Raytheon Corporate Jets BEECHJET 400 N691TA

The flight was level at Flight Level (FL) 400 for approximately 30 minutes. Air traffic control (ATC) cleared the flight to FL380, where it remained for about 15 minutes. Further clearance was issued for 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 elected to save battery power for navigation and landing. The crew executed a successful emergency landing. After landing, the Captain attempted to restart the engine, and did observe some rotation, however stopped when the temperature did not rise. Post incident examination of the aircraft, and review of crew actions indicated that the cause of 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 it was marginally below the recommended ratio, and 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 a mechanical binding of the rotating components of the engine. In-flight testing of the right engine of the incident airplane was unable to cause a core lock condition. This flight and other BE-400A engine flameout events before and subsequent to this incident occurred in similar environmental conditions. Pilot reports and meteorological records indicated the flights were in or near instrument meteorological conditions with convective weather cells in the area and the pilots had just retarded the power levers to initiate a descent. Research revealed that convective storms can lift significant amounts of water into the upper atmosphere and that the blowoff from the tops of these storms can contain significant amounts of ice crystals. A post-incident study showed that the ice crystals could partially melt passing through the low-pressure compressor of the JT15D-5 engine due to the increase in temperature of the air being compressed. Further, the study determined that with the engine anti-ice turned off, that it was possible for the ice crystals to accrete on the leading edges of the front inner compressor stator leading edges. The study also determined that if a significant build up of ice had occurred, any change in the airflow angle of incidence that would occur as power was reduced would cause any ice that had accreted on the leading edges of the stators to break away and would result in the engine surging or possibly flaming out. The study also revealed that after the engine had flamed out, the radiant heat from the oil tank, which is in the core of the engine between the low and high pressure compressors, could cause the ice on the front inner compressor stators to melt and the water could run back and refreeze in the high pressure compressor impeller, acting like a wedge to prevent engine rotation and restart. 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, much more rapidly than normal, and reduce flow to a level that would not support combustion. The orifice was added to eliminate trapped water from the line, however this orifice was small enough to be thermally overwhelmed in the incident conditions. Pilot interviews revealed that pilots do not perceive high altitude ice crystals to be a threat to the airplane or the engines. 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 that was distributed to the 400A community via a safety communiqué. The NTSB recommended that the guidance provided in the Raytheon safety communiqué be incorporated into the Beechjet 400A AFM as well as the AFMs of other JT15D-powered airplanes (A-06-57 and 58). The NTSB also recommended that the FAA require the operation of the engine anti-ice system whenever flights are near convective weather (A-06-56). Crew actions after the flameout were appropriate to attempt to restart the engines, notify and coordinate with ATC for an emergency diversion, and make a forced landing. In fact, investigators found the crew's coordination and actions to be exemplary. Investigators also found ATC's assistance to the crew was also exemplary and contributed to the successful outcome of the flight.

Quoted verbatim from the NTSB record.