From the NTSB Pinnacle Airlines report
1.18.2 Core Lock
During the investigation of this accident, the Safety Board learned that GE CF34-1 and CF34-3 engines had a history of failing to rotate during in-flight restart attempts on airplanes undergoing production acceptance flight testing at Bombardier. The manufacturers referred to this condition as “core lock.” Bombardier first identified this problem in 1983 during Challenger certification tests, and GE attributed the problem to interference contact at an air seal in the high pressure turbine.
The CF34 high pressure turbine air seals are designed to control cooling and
balance airflow. The seals include teeth on the rotating components that grind operating grooves into abradable surfaces on the stationary components. The efficiency of these seals significantly affects engine performance, so the seals are designed to operate with minimal clearances.
Bombardier added a procedure that screened for core lock to the production
acceptance flight tests for its airplanes powered by CF34-1 and CF34-3 engines. At the time of the accident, this screening procedure was as follows:
1. Climb to 31,000 feet.
2. Retard the test engine throttle to idle and stabilize for 5 minutes.85
3. Shut down the test engine.
4. Descend at 190 knots.
5. Slow the aircraft until N2 is reduced to 0 percent.
6. At 8 1/2 minutes from shutdown, push over to 320 knots.
7. If N2 is 0 rpm at 21,000 feet, the engine is declared to be core locked.
Engines that are found to be core locked are reworked using an in-flight “grind-in” procedure that was designed to remove seal material at the interference location. Engines that undergo grind-in rework are then rescreened for core lock. The grind-in procedure is as follows:
1. ATS cross-bleed start.
2. Ascend to 31,000 feet.
3. Repeat core lock screening procedure but descend at an airspeed of
about 240 knots to establish 4 percent N2.
4. Maintain 4 percent N2 for at least 8 1/2 minutes.
5. Confirm that no core lock exists by repeating screening procedure.
As testimony during the Safety Board’s June 2005 public hearing on the Pinnacle Airlines accident indicated, neither Bombardier nor GE considered core lock to be a safety-of-flight issue. The manufacturers claimed that engines that passed the screening procedure, with or without grind-in rework, would not core lock as long as the 240-knot airspeed was maintained.
Bombardier’s core lock screening procedure requires a cool-down period before
engine shutdown to stabilize internal temperatures and clearances. However, this procedure does not produce the more severe thermal distress associated with the high power, high altitude flameouts that were experienced during the accident flight. As stated in the Safety Board’s November 20, 2006, safety recommendation letter to the FAA the successful demonstration of Bombardier’s flight test procedure might not ensure that an engine will
not experience core lock if the core is allowed to stop rotating after a high power, highaltitude flameout. In its letter, the Board noted that the No. 1 accident engine had successfully passed the screening procedure during initial production acceptance testing.The Board further stated that the successful demonstration of Bombardier’s flight test procedure might not ensure that slowing the airplane to an airspeed of 170 to 190 knots is
sufficient to maintain core rotation during an attempted APU-assisted restart
T'Bug