Date: 201901
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Mission: Test Flight / Demonstration
Flight phase: Climb
Aircraft component: Wing Access Panel
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Maintenance
When detected: Other Post Flight
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: This was a [Test Flight] on Aircraft X after heavy maintenance. As the Pilot Flying; after takeoff; I noticed the airplane required left aileron input in order to maintain heading. Continued the climb to 15;000 and leveled at 250 kias (Knots Indicated Airspeed). Aircraft had slight vibration noted by maintenance observers in the passenger cabin as well as myself and the First Officer. We started performing the normal trim checks per the FCF profile and noted as the flaps were being extended to each setting the aircraft required more and more left rudder trim in order to fly straight and the vibration became considerably worse. We elected to terminate the trim checks; cleaned up the aircraft; and decided to return to ZZZ and land using flaps 15. During post flight maintenance observers noted a panel on the right wing approximately 3'x2' was missing near the fuselage. We contacted ZZZ Tower to let them know we had lost a panel off of the aircraft and they said they would send airport operations out to check the runway. Approximately 30 minutes later maintenance informed us that ZZZ Airport Operations had found the panel near the approach end of Runway XXL; which was the runway we used for departure. I suspect the panel departed the aircraft as we began our takeoff roll.


Second narrative: Aircraft was on a [Test Flight] following Heavy Check. After takeoff; the Captain/PF noted that the aircraft was requiring about 2.0 units left wing down aileron control wheel input to maintain wings level thru aircraft cleanup and climb to 15;000. We climbed out at 250 IAS and established ourselves in a block altitude of 14;000 to 16;000 and starting performing normal Flight Check profile checks. We also noticed that aircraft had a mild tactile vibration thru out the flight deck and cabin during the climb. As the Captain started performing normal Flight Check trim checks; he noticed that the aircraft was requiring greater and greater left rudder trim inputs to achieve trimmed conditions at each new flap position selected; plus the tactile vibration noted in the cockpit and cabin was increasing. By flaps 10 it was evident that the status of the aircraft was unknown and the decision to terminate the Flight Check was made. We cleaned the aircraft up at this time and started back to ZZZ to land. The Captain briefed the use of flaps 15 for the approach and landing; we configured early and the approach and landing were uneventful. Following engine shutdown; while the flight crew was still on the flight deck; maintenance personnel came to the cockpit to tell us that we had a panel missing on top of the right wing. The panel's location adjacent to the fuselage structure made it impossible for our tech crew chiefs cabin observers to see from the cabin during the flight. Following the discovery; we notified the ZZZ airport tower personnel that we had lost a panel from the aircraft and that they needed to do a runway inspection for it. About 30 minutes later the panel was found near the approach end of Runway XX; it evidently had departed during our initial takeoff roll. The event occurred because the panel was not reattached and properly inspected during the heavy check bill of work. Maintenance should review and update sign offs for panel removal and re-installations.


Synopsis: B737 flight crew reported lateral trim problems during a post maintenance check flight. Post flight inspection revealed a missing wing panel.