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N24HM accident description

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Crash location Unknown
Nearest city Mayville, WI
44.989135°N, 90.373746°W

Tail number N24HM
Accident date 24 Jul 2008
Aircraft type Bell 47G-4A
Additional details: None

NTSB description

On July 24, 2008, about 0652 central daylight time, the pilot of a Bell 47G-4A, N24HM, received fatal injuries when he was stuck by the main rotor blade while exiting the helicopter near Mayville, Wisconsin. The pilot had shut down the engine and exited to inspect the helicopter. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 agricultural application flight was not operating on a flight plan.

The ground loader heard the engine running rough when the helicopter took off next to the field that was going to be sprayed. The helicopter flew in a southerly direction for about 200 yards before landing. During landing, the left skid became entangled in the surrounding vegetation and the helicopter turned towards the north breaking off the left skid. The helicopter landed in a field about 3/8 mile from the loading site and was resting left side low. The pilot exited and walked around the helicopter with the main rotor blades still rotating. The pilot was then fatally injured when he was struck by the rotating blades.


The helicopter was powered by a Lycoming VO-540-B1B3 engine, serial number L-2450-43, that had a total time in service of 3,378.6 hours and a total time since overhaul of 395 hours. There were no markings identifying all the cylinder heads and barrels as Parts Manufacturer Approval (PMA) parts. The return to service tags associated with the engine cylinder assemblies were not received by the National Transportation Safety Board or Federal Aviation Administration (FAA).

Textron Lycoming Mandatory Service Bulletin 388 calls for 300-hour interval inspections if engine valve sticking is suspected. Also, Engine Components, Inc. (ECI) Service Instruction 03-11, which references Textron Lycoming Service Bulletin 388 also calls for similar inspections on ECI Titan Cylinder assemblies. There were no engine logbook entries showing that such inspections were performed.

The maximum gross weight of the helicopter was 3,200 lbs and the operator reported weight at the time of the accident was 3,100 lbs. The helicopter was loaded with 80 lbs of chemical applicant.


An autopsy of the pilot was conducted by the Dodge County Medical Examiner on July 24, 2008. The cause of death was listed as blunt force trauma.

The FAA's Final Forensic Toxicology Fatal Accident Report on the pilot states that no carbon monoxide was detected in blood, no cyanide was detected in blood, no ethanol was detected in blood, metoprolol was detected in blood and urine, and naproxen was detected in urine.


A compression check of the engine revealed that the number one cylinder had a compression ratio of 45/80 with an exhaust valve leak. The number one cylinder assembly contained Superior Air Parts, Inc. Nimonic exhaust valve, part number SL16470AK, and Engine Components, Inc. (ECI) exhaust valve guide, part number AEL 75838 P030. The exhaust valve exhibited four fractures and a relative burnt appearance. The valve guide displayed excessive wear (bell-mouthing). The exhaust valve had an outside diameter of 0.496 inch; the exhaust valve guide interior diameter was 0.540 inch, and a clearance 0.044 inch (0.006 inch nominal).

Comparative examination of the number three cylinder exhaust valve also showed a burnt and worn appearance but to a lesser degree than the number one cylinder. No visible cracks of the exhaust valve were noted.

(c) 2009-2018 Lee C. Baker / Crosswind Software, LLC. For informational purposes only.