{"id":"F2006B06716","name":"AD/S-76/35 - Tail Rotor Controls Centering Quadrant - Modification","slug":"ad-s-76-35-tail-rotor-controls-centering-quadrant-modification","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":114533,"registerId":"commonwealth-F2006B06716-current","compilationNumber":null,"startDate":"2026-04-03","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/S-76/35 - Tail Rotor Controls Centering Quadrant - Modification","content":"COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n![](image.001.png)  \nSikorsky S-76 Series Helicopters\n\nAD/S-76/35 Tail Rotor Controls Centering Quadrant - Modification\n\n  \n10/84\n\nApplicability: All models S-76A.\n\nRequirement: Sikorsky CSN No. 76-133 Revision A.\n\nNote: FAA AD 84-15-04 Amdt. 39-4893 refers.\n\nCompliance: Unless already accomplished, within 30 days after 31 October 1984.\n\nBackground: There have been two reports of tail rotor cable failure. Both failures have been caused by misrigging of the control cables over the pulley guide pin at the lower left-hand pulley, located under the main gearbox. The misrigging has resulted in accelerated wear and subsequent failure. The modification incorporates a centering quadrant which returns the tail rotor pitch controls to the basic rigging position if tail rotor control cable fails.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The AD applies to all Sikorsky S‑76A helicopters and requires installation of the centering quadrant per Sikorsky CSN No. 76‑133 Revision A within the stated compliance window. The instrument’s scope is consistent with the stated purpose in the text (applicability, required action and timeframe). No change of scope from the stated requirement is apparent in the document supplied (Applicability; Requirement; Compliance; Background)."},"complexity_factors":["Narrow technical scope: single, well-specified mechanical modification (centering quadrant) (Requirement; Background).","Clear applicability: all Sikorsky S‑76A helicopters only (Applicability).","Reference to external technical document (Sikorsky CSN No. 76‑133 Rev A) that must be followed for implementation (Requirement).","Short, fixed compliance window (30 days after 31 October 1984) creates scheduling pressure (Compliance).","Safety-critical nature increases operational consequences of non-compliance, despite procedural simplicity (Background).","Cross-reference to an FAA AD introduces an external regulatory link to consider (Note)."],"plain_english_summary":"What the instrument changes mechanically\n\n- This Airworthiness Directive requires installation of a tail-rotor \"centering quadrant\" on Sikorsky S‑76A helicopters. The installation must follow Sikorsky Customer Service Notice (CSN) No. 76‑133 Revision A (Requirement). The centering quadrant is a mechanical part that returns the tail-rotor pitch controls to their basic rigging position if a tail-rotor control cable fails (Background).\n\nWho it affects and what must happen\n\n- It applies to all Sikorsky S‑76A helicopters (Applicability). Owners/operators must have the modification carried out in accordance with Sikorsky CSN No. 76‑133 Revision A (Requirement). The AD set a compliance window of 30 days after 31 October 1984 for aircraft that had not already been modified (Compliance).\n\nWhy the change was issued (stated reason)\n\n- The published reason is that there were two reported tail-rotor cable failures caused by misrigging of control cables over the lower left-hand pulley under the main gearbox. That misrigging produced accelerated wear and eventual cable failure. The centering quadrant modification is described as a means to return controls to the basic rigging position if a cable fails, reducing the risk associated with those failures (Background). The AD also cross-references an FAA airworthiness directive (Note).\n\nCosts, incentives and trade-offs (mechanical and operational effects)\n\n- Who pays: owners/operators of the affected S‑76A helicopters are required to have the modification installed, and therefore will bear the direct costs of parts, labour and any aircraft downtime needed to comply (Applicability; Requirement; Compliance).\n- Who decides: the Civil Aviation Safety Authority issued this Airworthiness Directive under the Civil Aviation Regulations Part 39 framework (header). The AD prescribes compliance by reference to the Sikorsky CSN; the manufacturer’s service instructions determine the technical method (Requirement).