Incident Overview
A Beechjet 400A was cruising at Flight Level 400 for approximately 30 minutes before air traffic control cleared the flight to FL380, where it remained for about 15 minutes. Further clearance was issued for a pilot's discretionary descent to FL330. The flight was operating in visual meteorological conditions in the vicinity of cumulonimbus buildups. Upon reducing power to initiate descent, the crew heard a "popping" noise and both engines rolled back. The crew declared an emergency, elected to divert to Jacksonville Airport, and initiated emergency procedures. All attempts to restart the engines were unsuccessful, and the crew chose to conserve battery power for navigation and landing. The crew executed a successful emergency landing. After landing, the captain attempted to restart the engine and observed some rotation, but stopped when the temperature did not rise.
Post-Incident Examination
Post-incident examination of the aircraft and review of crew actions indicated that the dual-engine flameout was not due to fuel exhaustion or fuel starvation. There were no open maintenance items, and post-incident evaluation and flight test of the aircraft revealed no mechanical malfunctions. The chemical composition of the fuel was correct; anti-ice additive was present, although marginally below the recommended ratio. No contamination or excess water content was found. There was no evidence of core lock—a phenomenon in which differential cooling of engine components following a rapid in-flight rollback causes mechanical binding of rotating components. In-flight testing of the right engine of the incident airplane was unable to cause a core lock condition.
Research and Findings
The incident occurred under similar environmental conditions to other BE-400A engine flameout events before and after this incident. Pilot reports and meteorological records indicated flights were in or near instrument meteorological conditions with convective weather cells, and pilots had just retarded power levers to initiate descent. Research revealed that convective storms can lift significant amounts of water into the upper atmosphere, and blowoff from storm tops can contain significant amounts of ice crystals. A post-incident study showed that ice crystals could partially melt when passing through the low-pressure compressor of the JT15D-5 engine due to temperature rise from compression. With engine anti-ice turned off, ice crystals could accrete on leading edges of front inner compressor stator leading edges. The study determined that if significant ice buildup occurred, any change in airflow angle of incidence during power reduction would cause ice to break away, leading to engine surging or flameout. After flameout, radiant heat from the oil tank (located between low and high pressure compressors) could melt ice on front inner compressor stators; the resulting water could run back and refreeze in the high pressure compressor impeller, acting as a wedge to prevent engine rotation and restart.
Further research, flight tests, and bench testing revealed that a temporary ice blockage in an orifice in the combustion chamber pressure signal line could cause a sudden drop in fuel flow, more rapidly than normal, reducing flow to a level insufficient for combustion. The orifice was added to eliminate trapped water, but it was small enough to be thermally overwhelmed in the incident conditions.
Crew and ATC Actions
Crew actions after the flameout were appropriate for attempting restarts, notifying and coordinating with ATC for diversion, and executing a forced landing. Investigators found the crew's coordination and actions exemplary. ATC assistance was also found to be exemplary and contributed to the successful outcome of the flight.
Post-Incident Guidance
Pilot interviews revealed that pilots did not perceive high altitude ice crystals as a threat. The Beechjet's airplane flight manual (AFM) and flight crew training did not address high altitude ice crystals prior to the dual-engine flameout events. Post-incident, Raytheon developed guidance for Beechjet 400A flight crews on high altitude ice crystals, distributed via a safety communiqué. The NTSB recommended that this guidance be incorporated into the Beechjet 400A AFM and AFMs of other JT15D-powered airplanes (A-06-57 and 58). The NTSB also recommended that the FAA require operation of engine anti-ice system whenever flights are near convective weather (A-06-56).