AD/B737/224 Amdt 4 - Horizontal Stabiliser Attachment Pins and Bolts - Inspection

This instrument is Amendment 4 to Airworthiness Directive AD/B737/224, issued by a delegate of the Civil Aviation Safety Authority (CASA) on 9 December 2014 with an effective date of 11 December 2014. It imposes a mandatory continuing airworthiness requirement on operators of certain Boeing 737 series aeroplanes. The directive addresses an unsafe condition identified in the horizontal stabiliser attachment structure. Specifically, it requires inspection, repair or replacement of the rear spar attach pins and front spar attach bolts that connect the horizontal stabiliser to the aircraft. The requirement is grounded in the technical provisions of the United States Federal Aviation Administration AD 2004-05-19 (as corrected in the Federal Register of 13 April 2004), which CASA adopts with modifications. The directive does not stand alone; it incorporates by reference the FAA AD and two Boeing Service Bulletins: SB 737-55-1086 (Original Issue, 11 December 2003, or later FAA-approved revisions) and SB 737-55-1093 (Original Issue, 9 April 2012). The background explains that reports were received of a significant number of aircraft with damaged bolts and pins, with wear, pits or corrosion damage. If undetected and uncorrected, such damage could lead to failure and consequent loss of the horizontal stabiliser. The directive thus mandates action to prevent a catastrophic structural failure. The compliance time is set by reference to the FAA AD as if it had an effective date of 13 May 2004, meaning operators must calculate initial and repetitive intervals from that date. Amendment 4 introduces new exclusions: aircraft with a line number from 1725 to 3534 inclusive that are fitted with front spar attach bolts part number (P/N) 180A1611-3 or 180A1611-5 are no longer required to perform the inspection, repair or replacement of those bolts; and aircraft with a line number of 3535 or higher are entirely excluded from the inspection, repair or replacement requirements for both rear spar attach pins and front spar attach bolts. These exclusions reflect CASA’s acceptance of FAA-approved alternative methods of compliance, effectively allowing certain aircraft to avoid the burden of the original directive based on their production line history and hardware configuration.

The directive operates through several interlocking technical concepts. An airworthiness directive (AD) is a mandatory regulatory instrument issued by CASA under the Civil Aviation Act 1988 to require the correction of an unsafe condition in an aircraft type. This AD adopts the technical substance of an FAA AD (specifically 2004-05-19, as corrected), which is the equivalent directive issued by the United States Federal Aviation Administration. The FAA AD sets out the inspection, repair and replacement actions for the horizontal stabiliser attachment pins and bolts. Horizontal stabiliser refers to the tailplane that controls pitch; its attachment to the fuselage is critical. The rear spar attach pins and front spar attach bolts are the mechanical fasteners that secure the stabiliser to the airframe. The directive references two Boeing Service Bulletins (SBs). A Service Bulletin is a manufacturer-issued document that describes a modification, inspection procedure or part replacement. SB 737-55-1086 covers inspection, repair or replacement of both rear spar attach pins and front spar attach bolts; SB 737-55-1093 covers only the rear spar attach pins. The AD states that compliance with SB 737-55-1086 Revision 1 (dated 16 December 2004) or a later FAA-approved revision is taken to be compliance with the original issue of that SB. Similarly, compliance with SB 737-55-1093 Original Issue is taken to comply with the FAA AD requirements for rear spar attach pins. Line number is the manufacturer’s production sequence number for each aircraft, a unique identifier that determines applicability. Part number (P/N) identifies a specific component design; here P/N 180A1611-3 and 180A1611-5 are particular front spar attach bolts. The AD introduces exclusions based on line number and part number, which are a form of alternative method of compliance (AMOC) already approved by the FAA and accepted by CASA. The concept of grace period appears in Amendment 1, which corrected the omission of the phrase “whichever occurs first” in the initial compliance time, ensuring operators use the earlier of two possible dates. The AD’s effective date (11 December 2014) is distinct from the compliance calculation date (13 May 2004) - a potential source of confusion. The instrument is signed by a Delegate of CASA, an officer authorised to issue ADs under the relevant part of the Civil Aviation Safety Regulations.