\n- Behavioural change required: operators must schedule and perform the specified modification within the compliance window, or have previously complied (Applicability; Requirement; Compliance). That changes maintenance planning and may temporarily remove helicopters from service while the work is done.\n- Incentives and risk allocation: the modification shifts some operational risk away from routine cable‑rigging failures by adding a mechanical centering feature. That creates an incentive for operators to install the part promptly to reduce risk of in‑flight cable‑related control problems (Background). The AD also creates a compliance cost concentrated on owners/operators, while the safety benefit is accrued to operators, crews and passengers.\n- Trade-offs and opportunity cost: installing the centering quadrant requires expenditure of maintenance resources and scheduling effort; those resources cannot be used for other maintenance or operations during that time (Requirement; Compliance).\n- Implementation risks and compliance burden: the AD requires adherence to a specified Sikorsky CSN. Incorrect installation or omission would leave the original failure mode unaddressed; following the manufacturer’s procedure is necessary to realise the intended safety effect (Requirement; Background). The AD gives a short compliance window (30 days) which increases scheduling pressure (Compliance).\n\nSummary statement\n\n- In short: CASA mandated a manufacturer-specified modification (centering quadrant) for all Sikorsky S‑76A helicopters to address reported tail-rotor cable failures caused by misrigging. The directive requires operators to install the modification within a defined short period, shifting immediate costs and scheduling obligations to owners/operators while aiming to reduce the mechanical risk described in the AD (Applicability; Requirement; Background; Compliance)."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, time-bound safety directive addressing a specific mechanical defect in a specific aircraft model. It has not expanded beyond its original purpose of mandating a centering quadrant modification to prevent tail rotor control failures."},"complexity_factors":["Single, discrete technical requirement (one specific modification)","No defined terms section or interpretation provisions","No conditional logic or nested exceptions — straightforward applicability ('All models S-76A')","Single compliance pathway with clear deadline","Cross-reference to only one external technical document (Sikorsky CSN) and one foreign equivalent (FAA AD)","Extremely short — approximately 150 words of substantive content"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for aircraft.\n\n**What it does:**\nOrders owners and operators of **Sikorsky S-76A helicopters** to install a specific safety modification to the tail rotor controls. The modification adds a \"centering quadrant\" — a mechanical device that ensures the tail rotor pitch controls return to a safe, neutral position if the control cables ever fail.\n\n**Why it matters:**\nTwo helicopters had already suffered **tail rotor cable failures** caused by cables being incorrectly routed over a pulley guide pin. This misrouting caused rapid wear and sudden cable failure. A failed tail rotor cable means the pilot loses directional control of the helicopter, which can be catastrophic. The centering quadrant acts as a failsafe to prevent loss of control.\n\n**Who must comply:**\nAnyone operating an S-76A model helicopter in Australia. They had 30 days from 31 October 1984 to complete the modification unless already done.\n\n**Key details:**\n- **Compliance deadline:** 30 days after 31 October 1984\n- **Technical standard:** Sikorsky CSN No. 76-133 Revision A (the manufacturer's service bulletin specifying the fix)\n- **US equivalent:** FAA AD 84-15-04 (showing this was a coordinated international safety response)"}},"importantCases":[],"_links":{"self":"/api/acts/ad-s-76-35-tail-rotor-controls-centering-quadrant-modification","history":"/api/acts/ad-s-76-35-tail-rotor-controls-centering-quadrant-modification/history","analysis":"/api/acts/ad-s-76-35-tail-rotor-controls-centering-quadrant-modification/analysis","conflicts":"/api/acts/ad-s-76-35-tail-rotor-controls-centering-quadrant-modification/conflicts","importantCases":"/api/acts/ad-s-76-35-tail-rotor-controls-centering-quadrant-modification/important-cases","documents":"/api/acts/ad-s-76-35-tail-rotor-controls-centering-quadrant-modification/documents"}}