power poles blocked his landing approach, and he therefore did not have sufficient speed or space to perform an adequate flare, and the helicopter landed hard. The helicopter came to rest on a street at an elevation of 672 ft mean sea level, in a densely populated residential neighborhood between the Los Angeles River and the 101 Freeway, about 3 miles southeast of VNY (See Figure 1). The landing skids had spread during the impact, and were level with the belly, and the emergency pop-out floats had deployed. The entire airframe exhibited buckling damage, and the belly was crushed. The tail rotor gearbox struck a fence, and had partially detached. Both main rotor blades remained attached to the mast, but one was bowed downward and the other was bent slightly upward. Figure 1 – Helicopter at Accident Site The pilot stated that at no point during the flight did he receive any instrument panel annunciations or indications of a problem. He did not scan the instrument panel during the descent because his focus was on flying the helicopter and looking for a landing spot. He reported that the engine continued to operate during the descent, and that although he has trained for emergency events, he was shocked at how quickly everything transpired. The floats had been armed for the flight, and they appeared to have deployed during the hard landing. All the doors opened on impact, and the occupants were able to egress on their own. Prior to exiting the helicopter, the pilot secured the helicopter by pulling the fuel mixture control to the out position, and turning off the magnetos and the fuel selector valve. WRECKAGE AND IMPACT INFORMATION Examination The engine and airframe were examined following the accident by the NTSB investigator-in-charge, and technical representatives from Robinson Helicopters, and Lycoming Engines. Complete examination reports are contained in the public docket. No anomalies with the flight controls, drive train or engine were noted, the governor switch was found in the "ON" position, and witness signatures indicated that the engine was producing power at the time of ground impact. The low rotor RPM warning horn was tested by the application of electrical power directly to its input, and it did not produce a sound. The filament of the low rotor RPM warning light on the instrument panel was still intact and operational. The magnetos were removed, along with the clutch actuator assembly, overrunning clutch, tachometer voltage regulators, engine and rotor tachometer, and governor control unit for examination and testing at the facilities of Robinson Helicopters. All components performed nominally. Examination of the tachometer breaker point wires in the right magneto revealed that they were twice the designed length, and made of slightly thicker gauge wire, which did not have any identification on its sheath. The female spade connectors which connected the tachometer breaker points to the wires were of slightly heavier gauge than standard. Neither had any strain relief tabs, and closer examination revealed that the wires had been soldered to the spade connectors, rather than crimped. The wire on the moving side of the points had completely separated at the solder joint, with soldered strands still attached to the connector (See Figure 2). The wire on the fixed side of the points was still attached to the spade, however about 1/3 of its strands had frayed and separated in a similar manner. The helicopter was fitted with the fuel tank bladders required in Robinson Helicopters Service Bulletin SB-78B. The bladders were not compromised during the landing. Figure 2 – Separated Tachometer Points Wire Maintenance History The helicopter was maintained by mechanics employed by National Helicopter Service and Engineering Company at their base on Van Nuys Airport. The mechanics stated that about two weeks before the accident the rotor and engine tachometer needles were intermittently binding against each other, particularly on startup as the needles climbed (the needles were housed in a dual gauge), but they would then usually unbind and work correctly once operating speeds had been reached. The tachometer was therefore replaced at the 100-hour inspection. However, as soon as they started the helicopter, neither the engine tachometer nor the governor worked. They performed some troubleshooting steps and determined the right magneto, which contained the tachometer breaker points, was the problem, so they replaced the magneto with a unit loaned by a maintenance facility in Oxnard. It was their understanding that the replacement magneto had accrued about 5 hours of total time since overhaul. Maintenance records indicated that the inspection and magneto replacement was complete on July 7, 2017, 26.2 flight hours before the accident. Both mechanics from National Helicopter Service stated that they did not replace the tachometer wires at any time, and the owner of the maintenance facility who loaned the magneto stated that he was not aware of the cables ever being replaced while the unit was in his possession, and added that he would not have used solder to connect the wires. During the investigation, the right magneto was taken to the facility who performed the last overhaul. The owner of the facility inspected the magneto, and confirmed that the breaker points cable and spade connectors were not the type he used, and that the cables were longer than standard. He then provided exemplar copies of the cables and connectors, and demonstrated how the tachometer points are typically assembled and wired. None of the methods matched those observed in the accident magneto. He stated that wires were always crimped, and solder was never used. Governor and Tachometer System The collective control for the R44 II series is conventional, and includes a twist grip throttle. When the collective control is raised, the engine throttle is opened automatically by an interconnecting linkage. In addition, the helicopter is equipped with an engine governor system, which senses engine RPM and applies corrective input forces to the throttle to maintain engine RPM as needed. The governor system is composed of a solid-state electronic controller, which determines engine RPM from the tachometer points in the engine's right magneto. When the governor senses the need to adjust engine RPM it activates a motor which drives the throttle directly through a worm gear drive and friction clutch. The clutch can be overridden by the pilot through the throttle twist grip. The governor is engaged