AD/CESSNA 750/2 Amdt 1 - Chafing of APU Fuel Tube Assembly

The central concept is an airworthiness directive (AD) , a legally enforceable instrument issued under the Civil Aviation Act 1988 and associated regulations. An AD mandates actions to ensure an aircraft continues to meet its type design standards and remains safe to operate. Non-compliance renders the aircraft un-airworthy and prohibits its operation. The specific problem addressed in this AD is chafing of the APU fuel tube assembly. The APU (auxiliary power unit) is a small turbine engine in the tail cone that provides electrical and pneumatic power when the main engines are off. The fuel tube in that area, according to the background, has been found to interfere with surrounding components: elevator flight control cables, hydraulic lines, and high temperature bleed air couplings. Vibration and relative movement cause the fuel tube to rub against these items, wearing through the tube wall. That leads to a fuel leak in a confined space where hot surfaces, electrical arcs, and bleed air couplings create ignition sources. Critically, the tail cone area has no fire detection or extinguishing system, so a small leak can escalate into an uncontrollable fire. The AD uses two types of compliance measures: repetitive inspections and terminating action. The repetitive inspection must be performed at intervals not exceeding 250 flight hours or 3 months, whichever comes first. The inspection, detailed in Alert Service Letter ASL750-49-09 Revision 2, is a "detailed inspection" meaning a visual check with appropriate lighting and possibly mirrors to verify clearance and detect signs of chafing. If chafing is found, corrective action is required, following the same service letter. The terminating action is a one-time modification that changes the fuel line routing, adds new clamps and fairleads, and installs updated parts per SB 750-49-12 Revision 1. Once accomplished, the repetitive inspections cease. For certain serial numbers (-0001 to -0031 and -0033 to -0107) there is a mandatory replacement of the original APU fuel tube with a redesigned part (P/N 6756605-23) before the first inspection. This indicates that early production aircraft had a tube design more susceptible to interference.

The primary persons affected are the registered owners and operators of Cessna 750 (Citation X) aeroplanes bearing manufacturer serial numbers -0001 through -0256 inclusive. That amounts to 256 airframes, many of which are in service with corporate flight departments, charter operators, and private owners. The amendment adds serials -0241 to -0256, so owners of those 16 aircraft now face the full set of requirements for the first time, and must comply before further flight after the effective date (15 March 2007). For the 240 aircraft already subject to the original directive, the amendment does not impose new initial requirements but provides a new option: performing the terminating action to stop the cycle of repetitive inspections. However, the amendment also renders the repetitive inspection requirements non-terminable without that modification, heightening the cost of non-compliance over the long term. Maintenance organisations that hold CASA approvals (e.g. Part 145 or CAR 30 certificates) must have access to the service documents and be competent to perform the detailed inspection, replacement, and modification. Personnel must be familiar with the ASL and SBs and with the specific access procedures for the tail cone area. Parts suppliers are affected insofar as the replacement tube (P/N 6756605-23) and the modification kit (implied by SB 750-49-12) must be available. If parts are on backorder or are expensive, operators face extended downtime or pressure to accept used parts. Insurers will consider compliance with ADs when assessing risk; failure to comply may void coverage for fire damage. Future buyers of these aeroplanes will need confirmation that the AD actions are recorded correctly, particularly whether the terminating action has been embodied, as that affects the residual value and continuing compliance costs. The AD also indirectly affects passengers and crew by aiming to improve safety, though this imposes a cost on operators that is ultimately reflected in ticket prices or charter rates. For businesses operating these aircraft, the compliance cost is a direct burden on productivity and profitability. The inspection interval (250 hours or 3 months) is relatively frequent, and for a typical business jet flying 400-500 hours per year, that means two to three inspections annually, each requiring hangar access, labour, and possibly parts. The terminating action, while a larger one-time expense (parts, labour, testing), eliminates these recurring costs.

