What happened
On December 11, 1999, a British Aerospace ATP, registration CS-TGM, operated by SATA Air Açores, was performing a scheduled flight from Ponta Delgada to Horta. The flight, carrying 35 people (4 crew and 31 passengers), encountered severe weather conditions characterized by a cold front, heavy rain, and strong turbulence.
During the flight, the aircraft drifted approximately 14 nautical miles to the right of its planned route, likely an attempt to avoid the approaching weather system. While descending to 5,000 feet as authorized by Horta Tower, the crew failed to notice they were no longer over the intended channel between the islands of Pico and São Jorge. The crew did not cross-check the discrepancies between the barometric altimeters and the radio altimeter.
As the aircraft descended through heavy turbulence, the Ground Proximity Warning System (GPWS) activated with "SINK RATE" and "TERRAIN" alerts. The crew attempted an evasive maneuver, but the aircraft' and the left wing struck the slope of Pico da Esperança. The aircraft subsequently entered an inverted state and struck a watercourse approximately 200 meters from the initial impact site. All 35 fatalities occurred during the impact.
The investigation
The GPIAAF investigation examined the flight history, weather reports, aircraft maintenance records, and cockpit voice/flight data. The investigation focused on the crew's navigation techniques, the use of onboard radar, and the lack of cross-checking between altitude instruments. Investigators also reviewed the effectiveness of the GPWS response and the availability of radio navigation aids in the Azores region at the time of the accident.
Findings
- The primary cause of the accident was the failure to maintain safe altitude, resulting in a controlled flight into terrain.
- Navigation was imprecise due to reliance on dead reckoning and the lack of autonomous navigation aids to confirm the aircraft's exact position.
- The crew failed to perform a cross-check of the radio altimeter against the barometric altimeters.
- The crew's use of the onboard radar for checklists inhibited the use of the ground map function, which could have revealed the deviation from the route.
- Severe weather, including heavy rain and turbulence, significantly reduced visibility and contributed to the loss of situational awareness.
- The crew's evasive maneuver was performed too late and included a turn to the left that directed the aircraft toward the terrain.
Safety action
The investigation resulted in several recommendations for SATA Air Açores, including the installation of autonomous navigation equipment, the implementation of improved Standard Operating Procedures (SOPs) for descent and approach, and the reinforcement of training regarding GPWS alert responses.