Date: 201405
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Horizontal Stabilizer
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Inspector
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Communication Breakdown; Situational Awareness
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Other Post Maintenance
Result: General Maintenance Action
Contributing factors: Aircraft; Procedure; Incorrect / Not Installed / Unavailable Part; Human Factors
Primary problem: Human Factors

Narrative: A B737-700 aircraft; Aircraft X; was in ZZZ for a Heavy C-1 (HC1) Check. I; Inspector X; was assigned to Heavy Tail Inspection. I found two each cracks on the Left-Hand (L/H) Horizontal Stabilizer lower inspar skin. Mechanics started the repair per Structural Repair Manual (SRM) XX-XX-01-2R-11; Repair 11.  Per the SRM repair; Table 201 calls out the Repair Doubler to use 2024-T3 and Table 202 calls out for the Doubler to be .090' thousandths of an inch thick. As the night went on I stopped by to verify metal type and thickness and saw the X-colored Tag (Form) showing [2023]-T3; .090' thick on the 16'x16' inch piece on metal the mechanics had. I told them to carry on with the repair and parts fabrication (fab). During the fab of the repair parts; it was time to turn over [the work] to dayshift. At that time; I and the mechanics signed-off the steps on the (Form). The graveyard shift ended and the mechanics went home. I stayed and worked the aircraft. Dayshift mechanics took over the repair part fabrication and continued to install. At one point during the dayshift; the mechanics working the repair asked if the Doubler was in fact [2024]-T3; I responded; 'Yes.'  I saw the X-colored tag (form) during the night shift.  Mechanic X stated that it didn't appear correct; but with the information I told him; he continued on with the repair. With all solid fasteners installed and waiting on an Engineering Authorization (E/A) from Engineering for two blind fasteners to be installed; I then turned the aircraft over. [Several shifts later]; the dayshift Mechanic X that installed the repair on L/H Horiz Stab lower inspar skin on Aircraft X; needed some metal from the Sheet Metal (S/M) Shop. He then noticed that a sheet of [2024]-0 was mislabeled as [2024]-T3. He brought this to my attention and we realized that this was the same metal used on Aircraft X. At this time; we called the company Hot Line and the aircraft was grounded. [Recommend] better Vendor Quality Control (Q/C).


Second narrative: I; Mechanic X; took a [Maintenance] Turnover from the grave shift B/C Check [Crew] on Aircraft X; a B737-700 aircraft for a Left-Hand (L/H) Horizontal Stab lower skin crack. The mechanics that worked it [on grave shift] had the Repair Doubler fabricated; all I had to do was countersink the fastener holes and install the repair. After installation; I noticed that the Repair Doubler had dimpled when I shot it on. I found this to be odd. I brought this to the attention of the Inspector X; he assured me that he had checked the [X-colored] Tag on the metal and it was [2024]-T3. [Several shifts later] while working on another aircraft; I noticed a piece of [2024]-O material in the Sheet Metal (S/M) Shop that was mislabeled [2024]-T3. At that time I realized that the wrong metal was installed on Aircraft X on the L/H Horizontal Stab lower skin. I then informed Inspector X that I had worked with on Aircraft X; and we contacted Maintenance Control and had the aircraft taken out of service. Upon further investigation; we found the Certifications (Certs) from the metal Vendor were wrong. The Certs said the metal was [2024]-T3; when in fact it was [2024]-O. Dayshift. [Recommend] while selecting metal for a repair; verify Tag and stencil on metal match.

Second callback: Reporter stated the Structural Repair Manual (SRM) requires 2024-T3 ALCLAD Aluminum material for a Repair Doubler fabrication to be installed over cracks found on the L/H Horizontal Stab (H/S) lower inspar skin. The inspar area is the upper and lower sections of the H/S between the Forward and Aft spars and runs outboard from the fuselage to the Horizontal Stab tip. The leading and trailing edges of the H/S are not part of the inspar area.Reporter stated that cracks are being found on the Horizontal Stab lower skin surfaces of many B737-700 aircraft. A repair video showed how the H/S skin tends to flex and bow similar to a shock wave moving across a surface.  The metal skin is Chem-milled and approximately .071' thousandths of an inch thick. The cracks found on the B737-700 aircraft he was working were located about two and a half feet outboard of the fuselage. The Repair Doubler they installed that covers the area of the cracks requires .090' thick 2024-T3 material; but is considered a Temporary Repair requiring Repetative Inspections every 50;000 cycles. A larger repair modification can also be installed using even thicker metal that covers the H/S skin from the fuselage outboard approximately 200' inches; between the H/S Forward and Aft spars.Reporter stated he originally had a gut feeling something didn't appear right with the Repair Doubler metal he was working; but the Material Tag had all the correct information and the Inspector seemed confident they were using 2024-T3 aluminum. The -0 section of 2024-0 stenciling was not visible on the part that was cut for the Repair Doubler.  When he later noticed in the Sheet Metal Shop that a much softer aluminum sheet of 2024-0 had been mis-labeled as 2024-T3; he realized his earlier gut feeling had been correct.  In hindsight; he wished he had pursued further verification by performing a Conductivity Test of the Repair Doubler metal which would have clearly determined the hardness of the aluminum material.Reporter stated the mis-labeled aluminum metal had gone through five levels of verification that included the Vendor certifying the material; his Air Carrier Receiving Inspector; the Mechanic cutting the part; the Inspector involved with the Repair Doubler installation and himself.  The aircraft was grounded and than ferried back to their Maintenance Base for a 'Redo' of the repair. His Air Carrier has changed their Receiving Inspection protocall by adding another Inspector to verify the paperwork matches the material and the material matches the paperwork. He has 23 years in Maintenance and prefers to work Sheet Metal repairs. He was trained by an older Sheet Metal Mechanic who was very knowledgeable and had taught him a lot about sheet metal skills.

Synopsis: An Aircraft Maintenance Inspector; a Parts Receiving Inspector; and an Aircraft Maintenance Technician (AMT) report about a Structural Repair Doubler that was inadvertently fabricated and installed from a mislabeled sheet of 2024-0 aluminum instead of the harder 2024-T3 ALCLAD aluminum material. Doublers are being installed over the Horizontal Stab skin cracks found on many B737-700 aircraft.