{"id":"F2006B01987","name":"AD/A320/98 - Nose Landing Gear (NLG) - Steering","slug":"ad-a320-98-nose-landing-gear-nlg-steering","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41209,"registerId":"commonwealth-F2006B01987-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/A320/98 - Nose Landing Gear (NLG) - Steering","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)  \nAirbus Industrie A319, A320 and A321 Series Aeroplanes\n\nAD/A320/98 Nose Landing Gear (NLG) \\- Steering 6/99\n\nApplicability: All Airbus Industrie model A319, A320 and 321 aircraft without Airbus Industrie modification No. 23740 (in production from 03 April 1999) or Airbus Industrie Service Bulletin A320-32-1197 (embodied in service).\n\nRequirement: 1. Perform corrective actions in accordance with instructions in Airbus Industrie Service Bulletin A320-32-1197.\n\n2. Ensure that replacement Hydraulic Control Units are not Part Number C24736000 and C24736001. All of these units are to be reworked in accordance with Airbus Industrie Service Bulletin A320-32-1197 prior to re-identification and installation on any aircraft.\n\nNote: DGAC 1999-124-129 (B) dated 24 March 1999 refers.\n\nCompliance: 1. Before 03 April 2000.\n\n2. Prior to fitment of a replacement Hydraulic Control Unit.\n\nThis Airworthiness Directive becomes effective on 17 June 1999.\n\nBackground: One operator experienced three events when the aircraft landed with the nose wheel turned to 90 degrees. Investigation revealed that the two O-ring seals fitted to the electro hydraulic distributor which controls the hydraulic supply to the steering control module 6GC were extruded due to ageing or the absence of a back up ring. This defect allowed permanent pressurisation of the steering actuator through the steering selector valve and the possibility of the nose wheel rotating through 90 degrees. This directive introduces new seals into hydraulic control module 6GC to prevent an occurrence of such a defect.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":2,"scope_assessment":{"changed":false,"description":"The directive is narrowly focused on correcting a specific hardware failure mode (extrusion of O-ring seals in the electro-hydraulic distributor feeding steering control module 6GC). It does not broaden regulatory reach beyond that technical fix: it requires rework per the Airbus service bulletin and limits installation of two identified HCU part numbers unless reworked. There is no indication in the text that the AD expands into unrelated areas or substantially alters the original safety purpose."},"complexity_factors":["Short, single-issue Airworthiness Directive (one clear safety problem and a single corrective action)","Applicability includes a simple exclusion (aircraft with Airbus modification No. 23740 or SB A320-32-1197 already embodied are excluded)","Two substantive requirements (perform rework per SB; do not fit two specific HCU part numbers unless reworked)","One external cross-reference to Airbus Service Bulletin A320-32-1197 (the SB contains the technical detail)","Two compliance timing points (before 03 April 2000; prior to fitment of replacement HCUs)","Minimal definitions and few conditional branches or nested exceptions"],"plain_english_summary":"**What this directive does (mechanically)\n\n- Requires operators of Airbus A319, A320 and A321 aircraft that have not already incorporated Airbus modification No. 23740 or Airbus Service Bulletin (SB) A320-32-1197 to have a specific corrective action carried out on the nose landing gear (NLG) hydraulic steering system.  The corrective work must follow the instructions in Airbus Service Bulletin A320-32-1197.\n\n- Prohibits fitting replacement Hydraulic Control Units (HCUs) with part numbers C24736000 and C24736001 unless those units have first been reworked in accordance with Airbus SB A320-32-1197 and re-identified as reworked.\n\n- Sets deadlines: the rework required by item 1 must be done before 3 April 2000; the prohibition on un-reworked replacement HCUs applies prior to any fitment.\n\n- Becomes effective 17 June 1999.\n\nWho it affects\n\n- Aircraft operators and owners of Airbus A319, A320 and A321 aeroplanes that do not already have mod No. 23740 or have not already embodied Airbus SB A320-32-1197.\n- Maintenance organisations, parts suppliers and maintenance personnel who service or replace the NLG hydraulic control modules (HCU / electro-hydraulic distributor and associated seals).\n\nWhy it matters (official rationale and practical consequences)\n\n- The AD responds to recorded events where aircraft landed with the nose wheel frozen at a 90° deflection. The investigation found the two O-ring seals in the electro-hydraulic distributor (which supplies the steering control module 6GC) were being extruded because of ageing or missing a backup ring. That allowed hydraulic pressure to hold the steering actuator engaged through the selector valve, producing the risk of the nose wheel rotating to 90° on landing. The Airbus SB introduces new seals and required rework to prevent this failure mode.