Several Beechjet flameouts have led the NTSB to make recommendations to prevent recurrences. The final recommendation, if adopted, would have wide implications: require the FAA and industry to pursue research to develop an ice detector that would alert pilots to internal engine icing and require that it be installed on new production turbofan engines and retrofitted to existing turbofan engines.
On August 25 the NTSB issued several recommendations–one classified as urgent–to the FAA aimed at preventing dual engine flameouts such as the ones that occurred on three Beechjet 400s in the past two years. As noted in last month’s issue of AIN (“Third dual flameout raises questions about Beechjet,”), all three aircraft landed safely without injury, but only two of the crews were able to restart at least one engine; the third landed without any engine power. The Beechjet is powered by two Pratt & Whitney Canada JT15D-5 turbofans.
According to the NTSB, all the airplanes involved in the incidents were operating between FL380 and FL400 near convective activity and all the flameouts were preceded by a power reduction. The flameouts happened on July 12, 2004, near Sarasota, Fla.; Nov. 28, 2005, near Jacksonville, Fla.; and this past June 14 near Norfolk, Va. A similar case–also involving a Beechjet 400–occurred on April 23, 2000, in Brazil.
NTSB chairman Mark Rosenker said, “Dual engine flameout is an unacceptable risk that needs to be addressed as quickly as possible.” However, “quickly” is a relative term since it took the agency more than two years to formally address this issue.
An FAA specialist on engine icing told the NTSB that convective storms can pump large amounts of moisture into the upper atmosphere and the blowoff from the tops of these storms can contain significant amounts of ice crystals. A P&WC study concluded that with engine anti-ice turned off, it was possible for ice crystals to build on the leading edges of the JT15D-5 engine’s front inner compressor stator and that the buildup could lead to a compressor surge and/or flameout.
The Safety Board’s urgent recommendation to the FAA would require Beechjet 400 pilots to activate ignition and anti-ice systems–before any power reduction–at high altitude whenever they are in or near visible moisture or near convective storm activity. These operational procedure changes stem from the NTSB’s belief that many pilots are not aware of the risk posed by ice crystals at high altitudes.
Two other NTSB recommendations pertain to procedural changes that Raytheon Aircraft outlined in Safety Communiqué No. 269, released in April. One would require Raytheon to incorporate the information regarding anti-ice operation and ice formation contained in the communiqué into the Beechjet 400 airplane flight manual (AFM). The other would mandate incorporating the same information into the AFMs of other JT15D-powered airplanes, which includes several Cessna Citation models, the Aerospatiale Corvette, Mitsubishi Diamond 1/1A and Hawker 400XP.
Earlier last month, Raytheon issued Safety Communiqué No. 272, which broadens the parameters requiring the use of engine anti-icing and removes the 90-percent N1 limitation to turn on this system. The letter expands the definition of engine icing conditions to include: illumination of the icing annunciator, if installed; airframe ice accumulation; at all altitudes and ram air temperatures (RAT) of 10 degrees C or less in visible moisture (any obstruction to visibility should be considered visible moisture unless it can be determined to be smoke, dust or ash); or above 20,000 feet msl and RAT of 10 degrees C or less and any of the following conditions: within approximately 5,000 feet above visible moisture associated with convective activity; within approximately 10 nm horizontally of visible moisture associated with convective activity, in visible moisture or at any time that the conditions listed above cannot be verified.
Further, the document states, “As was noted in Safety Communiqué No. 269, no lower temperature limit exists for the operation of anti-ice systems. Operators should be aware that air moving through the engine experiences a significant temperature increase as it passes through the compressor section. This increase could bring the air temperature to a range where internal engine ice formation might occur if engine anti-ice were not operating. Operators should not assume ice formation to be impossible at very low ambient temperatures (i.e. -30 degrees C or colder).
“In addition, Safety Communiqué No. 269 states that activation of engine anti-ice is not recommended above 90-percent N1 to prevent transient exceedance of engine ITT limits. This procedure has been reviewed by [Raytheon Aircraft] and P&WC and it has been determined that reduction of power below 90-percent N1 is not required. Engine gauges should be monitored and thrust levers adjusted (if required) to maintain engine ITT limits following activation of engine anti-ice.”
Raytheon Aircraft “will be releasing a temporary change to all applicable model AFMs, with subsequent AFM revision, to incorporate this information. Additionally, [Raytheon Aircraft] intends to request an Airworthiness Directive from the FAA to introduce and distribute this change both domestically and internationally.”
According to Raytheon, additional information on operating in or near icing conditions can be found in Section V of the applicable pilot’s operating manual [sic] under “General Information on Specific Topics, Flight Operations, Flight in Icing Conditions.”
The NTSB is still investigating all three of the dual engine flameouts that struck N-registered Beechjets, and Raytheon Aircraft says it continues to assist the agency in this investigation and will provide additional information as required.