LONDON—The uncontained failure of the Pratt & Whitney PW1524G engine that substantially damaged Bombardier’s C Series CS100 prototype in May 2014 was triggered by the failure of a Teflon seal in the oil system, according to a Transport Safety Board of Canada report.
The failure of the low pressure (LP) turbine, which occurred during engine ground runs, followed heat soaking of the oil feed tube to the No. 4 bearing at the back of the engine. The heat specifically impacted the integrity of the feed tube’s Teflon C-seal after a series of engine “hot shutdowns.” The resulting airframe and engine damage prompted further delays to the C Series program, temporarily grounding the aircraft and preventing its much anticipated debut at the 2014 Farnborough International Airshow.
The damaged seal allowed engine oil to merge with the turbine rotor’s cooling airstream, leading to ignition of the resulting air-oil mixture in the cavity around the base of the first stage of the three-stage LP turbine. “The ensuing combustion heated the low-pressure turbine rotor to the point of failure,” says the report, which adds the resulting disintegration of the rotor “was uncontained, and resulted in major damage to the engine, nacelle and wing.”
Engine debris damaged the wing’s lower surface, wing-to-fuselage fairing, leading-edge slats and flap fairings, as well as the landing-gear door panels and strut. It also damaged parts of the fuel tank inerting system, which is designed to flood the tank’s ullage space with inert, nitrogen-enriched gas to inhibit combustion. The system “functioned as designed” to prevent a far more serious fuel fire, despite a 38-inch, hot section of the LP turbine rotor disk penetrating the center fuel tank and wedging in the upper wing skin. At the time of the event, the center tank was almost half-full with 12,200 lb. of fuel.
Following the incident, Pratt instituted a series of interim measures to enable flight tests to resume, including a revised cooldown procedure with an increased pre-shutdown cooling period of 20 min. It also added a metallic face seal, in addition to the Teflon C-seal, on the No. 4 bearing oil-feed tube mounting flange, and changed the material of the mounting bolts of the flange to enable higher torque on the bolts. Thermocouples were also added to permit real-time monitoring of the LP turbine cavity temperature, and the oil-seal temperature was operationally limited to 500F. Pratt also instituted postflight daily oil-consumption monitoring and increased daily borescope inspections.
For the production-standard PW1500G, Pratt changed the design of the oil-supply tube and cooling-airflow areas to physically separate the turbine-rotor-cooling airflow from the bearing compartment, to prevent any chance of a repeat occurrence.