AD/HS 125/179 - UAM100 Current Limiters

Airworthiness Directive (AD). This is a mandatory instrument issued under the Civil Aviation Safety Regulations (CASR). Compliance is not optional; failure to meet the requirements renders the aircraft unairworthy. ADs are typically adopted from foreign airworthiness authorities (here the FAA AD 2006‑12‑20) with any necessary modifications for Australian conditions. This one is a “directive” in the sense of a legislative instrument that creates binding obligations.

Current limiter. An electrical device that limits current to protect wiring and equipment. Here, P/N UAM100 is the primary bus feed for the PE, PS1 and PS2 busses. The background explains that certain units have opened within two to four hours after installation, including in flight. The specified resistance range (0.46-0.56 milliohms) is derived from the service bulletin. The term “milliohm” is one‑thousandth of an ohm; the precision required (0.01 milliohm tolerance) necessitates accurate measurement equipment and technique.

Picking tag purchase order. A manufacturing batch identifier. Two suspect POs (4501760749 and 4501743706) were identified by the supplier as containing sub‑specification current limiters. The directive prohibits installation of parts from these batches. This concept is important because it creates an alternative compliance pathway: if records can conclusively trace a current limiter back to a non‑suspect PO, the physical measurement may be avoided.

Service bulletin. Raytheon SB 24‑3793 (dated May 2006) is the manufacturer’s recommended corrective action. The AD incorporates the accomplishment instructions of that bulletin by reference. The bulletin includes a table of applicable current limiters, the measurement procedure, and the report fax form.

Time in service. Counting from the effective date, either 30 calendar days or 25 hours of flight time, whichever comes first. This dual trigger is typical for ADs to ensure that aircraft that fly infrequently are not given indefinite grace, while high‑utilisation aircraft are caught by the hours limit.

Before further flight. For Requirement 2 (replacement of out‑of‑spec limiters), the aircraft must not be flown again until the replacement is completed. This is the strictest possible compliance deadline and maximises safety but also imposes immediate disruption if a measurement is taken and fails.

Records review in lieu. A concession that reduces compliance burden where the operator has complete and accurate records. The records must “conclusively” establish the dates and the PO. The adverb is important: mere probability is insufficient. If any ambiguity exists (e.g., the PO tag is missing or illegible), the measurement must be performed.

Effective date. The AD takes effect on 3 July 2006. All compliance deadlines run from this date, except that measurements performed before the effective date can be used for the reporting requirement, provided the report is submitted within 10 days after the effective date. This allows operators who acted proactively to be credited.

Replacement part provenance. The directive goes beyond requiring a new part with correct resistance; it also mandates that the new part not be from a suspect PO. This imposes an additional verification step on the supply chain, even for new parts. The burden falls on both the installer and the parts distributor.

The directive applies directly to owners and operators of the listed aeroplane models registered in Australia. The applicability clause covers: Model HS.125 series 700A and 700B aeroplanes on which Raytheon Modification 252885 or BAe 125 Service Bulletin SB24‑239‑2885 has been incorporated; all Model BAe.125 series 800A (including C‑29A and U‑125 variants), 800B, 1000A and 1000B aeroplanes; and all Model Hawker 800 (including U‑125A) and 1000 aeroplanes, together with Model Hawker 800XP aeroplanes with serial numbers 1 through 258768 inclusive.

The scope is narrow: only Australian‑registered aircraft of those types are covered. Fleet operators (charter, corporate, government) with these aeroplanes are directly affected. Individual owners of a single BAe 125 or Hawker 800 series jet must also comply.

Maintenance organisations (Part 145 or LAME‑approved) will be engaged to perform the resistance measurement, the records review, or the replacement. They bear the liability for correct compliance and must ensure the new part is from an approved source. The directive may also affect suppliers and distributors of UAM100 current limiters, as they need to manage stock and provide traceability of picking tag POs.

Raytheon Aircraft Company (the manufacturer) is affected insofar as it receives the mandatory reports. The directive does not impose obligations on Raytheon, but it creates a data‑gathering pathway for the manufacturer to monitor the fleet.

