What happened
On 26 December 2011, a Boeing 737-500, registration PK-MDT, operated by PT. Merpati Nusantara Airline, was performing a scheduled passenger flight from Manado to Makassar. The flight, carrying 64 individuals, was conducting an ILS approach to runway 13 amidst heavy rain and thunderstorm activity.
During the approach at approximately 550 feet, the Second in Command (SIC), acting as the pilot flying, suggested a go-around due to discomfort with the weather conditions. However, the Pilot in Command (PIC), acting as the pilot monitoring, decided to continue the approach, took control of the aircraft, and manually flew the landing. Upon touchdown, the aircraft experienced a loss of directional control, veering to the right. The aircraft struck a taxiway signboard and runway lights, traveling approximately 375 meters on the runway shoulder before returning to the pavement. There were no injuries to the passengers or crew, though the aircraft's right engine cowl sustained damage.
The investigation
The KNKT examined flight data recorder (FDR) and cockpit voice recorder (CVR) data to reconstruct the landing roll. The investigation focused on the transition of control between the two pilots and the mechanical application of deceleration forces. Investigators analyzed the engine parameters, specifically the N1 values, and the application of braking pressure during the period when the aircraft deviated from the centerline.
Findings
Data from the FDR revealed that the left thrust reverser deployed one second before the right thrust reverser. This timing discrepancy created a period of asymmetric reverse thrust. Furthermore, the N1 of the right engine reached a level 35% higher than the left engine, creating significant asymmetric drag that pulled the aircraft toward the right.
Additionally, the investigation found that 12 seconds after touchdown, the braking was applied asymmetrically, with the left brake pressure averaging 200 psi higher than the right. The pilot's attempt to correct the heading was not in accordance with established Flight Crew Training Manual (FCTM) procedures. The pilot monitoring also failed to alert the pilot flying to the significant differential in engine N1 values during the reverse thrust application.
Safety action
Following the incident, the operator issued an Aviation Safety Notice to flight crews regarding precautions for landing on wet runways at Sultan Hasanuddin International Airport.