Norfolk, VA, USA
N440DS
Raytheon Corporate Jets Beechjet 400A
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.
HISTORY OF FLIGHT On June 14, 2006, about 1615 eastern daylight time, a Raytheon Beechjet 400A airplane, N440DS, experienced a dual-engine flameout in both of the installed Pratt & Whitney Canada (PWC) JT15D-5 engines after the pilots reduced power to activate the engine anti-ice system while the airplane was in cruise flight at flight level (FL) 380 about 70 miles south of Norfolk, Virginia. After the engines restarted, the airplane landed at Norfolk without further incident. Daytime visual meteorological conditions (VMC) prevailed at the time the engines lost power. The airplane was operating on an instrument flight rules flight plan under 14 CFR Part 91 from Quonset Point, Rhode Island to Charleston, South Carolina. The two pilots on board were not injured. The pilots stated that they were cruising at FL 380 in VMC and air traffic control gave them a heading change that was towards an upsloping cloud deck that was known to be remnants of tropical storm Alberto. The pilots stated that they were unsure whether they could remain in VMC, so they decided to turn on the engine anti-ice system. In accordance with the Beechcraft 400A FAA-approved airplane flight manual (AFM) procedures at the time of the incident, the pilots reduced the engine power from 101.5 to 89.5 percent N1 rpm, but both engines flamed out before they could turn on the engine anti-ice. The pilots stated that they put on their oxygen masks, declared an emergency, and turned towards Norfolk. The pilots stated that they maintained FL 380 until the airplane had slowed to 180 knots before they began to descend. The pilots stated that during the descent, they maintained between 220 to 250 knots. They also stated that the cockpit engine indicators for both engines showed engine rotation. The pilots stated that the left engine restarted on its own at about FL 300 and the right engine started on its own at about FL 240. TESTS AND RESEARCH A review of the airplane's maintenance records showed that maintenance personnel at Norfolk had checked the airplane and engines after the event and no discrepancies were noted. The maintenance records also showed that the maintenance personnel drained all the fuel from the airplane and no water or contamination was observed. A gallon of fuel that was drained from the airplane was forwarded to a petrochemical laboratory for testing. The tests revealed that the fuel conformed to the requirements for Jet A. In addition, the tests showed that the concentration of the fuel system icing inhibitor (FSII) was 0.13 percent. FSII is not a requirement for jet fuel, but the Beechjet 400A AFM states that the airplane's fuel must have an FSII in concentrations of 0.1 to 0.15 percent. At the time the airplane's engines flamed out, the airplane was approaching clouds that were known to be remnants of tropical storm Alberto. Weather satellite imagery showed that there embedded convective weather cells in the area where the airplane's engines flamed out. According to the Federal Aviation Administration's (FAA) engine icing expert, convective weather can lift significant amounts of moisture into the upper atmosphere and the blow off from the tops of these convective storms can contain significant amounts of ice crystals. PWC did a study to determine if high altitude ice crystals could adhere to the inside of a JT15D-5 engine to the point that it would flame out. The study revealed that it was possible for ice crystals to partially melt passing through the fan and with the anti-ice turned off, to accrete on the leading edges of the front inner compressor stator vanes. Any change in engine speed would change the airflow's angle of incidence over the stators causing any accreted ice to be blown off of the stator vane. The study showed that if a significant amount of ice had accreted on the stator vanes and was blown off, it could result in the engine surging and possibly flaming out. In addition, any ice that may have accreted within the engine would degrade the engine's compressor efficiency, surge margin, and relight capability. Additionally, the ice that was blown off of the stator vanes would melt passing through the high pressure compressor that could be then picked up by the P3 (compressor discharge pressure) line that goes to the fuel control. Raytheon conducted a flight test of a Beechjet 400A airplane that had thermocouples installed on the P3 line adjacent to the fuel control on one engine. The testing revealed that the temperature of the line would go below freezing at low power settings when the engine was operated in ambient conditions like those when the airplane's engines flamed out. PWC developed a rig test to simulate the effect of a blocked P3 line. The testing revealed that if the line was blocked when the power lever was retarded, the electronic engine control would reduce the fuel flow even though the pressure was maintained in the fuel control P3 bellows. When the line became unblocked, either by the pressure differential or the melting of an ice blockage, the sudden drop in the P3 bellows pressure resulted in the fuel flow dropping faster than in a normal rapid engine deceleration that could result in the engine flaming out. The Safety Board issued Urgent Safety Recommendation A-06-56 that requested the FAA to Beechjet 400 pilots to activate the engine anti-ice at high altitudes whenever they were in or near visible moisture or convective storm activity. On September 15, 2006, Raytheon issued a temporary change to the Beechjet 400 AFM that provided guidelines for turning on the engine anti-ice at high altitudes when in the vicinity of visible moisture and convective storm activity. The FAA issued Airworthiness Directive 2006-21-02 that required operators of Beechjet 400 airplanes to incorporate the temporary revision into their airplanes AFM.
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.
Source: NTSB Aviation Accident Database
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