What happened
On October 31, 1996, at approximately 08:26 UTC, a Fokker 100, registration PT-MRK, operated by TAM, was performing a scheduled passenger flight from São Paulo (Congonhas) to Rio de Janeiro (Santos Dumont). Immediately after takeoff, the flight crew encountered an abnormal situation involving the aircraft's auto-throttle system. Witnesses observed the right engine's thrust reverser cycling between the deployed and stowed positions.
In an attempt to correct the anomaly, the crew's actions, combined with the mechanical malfunction, led to a loss of control. The aircraft drifted to the right at a low altitude and low airspeed, eventually developing a steep right bank. The aircraft struck buildings in a residential area of the Jabaquara neighborhood before impacting the ground and catching fire. The accident resulted in 95 fatalities (89 passengers, 6 crew) and 4 fatalities among people on the ground, with the aircraft suffering a total loss.
The investigation
CENIPA investigators examined the aircraft's engines, hydraulic systems, and electrical components. The investigation focused heavily on the thrust reverser mechanism. Testing of the Rolls Royce Tay 650-15 engines showed they were fully operational and producing power at the time of impact. However, the investigation scrutinized the electrical and hydraulic components of the thrust reverser system.
Technical analysis revealed that the secondary lock actuator on the right engine exhibited extremely high electrical resistance (up to 357 ohms, compared to the standard maximum of 0.7 ohms). Furthermore, investigators analyzed the impact of a post-modification electrical configuration. This modification, intended to save energy by de-energizing the stow solenoid, introduced a potential for a dormant failure in the secondary lock relay, which could inhibit cockpit warnings during an uncommanded deployment.
Findings
- The primary contributing factor was the unintended deployment of the thrust reverser during the takeoff phase.
- A dormant failure in the secondary lock relay prevented the crew from receiving an appropriate cockpit warning regarding the unlocked reverser.
- High electrical resistance in the secondary lock actuator contributed to the malfunction.
- The aircraft's electrical configuration following a service bulletin made the system more susceptible to uncommanded deployment and reduced the reliability of the warning systems.
- The crew's attempt to execute corrective actions during a critical phase of flight, following the perceived auto-throttle failure, contributed to the loss of control.