United Airlines flight UA3872 experienced a tailstrike while taking off from San Francisco International Airport (SFO), San Francisco, California. The flight was a local maintenance test flight. The purpose of the test flight was to validate maintenance conducted after a write-up for abnormal flight deck noises that occurred on a previous flight. According to the flight crew, they conducted a flaps 5, reduced power takeoff from runway 28R. The captain recalled the aircraft accelerated faster than normal and had the propensity to lift off prior to the rotation speed of 127 knots. At rotation speed, the captain began a normal aft pull on the control yoke and the pitch attitude increased faster than expected. The captain just called ?positive rate’ when the crew heard and felt a ?boom.’ As this flight was a test flight, it was not loaded with passengers and cargo to a takeoff weight typical of flights the pilot flying normally flew. The pilot flying rotated at a rate ?normal’ to his experience, however because the accident aircraft was loaded lighter than normal, the normal rotation inputs resulted in an over-rotation. The crew assessed the instruments and controllability and decided to continue their climb to flight level (FL) 190 [19,000 feet] where they conducted the tailstrike checklist. They returned to SFO and conducted a flaps 30 landing with no further incident. Prior to leaving the flight deck after the flight, the first officer received the pitch report for takeoff which showed that tail contact occurred about 12.2 degrees during departure and the actual pitch for this departure reached 15.3 degrees. Inspection of the aircraft showed that the tail strike damaged the chord, web, and stiffeners of the auxiliary power unit firewall bulkhead. Because the damage would adversely affect the structural strength of the bulkhead and require major repair or replacement, the damage was considered substantial. The investigation determined that the maintenance conducted prior to this test flight was not related to the accident. The aircraft was repaired and resumed regular service on 23 March 2024. Probable Cause: The crew’s over-rotation of the aircraft pitch during takeoff which resulted in a tailstrike.