Van Nuys, CA, USA
N3254E
ROBINSON HELICOPTER COMPANY R44 II
**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 via a switch located on the pilot's collective control. A "GOV-OFF" warning light in the instrument cluster will illuminate if the governor has been turned off by this switch. The light will not indicate a governor failure or error. According to the maintenance manual, "The governor is active from 79% - 111 % engine RPM and can be switched on or off by the pilot using the toggle switch on the end of the right seat collective control. The governor system is designed to assist the pilot in controlling the RPM in the normal operating range. It may not prevent over- or under-speed conditions generated by aggressive flight maneuvers. Within the active range there is a 1 %-wide deadband from 101 % - 102%> where the governor will not take action provided the RPM is steady." The helicopter is equipped with one electronic dual (engine and rotor) tachometer. The sensor for the engine tachometer is the same set of magneto breaker points used by the governor. The sensor for the rotor tachometer is an electronic Hall Effect device which senses passage of two magnets attached to main rotor gearbox input yoke assembly. The engine and rotor tachometer circuits have separate circuit breakers and are completely independent from the other. According to technical representatives from Robinson Helicopters, if the magneto breaker point signal is lost completely, the governor controller will no longer send power to the governor motor, which will therefore remain at the last set position. Should the signal be partially interrupted, the governor may interpret the signal as a command to increase or decrease engine RPM, depending on the intermittency of the failure. None of these failure modes would trigger the "GOV-OFF" indicator light. ADDITIONAL INFORMATION Onboard Recording System The helicopter was equipped with a "Tourmaster" GPS enabled video recording unit, manufactured by Rugged Video LLC. The unit was typically used by air tour operators to record video, audio, and GPS data for customers as a memento of their flight. The accident helicopter was configured to capture alternating video images from two cameras. One camera was facing aft, with a view of the pilot and passengers, and the other facing forward showing the terrain the helicopter was flying towards and over. The audio consisted of conversations picked up by the intercom system, air traffic control transmissions, and a musical soundtrack. The memory card from the unit was sent to the NTSB vehicle recorders division for data extraction and review. A complete report is contained in the public docket. The video recording included the enroute portion of the flight and began shortly after takeoff. At 12:03:30, the helicopter was about 3 ½ miles southeast of VNY traveling on a westbound heading at a ground speed of about 105 knots and a GPS altitude of 1,490 ft (750 ft above ground level). About that time, the pilot requested a transition through the VNY airspace, which was granted, and about 30 seconds later, at 12:04:10, the front facing camera captured the helicopter yawing to the left and then back to the right. The recording then picked up someone exclaiming "whoa", and 44 seconds later the passenger in the rear left seat could be seen looking toward the instrument panel, and asking, "what's the low rpm thing?". By that time, the helicopter had descended to 1,100 ft, and slowed to 50 knots. A few seconds later at 12:05:56, the pilot made a mayday call to the VNY tower, and reported that he was going down just north of the 101 Freeway. Over the next 12 seconds the helicopter continued on the same heading, descended about 75 ft, and slowed to a speed of 37 knots. The descent rate then rapidly increased, and the recording ended as the helicopter collided with the street 20 seconds later at 12:05:24. The audio did not record any unusual sounds prior to the event as reported by the pilot, and no audible warning tone similar to a low rotor RPM warning horn could be heard at any time during the recording. Audio Study The audio portion of the video recording was evaluated in an attempt to determine the engine, rotor, and transmission characteristics during the accident sequence. A spectrogram was generated showing the frequency content of the sound and how it changed over time. Review of the data indicated that the sampling rate was 8,000 Hz, but the effective bandwidth of the intercom and soundtrack recording appeared to be limited to 2,400 Hz. Because of the bandwidth limitation, and multiple periods where the intercom and soundtrack audio saturated the dynamic range, a continuous visual trace of background tone was not possible. However, there appeared to be a distinct tone evident during the periods between the intercom and soundtrack saturation. The data revealed an initial tone at a frequency of about 2,360 Hz, which then dropped to 2,165 Hz at 12:04:10, coincident to the helicopter yawing left and right. Over the next 7 seconds, the tone began a series of frequency oscillations, varying between 2,210Hz and 2,173 Hz. The tone then recovered back to about 2,360 Hz, where between intercom and soundtrack saturation, it appeared to remain. At 12:04:55, about the time that the pilot made the mayday call to the VNY tower, the tone frequency dropped off significantly until it reached its lowest value of 1,734 Hz, 26 seconds later and just before the recording ended. According to technical representatives from RHC, the frequency of the observed background tone did not match directly with any known rotor, engine, or transmission frequencies. However, the tone had a sound very similar to "electrical crosstalk" often encountered when an electrical signal on one circuit creates an undesired effect on another, such as the audio recording system. The Emergency Procedures section of the helicopters Pilot's Operating Handbook states the following regarding tachometer and governor failures: "TACHOMETER FAILURE If rotor or engine tach malfunctions in flight, use remaining tach to monitor RPM. If it is not clear which tach is malfunctioning or if both tachs malfunction, allow governor to control RPM and land as soon as practical. NOTE Each tach, the governor, and the low RPM warning horn are on separate circuits. A special circuit allows the battery to supply power to the tachs with the battery and alternator switches both OFF. GOVERNOR FAILURE If engine RPM governor malfunctions, grip throttle firmly to override the governor, then switch governor off. Complete flight using manual throttle control." "LOW RPM HORN & CAUTION LIGHT A horn and an illuminated caution light indicate that rotor RPM may be below safe limits. To restore RPM, immediately roll throttle on, lower collective and, in forward flight, apply aft cyclic. The horn and caution light are disabled when collective is full down."
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.
Source: NTSB Aviation Accident Database
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