ASN Wikibase Occurrence # 196807
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| Date: | Friday 21 July 2017 |
| Time: | 12:11 |
| Type: |  Robinson R44 II |
| Owner/operator: | National Helicopter Service |
| Registration: | N3254E |
| MSN: | 11702 |
| Year of manufacture: | 2007 |
| Total airframe hrs: | 2661 hours |
| Engine model: | Lycoming IO-540-AE1A5 |
| Fatalities: | Fatalities: 0 / Occupants: 4 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Sherman Oaks, CA -
 United States of America
|
| Phase: | En route |
| Nature: | Passenger - Non-Scheduled/charter/Air Taxi |
| Departure airport: | Van Nuys, CA (VNY) |
| Van Nuys, CA (VNY) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
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 dual 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.
Probable Cause: An improper repair of the right magneto's tachometer and governor signal wire, which failed during cruise flight. Contributing to the accident were the design of the helicopter, which allowed for a single failure to affect both the engine tachometer and governor control system, and the lack of guidance on how to address the simultaneous failure of both.
Accident investigation:
 |
|
Sources:
NTSB
http://aerossurance.com/helicopters/r44-improper-repair-mx-error
Location
Media:
Helicopter crashes in Sherman Oaks, multiple people injured, USAhttps://t.co/v3bMgpLREJ pic.twitter.com/gKKr1QMd4m
— Planes Of Legend (@PlanesOfLegend) 22 juillet 2017
An improper repair failed and this R44 #helicopter pilot was faced with "a confusing set of cues and no definitive procedures for corrective action" say #NTSB. The pilot was "was shocked at how quickly everything transpired".https://t.co/qd9DwRnWgd #flightsafety pic.twitter.com/2OqiZQGVer
— Aerossurance (@Aerossurance) March 15, 2020
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 22-Jul-2017 07:00 | gerard57 | Added |
| 22-Jul-2017 07:59 | CTYONE | Updated [Cn] |
| 22-Jul-2017 08:01 | Iceman 29 | Updated [Cn, Source, Embed code] |
| 22-Jul-2017 08:10 | Iceman 29 | Updated [Cn, Operator, Embed code] |
| 22-Jul-2017 17:02 | Aerossurance | Updated [Aircraft type, Operator, Location] |
| 22-Dec-2019 14:16 | ASN Update Bot | Updated [Time, Operator, Nature, Departure airport, Destination airport, Source, Narrative, Accident report, ] |
| 15-Mar-2020 15:24 | Aerossurance | Updated [Nature, Source, Narrative] |
| 15-Mar-2020 17:45 | Aerossurance | Updated [Embed code] |
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