The AD imposes several mandatory duties on the operator and the person responsible for the aircraft's continuing airworthiness. The first duty is to perform a detailed inspection of the APU fuel tube assembly for clearance and chafing, using the procedures in Cessna Alert Service Letter ASL750-49-09 Revision 2 or later FAA approved revision. The inspection must be carried out within the compliance times specified: 25 flight hours or 48 days (whichever first) for serials -0001 through -0240 after the original AD date; for serials -0241 through -0256, before further flight after the effective date of this amendment. Thereafter, the inspection must be repeated at intervals not exceeding 250 flight hours or 3 months, whichever occurs first. The second duty is, if chafing is detected during any inspection, to carry out corrective action per the same service letter. The third duty, specifically for aircraft with serial numbers -0001 through -0031 and -0033 through -0107, is to replace the APU fuel tube with a new tube, part number 6756605-23, in accordance with Cessna Service Bulletin 750-49-05 Revision 1 (or later revision). This replacement must be completed within 25 flight hours or 48 days after the effective date of the AD, but before carrying out the inspection. That sequencing is important: the replacement occurs first, then the inspection must also be done (unless the termination action is already accomplished). The operator also has rights. The primary right is to perform the terminating action described in Cessna SB 750-49-12 Revision 1 (or later revision), which modifies the fuel line by installing new lines, fairleads, and clamping configurations. Once this work is certified as complete, the repetitive inspection requirements cease. The operator may use later FAA-approved revisions of the service documents, which gives flexibility if Cessna publishes improved instructions. The operator also has the right to apply to CASA for an alternative means of compliance (AMOC) under the relevant regulation, though the AD does not mention this explicitly. Operationally, the operator has the right to operate the aircraft only if it is in compliance with this AD; non-compliance makes the aircraft unairworthy and any flight in that condition may breach the Civil Aviation Act. The duty to comply rests ultimately with the registered operator, who may delegate to a maintenance organisation, but the legal responsibility cannot be contractually shifted to avoid liability.

Failure to comply with an airworthiness directive is a breach of the Civil Aviation Act 1988 and the Civil Aviation Regulations (or the Civil Aviation Safety Regulations, depending on the aircraft's certification basis). Under the Act, operating an aircraft that does not meet applicable airworthiness directives is unlawful. The potential consequences are serious. Civil penalties apply: at the time of this AD (2007), the maximum penalty for an individual was $13,200 per offence, and for a body corporate $66,000. Those amounts are indexed and likely higher now. Each day of continuing non-compliance can constitute a separate offence. Criminal prosecutions are possible if the non-compliance is reckless or results in danger to life or property; penalties can include imprisonment. In practice, CASA enforcement action often begins with a verbal or written direction to ground the aircraft, followed by a show cause notice and possibly suspension or cancellation of the aircraft's Certificate of Airworthiness. The aircraft cannot be lawfully operated while the AD requirements are outstanding. If an operator attempts to fly a non-compliant aircraft, CASA's enforcement officers (or the Australian Federal Police) can detain the aircraft. Maintenance organisations that certify aircraft as airworthy when they are not in compliance with ADs risk their approval being suspended or revoked, and individual certifying staff may face disciplinary action including suspension of their licences. Insurance consequences are also significant: a policy typically excludes cover for loss arising from non-compliance with a mandatory directive. If an aircraft suffers a fire caused by an unaddressed APU fuel leak, the insurer may deny the claim, leaving the operator liable for the full hull value and any third-party damages. The AD itself does not prescribe penalties; rather, the background legislation does. The risk of enforcement is heightened for this AD because of the safety rationale: an undetected chafed fuel tube can lead to an unannounced catastrophic fire with no on-board means of detection. CASA and the Cessna service bulletins treat this as a high-risk item. The strict compliance deadline of "before further flight" for newly added serial numbers indicates that any delay in compliance is considered unacceptable.

This airworthiness directive sits within a layered regulatory framework. The primary enabling legislation is the Civil Aviation Act 1988 (Cth). Under that Act, the Civil Aviation Safety Authority issues Airworthiness Directives as delegated legislation, typically under the Civil Aviation Regulations 1988 (specifically CAR 37 or 39) or the Civil Aviation Safety Regulations 1998 (Part 39). The AD also interacts with CASA's Continuing Airworthiness Manual and the Maintenance Control Manual requirements for aircraft registered in Australia. The AD cross-references FAA AD 2006-24-01 (dated 3 November 2006). This is common for US type-certificated aircraft: CASA harmonises with FAA directives, but the Australian AD is a separate legal instrument. Compliance with the FAA AD alone does not guarantee compliance with the CASA AD if there are differences; here the CASA AD supersedes an earlier AD 2005-09-01 and the FAA AD is referenced only for background. However, the service documents (ASL750-49-09, SB 750-49-05, SB 750-49-12) are FAA-approved revision controlled, meaning the operator must use the revision specified or later FAA-approved revisions. This ties the AD to FAA approval processes. If the FAA issues a later revision of these documents, the CASA AD accommodates it without needing a separate amendment. The AD also interacts with contractual obligations under aircraft lease agreements, purchase agreements, and insurance policies. Leases typically require the lessee to maintain the aircraft in compliance with all applicable ADs. A failure to perform the terminating action could be a default. Cross-border operations are affected: an Australian-registered Cessna 750 flying into the United States must comply with FAA ADs as well; since the CASA AD mirrors the FAA AD for the most part, dual compliance is achieved. However, an aircraft imported into Australia from the US may have had only FAA compliance; the operator must ensure that the CASA AD requirements are also satisfied. The AD also interfaces with the Cessna maintenance manual and the Aircraft Flight Manual. Any deviation from the approved maintenance data would require CASA approval. Finally, work health and safety laws apply to maintenance personnel entering the tail cone area; they must follow safe working practices when handling fuel tubes, inspecting in confined spaces, and working near flight controls and hydraulic lines.

