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Lack of inspection method to detect fracture

Issue number: AO-2017-032-SI-01
Who it affects: All owners and operators of GE CT7-5A2/-5A3/-9B/-9B2 series engines
Issue owner: General Electric (GE)
Operation affected: Aviation: Air transport
Background: Investigation Report AO-2017-032
Date: 10 October 2018

Safety issue description

The engine manufacturer did not have specific inspection procedures in the maintenance documents of the propeller shaft to detect a fatigue crack originating from the dowel pin hole.

Proactive Action

Action organisation: General Electric
Action number: AO-2017-032-NSA-004
Date: 10 October 2018
Action status: Closed

On 16 May 2017, the engine manufacturer released service bulletin SB 72-0530 to four main operators of the CT7-5A2/-5A3/-9B/-9B1/-9B2 series engines. The intent of the service bulletin was to conduct a detailed inspection of the propeller reduction gearbox (PGB) shaft, for the detection of cracking, corrosion, wear damage, and other abnormalities to the propeller hub flange for high-time PGBs. The inspection addressed 12 ‘fleet-leader’ PGBs from each operator and included the following criteria:

  • non-destructive fluorescent penetrant inspection for cracking and corrosion of the PGB hub flange region that surrounded the dowel pins
  • wear measurements of the hub flange
  • detailed visual inspection and documentation of the PGB hub flange physical condition.

The inspection results were forwarded to GE Aviation for assessment, within the requested timeframe of one calendar month. GE Aviation reported to the ATSB that no other instances of cracking were identified through the ‘fleet-leader’ inspection program.

Proactive Action

Action organisation: General Electric
Action number: AO-2017-032-NSA-005
Date: 10 October 2018
Action status: Closed

On 22 June 2017, the engine manufacturer released service bulletin SB 72-0531 to all operators of CT7-5A2/9B/-9B1/-9B2 series engines. The SB applied to all PGBs that had accumulated over 30,000 hours’ time-in-service if the PGB main propeller shaft had not been replaced within 10,000 hours.

The SB was focused on the hub flange region and required an inspection for cracking, corrosion and wear damage using both detailed visual and non-destructive methods. The following compliance timeframes were recommended for operators to follow:

  • for PGBs that had accumulated over 46,000 hours service, compliance should be within one month, but before accumulating an additional of 150 flight hours
  • for PGBs that had accumulated between 40,000 and 46,000 hours, compliance should be within four months, but before accumulating an additional 500 flight hours
  • for PGBs that had accumulated between 30,000 and 40,000 hours, compliance should be within eight months, but before accumulating an additional 1,000 flight hours.

At the time of the release of this report, there had been six PGB shaft rejections as a result of the inspections in accordance with SB 72-0531. The rejections were related to corrosion and fretting around the dowel pin holes and the face of the flange. There were no reports of any cracking on any of the inspected shafts.

Proactive Action

Action organisation: General Electric
Action number: AO-2017-032-NSA-006
Date: 10 October 2018
Action status: Closed

In September 2017, GE made changes to the maintenance manual, including the introduction of fluorescent penetrant inspection of the dowel pin areas, a clarification of the inspection criteria for corrosion, and the addition of photographs of the dowel pin area condition.

Proactive Action

Action organisation: General Electric (GE)
Action number: AO-2017-032-NSA-007
Date: 10 October 2018
Action status: Closed

The United States Federal Aviation Administration (FAA) released Airworthiness Directive AD 2018-03-13 on 14 February 2018, for certain General Electric CT7-5A2/-5A3/-7A/-7A1/-9B/-9B1/-9B2/-9C/-9C3 model turboprop engines. The AD required initial and repetitive visual inspection and fluorescent-penetrant inspection (FPI) of the main propeller shaft.

   
Current issue status: Adequately addressed
Status justification:

The release of the service bulletins and airworthiness directives introduced a targeted inspection of the PGB flange and shaft area, along with an additional inspection method. As of June 2018, 168 inspections had been completed, with six rejections and no cracking. Additionally, the new inspection criteria has been incorporated into the maintenance manual, so inspection of the critical area is now part of the ongoing maintenance for the PGB propeller shaft.

 
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Last update 10 October 2018