Report into Circuit City Cessna Citation 560 accident reinforces US safety agency's concerns about certification
The US National Transportation Safety Board (NTSB) is again pressuring the Federal Aviation Administration to change the way it certificates aircraft cleared to fly in icing conditions. The plea came last week as the board revealed its findings on the fatal crash of a Circuit City-owned Cessna Citation 560 business jet on approach to Pueblo Memorial airport in Colorado on 16 February 2005. The NTSB has made similar calls for enhanced icing certification since 1981, and the topic has been on its "most wanted" list since 1997.
Although it ruled that poor airmanship was principally to blame for the crash, the NTSB says the contributing factor was the FAA's "failure to establish adequate certification requirements for flight into icing conditions, which led to the inadequate stall warning margin provided by the airplane's stall warning system". Killed in the accident were the two pilots, employees of charter company Martinair, and six Circuit City staff and contractors on their way from Richmond to Santa Ana. The aircraft was stopping in Pueblo for fuel.
Investigators say the crew, in the midst of numerous distractions - icing, a switch in expected runways and ill-timed missed approach procedure discussions - failed to monitor the aircraft's speed. As a result, the captain allowed the airspeed to steadily decrease to well below target approach speeds for flight in icing conditions. In addition, both pilots failed to heed aircraft operating manual and company guidance to activate the wing leading edge pneumatic de-icing "boot" system when the aircraft is in landing configuration and ice is detected on the wings.
Investigators said the Citation stalled about 1,500ft (460m) above the terrain - with no advance notice from the "stick shaker" low-speed warning device - and rolled sharply to the left in a dive that was probably unrecoverable. Their findings were based on the cockpit voice recorder and limited performance data from memory in the aircraft's Enhanced Ground Proximity Warning System. The aircraft did not have a flight data recorder, nor was it required to.
Analysis of the data revealed that the aircraft was travelling at about 90kt (165km/h) when it stalled. Martinair procedures would have called for an approach speed of at least 101kt, plus an extra 8kt buffer for the potential performance degradation due to icing. The NTSB determined that the Cessna's stick shaker would not have activated until the speed was down to 86kt, based on a "contaminated" stall speed of 81kt and a 5kt buffer that Cessna adds in for the possible effects of ice.
Daniel Bower, the NTSB engineer responsible for the aircraft performance analysis, estimated that the aircraft had accumulated about 4mm of super-cooled large droplets (SLD), or freezing rain, in the clouds before the accident, a factor that possibly increased the stall speed beyond the built-in buffer for the stick shaker. "SLD droplets are very rare, but it is a much different type of ice and can have a serious performance penalty," says Bower.
He formulated the icing estimate with help from the National Center for Atmospheric Research. SLD has been implicated in other crashes, including a Comair Embraer EMB-120RT accident in Monroe, Michigan on 9 January 1997.
Source: Flight International
