Incident
A Raytheon Beechjet 400A was descending through flight level 390 when both engines flamed out and lost all power. The aircraft was operating in instrument meteorological conditions with convective storms in the area. After several attempts, the pilots were able to restart one engine as the airplane descended through 10,000 feet and diverted to a nearby airport.
Immediate Findings
Testing of the engines immediately after the incident revealed that they were operating correctly. Analysis of the fuel showed it conformed to specifications for Jet A, but the fuel system icing inhibitor (FSII) was significantly below the required concentration.
Subsequent Investigations
The investigation of two subsequent dual-engine flameout incidents determined that the FSII concentration was not a factor in those events.
Research and Testing
Convective weather can lift significant amounts of water into the upper atmosphere, which then form ice crystals. Research indicates that these ice crystals can partially melt as they pass through the JT15D-5 engine fan and then accrete onto an internal compressor stator. When any accreted ice sheds from the stator vanes, it can pass through the high pressure compressor and be picked up by the pressure sense line to the fuel control.
Flight tests confirmed that the sense line could go below freezing at low power settings at high altitude. Other tests showed 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. However, when the blockage was cleared, the sudden reduction in the sense pressure in the fuel control resulted in a reduction of the fuel flow at a rate much faster than a normal rapid power reduction, which could result in the engine flaming out.