McDonnell Douglas 369E N504MP

Summary

On 8 June 2014 at about 07:56 local time, a McDonnell Douglas 369E registered N504MP, operated by City of Mesa, was involved in an accident near Mesa, Arizona, United States. 2 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

8 June 2014 at 07:56

Classification

Accident

Location

Mesa, Arizona, United States

Nearest airport

Falcon Fld (FFZ)

Coordinates

33.3772, -111.7953

Aircraft

McDonnell Douglas 369E

Registration

N504MP

Category

Helicopter

Year built

1995

Engines

1

Operator

City of Mesa

Operating rule

Public Use

Phase of flight

Not recorded

Route

Mesa → Mesa

Aircraft damage

Substantial

Weather

VMC

Light

Night

NTSB number

WPR14TA236

Nobody was injured.

The loss of engine power during cruise flight due to the decoupling of the engine’s turbine and compressor sections. Contributing to the decoupling was the excessive wear of the turbine-to-compressor coupling components due maintenance personnel’s placement of an incorrect shim during a compressor section overhaul and a latent misalignment within the exhaust collector.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

The commercial pilot was conducting a nighttime law enforcement patrol flight in the helicopter. The pilot reported that, about 1.4 hours into the flight, the engine lost all power, and he subsequently performed an autorotation to an unlit empty field. The helicopter landed hard, and the main rotor blades subsequently struck and severed the tailboom. Postaccident examination of the engine revealed excessive wear in the turbine-to-compressor coupling components. The turbine-to-compressor coupling had moved, which caused its forward splines to contact the spur adapter gear shoulder and its rear splines to disengage from the turbine-splined adapter. The disengagement caused the decoupling of the turbine section from the compressor section and the subsequent loss of engine power. The excessive wear was determined to be the result of a misalignment of the engine's centerline shafting components. Two areas of misalignment were found. The first misalignment was caused by the incorrect placement of two shims at the interface between the rear diffuser and gearbox. A review of the engine maintenance logbooks revealed that maintenance personnel had placed these shims during the last compressor section overhaul, about 2 years or 1,432.1 flight hours before the accident. The second misalignment was found between the forward and rear pilot diameter bores of the exhaust collector assembly, which exhibited excessive runout when compared to engineering drawings. The maintenance records showed that, about 9 months before the accident, the exhaust collector assembly was reworked in accordance with the engine manufacturer's maintenance manual, which did not call for a runout check of the forward-to-rear pilot diameters but rather a runout check of both pilot diameters independently with reference to the bearing bore. At the time of rework, the runout-to-bearing bore values were within tolerance. Therefore, it is likely that the discrepancy was latent, predating the accident by a long period of time, and was not significant enough to cause previous shafting misalignment. Although neither of the misalignments individually were likely significant, cumulatively, they produced sufficient misalignment to cause the observed wear.

Quoted verbatim from the NTSB record.