\n\nTesting the purpose-claim against implementation costs and incentives\n\n- Who pays: aircraft operators pay the direct costs — labour, potential parts, and aircraft downtime while the rework is performed.\n- Who decides and enforces: the Civil Aviation Safety Authority (CASA) issues and enforces the AD; Airbus provides the technical procedure (the SB) which maintenance organisations must follow.\n- Behavioural change required: operators must schedule and complete the mandated rework by the compliance date; they must not fit the two specified HCU part numbers unless pre‑reworked and re-identified; maintenance providers must follow the SB procedures when reworking HCUs.\n- Costs and opportunity cost: there is a short-term cost in maintenance man-hours and possible aircraft out-of-service time. The deadline compresses scheduling demand for authorised rework facilities, which could increase short-term costs or delay fleet availability.\n- Implementation risks: availability of spare HCUs and capacity at maintenance/repair facilities to carry out the SB rework could be a bottleneck. If spare HCUs with the listed part numbers are held in stock but cannot be reworked promptly, operators may face aircraft dispatch limitations.\n- Compliance burden and market effects: the AD channels corrective work through Airbus’ SB. That central technical procedure can make Airbus’ authorised repair network (or shops able to follow the SB) the logical supplier of the required work, which concentrates demand for that rework among qualified providers. The text does not state any exclusive authorization requirement, but following the SB will be necessary to demonstrate compliance.\n- Trade-offs: the directive imposes immediate maintenance cost and scheduling burden in exchange for reducing a low-probability but high-consequence failure (nose-wheel misalignment on landing). The AD is narrowly targeted to a specific component and failure mode.\n\nConcentrated benefits and diffuse costs\n\n- The primary and concentrated benefit accrues to each operator that completes the rework through reduced risk of the described steering failure and avoided accident/repair costs. The costs are borne by those same operators and their maintenance suppliers.\n\nNotes on scope and mechanism (what the AD does not do)\n\n- The AD does not require replacement of the entire steering module or changes to operational procedures beyond the hardware rework requirement.\n- It does not prescribe specific maintenance providers or impose broader design changes beyond the sealing improvement set out in the Airbus SB.\n\nSource references in the instrument\n\n- Applicability, required action and timing are set out under the AD headings \"Applicability\", \"Requirement\" and \"Compliance\". The corrective action is specified by reference to Airbus Service Bulletin A320-32-1197; part numbers C24736000 and C24736001 are identified as the units that must not be fitted without rework. The Background paragraph summarises the failure events and the technical cause (extruded O-rings and missing backup ring) that the rework addresses."},"flash_summary_failed":{"failed":true,"reason":"Unauthenticated. Configure AI_GATEWAY_API_KEY or use a provider module. Learn more: https://ai-sdk.dev/unauthenticated-ai-gateway","source":"analysis-cron"},"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":false,"description":"This is a targeted safety directive addressing a specific mechanical defect in a defined aircraft fleet. It has not expanded beyond its original purpose of preventing nose wheel steering failures."},"complexity_factors":["Single, specific technical issue with clear mechanical cause","Only 2 compliance requirements with straightforward deadlines","Limited applicability (3 specific aircraft models, excluding already-modified planes)","References external technical documents (Airbus Service Bulletin) rather than reproducing instructions","No nested exceptions or conditional logic — just 'if not modified, then fix'","Short document (approximately 250 words) with minimal defined terms"],"plain_english_summary":"This CASA Airworthiness Directive requires operators of Airbus A319/A320/A321 aircraft to fix a dangerous nose landing gear steering fault caused by failing hydraulic seals. It applies to aircraft not already modified by Airbus modification 23740 or SB A320-32-1197.\n\nOperators must install new steering seals by 3 April 2000 and must not use certain replacement hydraulic control units (parts C24736000/C24736001) unless repaired. The measure prevents nose wheels turning 90° on landing, which risks loss of control and gear collapse."}},"importantCases":[],"_links":{"self":"/api/acts/ad-a320-98-nose-landing-gear-nlg-steering","history":"/api/acts/ad-a320-98-nose-landing-gear-nlg-steering/history","analysis":"/api/acts/ad-a320-98-nose-landing-gear-nlg-steering/analysis","conflicts":"/api/acts/ad-a320-98-nose-landing-gear-nlg-steering/conflicts","importantCases":"/api/acts/ad-a320-98-nose-landing-gear-nlg-steering/important-cases","documents":"/api/acts/ad-a320-98-nose-landing-gear-nlg-steering/documents"}}