This directive directly affects the owners and operators of all Boeing model 737-600, 737-700, 737-700C, 737-800 and 737-900 series aeroplanes on the Australian civil aircraft register. It also affects any foreign-registered aircraft operated by Australian operators under Australian regulatory oversight. The applicability is broad, covering the most common variants of the Next Generation 737 fleet. The directive imposes obligations on any person responsible for the continuing airworthiness of these aircraft, which under the Civil Aviation Safety Regulations is typically the registered operator or a Part 42 continuing airworthiness management organisation (CAMO). The actual physical work of inspection, repair or replacement will be performed by approved maintenance organisations (AMOs) that hold the necessary approvals under Part 145. The directive also affects suppliers of replacement parts, particularly the bolts P/N 180A1611-3 and 180A1611-5. The exclusions based on line number mean that operators must know their aircraft’s line number; those with line number 3535 or higher are not required to do anything under this AD (but remain subject to other ADs). Operators with aircraft line numbers 1725 to 3534 must determine the front spar attach bolt part number; if those bolts are the specified part numbers, they are exempt from the front spar bolt inspection but still must comply with rear spar pin requirements. Operators of aircraft with line numbers below 1725 face the full requirement. The compliance burden includes the cost of obtaining the referenced Boeing SBs and the FAA AD, performing the inspections at prescribed intervals, documenting results, and carrying out any necessary repairs or replacements. The repetitive nature of the inspections means ongoing cost and aircraft downtime. The directive also indirectly affects lessors, financiers and insurers who rely on aircraft being continuously airworthy. Anyone buying or selling affected aircraft will need to verify compliance history as part of due diligence. The directive does not affect Boeing 737-100, -200, -300, -400, -500 or MAX series, nor other models such as the 757 or 767.

The primary duty is that subject to the exceptions, action must be taken in accordance with the technical requirements of FAA AD 2004-05-19 as corrected. This duty is a mandatory obligation under Australian aviation law; it is not optional. The specific actions required are set out in paragraphs (a), (b), (c), (d) and (e) of the FAA AD, which the Boeing SBs operationalise. Those actions involve inspection of the rear spar attach pins and front spar attach bolts for damage (wear, pits, corrosion), followed by repair or replacement if damage is found. The duty includes performing these actions at the time specified in the FAA AD, with the initial compliance time calculated from 13 May 2004. There is also a duty to use the correct, approved documentation: either the original Boeing SB 737-55-1086, a later FAA-approved revision, or for the rear pins SB 737-55-1093. A key right conferred by the directive is the ability to use alternative methods of compliance (AMOCs) that have been approved by the FAA and accepted by CASA. The amendment history confirms that CASA accepts FAA-approved AMOCs as equivalent compliance, which gives operators flexibility if they can obtain an FAA alternative. The right to use later revisions of the SB (if FAA-approved) is explicitly stated. Another right is the exclusion for certain aircraft: if the aircraft’s line number is 3535 or higher, the operator is relieved of all inspection, repair and replacement duties under this AD. Operators with line numbers 1725-3534 fitted with the specific bolt P/Ns are relieved of the front spar bolt duties, but not the rear spar pin duties. The directive also implicitly confers the right to rely on the FAA AD and SBs as definitive statements of what must be done, providing a clear compliance pathway. There is no duty under this AD to inspect aircraft that fall within the exclusions, but operators retain the broader duty under other regulations to maintain continuing airworthiness. The directive does not grant any right to deviate from the required actions without obtaining an approved alternative; unauthorised deviations would constitute non-compliance.

The text of this airworthiness directive does not itself prescribe specific penalties or enforcement measures. However, it is issued under the authority of the Civil Aviation Act 1988 (Cth), and compliance is mandatory. Non-compliance with an AD is a serious breach of airworthiness regulations. In Australia, the applicable legislation would be the Civil Aviation Safety Regulations (CASR), particularly Part 39 (Airworthiness Directives) and Part 42 (Continuing Airworthiness). Under that framework, operating an aircraft that does not comply with applicable ADs renders the aircraft unairworthy. The consequences can include: grounding of the aircraft by CASA (either by direction or by revocation of the airworthiness certificate), civil penalties (financial penalties that can be substantial), and in severe cases criminal prosecution. The background to the directive states that the unsafe condition - damage to the bolts and pins - could lead to failure and consequent loss of the horizontal stabiliser. That makes non-compliance a safety-critical issue, likely to attract the highest level of enforcement priority. CASA has power to issue enforcement orders, suspend or cancel certificates, and impose fines. The AD’s amendment history shows that CASA has progressively refined the requirements, but that does not signal any relaxation of enforcement for aircraft still within the scope. Operators must also consider that insurance policies may exclude cover if an aircraft is operated in breach of an AD. The directive itself does not list specific penalty amounts, but practitioners should refer to the Civil Aviation Act and CASR Part 39 for details such as maximum fines per offence. The structure of the directive (with mandatory "must" language and defined compliance times) underscores that CASA intends strict compliance. Because the AD incorporates the FAA AD, non-compliance could also affect operations under bilateral agreements, potentially impacting an operator’s ability to fly into US airspace or have FAA acceptance of their maintenance.

