What happened
The flight crew operating Continental Flight 1223, a Boeing 727, reported experiencing severe floor vibration between passenger rows 21 and 26 while cruising at flight level 310. The aircraft was maintaining a speed of 0.8 Mach at the time the issue was detected. In response to the abnormal condition, the crew reduced airspeed to 280 knots indicated airspeed and descended to flight level 240. The flight continued to Los Angeles without further difficulty.
The investigation
Following the landing, maintenance personnel inspected the aircraft structure. Inspectors identified a continuous crack measuring 15 1/2 inches along a row of rivets at the floor line within the right wheel well. This damage was located in close proximity to two previous repairs on the panel. The repair history indicated that the original fix involved installing a double plate over the affected region.
Findings
The structural analysis determined that the cracks appeared to be the result of fatigue. The location and nature of the damage were consistent with stress concentrations associated with the earlier repair work.