Introduction
On 2 November 2003, a Boeing 777-200B, registration N795UA, operated a scheduled passenger flight from Frankfurt, Germany, to Washington, USA. The aircraft was manufactured in 1997 and powered by two Pratt & Whitney PW4090 turbofan engines. The crew consisted of 15 persons, and there were 261 passengers on board. No injuries were reported, and no damage occurred.
Incident Details
Before departure from Frankfurt, after engine start and pushback, the flight crew received a 'PACK MODE L' message on the Engine Indication and Crew Alerting System (EICAS), indicating a possible fault in the left air conditioning pack. Engineering assistance identified a defective left air cycle machine (ACM), which was deactivated. The defect was deferred per the operator's Minimum Equipment List (MEL), and the aircraft departed about four hours behind schedule.
Approximately one and a half hours into the flight, while at Flight Level 330 about 100 nautical miles north-west of Glasgow, Scotland, the cabin crew reported 'heavy smoke' in the cabin, which was intensifying. The relief pilot, on rest break, also noted a faint smoke smell. The commander and co-pilot decided to divert immediately. The commander instructed the cabin crew to 'fight the fire,' transmitted a MAYDAY call, and air traffic control cleared the aircraft to descend to Flight Level 100 and vectored it toward Glasgow.
During descent, the relief pilot assisted the cabin crew in searching for the smoke source, including removing ceiling panels in the rear galley. No fire or hot spots were found, and the smoke dissipated. After leveling at FL100, a 'PACK MODE R' message appeared, indicating a possible fault in the right air conditioning pack. A very faint smoke presence was noted on the flight deck, but the crew deemed it insufficient to don oxygen masks. Despite failures in both packs, conditioned air and pressurization were maintained.
The approach and landing at Glasgow were uneventful, though the aircraft touched down about 28,000 lbs overweight despite fuel jettisoning. After landing, the aircraft taxied to stand accompanied by the Airport Fire Service. The fire service boarded and confirmed no signs of fire; passengers disembarked without incident.
Investigation Findings
The aircraft interior and air conditioning system were inspected, but no heat damage was found. Engineering investigation revealed the right ACM was defective, and both the left and right ACMs were replaced. The replacement also required changing the respective re-heaters, which had been fitted for over 4,000 hours. Additionally, the right air conditioning pack's 2nd stage turbine bypass valve was suspected seized in the closed position, and the economy cooling valve was not in the commanded position; both were replaced. An overweight landing inspection found no faults.
The failures in the two ACMs were attributed to blockages in the condenser/reheater units in each air conditioning pack. Both units had been fitted since the aircraft was built. These blockages reduced water separation efficiency, allowing excessive moisture to enter the first turbine stage of the ACMs, where air temperature is significantly reduced. Water collected on turbine blades and froze, causing an imbalance in the rotating machinery. This imbalance led to the failure of the ACMs' air bearings, which was the probable source of the smoke in the cabin.
The aircraft manufacturer identified that blockage and deterioration of condenser/reheater units result from particulates, moisture, or oil adhering to inlet surfaces, depending on the operating environment. A Service Letter issued in September 2001 described the potential for blockage and its effect on ACMs, and informed operators of a 'Pack Flow Restriction Test Procedure' to monitor health of the condenser/reheater. This procedure was added to the Aircraft Maintenance Manual and Fault Isolation Manual.
The supplier introduced a modification involving a coating on inlet surfaces of heat exchangers, described in a Service Bulletin published in May 2001. Older units could be returned for coating.
Operator Actions
The operator concluded that both sets of reheater/condenser likely became blocked almost simultaneously due to identical service time. Since the incident, the operator has cleaned all reheater/condenser units on its Boeing 777 fleet, instigated a modification program to add corrosion protection to unit cores (as corrosion can also cause blockage), and established a maintenance program to clean reheater/condensers at 30-month intervals.