This directive operates within a hierarchical framework of Australian and international aviation legislation. It is issued under the Civil Aviation Act 1988 and, more specifically, under the Civil Aviation Safety Regulations (CASR). It would typically be classified under CASR Part 39, which governs the issuance of airworthiness directives. The directive adopts by reference an FAA AD and two Boeing Service Bulletins; those documents are not reproduced verbatim in the directive, so operators must obtain them separately. The FAA AD is a foreign regulatory instrument, but CASA has accepted it as the technical standard for compliance in Australia. This creates an interaction with US law: any amendment to the FAA AD could affect the Australian obligation, and CASA has a practice of issuing amendments (as seen in the four amendments to AD/B737/224) to align with FAA changes. The directive also interacts with the continuing airworthiness regulations (CASR Part 42) and maintenance regulations (CASR Part 145). Work performed under the directive must be carried out by an approved maintenance organisation (AMO) holding the appropriate ratings, and the maintenance release must be properly signed. Record-keeping obligations under Part 42 require that compliance with ADs be documented in the aircraft’s continuing airworthiness records. The directive also interacts with other CASA airworthiness directives that apply to the Boeing 737 series; this AD is specific to the horizontal stabiliser attachment, but other ADs may address different components. The exclusions in this AD may not relieve the operator from other ADs that impose similar inspections. Additionally, the directive interacts with the aircraft’s type certificate data sheet and any supplementary type certificates if modifications have been installed. For aircraft imported from other jurisdictions, there may be an interaction with the state of design’s airworthiness requirements; CASA accepts the FAA’s determination of the safe condition, which is typical for US-type-certificated aircraft. Finally, the directive interacts with private contracts, such as lease agreements, which typically require the lessee to comply with all applicable ADs. Failure to comply could constitute a default under the lease.

AD/B737/224 has been amended four times since its original issuance. The original AD became effective on 13 May 2004. It was issued in response to reports of a significant number of aircraft with damaged horizontal stabiliser attachment bolts and pins, including wear, pits and corrosion damage. The first amendment (Amdt 1) became effective on 25 November 2004. The background explains that the original FAA AD inadvertently omitted the phrase “whichever occurs first” in the grace period for the initial compliance time. This omission was corrected by the FAA, and CASA issued Amdt 1 to align with the corrected FAA AD. The effect was to fix the calculation of the initial inspection threshold. The second amendment (Amdt 2) became effective on 9 June 2005. It allowed operators to use Boeing SB 737-55-1086 Revision 1 (dated 16 December 2004) or a later revision approved by the FAA, as an alternative method of compliance with the original SB requirement. This provided more flexibility as the manufacturer improved the inspection instructions. The third amendment (Amdt 3) does not have a specific effective date stated in the source text, but it is referenced as having accepted FAA-approved alternative methods of compliance and introduced the exclusion of aircraft with line number 3535 and higher from the requirement to conduct the inspections required by paragraphs (a) and (b) of the FAA AD for both rear spar attach pins and front spar attach bolts. The fourth amendment (Amdt 4), which is the subject of this deep dive, became effective on 11 December 2014. It further refined the exclusions: aircraft with line numbers from 1725 to 3534 inclusive that are fitted with front spar attach bolts P/N 180A1611-3 or 180A1611-5 are excluded from the inspection, repair or replacement requirements for those bolts. The amendment history shows a trend of narrowing the scope of mandatory action as the manufacturer and regulators gained more understanding of which aircraft actually exhibited the unsafe condition. Each amendment was driven by FAA actions, indicating close harmonisation between CASA and the FAA on this issue.

The text of this airworthiness directive does not reference any litigation, court proceedings, or administrative appeals. Airworthiness directives of this kind are typically technical regulatory instruments issued without judicial involvement. They arise from engineering investigations and incident reports, not from lawsuits. In Australia, an affected operator could theoretically seek merits review of a CASA decision to issue or amend an AD under the Administrative Appeals Tribunal Act, or judicial review under the Administrative Decisions (Judicial Review) Act 1977. However, such challenges are uncommon for ADs because the safety rationale is usually compelling and the regulator has broad discretion. No evidence appears in the source text that AD/B737/224 was ever contested in court. The directive itself is a consolidation and further amendment of a directive originally issued in 2004, suggesting that it has been in force for over a decade without reported legal challenge. The exclusions introduced by Amendments 3 and 4 may have been influenced by industry representations, but the source does not mention any formal dispute resolution. The absence of litigation history is consistent with the nature of these instruments: they are routinised safety measures that operators generally comply with to maintain airworthiness and avoid enforcement. Practitioners should note that while this specific directive has no listed litigation, any enforcement action for non-compliance could give rise to legal proceedings in which the validity or interpretation of the AD might be tested. That said, the directive is drafted in clear mandatory terms with explicit incorporation of the FAA AD, leaving little room for interpretive ambiguity. The litigation history relevant to this directive is thus nil from the source material, and practitioners should confine their advice to the text of the AD and the underlying regulations.

