Date: 201304
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: APU Pneumatic System & Ducting
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Situational Awareness; Communication Breakdown; Confusion
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Person Maintenance
When detected: Aircraft In Service At Gate
Result: General Maintenance Action
Contributing factors: MEL; Logbook Entry; Human Factors; Company Policy; Aircraft
Primary problem: MEL

Narrative: Maintenance Manager advised me that Company Engineering has approved a deferral for cracked APU [pneumatic] manifolds on B757s. Manager cited the recently issued B757 MEL 49-X 'APU Pneumatic Functions' as the authority for deferring a cracked APU duct. This is contrary to everything I've ever been told. It's my understanding that there is a strict interpretation of the MELs. It's unclear to me how a Chapter-36 component can be deferred in Chapter-49 of the MEL without being referenced. Supposedly Company Engineering has left a trail of [information] to support the use of this vague MEL to defer cracked APU ducts. We [were] warned at ZZZ approximately two years ago not to use a loose interpretation of the MEL and the scenario they used was deferring an APU [pneumatic] manifold with the APU Bleed MEL. I see no distinction in using the 'APU Pneu Functions' over the 'APU Bleed' in a cracked manifold situation.  If [our] air carrier wants a deferral from the spat of recent cracked ducts then they need precise verbiage rather than parceling with words.

Callback: Reporter stated the B757 APU Pneumatic Bleed Air manifolds have a history of cracking at the weld bead on the pneumatic duct sections. There are five sections of pneumatic manifold ducts from the APU in the tail area; connected along the lower fuselage sidewall to the forward bulkhead of the aft cargo pit. Most of the cracked ducts are found in that area. The APU duct then crosses over to a center line position and runs forward along the fuselage keel beam between the Left and Right Main Landing Gear (MLG) wheel well and continues forward between the Left and Right Air Conditioning packs. Reporter stated that his air carrier has noted the B757 reliability was being hurt by the number of APU manifold pneumatic leaks due to cracks at the weld bead. The cracks and resulting pneumatic leaks are a No-Go item unless the duct is replaced or the APU pneumatic bleed air is basically deactivated per the MEL. The potential for an explosive blowout of a pressurized duct in flight is the concern.  When the APU Bleed Valve is open and pressurizing the pneumatic manifold; one can hear a distinct muffled whistle noise below the Cabin floor deck if a manifold duct is cracked.  Since the B757 is no longer in production; new replacement manifold ducts are not available; so cracked ducts have to be re-welded over and over again.  Line Stations like his cannot perform a re-weld on the ducts due to a lack of facilities to satisfy an FAA Airworthiness Directive (AD) that requires APU pneumatic ducts to be pressure checked in water after repairs.    Reporter stated they previously used MEL Chapter-36 Pneumatic System Bleed Valve for deferral of the APU Pneumatic System.  But the FAA's strict enforcement of the MEL noted that the APU Bleed Valve was not the problem; the cracked pneumatic manifolds were the problem. So his Company Engineering wrote an addition to their MEL APU Chapter-49 to include pneumatic functions. But using the deferral under Chapter-49 makes no reference to a cracked duct; neither on the cockpit placard or in the Logbook.  Also; the APU Bleed Valve still has to be verified in the 'Closed' position; not operated; similar to MEL Chapter-36; to prevent pneumatic pressurization by the APU; of a deferred cracked manifold duct. He does not know why his Engineering group did not specifically identify APU pneumatic duct in their add-on language to Chapter-49.



Synopsis: A Line Mechanic reports his concerns for maintaining precise language in their MEL Manual when deferring cracked APU pneumatic manifold ducts on their B757 aircraft.  Wording added to their MEL for APU Chapter-49 still does not specifically reference an APU cracked pneumatic duct.