CASA is the enforcing authority. It will have to process any reports, monitor compliance, and potentially take enforcement action. The Australian Transport Safety Bureau may receive information if failures occur.

The directive also has an extraterritorial effect: although it is an Australian AD, the underlying FAA AD (2006‑12‑20) applies to US‑registered aircraft. Australian operators who also operate abroad may encounter the same requirement in other jurisdictions. This alignment reduces cross‑border compliance complexity, but the Australian AD adds the reporting requirement directly to Raytheon, which the FAA AD may already require (the AD text says “paragraph (f) of the corresponding FAA AD” - this Australian AD effectively incorporates that requirement and adds the recipient address).

Economic impact falls on a small pool of operators. The cost of measurement is relatively low (a few hours of labour and equipment use), but the cost of replacement parts and the risk of grounding if a part fails the measurement are higher. The availability of the records review option can reduce this cost significantly for operators with good paper trails.

The core duties are:

1. Duty to measure or review. Within 30 days or 25 hours after the effective date, the operator must measure the resistance of the current limiters for the PE, PS1 and PS2 busses per SB 24‑3793, or perform a conclusive records review. This is a time‑sensitive obligation that cannot be deferred.

2. Duty to replace. If the measured resistance is outside 0.46-0.56 milliohms, the part must be replaced before further flight. The replacement part must have a correct resistance and come from a non‑suspect picking tag PO.

3. Duty to report. Within 10 days after the measurement (or within 10 days after the effective date if measurements were done before), the operator must submit the Service Bulletin/Kit Drawing Report Fax to Raytheon at the specified address. The report must include results, owner/operator names, registration and serial numbers, and the number of landings and flight hours.

4. Duty to verify replacement parts. From the effective date, no UAM100 current limiter may be installed as a replacement unless the PO is conclusively determined to be non‑suspect, or the resistance is measured and correct. This duty applies every time a part is installed, not just for the initial compliance.

Rights include:

• Right to use records review. Instead of performing the measurement, the operator may rely on maintenance records to show that (a) no inspection or replacement occurred between 1 February 2006 and the effective date, and (b) the current limiters are not from suspect POs. This is a significant right because it saves direct labour and potential discovery of out‑of‑range parts.

• Right to use new parts from the measured batch. A replacement part that has been resistance‑tested and found correct can be installed even if its PO is unknown (provided it does not come from a suspect PO). The directive gives two alternative pathways to qualify a part.

• Right to rely on the report form provided in the service bulletin. The directive does not impose a unique format; it incorporates the form from SB 24‑3793, which reduces administrative burden.

• Right to submit reports by fax. The directive specifies a fax number as the sole means of reporting. This may be outdated, but it grants the right to use that method.

The duties are non‑delegable in the sense that the operator remains ultimately responsible for compliance, even if maintenance is contracted out. However, the operator can authorise a maintenance organisation to perform the work and file the report. The directive does not require CASA notification; it only requires notification to the manufacturer.

The records review alternative carries the risk that if an error is later discovered (e.g., the records were not conclusive), the operator may be found in breach. The directive does not provide for any waiver or extension. The compliance deadlines are absolute.

The directive itself does not specify penalties; it is a legislative instrument that creates obligations under the Civil Aviation Safety Regulations (CASR) 1998. The primary enforcement mechanism is through CASR Part 21 (certification and airworthiness) and the general provisions of the Civil Aviation Act 1988. Under CASR 21.184, an operator must not fly an aircraft unless it complies with all applicable airworthiness directives. Breach is an offence punishable by a maximum penalty of 50 penalty units for an individual (currently $12,540 as of 2023) and 250 penalty units for a body corporate ($62,700). CASA may also issue a direction to ground the aircraft until compliance is achieved, seize the aircraft’s certificate of airworthiness, or impose conditions on its operation.