Several aspects of this directive can trap the unwary. First, the compliance time is calculated from the effective date of the FAA AD (13 May 2004), not from the effective date of this amendment (11 December 2014). That means for any aircraft that was not previously subject to the full requirement and is now brought back in (or for new aircraft entering the fleet), the initial inspection interval must be determined as if the clock started in 2004. That may require complex calculation of hours or cycles since that date. Second, the term “line number” is not defined in the directive itself. Operators may not have ready access to Boeing’s line number for each aircraft, especially if records are incomplete. The line number is a production sequence identifier, not the registration mark or serial number. Third, the part number for the front spar attach bolts must be verified. If an aircraft has been modified and the bolts replaced with different part numbers, the exclusion for line numbers 1725-3534 may not apply. Fourth, the directive says compliance with Boeing SB 737-55-1086 Revision 1 or a later FAA-approved revision is “taken to be compliance” with the original SB. That means if an operator uses an older revision or an unauthorised revision, they may not satisfy the requirement. Fifth, the directive does not explicitly state the inspection method or detailed steps; it incorporates by reference the FAA AD and SBs. Failure to obtain and follow those documents correctly is non-compliance. Sixth, the exclusions for line number 3535 and higher apply only to this specific requirement; other ADs may still require inspections of the same components. Seventh, the directive applies to all Boeing 737-600, -700, -700C, -800 and -900 series aircraft regardless of age or registration. Even if an aircraft has never been in Australia, once on the Australian register it falls under this AD. Eighth, the amendment history shows that earlier versions may have imposed different requirements; operators relying on previous compliance actions must ensure they meet the latest version, or risk a gap. Ninth, the directive is signed by a Delegate of CASA, not the Minister; its legal validity depends on proper delegation under the Act. Any challenge on that ground would require examining the delegation instrument. Tenth, the directive does not contain a sunset or review date, so it remains in force indefinitely unless superseded or withdrawn.

Compliance follows a step-by-step process based on the aircraft’s configuration. Step one: determine applicability. Confirm the aircraft is a Boeing 737-600, -700, -700C, -800 or -900 series. If not, this directive does not apply. Step two: obtain the essential documents. These are FAA AD 2004-05-19 (as corrected in the Federal Register of 13 April 2004), Boeing SB 737-55-1086 (any FAA-approved revision, at least Revision 1), and Boeing SB 737-55-1093 (Original Issue). These documents contain the detailed inspection procedures, thresholds, intervals, and repair/replacement methods. Step three: identify the aircraft’s line number. This may require consulting the manufacturer’s delivery records, the aircraft’s technical log, or an online database. Step four: if the line number is 3535 or higher, no further action is required under this specific directive. However, maintain a record showing that the exclusion applies. Step five: if the line number is below 1725, the full requirements of the FAA AD apply. Comply with the inspection intervals as originally set, using the approved SBs. Step six: if the line number is 1725 to 3534 inclusive, identify the part number of the front spar attach bolts. Check the aircraft parts list or physically inspect if necessary. If the bolts are P/N 180A1611-3 or 180A1611-5, then only the rear spar attach pin inspections are required (using SB 737-55-1093 and the relevant parts of SB 737-55-1086 for the rear pins). If the bolts are any other part number, both front spar attach bolt and rear spar attach pin inspections are required. Step seven: schedule the initial inspection at the time specified in the FAA AD, calculated from 13 May 2004 using flight cycles, hours or calendar time as per the AD. Then set the repetitive inspection intervals. Step eight: perform the inspections in accordance with the approved SBs. If damage is found, repair or replace using the procedures and parts specified in the SBs. Step nine: document all actions in the aircraft’s continuing airworthiness records, referencing this AD number and the specific paragraphs complied with. Keep copies of the SBs and FAA AD used. Step ten: monitor for further amendments. The directive may be superseded or amended again; operators should check the CASA website or the Airworthiness Directive database regularly. Also ensure that any approved alternative methods of compliance (AMOCs) from the FAA are recognised by CASA; the directive states CASA accepts FAA-approved AMOCs, but operators should confirm in writing if in doubt. Engage an approved Part 145 maintenance organisation for the physical work, and a Part 42 continuing airworthiness management organisation for oversight if the operator does not manage airworthiness in-house.