Swiss RJ1H near Zurich on Jul 2nd 2012, loss of cabin pressure

The Schweizerische Untersuchungsstelle (SUST) released their final report concluding the probable cause of the serious incident was:

The serious incident is attributable to the fact that after a defect in the right bleed air system the cabin pressurisation system failed completely at FL 244 (7437 m) and the crew were forced to make an emergency descent to a safe altitude.

The fact that airconditioning pack 1 had been deactivated prior to the flight contributed to the serious incident.

The SUST reported that during the climb through FL244, cleared to climb to FL300, the "R ZONE HI TEMP" indication illuminated together with a number of other indications. The indication prompted the automatic shut down of the right hand air conditioning system, the left hand air conditioning system had been inoperative and the aircraft been dispatched under minimum equipment list requirements. Although the outflow valves was completely closed the cabin "climbed" at about 2000 feet per minute. The crew radioed they had a pressurization problem and requested a descent, the air traffic controller however had not understood the callsign and radioed "station calling, say again", the crew radioed their call again however using an incomplete callsign, immediately afterwards the frequency was blocked by other transmissions with two aircraft. In the meantime the crew donned their oxygen masks, the "CABIN HI ALT" warning activated at FL259, the crew radioed "Mayday Mayday Mayday, emergency descent to FL100", due to the oxygen masks the transmission was not understood and the controller queried again. The crew set the emergency transponder code, the controller radioed: "Swiss four seven three foxtrot we have big difficulties to understand you, so you may descend to flight level one hundred, if you have to."

The aircraft descended at an average rate of descent of 3600 feet per minute, the passenger oxygen masks were automatically released when the cabin "climbed" through 13250 feet, all passengers donned their masks. The SUST stated: "As a result of a chemical reaction, the oxygen generators heated up and a visible bluish mist was produced. For a few passengers this caused some insecurity and slight eye irritation. According to statements from the cabin crew there was no panic."

A bleed air leak at the engine pylon #4 (outboard right hand) had triggered the temperature warning and associated shutdown of the bleed air and air conditioning system. The leak was caused by a broken clamp.

The broken clamp causing the air leak (Photo: SUST):