The Aviation Safety Council published the final investigation report on an UH-1H helicopter, belonging to the National Airborne Service Corps (NASC), Ministry of the Interior, due to the hoist device had a wrong measurement for the axial gap value, which caused the axial gap of the coupling oversized after the assembly. The drive gear could not take the load of the coupling stress from the driven gear and began to slip, which led the hoisted personnel to fall off during recurrent rescue hoist training.

On May 21st 2011 National Airborne Service Corps (NASC), Ministry of Interior, an UH-1H helicopter, registration No. NA-511, had recurrent rescue hoist training at Longxiang Riverside Park at Ligang Township, Pingtung County. During the operation at 30ft from the ground the flight engineer felt the rope retrieving process was stopped and the rope started to slip down which led the rope to slip downwards and the hoisted personnel falling off to the ground and was severely injured with fracture.

The ASC launched investigation according to the Aviation Occurrence Act after the occurrence. Several departments were invited for joint investigation, including: NASC, Ministry of Interior, National Fire Agency, Ministry of Interior, Air Asia Co. Ltd, the hoist manufacturer Breeze-Eastern Corp., and National Transportation Safety Board(NTSB). The investigation report was published after approval by the ASC council members on May 29, 2012, at the 1st Council Meeting.

Finding related to probable causes：During overhaul the hoist device had a wrong measurement for the axial gap value, which caused the axial gap of the coupling oversized after the assembly. During hoist operation when the coupling face length of the drive gear decreased from 0.074 inch to 0.015 inch, the maximum stress the drive gear wall could take was over the yield strength and damaged; the drive gear could not take the load of the coupling stress from the driven gear and began to slip. The driving gear was over-driven by the load to rotate in the opposite direction, which led the hoisted personnel to fall off.

Findings related to risks：

1. SN 388 and 387 were the first two hoist devices that overhauled by the maintenance crew. The thickness value of the implanted shim was not obtained by following proper procedures and the recorded shim thickness was not correct. Afterwards the over size of the axial gap caused a significant incident after overhaul, which showed that Air Asia did not meet the standard of the manufacturer’s overhaul manual for the operation of the axial gap measurement during the hoist device overhaul.
2. During the overhaul of the hoist device the technician used a non-standard bearing positioning tool that easily made the lateral of the bearing get stuck, which was leading to be mistaken as if the bottom of the bearing had been contacted; then an erroneous value was measured and caused the axial gap to become over size.
3. During the overhaul the operation of the axial gap measuring has not yet met the standard of manufacturer’s overhaul manual.
4. The personnel who overhauled the hoist device had taken professional training provided by Air Asia, but still failed to get accurate axial gap value; which showed that this training was not completely implemented and improvement was still required.
5. The OEM’s overhaul manual for the rescue hoist device did not define the visual inspection and measurement inspection of drive gear and driven gear, so that the wear of the contact between drive gear and driven gear failed to be evaluated and discovered promptly.

The Aviation Safety Council issued a total of 10 Safety Recommendations.

Safety Recommendations to NASC：Supervise Air Asia to overhaul hoist device by following precisely overhaul manual to meet the standard requirement for the axial gap. Inspect and establish relevant training and audit mechanism for flight engineers. Reinforce hoist operation procedures for each fleet, provide operation limit and inspection procedures from hoist device OEM’s manuals and implement personnel training.

One Recommendation to National Fire Agency, Ministry of Interior: Establish the guideline or relevant mechanism of sending the injured personnel during training to hospital for Special Rescue Team to follow.

Four Recommendations to Air Asia: Use standard tool for bearing positioning during hoist device overhaul and establish a method to ensure the bearing in position. Reinforce audit mechanism of overhaul training for this type of hoist device and ensure trainee’s skill meet overhaul standard. Improve to have figures in agreement with text description concerning work orders of hoist device overhaul. Improve the control operation of scheduled inspection items for rescue hoist device and do not have overdue items again.

Two recommendations to National Transportation Safety Board, U.S.A:
Please advice BREEZE-EASTERN Corporation following safety recommendations:

1. Revise visual inspection and measurement test standard of drive gear and driven gear in OEM’s overhaul manual.
2. Improve the defect of the manufacturer’s overhaul manual that figures do not agree with text description. 

The full investigation report is available for download at http://www.asc.gov.tw (Chinese version only)

Sherry Liu, Engineer
Tel: 89127388-330 
Email:sherry@asc.gov.tw