Date: 201207
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Mission: Passenger
Flight phase: Parked
Aircraft component: Fuselage Skin
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Situational Awareness
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance
Detector: Person Maintenance
When detected: Aircraft In Service At Gate; Routine Inspection
Result: General Maintenance Action
Contributing factors: Human Factors; Procedure; Manuals
Primary problem: Manuals

Narrative: It has come to my attention that I may have inadvertently used a Structural Repair Manual (SRM) repair that is not applicable to the area that was repaired. I became aware of this a few days after completing the repairs after receiving a company Maintenance Alert. The Maintenance Alert was titled B737-700/800 SRM Allowable Damage and Repairs on Bilge Skins; (posted July 2012). The Maintenance Alert noted the following: the SRM's for both -700/-800 fleet types have always included a statement in SRM 53-30-01 and 53-60-01 stating that the Allowable Damage and Section-53 General Repairs may not be used in the bilge skin; however; the area comprising the bilge skin was not clearly defined. In the most recent Boeing SRM Revisions; the SRM's have been revised to clearly define the term Bilge Skin: Section 43 Bilge Skin: Frame Stations (FS) FS 360-540; Stringer (S) S-24L to S-24RSection 46 Bilge Skin: FS 727-887; S-23L to S-23R. I realized after reading this Maintenance Alert that I had used the SRM 53-00-01 Allowable Damage limits and repair on our company B737-800 aircraft two days prior to the Maintenance Alert being posted. So the damage locations stuck in my mind and I realized that the repair locations that I had repaired fell into the locations in the Maintenance Alert. Due to the [fuselage] 'bilge areas' not being clearly defined and to the placement of the 'Note' in the SRM 53-60-01 on Page 101; Figure 101; 'REFER TO SRM 53-60-70 FOR THE FAIRING SKIN ALLOWABLE DAMAGE. THERE IS NO ALLOWABLE DAMAGE FOR BILGE SKIN IN THE STRUCTURAL REPAIR MANUAL AT THIS TIME'. At the time I made the repairs on the B737-800 aircraft; I was not aware of the exact location of the 'bilge area' and since the above underlined sentence had followed the 'Note' for the fairing skin; I thought the bilge area was only the area located under the fairings. As of writing this letter I have contacted the following people to make them aware of this: my Lead Mechanic; my Duty Manager; my Station Manager; Company Maintenance Control and FAA/NTSB Company Liaison notified me that he would contact Company Lead Engineer to see what would need to be done in this matter.

Callback: Reporter stated there were several scratches in the external skin of the lower fuselage aft of the Main Landing Gear (MLG) wheel wells; under the aft pit. He had burnished out the fuselage skin to remove the sharp edges of the scratches. But after reading the company Maintenance Alert two days later regarding the bilge skin areas and discussing with Engineering; a decision was made to have Inspection perform an Ultra Sound Non-Destructive Testing (NDT) for skin thickness loss and scratch damage depth. A SRM Skin Repair was required for one of the scratched areas.



Synopsis: A Line Mechanic reports he decided to revisit the lower fuselage external skin scratches that he had previously burnished out on a B737-800 aircraft after reading a company Maintenance Alert about revisions in the Structural Repair Manual (SRM) that clarified skin areas considered to be 'bilge skin'.