This document is Amendment 1 to AD/CESSNA 750/2. The original AD/CESSNA 750/2 became effective on 25 May 2005. That original directive applied to serial numbers -0001 through -0240 and required the initial and repetitive inspections, corrective action if chafing found, and for certain serial numbers the replacement of the APU fuel tube. It did not include a terminating action. This Amendment 1, signed on 2 February 2007 and effective 15 March 2007, makes three principal changes. First, the applicability is extended from serials -0001 through -0240 to -0001 through -0256, adding 16 aircraft built between serial numbers -0241 and -0256. For those aircraft, the initial compliance requirement is "before further flight," stricter than the 25 hours/48 days for earlier aircraft. Second, it introduces a terminating action in the form of modification per Cessna SB 750-49-12 Revision 1. This means operators who choose to perform that modification will no longer be bound by the recurrent inspection interval. The terminating action is not mandatory; it is an alternative route to compliance. Third, it supersedes FAA AD 2005-09-01 (which seems to have been the earlier referenced directive for the original CASA AD) and now references FAA AD 2006-24-01. The amendment does not alter the compliance times for aircraft already covered by the original AD; those remain unchanged. There is no indication of any further amendments to this AD; the source is a standalone document. However, because the AD allows later FAA-approved revisions of the service documents, subsequent FAA revision history may effectively update compliance methods without a formal CASA amendment. Operators should monitor the CASA AD website for any future amendments.

The source document does not record any litigation related to AD/CESSNA 750/2 or its predecessor. This is unsurprising. Airworthiness directives are rarely the subject of judicial review in Australia. The Civil Aviation Act provides limited merits review of CASA decisions, but a directive is legislative in character, not a reviewable decision. Any challenge would likely be via an application for judicial review under the Administrative Decisions (Judicial Review) Act 1977 or the High Court's original jurisdiction. To succeed, an applicant would need to show that the delegate acted beyond power, that the directive was procedurally flawed, or that it was so unreasonable that no reasonable delegate could have issued it. Grounds related to economic impact alone would not suffice. There are no publicly reported judgments concerning this specific AD. This does not mean no enforcement proceedings have occurred; CASA may have issued infringement notices or commenced prosecutions for non-compliance, but those would be administrative or criminal matters, not litigation about the validity of the AD. The absence of litigation suggests that operators have generally complied or at least not challenged the directive in court. The safety rationale (risk of fire with no suppression) is strong, making a legal challenge difficult. It is also possible that disputes over costs led to private settlement or alternative means of compliance, but those would not be public. For the practising lawyer or compliance professional, the key point is that the AD is presumed valid until set aside, and the safer course is to comply and then, if aggrieved, seek an AMOC or informal guidance from CASA rather than litigate.

