{"id":"F2006B02441","name":"AD/APU/9 - PW901A APU Idler Gear and Cooling Fan Shaft","slug":"ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41697,"registerId":"commonwealth-F2006B02441-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/APU/9 - PW901A APU Idler Gear and Cooling Fan Shaft","content":"COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 107 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n![](image.001.png)  \nAuxiliary Power Units\n\nAD/APU/9 PW901A APU Idler Gear and Cooling Fan Shaft 10/93\n\nApplicability: Pratt and Whitney Canada PW901A Auxiliary Power Units (APU) installed in, but not limited to, Boeing 747-400 series aircraft.\n\nRequirement: Action in accordance with the requirements of Pratt and Whitney Canada Alert Service Bulletin Number A16159 Rev. 1.\n\nNote: Transport Canada Airworthiness Directive CF-93-09 and FAA Airworthiness Directive T93-10-51 refer.\n\nCompliance: Effective 16 September 1993: At the first main base transit check with an aircraft layover time exceeding 3 hours.\n\nBackground: This Directive has been issued to address a possible smoke in cabin situation. The requirement follows advice of two incidents in which, fracture of the PW901A APU idler gear and cooling fan shaft, resulted in debris flooding the load gearbox allowing oil to enter the load compressor air system.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The directive’s scope remains directed at PW901A APUs installed in aircraft, with Boeing 747‑400 series cited as an example but not limiting application (Applicability). It mandates following Pratt & Whitney Canada Alert Service Bulletin A16159 Rev. 1 at the specified maintenance opportunity (Requirement; Compliance) to address the fracture risk described in the Background. There is no indication in the text that the scope has been broadened or narrowed from its original stated purpose."},"complexity_factors":["Narrow technical scope limited to Pratt & Whitney Canada PW901A APUs (Applicability)","Mandatory reliance on an external manufacturer document (Alert Service Bulletin A16159 Rev. 1) to define the required technical actions (Requirement)","Timing condition tied to a specific maintenance opportunity (first main base transit check with aircraft layover > 3 hours) rather than an absolute calendar interval (Compliance)","Cross‑references to foreign airworthiness directives (Transport Canada CF‑93‑09; FAA T93‑10‑51), which may require operators to reconcile multiple documents (Note)"],"plain_english_summary":"This AD (effective 16 Sep 1993) requires operators of Pratt & Whitney Canada PW901A APUs to perform the inspections/repairs specified in Pratt & Whitney Canada Alert Service Bulletin A16159 Rev. 1 to prevent smoke‑in‑cabin caused by fractured idler gear and cooling fan shaft. \n\nCASA mandates compliance at the first main base transit check with a layover over 3 hours; operators and maintenance providers bear the cost and scheduling impact."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive addressing a specific mechanical failure mode. It has not expanded beyond its original purpose of preventing APU shaft fractures in the identified engine type."},"complexity_factors":["Single, specific mechanical component targeted (idler gear and cooling fan shaft)","Only one compliance pathway (follow the referenced service bulletin)","Single applicability criterion (PW901A APU in 747-400 series)","No nested exceptions or conditional logic — straightforward time-based compliance trigger","Short document (approximately 150 words)","Only 2 defined references (the service bulletin and related foreign ADs)"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order for aircraft.\n\n**What it does:**\nOrders airlines and aircraft operators to inspect and fix a specific mechanical problem in Pratt & Whitney Canada PW901A Auxiliary Power Units (APUs). An APU is a small engine at the back of large aircraft that provides power when the main engines are off.\n\n**The specific problem:**\nThe idler gear and cooling fan shaft inside these APUs can fracture (break). When this happens, metal debris floods into the gearbox, allowing oil to leak into the air system — which has caused smoke to fill the aircraft cabin in at least two incidents.\n\n**Who must comply:**\nOperators of Boeing 747-400 aircraft (and potentially others) fitted with these specific PW901A APUs.\n\n**What they must do:**\nFollow the repair instructions in Pratt & Whitney Canada Alert Service Bulletin A16159 Revision 1.\n\n**When:**\nBy the first scheduled maintenance check where the aircraft sits on the ground for more than 3 hours, starting from 16 September 1993.\n\n**Why it matters:**\nThis prevents in-flight smoke emergencies that could endanger passengers and crew, and potentially force an emergency landing."}},"importantCases":[],"_links":{"self":"/api/acts/ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft","history":"/api/acts/ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft/history","analysis":"/api/acts/ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft/analysis","conflicts":"/api/acts/ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft/conflicts","importantCases":"/api/acts/ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft/important-cases","documents":"/api/acts/ad-apu-9-pw901a-apu-idler-gear-and-cooling-fan-shaft/documents"}}