What happened
On January 9, 2016, a Boeing 737-400 was climbing through approximately 6,000 feet when the aircraft exceeded its maximum allowable speed. The incident was identified through Flight Data Monitoring (FDM) analysis. During the transition to level flight, the autothrottle increased engine power, setting the speed near Vmo. Although the crew subsequently disconnected the autothrole and reduced power, the aircraft's speed reached a maximum of 343 knots, exceeding the structural limit of 340 knots.
The overspeed condition lasted approximately one to two seconds, with a speed excursion of only one to two knots above the limit. Flight data recorder analysis showed that the engines maintained a constant N1 of 91.5% from the moment flaps were retracted until the speed limit was breached. The aircraft's speed increased linearly from 210 knots to 343 knots over a period of 80 seconds.
The investigation
The investigation examined flight data recorder parameters, maintenance records, and crew performance. The investigation established that the climb initially began in TO/GA mode. At 3,800 feet, the autopilot and autothrottle were transitioned to LVLCHG mode. At 5,100 feet, the crew switched to Vertical Speed (V/S) mode.
Technical inspections of the aircraft following the landing revealed no mechanical defects or malfunctions. A review of the previous seven days of flight data and maintenance logs from the operator's technical base showed no prior instances of similar speed excursions or autothrottle issues. The investigation also noted that the aircraft type lacks an automatic system to prevent exceeding maximum speed limits.
Findings
- The primary cause of the incident was incorrect speed selection by the crew, followed by a failure to monitor the increasing airspeed and a delayed corrective response.
- Contributing factors included the frequent switching between different autopilot and autothrottle modes in a short timeframe.
- High workload during the post-takeoff phase contributed to the lack of effective monitoring.
- The use of a high Cost Index (CI) for performance calculations meant the target speed was very close to the maximum allowable speed, making the aircraft susceptible to overspeeding even with minimal turbulence.
- The crew was found to be fully qualified with all required flight documentation up to date.