Enforcement is risk‑based. CASA may proactively audit compliance through spot checks of aircraft maintenance records. Non‑compliance may be detected during scheduled maintenance inspections, through reports from maintenance organisations, or as a result of incident investigations. The reporting line to Raytheon creates a paper trail that CASA could potentially access, though the directive does not explicitly grant CASA access to those reports.

The “before further flight” requirement for out‑of‑range parts is the strictest: an aircraft flown with a known out‑of‑spec current limiter would be in breach even if no incident occurs. This can attract enforcement action immediately.

The directive does not provide an appeal mechanism within itself. An operator aggrieved by a CASA decision (such as a refusal to approve an alternative means of compliance) may apply to the Administrative Appeals Tribunal for merits review under the Civil Aviation Act. However, the terms of the directive are prescriptive and leave little room for discretion.

Penalties are not limited to financial fines. Continued non‑compliance can lead to suspension or cancellation of an airworthiness certificate, effectively grounding the aircraft indefinitely. This economic consequence usually outweighs the cost of compliance.

The directive’s adoption of an FAA AD as its technical basis means that any action taken by the FAA to suspend or cancel US airworthiness certification would indirectly affect Australian aircraft if they operate under a bilateral agreement. However, enforcement is solely CASA’s responsibility.

There is no private right of action; passengers or third parties cannot sue for breach of the directive itself, but the existence of the AD would be evidence of standard of care in a tort claim for negligence.

AD/HS 125/179 explicitly references FAA AD 2006‑12‑20 (Amdt 39‑14646). The Australian AD is issued under CASA’s own authority but mirrors the technical requirements of the FAA AD. For operators of aircraft registered in both Australia and the United States, compliance with one likely satisfies the other, provided the reporting requirement to Raytheon is satisfied for the Australian AD. The Australian AD adds the requirement to send the report to Raytheon’s fax number, which may duplicate the FAA requirement but does not conflict.

The directive is a “legislative instrument” for the purposes of the Legislation Act 2003. It is registered on the Federal Register of Legislation. It sits within the hierarchy of CASR Part 21 and must be read together with the appropriate regulations covering airworthiness. For example, the general obligation to maintain an aircraft in an airworthy condition under CASR 42.215 is fleshed out by compliance with ADs.

The directive does not interact directly with the Work Health and Safety Act, but an employer’s duty to provide a safe system of work may reinforce the obligation to comply with ADs. However, this is an indirect effect.

The directive applies concurrently with any applicable service bulletins from the manufacturer. Complying with the service bulletin alone is not sufficient if it is merely recommendatory; the AD makes it mandatory. Conversely, if the service bulletin is superseded or revised, the AD may still required the original version unless CASA amends the AD.

There is no interaction with privacy law of significance, except that the report requires disclosure of owner and operator names and aircraft identifiers, which is routine for safety reporting and exempted from the Privacy Act 1988 to the extent it is required by law.

The directive does not reference the Australian Consumer Law; however, if the suspect current limiters were sold as part of a contract for maintenance, the purchaser may have rights against the supplier for breach of a guarantee that parts are of acceptable quality. The AD indirectly evidences that the parts were defective from the supplier.

The directive does not interact with taxation laws in any specific way, but the costs of compliance (measurement, replacement parts, reporting) are likely deductible as business expenses or capital costs depending on the circumstances.

The directive may interact with insurance policies. Failure to comply with an AD could give an insurer grounds to deny a claim if non‑compliance contributed to a loss. Conversely, compliance demonstrates due diligence and may strengthen a claim.

The directive does not override contractual obligations, but it may give either party a right to terminate or vary a maintenance contract if compliance imposes unforeseen costs. However, those are private law matters outside the directive’s scope.

The directive as provided does not contain any amendment history. It is an original directive issued on 22 June 2006, effective 3 July 2006. There is no indication that it supersedes or amends any earlier AD on the same subject. It is a stand‑alone instrument.

