History of Flight
On March 17, 2011, at about 1640 UTC, a Colgan Air DeHavilland DHC-8-400Q, registration N339NG, operated as a scheduled passenger flight under 14 Code of Federal Regulations Part 121, from Cleveland Hopkins International Airport to Baltimore-Washington International Airport. While cruising at FL230, the flight crew reported that the left engine oil pressure master warning activated and the left engine oil temperature rapidly increased. The left engine subsequently lost power, and the engine fire warning activated. The crew declared an emergency and diverted to Pittsburgh International Airport, where a single-engine landing was performed without incident. There was minor damage to the airplane, and no injuries to the two flight crew, two cabin crew, and 36 passengers.
Aircraft and Engine Examination
Post-flight inspection revealed a hole in the left engine gas generator case. The airplane had accumulated 551 hours and 551 cycles since new, with no maintenance discrepancies. The left engine and two pneumatic fire/overheat detection elements from the left nacelle were removed for investigation. Examination at a Pratt & Whitney Canada facility in St-Hubert, Quebec, found a burn-through hole approximately 3.5 inches axially by 4.5 inches circumferentially in the gas generator case at the 6 to 7 o'clock position, with charred edges and deposits of molten material indicative of internal burning. The engine oil was heavily contaminated with metal particles. The oil pump was seized, and its drive shaft had separated at the shear point. The No. 1 bearing (power turbine rotor rearward thrust support) was destroyed, with the outer race having failed in fatigue from a crack in a spalled area on the raceway. The root cause of the spalling could not be determined due to the severity of damage.
The No. 4 and No. 5 main bearings showed signs of operation without adequate lubrication, with dry cavities and wear damage. The No. 5 bearing area was extensively thermally damaged: the housing assembly was destroyed, and components were partially consumed or melted. The integral diaphragm section of the gas generator diffuser assembly was entirely consumed, and large sections of the combustion section exit support duct were melted. The thermal damage was localized, with no evidence of high-velocity 360-degree molten particle distribution. Left oil pump teardown revealed that a vane in the scavenge pump element for the No. 2/2.5 bearing cavity had not fully retracted due to metal contamination, striking the rotor and arresting pump rotation, leading to torsional overload of the driveshaft and loss of oil supply.
Fire and Detection Systems
Both left nacelle fire suppression bottles were discharged. After landing, airport fire and rescue personnel confirmed no fire was present in the nacelle. The left nacelle fire warning remained active until aircraft power was shut down. Examination of the pneumatic fire/overheat detection elements found one element stuck in the alarm state due to a permanently deformed detector switch diaphragm. Similar diaphragm failures had occurred in other DHC-8 series 400 airplane fire events.
Additional Investigation Findings
PWC analyzed titanium fire mechanisms by examining a No. 5 bearing seal runner and flexible support from a non-fire-damaged PW150 engine that had operated without oil pressure for a similar duration but at a lower power setting. Dimensional checks showed the seal runner was radially deformed (coned), potentially allowing contact with the flexible support during operation. The thin-walled ribs of the flexible support exhibited rubbing and erosion consistent with oxidation. The seal runner showed microstructure consistent with exposure to excessive temperatures. According to PWC, the seal runner's material properties can be exceeded when No. 5 bearing cavity temperatures are abnormally elevated.