What happened
On 20 October 2008, a Merpati Nusantara Airlines Boeing 737-400, registration PK-MDO, was performing a scheduled passenger flight from Sultan Hasanuddin Airport in Makassar, Indonesia, destined for Timika. The aircraft was carrying 172 individuals, including 165 passengers and 7 crew members.
During the takeoff roll, as the aircraft reached approximately 130 knots, the pilot in command heard a loud blast and observed the aircraft veering sharply to the left. The pilot immediately initiated an aborted takeoff by reducing thrust and activating thrust reversers. As the aircraft decelerated, the crew noted the extension of the speed brake and the illumination of the auto-brake disarm light. The pilot managed to maintain the centerline using the rudder until the aircraft reached taxi speed. An airport security officer signaled the aircraft to stop on the runway turning area.
While there were no injuries to any occupants, the aircraft sustained significant damage. The left main landing gear tires were severely damaged, and the left outer landing gear door became detached. Additionally, the right main landing gear tires deflated because the safety fuses melted due to intense heat during the heavy braking process.
The investigation
The investigation focused on the sequence of the tire failures and the condition of the wheel assembly. Investigators determined that the left outer wheel (number one) had deflated prior to the aborted takeoff because four of the sixteen bolts holding the wheel hub halves together had broken. This sudden loss of pressure led to an overload on the adjacent tire (number two), causing it to burst. This chain of events created the directional instability that forced the pilot to abort the takeoff.
Examination of the broken bolts revealed that the failures were caused by premature fatigue. The investigation also looked into the maintenance history of the wheel hubs. It was discovered that the aircraft was using wheel hubs intended for older 737 models, which had a lower load rating than the specialized high gross weight (HGW) hubs designed for the 737-400.
Findings
- The primary cause of the tire failure was the fatigue-induced breakage of four bolts in the left main wheel assembly.
- The fatigue cracks likely originated from corrosion caused by a lack of proper maintenance; specifically, the operator failed to perform required cadmium re-plating on the bolts after ten hub assemblies, as mandated by the Component Maintenance Manual.
- The failure of the first tire led to an immediate overload and subsequent burst of the second tire, causing the aircraft to veer left.
- The intense braking required to stop the aircraft caused the right main wheels to overheat, resulting in melted safety fuses and subsequent deflation.
Safety action
The investigation identified several necessary improvements for the operator's maintenance program, specifically regarding the management of wheel hub tie bolts and the implementation of a life-limit replacement plan to prevent future fatigue-related failures.