Under CASA’s process, ADs are numbered in a series (here AD/HS 125/179). The number suggests it is the 179th AD for the HS 125 series aeroplane type. Earlier ADs may have dealt with other airworthiness issues, but this one specifically addresses UAM100 current limiters and does not amend or revoke any previous ADs. The record shows an FAA AD number (2006‑12‑20), but that is the US equivalent, not an amendment of an Australian AD.

CASA may issue later amendments to this AD if, for example, new batches of suspect parts are discovered, revised service bulletins are released, or compliance times need extension. However, no such amendments appear in the source.

In the absence of official amendment history, the reader should assume that the AD remains in force as originally issued unless expressly revoked or superseded. The Civil Aviation Safety Regulations require that a register of ADs be maintained, and any amendment would be published separately.

For the purpose of this deep dive, we note that the instrument is not part of a series of incremental changes; it is a single‑event directive triggered by an immediate safety issue. This means operators who have complied once are not required to repeat the measurement unless a new AD is issued.

The directive’s effect is time‑limited in one sense: once the 30‑day/25‑hour compliance period has passed, the initial measurement or records review must have been completed. The ongoing duty to install only non‑suspect parts continues indefinitely.

If future experience shows that the suspect batch did not reach certain aircraft, or that the manufacturing defect was limited to a specific production run, CASA may amend the AD to narrow applicability or extend deadlines. But no such amendment is documented here.

The absence of amendment history is significant for compliance: an operator must refer to the AD as originally published and cannot rely on any unofficial consolidations.

The source document contains no reference to any litigation related to this AD. There are no known Australian court decisions interpreting AD/HS 125/179. Given the narrow scope and the limited number of affected aircraft, it is unlikely that this directive has been the subject of legal challenge in the Administrative Appeals Tribunal or a court.

That said, airworthiness directives in general have been the subject of occasional judicial review or merits review in Australia. Operators have sometimes challenged CASA’s decision to issue an AD on grounds such as unreasonableness, lack of evidence, or failure to consult. However, the typical pattern is compliance under protest, with subsequent lobbying for amendment rather than litigation, given the high cost and urgency of the safety issue.

The directive’s adoption of the FAA AD’s technical rationale could be challenged if the underlying FAA determination were found to be flawed, but Australian courts would likely give considerable deference to the FAA’s expertise.

If an operator were to be prosecuted for non‑compliance, the court would consider whether the AD was validly made and whether the operator had a reasonable excuse. The strict liability nature of AD compliance means that a defence of “reasonable mistake of fact” might be available (e.g., if records suggested the part was from a non‑suspect PO but an error occurred), but the burden would be on the operator.

There is no recorded instance of the AD being struck down for procedural deficiency. The directive appears to have been signed by a delegate of CASA on 22 June 2006, which is regular.

Because the directive is a legislative instrument, it is subject to disallowance by either house of the Australian Parliament within 15 sitting days of tabling. There is no record of any disallowance motion for this AD.

In summary, the litigation history is blank. This is not unusual for a specific, non‑controversial AD. The risk of litigation is low, but the possibility exists for any enforcement action.

For the analyst, the lack of litigation should not be taken as an absence of controversy: the compliance burden may still be significant. But the legal risk is primarily from CASA enforcement, not from lawsuits.

Conclusive records review is strict. The alternative to measurement requires that a records review can “determine conclusively” that the current limiters are not from suspect POs. If the picking tag is not recorded, or if the records show a gap, the review is not conclusive, and the measurement must be performed. Operators with incomplete records cannot take the shortcut.

Time in service vs calendar days. The compliance trigger is “30 days or 25 hours time in service, whichever occurs first.” An aircraft that flies frequently may reach 25 hours before 30 days pass, giving a shorter window. An operator must track both, not just calendar days.

Before further flight. Requirement 2 (replacement of out‑of‑spec parts) must be done before the next flight after measurement. If the measurement is performed on a line at the gate and reveals a bad part, the aircraft cannot be dispatched. This can cause unexpected AOG (Aircraft on Ground) situations.

