What aircraft was involved and where did it happen?

The accident on 2009-12-12 involved a aircraft, registration PTWSI, at VÁRZEA GRANDE, MT, BR.

What was the probable cause of the accident?

The door was not correctly closed and secured prior to takeoff, a condition that went undetected due to a potentially misadjusted microswitch providing false 'locked' indications to the cockpit.

In-flight door separation on Beechcraft B 300

A Beechcraft B 300 lost its cabin door during the climb phase of a passenger flight, resulting in minor aircraft damage but no injuries.

What happened

On December 12, 2009, a Beechcraft B 300, registration PT-WSI, departed from Marechal Rondon Airport (SBCY) in Mato Grosso, Brazil, bound for Fazenda Simarelli (SWSG) for a passenger transport mission. During the initial climb, at an altitude of approximately 300 feet and a speed of 120 knots, a loud noise was heard, and the aircraft door became detached from the fuselage, falling near the Cuiabá River.

The pilot immediately recognized the emergency and declared an emergency with Cuiabá Tower. The pilot maintained control of the aircraft and performed an immediate landing on the opposite runway of the departure airport. All six occupants—the pilot and five passengers—remained uninjured. The aircraft sustained minor damage, while the door was completely destroyed, including a broken hinge.

The investigation

CENIPA investigators examined the door mechanism and the aircraft's electrical systems. The investigation confirmed that the aircraft's weight and balance were within limits and that the pilot was properly licensed and experienced. The aircraft's maintenance records were up to date, with the last annual inspection having been performed only three days prior to the event.

While the pilot had performed the required 'AIRSTAIR DOOR CHECK' before takeoff and the 'UNLOCKED DOOR' warning light remained extinguished, investigators found a small dent on a microswitch responsible for the door status indication. This suggested the switch might have been improperly adjusted, positioned too close to the locking hooks. This misalignment could have sent a false signal to the cockpit, indicating the door was locked when it was not.

Furthermore, the investigation noted that the door had been closed by a passenger (who was a pilot but not rated for this specific aircraft type) and visually checked by the aircraft owner. Investigators concluded that the door likely was not properly secured, and the faulty microswitch prevented the crew from detecting the error.

Findings

The primary contributing factor was the improper closure of the door, which allowed it to open under aerodynamic loads during the climb.
A secondary contributing factor was the potential misadjustment of the microswitch, which provided false positive indication of a locked door to the flight deck.
The involvement of unauthorized personnel in performing the door closure procedure increased the risk of operational error.