What happened
On 17 November 1999, a Fokker F 100, registration G-BXWF, was conducting a public transport flight after departing from East Midlands Airport. While climbing through approximately 4,000 feet near the 'SAPCO' reporting point, the crew heard a loud bang. Roughly one minute after this noise, smoke began to enter the flight deck.
After confirming with cabin crew that there was no fire in the galley area, the crew suspected an issue with the air-conditioning system, despite normal system indications. The commander issued a 'PAN' call and requested a diversion to Birmingham. During the descent, the density of the smoke fluctuornated, but it did not prevent the crew from seeing or operating the aircraft controls. The aircraft completed an overweight landing at Birmingham without further incident, and passengers were disembarked safely. There were no injuries to the 7 crew members or 81 passengers.
The investigation
Following the landing, inspections of the left and right air-conditioning bays were conducted. While no obvious external damage was visible, the crew noted a significant amount of water discharging from the left pack air cycle motor (ACM) drain when the APU was running.
Upon closer inspection of the left ACM, the coalescer bag from the water separator unit was found to be saturated and blackened, and the inlet screen from the cool air mixing duct had ruptured. The aircraft was subsequently returned to its maintenance base, where the ACM, water separator, and associated ducting were replaced. Subsequent examinations of the faulty components by the manufacturer revealed that the water separator assembly suffered from several defects, including a missing coupling nut, an unserviceable by-pass valve, and corrosion on the condenser shell.