The United States Federal Aviation Administration (FAA) is issuing an Airworthiness Directive (AD), addressing potential damage to the Boeing 737’s Auxiliary Power Unit (APU) fuel line shroud, located in the aft cargo area. The damage to the fuel line shroud could result in a potential failure and a leak of flammable fuel in an ignition zone.
The FAA issued the AD on March 27, 2023, addressing a potentially unsafe condition where the “placement of the pressure switch wire clamp assembly and its fastener” allowed “interference of the fastener against the APU fuel line shroud, possibly resulting in a damaged APU fuel line shroud and consequent failure of the APU fuel hose, which could result in a flammable fluid leak in an ignition zone”.
This AD is a final rule, and affects 1,919 aircraft registered in the US, ranging from the Boeing 737-600, -700, -700C, -800, -900, and -900ER (NextGeneration (NG) models), as well as certain 737 MAX-8 and 737 MAX-9s. It is effective from May 1, 2023. The FAA estimated that the total estimated costs are $326,230 for all US-based operators for one-time inspections, while on-condition costs, per product, can amount to $33,913.
Preventing potential APU fires
The on-condition costs, which means that operators have to do applicable actions as defined in the Accomplishment Instructions of Boeing Alert Requirements Bulletin 737-38A1072 RB and Boeing Alert Requirements Bulletin 737-38A1073 RB, dated February 25, 2022, are split into repairs and replacements.
The repair of the product should take up to 3 working hours, costing $85 per hour, while the replacement, including re-installation of the part, is split into labor that can take up as much as 300 work hours ($85 per hour) and part-related expenses of up to $8,158.
However, citing Boeing, the FAA stated that “some or all of the costs of this AD may be covered under warranty, thereby reducing the cost impact on affected operators”.
According to the AD, the two bulletins (737-38A1072 RB for the 737 NG and 737-38A1073 RB for the 737 MAX-8 and MAX-9), specify the procedures to inspect the APU fuel line shroud “in the area within 3 inches of the fastener of the pressure switch wire clamp for any damage (any crack or hole, any damage that exposes bare metal on the APU fuel line shroud, and any dent damage found that decreases the outside diameter of the shroud by more than 0.031 inch)”, as well as an inspection of “the pressure switch wire clamp to determine if the fastener of the pressure switch wire clamp is installed with the bolt head on top and the nut on the bottom, and that there is a minimum 1.5 inches of horizontal separation between the fastener of the pressure switch wire clamp and the APU fuel line shroud, and applicable on-condition actions”.
As such, if on-action conditions are required, airlines operating either the 737 NG or the two 737 MAX models will have to:
- Repair the existing APU fuel line shroud with a new or repaired shroud
- Repair any damage to the APU fuel line shroud
- Reinstall the fastener of the pressure switch wire clamp with the bolt head on top and the nut on the bottom
- Reinstall the pressure switch wire clamp assembly, ensuring a minimum of 1.5 inches of horizontal separation between the fastener of the pressure switch wire clamp and the APU fuel line shroud.
The FAA noted that it “has no way of determining the number of aircraft that might need these repairs, replacements, or re-installations.”
Supply chain woes?
With the agency inviting affected parties to comment on the proposed change prior to making the AD into a final rule as per the procedure, some airlines expressed concerns regarding part availability.
Among the stakeholders to comment were American Airlines, Delta Air Lines, Southwest Airlines, Boeing, Aviation Partners Boeing (APB), and the International Air Pilots Association (ALPA). ALPA supported the Notice of Proposed Rulemaking (NPRM) without change.
Delta Air Lines expressed concerns that the “information on MyBoeingFleet Part Page shows zero stock of MPN 346A2201-26 APU Fuel Line Shroud as of October 5, 2022, with one part expected to be available by June 25, 2023.” The carrier will have to inspect and potentially replace the APU fuel line shrouds on around 100 aircraft, indicating that the “lack of availability of this part could severely hamper their ability to comply with this AD in the time period proposed.” And even if Boeing advised the carrier that it has “an adequate number” of the part, they will be allocated by the manufacturer, making it hard to plan for maintenance in the case that a fuel line shroud will have to be replaced.
The FAA argued that when it “assesses unsafe conditions and establishes compliance times for ADs, the agency accounts for the practical aspects associated with compliance, including parts availability.”
Furthermore, Boeing informed the agency that the part supply would be “adequate.” The FAA argued it has no information about a potential shortage of the parts, which is why “the compliance time of three years is adequate for operators to acquire the necessary parts and accomplish the actions required by this AD.”
Southwest Airlines requested that it could use serviceable parts to replace damaged APU fuel line shrouds, namely parts that are not marked “new or repaired.” The FAA agreed, stating that the “service instructions should allow for serviceable parts, not just ‘new or repaired’ shrouds,” defining a serviceable APU fuel line shroud as a part “that has been maintained using methods acceptable to the FAA and identified by the FAA as airworthy.”
Delta Air Lines, American Airlines, and Boeing requested that the AD would be revised to include “service information that Boeing is currently drafting,” as the manufacturer is revising the two requirement bulletins to address the condition of a missing clamp. FAA deemed that while it “acknowledges the commenters’ concerns,” the “critical nature of the identified unsafe condition” makes it inappropriate to delay the final rule until Boeing completes drafting the two RBs.
Delta Air Lines made a further request to complete on-wing repairs to comply with the AD, citing that “the action of removing and re-installing the shroud represents a significant maintenance burden, estimated at up to 300 labor hours.”
The FAA answered that it was “unable to provide allowances for on-wing repairs of the damaged APU fuel line shroud since such repair procedures are not provided in the service information mandated by this AD.”
“However, if an alternative procedure is available that would provide an acceptable level of safety, such a procedure can be requested for FAA approval,” concluded the agency.