What happened
The aircraft was being pre-positioned for a scheduled flight when the incident occurred. During a visual approach to land, the captain (PIC) called for the flaps to be lowered in incremental settings of 16.5, 27, and 40 degrees. The flap handle was moved to the appropriate positions, but reportedly, neither of the pilots looked at the flap indicator.
The PIC stated that during the flare, the aircraft settled and contacted the runway quite hard. He immediately looked down and saw the flap indicator at 16.5 degrees, then noticed the flaps had extended to 40 degrees. The copilot saw the flaps at 30 degrees and moving to full down. During touchdown, the fuselage was damaged around the tail bumper/skid to the lower portion of the pressure bulkhead.
The investigation
On a subsequent flight for repair, the flaps over-ran the up-limit microswitch and then would not extend. The aircraft was equipped with a Model FPC-75 Ground Proximity Warning System in accordance with STC SA1176CE and was wired into the flap selector switch. It would have provided a warning if it had detected flap position discrepancies.
Findings
The primary factor was improper flap extension due to failure to monitor the flap indicator during the approach. This resulted in a hard landing that caused significant structural damage to the aircraft's rear fuselage area.