NTSB Identification: ENG06IA020.
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Incident occurred Wednesday, June 14, 2006 in Norfolk, VA
Probable Cause Approval Date: 01/31/2008
Aircraft: Raytheon Corporate Jets Beechjet 400A, registration: N440DS
Injuries: 2 Uninjured.
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
The Raytheon Beechjet 400A was cruising at flight level 380 when both of the engines flamed out after the pilots had reduced power prior to turning on the engine anti-ice systems. The airplane was in visual meteorological conditions (VMC), but were approaching clouds that were known to remnants of a tropical storm. The pilots were unsure if they could remain in VMC and had decided to turn on the engine anti-ice. In accordance with the airplane flight manual (AFM) procedures, the pilots reduced the power to 89.5 percent N1 and both engines flamed out. The left and right engines restarted on their own as the airplane descended through FL 300 and 240, respectively. Testing of the airplane and engines immediately after the incident revealed that they were operating correctly. Testing of the fuel revealed that it conformed to the specifications for Jet A and the concentration of the fuel system icing inhibitor was within limits. Weather satellite imagery showed that there were convective weather cells in the area where the airplane's engines flamed out. Convective weather can lift significant amounts of water into the upper atmosphere that then form ice crystals. Research indicates that the ice crystals can partially melt passing through the JT15D-5 engine fan that can then accrete to an internal compressor stator. When any accreted ice was shed from the stator vanes, it would pass through the high pressure compressor and could be picked up by the pressure sense line to the fuel control. Flight tests confirmed that sense line could go below freezing at low power settings at high altitude. Tests also show that if the sense line was blocked and the power levers were retarded, the electronic engine control would reduce the fuel flow to reduce the power, but the sense pressure within the fuel control would be maintained. But when the blockage was cleared, the sudden reduction in the sense pressure in the fuel control would result in a reduction of the fuel flow at a rate that was much faster than a normal rapid power reduction and could result in the engine flaming out.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: High-altitude ice crystals that had accreted on the compressor vanes and were ingested into the engine high pressure compressor when the pilots retarded the power levers causing compressor surges and the flameouts of both engines. Contributing factors were the lack of training on the hazards of high-altitude ice crystals to gas turbine engines and guidance to the pilots to activate the engine anti-ice system in conditions where high-altitude ice crystals may exist.
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