What happened
During the climbout phase of flight, the crew of the aircraft experienced an airframe vibration. At the time the vibration began, all airplane controls, gauges, readouts, and synoptic pages remained normal. The crew contacted their maintenance base to attempt troubleshooting, but they were unable to identify the source of the issue.
The vibration continued at a constant level until the aircraft leveled off at 7,000 feet during an approach. At that altitude, the crew felt a bump, after which the vibration ceased entirely. The airplane landed and taxied to the gate without further incident; the occupants were not injured.
The investigation
A post-flight inspection revealed that the number 1 exhaust fairing was missing and that a dent had been caused to the left, aft fuselage. Investigators found that all 30 bolts that normally secure the exhaust nozzle assembly to the exhaust frame were missing without a trace.
Records indicated that seven months prior to this event, the tailpipe had been found loose. During that previous maintenance, ten bolts were replaced and the remaining bolts were re-torqued. The most recent inspection of the aircraft had been performed approximately two weeks earlier, or 105 hours before the incident.
While company personnel had previously identified cracked bolts on exhaust flanges on other engines, laboratory examination of used bolts from a different engine showed no anomalies.