What happened
On 22 November 2018, a Federal Airlines Beechcraft B1900D, registered ZS-PHX, was performing a commercial charter flight from O.R. Tambo International Airport to Londolozi Aerodrome. While climbing through flight level 110, approximately 8.8 nm east of the airport, the crew observed a sudden drop in torque and RPM on the left-hand engine. Shortly thereafter, the left propeller entered an uncommanded feathered position.
The flight crew immediately declared a mayday and requested an emergency return to O.R. Tambo. Because the remaining engine parameters appeared normal, the crew elected to keep the left engine running rather than shutting it down. Following standard single-engine landing procedures, the crew instructed passengers to assume the brace position. The aircraft landed safely on runway 03L approximately 20 minutes after departure, with no injuries and no damage to the aircraft.
The investigation
SACAA AIID investigators examined the maintenance history of the aircraft and found a series of recent interventions regarding the engine's beta system. In the days leading up to the incident, the flight crew had reported various issues, including a leftward yaw during landing and pulling sensations when closing the power lever. Maintenance personnel had performed multiple rigging adjustments on the reverse/beta cable between 16 November and 22 November 2018.
Upon inspection of the reverse/beta system following the incident, investigators discovered that the clearance limits for the beta cable were exceeded by 3-4mm. This excessive clearance allowed the internal piston of the beta valve to be pulled into the open position, which triggered the propeller to feather.
Findings
- The primary cause of the incident was the incorrect rigging of the reverse/beta cable, which caused the beta valve to open and forced the propeller into a feathered state.
- The beta valve exhibited an excessive clearance of 3-4mm.
- Maintenance records indicated that on three separate occasions involving adjustments to the reverse/beta system, no duplicate inspections were performed, violating established maintenance requirements.
- The aircraft's recent maintenance history showed a pattern of repeated attempts to rectify engine yaw issues through cable adjustments without resolving the underlying mechanical cause.