Engine 'core lock' suspected in Pinnacle accident NTSB recommends restart tests be performed on some aircraft

New procedures and tests could be mandated for certain General Electric CF34-powered Bombardier aircraft as a result of the ongoing investigation into the October 2004 crash of a Pinnacle Airlines CRJ200. An engine "core lock" has been cited as a likely factor.

The recommendations from the US National Transportation Safety Board, if adopted by the Federal Aviation Administration, will affect over 1,000 CRJs (models -100, -200 and -440) as well as 550 Challenger 601 and 604 busines jets powered by the CF34-1 and -3.

The Pinnacle/Northwest Airlink CRJ200 crashed in Jefferson City, Missouri on a positioning flight, killing the two pilots on board. It suffered an aerodynamic stall at 41,000ft (12,500m) and the engines flamed out. Attempts to restart the engines failed. The airline's double engine failure checklist indicated pilots were to maintain a target airspeed of 240kt (444km/h) to keep the engine cores rotating at an appropriate speed for either a windmill restart or an APU-assisted restart.

Both FDR and cockpit voice recorder (CVR) data show that the crew did not accelerate the aircraft after the upset and that the engine core rotation slowed to zero-indicated rpm, according to the NTSB.

Data also shows that the crew did not achieve the 240kt airspeed before or after attempting to restart the engines.

Although the NTSB has not yet determined the probable cause of the accident, board chairman Mark Rosenker in a letter to FAA administrator Marion Blakey says the investigation "has revealed a safety issue regarding a condition that can preclude pilots from restarting an engine after a double engine failure". This phenomenon, known as core lock, is caused by differential cooling of static engine parts and the rotating core.

The NTSB notes that flameouts at high power and high-altitude conditions produce significant thermal distress, as internal temperatures are the hottest at high power settings and the air is colder at high altitudes. The increased thermal shock exacerbates the loss of component clearance and alignment.

"Because the [Pinnacle] engines flamed out under these conditions, axial misalignment caused the seal teeth, which were positioned aft of their normal grooves, to contact stationary abradable material when radial seal clearances closed down," says Rosenker.

"Once core rotation stopped, binding prevented core rotation from resuming during the windmill or APU-assisted restart attempts. Thus, the lack of core rotation on the accident airplane engines could be explained by the core lock phenomenon."

Saying that it is unaware of flight-test data that demonstrate 240kt is sufficient to keep the core rotating after the more severe thermal distress associated with a high-power, high-altitude flameout, the NTSB is recommending the FAA require manufacturers to perform high-power, high-altitude sudden engine shutdowns on aircraft equipped with CF34-1 and CF34-3 engines "to determine the minimum airspeed required to maintain sufficient core rotation, and demonstrate that all methods of in-flight restart can be accomplished when this airspeed is maintained". Several other recommendations have been issued (see box).

GE says the engine manufacturer will "work very closely and co-operate fully, as we have done all along" with regulators.

Bombardier points out that the NTSB's letter is not an FAA airworthiness directive, but contains recommendations that "still have to be reviewed by regulators and then viewed in the context of the final NTSB report".

© Bombardier   
The Northwest Airlink crash could result in new procedures for CRJs

Advice from the NTSB to the FAA 

The NTSB has also advised the FAA to:

  • ensure that information and data in flight manuals of CF34-1 and -3 powered aircraft be improved
  • review the design of turbine engines - other than the CF34-1 and CF34-3 powerplants addressed in the safety recommendation - to determine whether they are susceptible to core lock and, for those engines so identified, require manufacturers of aircraft equipped with these engines to perform high power, high altitude sudden engine shutdown tests (and where necessary improve flight manual information)
  • require manufacturers to determine if restart capability exists from a core rotation speed of zero indicated rpm after high power, high altitude sudden engine shutdowns and where necessary mitigate the hazard by providing design or operational means to ensure restart capability
  • establish certification requirements that would place upper limits on the minimum airspeed required and the amount of altitude loss permitted for windmill restarts.  

Source: Flight International