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Ice Detectors For All Jet Engines?

Air Safety Week, Oct 9, 2006

Recent Engine Flameouts Raise New Concerns About Icing

Double flameouts on Twinjets due to icing are rare. They're more often caused by heavy precipitation. However, they recently became less rare after three Beechjets fell victim to an as-yet undiagnosed phenomenon at height (see July 17 ASW, "The Price of No Prist").

A similar case had occurred on April 23, 2000, in Brazil. Even though the aircraft weren't in what's commonly accepted as "icing conditions" (see below), National Transportation Safety Board (NTSB) theorists are proposing engine inlet icing as the cause.

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Prior to this trio's trauma, the last known double flameout due to engine inlet icing occurred on a Fokker 70 of Austrian Airlines on January 5, 2004. It belly-flopped onto a snow-covered field 2.5nms short of the runway at Munich after totally running out of puff. In heavy icing conditions, ice had developed on the rotors of the low pressure compressors of both engines and the bonded joints of the ice impact panels on both engines then simultaneously failed due to strains caused by ice-induced vibrations of the engines. The loose ice impact panels became trapped in front of the outlet guide vanes of the low pressure compressor and affected the airflow in the bypass duct in such a way that the two engines could only produce a low idling thrust. That failing was addressed by FAA Airworthiness Directive (AD) 2004-03-03.

On August 25, the NTSB issued several recommendations to the FAA, the last one urgent, about the three Beechjet flameouts. Although all three aircraft landed safely without injury, only two of the crews were able to restart at least one engine; the third was landed masterfully without any engine power. The Beechjet sports the Pratt & Whitney Canada JT15D-5 turbofan.

The NTSB's all-encompassing recommendation, if adopted, would have wide implications. It would require the FAA and industry to pursue research and development of a new technology ice detector that could alert pilots to internal engine icing -- and then require that it be installed on new production turbofan engines and be retrofitted to existing turbofan engines. Air Safety Week thinks that the NTSB is drawing a mighty long bow here and its proposal is unlikely to come to pass.

The NTSB says that all airplanes involved in the incidents were operating between FL380 and FL400 near convective activity and that all the flameouts were preceded by a throttle-back. The flameouts happened on July 12, 2004, near Sarasota, Fla.; Nov. 28, 2005, near Jacksonville, Fla.; and this June 14 near Norfolk, Va.

NTSB chairman Mark Rosenker said, "Dual engine flameout is an unacceptable risk that needs to be addressed as quickly as possible." However, it's taken the NTSB more than two years to propose a solution.

FAA atmospheric specialists claim that large-scale convective activity can draw significant amounts of moisture into the upper atmosphere and that the blowoff from the tops of high-rising cumulus clouds can contain a significant quantity of ice crystals. A Pratt & Whitney analysis concluded that if engine anti-ice was not ON, it was possible for ingested ice crystals to build up on the leading edges of the JT15D-5 engine's front inner compressor stator.

After a power reduction, it was surmised that this buildup could lead to a compressor surge and/or flameout. That seems to go against the generally held view that a power increase, rather than a reduction, is much more likely to promote a compressor stall at height, which is why an instant throttle-back is the average turbine pilot's instinctive reaction to a surge or stall. Another name for a surge is "overfuelling" (the throttle-back removing fuel, reducing combustion back-pressures and allowing the airflow to re-establish normal axial- flow conditions throughout the ailing donk).

Getting The Word Out

NTSB's urgent recommendation would require Beechjet 400 pilots to select ignition and anti-ice systems just before any throttle back at any time they identify visible moisture nearby, or whenever in the vicinity of convective cloud tops at high level. The other recommendations would require Raytheon to issue the content of their April 2006 Safety Communique No. 269 as a flight manual amendment. Within a much broader scope, other manufacturers would also need to broadcast similar changes for other JT15D-powered airplanes (various Cessna Citation models, the Hawker 400XP, Mitsubishi Diamond 1/1A and Aerospatiale Corvette).

It seems somewhat incongruous that these airplanes aren't having similar flameout problems, but also need the remedial measures. An earlier Raytheon Communique simply advocated greater use of engine anti-icing and specified a generic environmental condition of wherever and whenever icing might be suspected possible, at any height. The emphasis was on visible moisture, proximity to cloud-tops and ram air temperatures of 10 deg C or less. The advice emphasized that compressor inlet icing could occur within a very wide range of outside air temperatures and advised that "Operators should not assume ice formation to be impossible at very low ambient temperatures (i.e., -30 degrees C or colder)."

Government Industry

Ice Detectors For All Jet Engines?

Air Safety Week, Oct 9, 2006

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