What happened
On August 15, 1997, at approximately 19:25Z, a Cessna 550 Citation II, registration PT-LML, was operating a flight from Porto Alegre to Rio de Janeiro. The aircraft was being operated by Riana Táxi Aéreo with two crew members on board. While cruising at FL 330, the aircraft began exhibiting variations in heading and level flight, prompting inquiries from air traffic control. Shortly thereafter, witnesses on the ground reported seeing an explosion in the sky near Cocal do Sul, Santa Catarina. The aircraft disintegrated in flight, with the largest section of the fuselage impacting the ground in a lateral attitude. The accident resulted in two fatalities, as both pilots died at the scene. The impact also caused partial destruction to a local residence.
The investigation
CENIPA investigators examined the wreckage and conducted flight simulations in Wichita, Kansas, to reconstruct the accident sequence. The investigation focused on the aircraft's pressurization system and the condition of the Air Cycle Machine (ACM). Investigators found that the pressurization selector was in the "OFF" position and that passenger oxygen masks had deployed, which occurs automatically above FL 130 during depressurization. Notably, the pilots' oxygen masks were found behind their seats, indicating they were not being used.
Technical analysis of the ACM revealed a failure in the air bearing system, where excessive friction caused the shaft to bind. While this failure does not cause immediate depressurization, it leads to significant temperature increases and noise. Flight simulations suggested two primary scenarios: either the crew suffered from hypoxia due to a slow decompression, leading to unconsciousness and subsequent loss of control, or the crew attempted an emergency descent due to the heat and noise from the ACM failure, inadvertently entering an uncontrolled spiral dive.
Findings
- A failure in the Air Cycle Machine (ACM) caused increased cabin temperature and noise.
- The crew may have experienced hypoxia due to a slow decompression, leading to a loss of consciousness and loss of aircraft control.
- The pressurization selector was found in the "OFF" position, which may have been a result of the crew attempting to manage the cabin environment or an error during pre-takeoff checks.
- The aircraft entered an uncontrolled descent and disintegrated due to structural overstress during a high-speed spiral dive.
- The crew did not utilize their onboard oxygen masks during the event.