Several features of this AD can trip up operators or compliance staff if not read carefully. The most important is the serial number gap: serial number -0032 is excluded from the requirement to replace the APU fuel tube in Requirement 3 (the replacement applies to -0001 through -0031 and -0033 through -0107). Aircraft -0032 exists but falls outside that paragraph; it still requires the inspection and, if chafing found, corrective action, but not the mandatory tube replacement. Operators of -0032 must confirm that their aircraft is correctly identified. Second, the sequencing of replacement and inspection for required serials: the replacement must be done "within 25 flight hours or 48 days ... after the effective date of this AD, but before carrying out inspection per Requirement 1 of this AD." That means the very first inspection after the AD takes effect must be preceded by the tube replacement. A maintenance facility that performs the inspection first and then replaces the tube will be out of sequence and technically in breach until the replacement is done. Third, the compliance time for newly added serials (-0241 to -0256) is "before further flight after the effective date of this AD" for the initial inspection and then repetitively at 250 hours/3 months. That is extremely tight: as soon as the AD becomes effective on 15 March 2007, any aircraft in those serials must be inspected before it flies again. Operators with aircraft based overseas or in remote locations may face logistical problems. Fourth, the terminating action is not mandatory. Many operators might continue with repetitive inspections, unaware that the modification is available and that it eliminates recurring costs. Over the life of the aircraft, the cumulative cost of inspections and potential corrective actions could exceed the one-time modification cost. The AD does not inform the operator of that cost comparison; it merely lists the option. Fifth, the service documents (ASL and SBs) are referenced as "or later FAA approved revisions". This means the operator must check whether the FAA has approved later revisions after the dates stated. Using an outdated revision could result in incorrect procedures, potentially missing chafing or introducing new issues. Sixth, the AD cross-references FAA AD 2006-24-01. Operators may mistakenly believe that compliance with the FAA AD alone suffices for CASA. It does not; the CASA AD is the legal instrument for Australian registered aircraft, and it may have different compliance times or scope. Finally, the liability for non-compliance rests with the registered operator, not the maintenance organisation. If a maintenance shop fails to perform the replacement or inspection correctly, the operator still bears the legal risk. Operators should verify that their maintenance providers are following the correct revision of the service documents and are authorised by CASA for that work.

Compliance with AD/CESSNA 750/2 Amdt 1 requires a systematic approach. The responsible person (registered operator or continuing airworthiness manager) must first identify the aircraft's serial number and confirm it falls within -0001 through -0256. If it does, the operator must obtain the required service documents: Cessna Alert Service Letter ASL750-49-09 Revision 2 (10 March 2005) or a later FAA approved revision; Cessna Service Bulletin 750-49-05 Revision 1 (17 January 2000) or later revision (for serials requiring replacement); and Cessna Service Bulletin 750-49-12 Revision 1 (3 August 2006) or later revision (for the terminating action). These documents are available from Cessna/Cessna Service Centers or through the FAA website. The next step is to determine the applicable compliance timeline. For aircraft with serial numbers -0241 to -0256, the initial detailed inspection must be performed before further flight after 15 March 2007. For all others (-0001 to -0240), the initial inspection was due within 25 hours or 48 days after 25 May 2005 (original AD); if not already done, it is overdue and the aircraft is grounded. However, this amendment does not reset that initial compliance; the operator must determine if the initial inspection was ever carried out. If the aircraft has been operating under the original AD, the repetitive inspection schedule (every 250 hours or 3 months) continues unless the terminating action has been performed. For operators of serials -0001 to -0031 and -0033 to -0107, the mandatory replacement of the APU fuel tube (per SB 750-49-05) must be accomplished within 25 hours or 48 days after the AD effective date but before the first inspection. That means the operator must schedule the replacement immediately, even if the inspection is not yet due. The replacement and inspection are separate tasks; after replacement, the inspection must still be done. After the replacement, the operator may choose to also perform the terminating action (SB 750-49-12) at any time. If the terminating action is completed, no further repetitive inspections are required. The terminating action is a modification of the fuel line installation, not a simple parts swap; it requires kit, fairleads, clamps, and adherence to the Accomplishment Instructions. The operator must ensure that the maintenance provider has the competency and approval to perform such structural modification on a transport category aeroplane. After each compliance action, the certifying maintenance engineer must make the appropriate entry in the aircraft maintenance records (the aircraft logbook or maintenance release) detailing the AD reference, the serial number, the action performed, the relevant service document revision, and the date. For the terminating action, it is crucial to clearly state "Terminating action per AD/CESSNA 750/2 Amdt 1 accomplished" so that future maintainers know the repetitive inspection requirement is lifted. Finally, the operator should monitor the CASA AD website and the FAA website for any further amendments or revisions to the service documents. If a later revision of ASL or SB is approved, the operator may use it, but must update internal tracking accordingly. For any difficulties (e.g., parts unavailability, AOG situations), the operator should apply to CASA for an alternative means of compliance or an extension, but such applications should be made well before the compliance deadline. Flying without compliance exposes the operator to enforcement and insurance voidance.