B407 Worn Throttle Detent Power Loss Accident (Air Methods N687AM)
On 4 September 2022 Air Methods Corp (AMC) Bell 407 N687AM from Mercy Air 22 air ambulance base in Hesperia, California crashed near Mount Baldy, California. The pilot was seriously injured.
The US National Transportation Safety Board (NTSB) issued their safety investigation report on 21 August 2024.
The Accident Flight
The NTSB lists the 58 year old pilot’s experience as 3447 hours total, 264 hours on type. In his statement the pilot explains he was tasked to support a public relations event for the Mount Baldy Fire Department and…
…consisted of flying water and other supplies and people from a base camp at Cow Canyon Saddle to the summit of Mount Baldy, dropping them off and returning to the base camp to pick up more supplies. The base camp is at approximately 4300 feet MSL. The summit of Mount Baldy is at 10064 feet MSL. The summit is approximately 4 nautical miles from the base camp.
The pilot was using the base’s spare aircraft. He anticipated 5 trips would be needed. After completing the second delivery to the summit…
I flew down-canyon to lose altitude and set up for a steep approach to the confined area landing zone at Cow Canyon Saddle.
I slowed from 60 knots to 40 knots as I turned final. Shortly after turning final I saw the low RPM warning light and heard the low RPM warning horn. There were no other warning lights illuminated. There were no other audio annunciations. The controls were responding normally. The engine did not sputter. I saw that the rotor RPM dropped to approximately 95%.
I lowered the collective and the rotor RPM went back up to 100%. I don’t remember looking at the throttle.
AMC B407 N687AM Collective Grip Twist Throttle Installed (Credit: NTSB)
I aborted the approach and turned left, down-canyon. I made a Mayday call to the firefighters at Cow Canyon Saddle. I told them I had a partial engine failure and that I was looking for a place to land.
I attempted to raise the collective and once again the low RPM warning light and horn came on. I lowered collective again and proceeded down-canyon with a descent rate of 600-700 feet per minute. I did not attempt to put the FADEC into Manual mode because I didn’t want to divert attention away from looking for a place to land.
I didn’t see any good places to land. I considered landing on the main road but decided against it because of the power lines along the road and the narrowness of the road. I saw a small paved road in a clearing at the bottom of the canyon and decided to land there. I did not autorotate. I decided to use what power the helicopter had for landing. There were trees and powerlines down-canyon from the clearing.
I was slightly too high to land in the clearing so I flared into the top of a tree on the downcanyon side of the clearing. The flare brought the helicopter to a stop. I started to level the helicopter with forward cyclic. I pulled full collective to cushion the impact. The helicopter came to rest on its right side.
The Safety Investigation
The NTSB explain that:
The engine control unit captured data consistent with main rotor impact. The data showed that during the 12 seconds before the fault, the power lever angle (PLA) readings were between 48° and 50° and the engine was operating consistent with those settings.
The normal PLA reading when the throttle is in the FLY position is 70°, and full engine governing is available down to 62° PLA. The pilot’s operating handbook states that the throttle should be in the FLY position for takeoffs and landings. It also cautions that failure to position and maintain the throttle in
the FLY detent position during normal flight operations can limit available engine power.
AMC B407 N687AM Throttle Assembly (Credit: Bell)
There is a detent in the throttle that is engaged by a ball plunger when the throttle is rolled to the FLY position from either the Idle or Max Ng positions. The throttle was manipulated by hand during post accident examination and, when the throttle was rolled down from the FLY position towards Idle, there was perceivably less tactile resistance than when the throttle was rolled from FLY to Max Ng.
Significantly:
Testing of the throttle determined the ball plunger would disengage from the FLY position detent when 4 lb force was applied to turn the throttle. The helicopter maintenance manual states 14 to 15 lb force should be required to move the throttle through the FLY detent position.
However, in a specialist report within the public docket it is also revealed that an exemplar throttle at the Bell Training Academy disengaged at only 5 lb force.
Remarkably this finding is not discussed further in the NTSB final report.
Disassembly of the collective assembly revealed the detent where the ball plunger in the throttle engages at the FLY position was worn, which likely reduced the force required to move the throttle out of the FLY position towards IDLE.
AMC B407 N687AM Throttle Detent Wear (Credit: Bell)
No other preimpact anomalies were found by investigators.
[The] throttle friction should have been verified in accordance with the manufacturer’s recommended inspection schedule and maintenance procedures.
NTSB say this would be at either Event No. 3 of the progressive inspection program (effectively conducted once per annum) or during the airframe periodic inspection (at a 12 calendar month or a 300-hour interval, whichever is first). An Event 3 was conducted 10 May 2022 (c 4 months earlier) at 6873 flying hours (70 hours earlier).
The procedures directed maintenance personnel to set throttle friction to the appropriate amount by adjusting the ball plunger on the throttle.
Although the ball plunger disengaged from the FLY position detent in post accident testing with less force than specified by the manufacturer, it could not be determined if maintenance personnel did not properly adjust the ball plunger in accordance with the manufacturer’s specified procedures, or if the worn detent reduced the ability of the ball plunger to maintain the correct torque after being set properly.
According to the public docket, one reason this could not be determined is that the Inspector was no longer employed by the operator.
Although the maintenance procedures did not require inspection of the throttle detent [itself], if the specified throttle friction forces were unable to be achieved during the inspection, this should have provided the opportunity necessary for maintenance personnel to identify and correct the worn
throttle detent.
NTSB Conclusion
NTSB concluded that:
The worn throttle detent bracket likely contributed to the pilot unknowingly rolling the throttle towards idle prior to commencing the approach to land and subsequently not recognizing that the throttle had moved.
Then without the throttle in the FLY detent position…
….the helicopter made an approach to land [with] reduced engine power, a low main rotor RPM condition, and subsequent impact with terrain.
NTSB make no safety recommendations to prevent a reoccurrence.
Our Observations on the Investigation
The problematic ‘arse about face’ formulation of the NTSB Probable Cause makes an unknowing action of the pilot “the cause” but the latent throttle detent bracket wear merely a “contributory factor”.
In light of the evidence from a Bell Training Academy B407 it is surprising there is no further consideration of the adequacy of the inspection regime. As is the lack of reference to another event we have discussed before, just 4 months earlier, also in California: Pilot Induced B407 HEC Power Loss (11 May 2022).
Neither the pilot’s statement or the record of an interview conducted by AMC are dated. The later contains questions whose answers feature in the pilot’s statement. This gives the impression that either the interviewer had not read the pilot’s statement first OR, more likely, that the interview deliberately or inadvertently primed the pilot to put certain comments in his statement. Both these situations should be avoided by investigators.
Again, there is no mention of the any flight data being analysed in this NTSB report. The FAA changed Part 135 in 2017 so that helicopter air ambulance operators had to comply with a new Flight Data Monitoring (FDM) System requirement, FAR 135.607:
After April 23, 2018, no person may operate a helicopter in air ambulance operations unless it is equipped with an approved flight data monitoring system capable of recording flight performance data.
FAA AC 135-14B Helicopter Air Ambulance (HAA) Operations explains the range of data to be recorded.
So while flight was classified as a Part 91 flight it would be very unusual to have deliberately turned the FDM system off for these flights just because it was not legally required and so useful data was presumably not analysed.
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
- Pilot Induced B407 HEC Power Loss (11 May 2022)
- B407 Worn Throttle Detent Power Loss Accident (4 Sept 2022)
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