What happened
On 9 November 2008, an Embraer 145, registration SE-DZB, was operating a scheduled commercial flight from Gothenburg/Landvetter to Prague. While cruising at FL370, the aircraft's warning system indicated a failure in the first air conditioning pack, which shut down automatically. As the crew began following emergency checklists, the second air conditioning pack also failed and shut down.
This secondary failure led to a rapid loss of cabin pressure. The crew donned oxygen masks and initiated an emergency descent. During the descent, the aircraft was transferred between air traffic control sectors. Due to poor radio communication quality and a misunderstanding during the handover, the second controller believed the aircraft intended to level off at FL150. However, the crew continued their descent toward FL100. This resulted in a loss of separation with an ATR72, which passed within 1.27 nm of the descending aircraft with an altitude difference of only 800 feet. The TCAS RA (Resolution Advisory) was activated, though the crew of the other aircraft maintained visual contact with the descending plane.
The investigation
The Swedish Accident Investigation Board (SHK) examined the technical state of the air conditioning system and the coordination between air traffic controllers. The investigation looked into the electrical connections of the air conditioning units and the effectiveness of the emergency checklists used by the crew. The board also reviewed the communication protocols and the handover process between the upper and lower airspace controllers.
Findings
- The primary cause of the first incident was deficiencies in the air computing unit (CPU) regarding automatic shutdown logic, which allowed the failure of one pack to trigger the shutdown of the second.
- The second incident, involving the loss of separation, was caused by a lack of coordination between air traffic controllers.
- Poor radio communication quality, exacerbated by the crew wearing oxygen masks, contributed to the misunderstanding of the aircraft's descent profile.
- The aircraft's emergency descent checklist lacked specific instructions to notify air traffic control of the descent and to set the transponder to the emergency code 7700.
- The air traffic management system (Eurocat) lacked a visual tool for transferring specific flight information between controllers, forcing them to rely on verbal handovers.
Safety action
SHK issued several recommendations to EASA and the Swedish Transport Agency, including:
- Ensuring air conditioning systems operate independently to prevent unjustified shutdowns.
- Investigating the implementation of quick-selection features for the 7700 emergency code on transponders.
- Updating air traffic controller training to assume that aircraft in an emergency descent will descend to FL100 or lower.
- Updating aircraft checklists to include mandatory steps for notifying ATC and setting the emergency transponder code.