Reporting deadline pre‑dating effective date measurements. If the operator measured the current limiters before 3 July 2006, they still must submit the report within 10 days after the effective date (i.e., by 13 July 2006). This may catch operators who thought they were done.

Only one reporting channel. The report must be faxed to the Raytheon number. No email, postal mail, or electronic portal is offered. Operators without a fax machine must find one.

No extension or exemption process stated. The directive does not provide a mechanism to request an extension of the 30‑day/25‑hour compliance time. In practice, CASA can issue an alternative means of compliance, but that requires a separate application. The AD itself offers no relief.

Applicability includes Hawker 800XP serial numbers 1 through 258768. The cutoff at 258768 means any aircraft with a serial number above that is not covered. Operators must verify their serial number exactly.

Only the specific models listed. The directive does not apply to other models of the BAe 125 series (e.g., earlier 600 series or the later Hawker 850) unless also listed by name. Operators must check applicability carefully.

Replacement part sourcing. New parts must not be from the suspect purchase orders. This imposes a due diligence obligation on the purchaser, even if buying from a distributor who claims the part is new. If the PO cannot be determined, the part cannot be installed.

Resistance tolerance. The acceptable range is 0.46-0.56 milliohms inclusive. Parts slightly outside (e.g., 0.45 or 0.57) are not allowed. Measurement equipment must be suitably calibrated.

No penalty immunity for records review. If the records review is relied upon and later found to be incomplete, the operator is non‑compliant from the original deadline, even if they were not at fault.

Step 1: Determine applicability. Check the aeroplane model and, where applicable, serial number (Hawker 800XP only). If the aircraft is not listed, no action is required. If it is listed, proceed.

Step 2: Check if the records review alternative can be used. The operator must locate maintenance records covering:

• The date of the most recent 24‑month “F” or “F7” inspection of current limiters.
• The date of the most recent replacement of current limiters.
• The picking tag PO of each current limiter installed.

From these records, determine:

• That no inspection or replacement occurred between 1 February 2006 and 3 July 2006.
• That the current limiters are not from PO 4501760749 or PO 4501743706.

If both criteria are conclusively established, step 3 can be skipped. If not, proceed to step 3.

Step 3: Perform resistance measurement. Using the accomplishment instructions of Raytheon SB 24‑3793, dated May 2006, measure the resistance of each current limiter listed in Table 1 of the bulletin. The measurement must be accurate within 0.01 milliohms. If any reading is below 0.46 milliohms or above 0.56 milliohms, that current limiter must be replaced before further flight.

Step 4: Replace any out‑of‑spec parts. Obtain a new current limiter P/N UAM100. Verify that its picking tag PO is not from the two suspect POs, OR measure its resistance to confirm it is within 0.46-0.56 milliohms (if the PO cannot be determined). Install per the service bulletin.

Step 5: Complete the Service Bulletin/Kit Drawing Report Fax. This includes results of measurements (or a statement that records review was used), owner/operator names, registration and serial numbers, landings and flight hours. Fax it to Raytheon Aircraft Company, Product Support Department, at (316) 676‑3400.

• If measurements were done after the effective date: within 10 days after inspection.
• If measurements were done before the effective date: within 10 days after 3 July 2006.

Step 6: Maintain records. Keep copies of the measurement records, replacement part traceability, and the report submitted. These should be retained as part of the aircraft’s permanent maintenance history.

Step 7: Ensure future part installs comply. Any time a UAM100 current limiter is installed after 3 July 2006, verify that either the PO is known and non‑suspect, or the part has been resistance‑measured and found correct. Document this in the aircraft records.

Ongoing obligations. The AD does not require recurrent testing unless a new AD is issued. However, operators should monitor for any superseding AD or revocation.

Risk management. If the records review is borderline, err on the side of performing the measurement to avoid later non‑compliance. If measurement equipment is not available, arrange it early; the 30‑day/25‑hour window is tight.

Professional help. If the aircraft is operated under a maintenance organisation, that organisation should be engaged to confirm compliance. The operator should ensure the organisation receives a copy of this AD.