Icing, Inadequate Airspeed Trigger Loss of Control of Saab 340
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FLIGHT SAFETY FOUNDATION Accident Prevention Vol. 58 No. 10 For Everyone Concerned With the Safety of Flight October 2001 Icing, Inadequate Airspeed Trigger Loss of Control of Saab 340 The flight crew was conducting a turn to enter a holding pattern at 15,000 feet when the airspeed decreased below the published holding speed. The airplane, which had accumulated ice, stalled and descended for 10 seconds before the crew regained control. FSF Editorial Staff About 1738 local time on Nov. 11, 1998, the flight • “The crew misinterpreted the pre-stall buffet as crew of a Saab 340A began a left turn to enter a propeller ice vibration; holding pattern at 15,000 feet over Eildon Weir, Australia. The airplane was being operated in • “The Saab 340 aircraft is capable of accreting ice instrument meteorological conditions and had deposits without visual clues being provided to accumulated ice. During the turn, a pre-stall buffet the flight crew; [and,] occurred. The crew believed that the vibration was caused by an icing-induced propeller imbalance. The • “The aircraft was not fitted with the Canadian autopilot disconnected, and the airplane rolled left, stall-warning system [which activates at lower pitched nose-down and descended 2,300 feet before angles-of-attack than the standard stall-warning control was regained by the crew. The flight attendant system]. If this had been fitted and activated, it received minor injuries; the two pilots and 28 would have … provided the crew with between passengers were not injured. The airplane was not 10 [seconds] to 18 seconds warning of the damaged. impending stall.” The Australian Transport Safety Bureau (ATSB) said, in its The airplane was being operated on a scheduled flight from final report, that the following significant factors were involved Albury to Melbourne. [The report did not identify the company in the incident: that operated the airplane.] • “The stall-warning system did not activate prior to the “The crew had earlier flown the aircraft from Melbourne to stall; Albury and described the departure from Albury and [the] following climb to cruising level as normal,” the report said. • “The crew allowed the aircraft’s speed to slow below the published holding speed; The captain had an air transport pilot (ATP) certificate and 13,486 flight hours, including 3,109 flight hours in type. He was hired • “The crew interpreted the ice deposit as being less than by the company in 1991 and served as a first officer in that specified in the aircraft flight manual [AFM] for [Swearingen/Fairchild] Metros and Saab 340s, and as a Metro activation of the wing-deice system; captain before becoming a Saab 340 captain in October 1995. The first officer had an ATP certificate and 5,460 flight hours, simulator, the flight-training maneuvering limits also were used including 365 flight hours in type. He was a flight instructor before during simulator training. being hired by the company as a Saab 340 first officer in 1998. The company’s simulator-training program included stall- During their Saab 340 transition training, both pilots received recognition-and-recovery training but not unusual-attitude- flight training to recognize and recover from stalls, and to recovery training. Nevertheless, training captains were allowed recover from unusual airplane attitudes. At the time, the to use time remaining at the completion of scheduled simulator company did not have use of a flight simulator, and flight training sessions to conduct exercises requested by pilots. The training maneuvers were limited. For example, to prevent fluids captain of the incident airplane had practiced unusual-attitude from spilling from the onboard toilet, pitch was limited to recovery in the simulator; both pilots had practiced stall- between 10 degrees and 20 degrees, and bank was limited to recovery in the simulator. 60 degrees. After the company later obtained use of a flight “The operator’s crews were not exposed to the presentation of the electronic attitude director indicator [EADI] at extreme attitudes during training and were, therefore, not familiar with the display presented to them at extreme attitudes,” the report said. The airplane operating manual (AOM) and the AFM did not contain information on recovering from unusual attitudes. The report said that the operator’s policy-and-procedures manual and operations manual contained minimal information on flight in icing conditions and on crew duties in holding patterns. “The operator’s operations manuals did not prohibit the use of the autopilot in icing conditions,” the report said. After departing from Albury, the crew was told by air traffic control (ATC) to fly to the Eildon Weir VOR (very-high- frequency omnidirectional radio) at Flight Level (FL) 150 (15,000 feet). Cloud bases were at 10,000 feet, and cloud tops were at 20,000 feet. The outside air temperature at 15,000 feet was −6 degrees Celsius (C; 21 degrees Fahrenheit [F]). The crew activated the engine anti-ice system and the propeller deice system; the crew did not activate the wing deice boots. Saab 340A “They reported [that] the only visible ice on the aircraft was a Saab-Scania (now Saab Aircraft) and Fairchild Aircraft light rime deposit on the leading edge of the wings and a small began development of a twin-turboprop transport aircraft buildup of ice on the [windshield] wiper arms,” the report said. designated as the SF-340A in 1980. The airplane made its “The crew’s interpretation of this ice deposit was that it did first flight in 1983. Saab assumed control of development and production of the airplane in 1985. The airplane was not meet the requirements in the [AFM] for the activation of designated as the 340A in 1987. deice boots and, consequently, they were not activated.” The airplane has accommodations for two pilots on the flight deck and up to 35 passengers in the cabin. Maximum takeoff At the time of the accident, the AFM recommended activation weight is 12,700 kilograms (28,000 pounds). Maximum of the deice boots when ice accumulated to about 0.5 inch (12 landing weight is 12,020 kilograms (26,500 pounds). millimeters) on the wing leading edges. (Saab Aircraft in Each of the General Electric CT7-5A2 turboprop engines is October 1999 revised the AFM and AOM with a requirement rated at 1,294 kilowatts (1,735 shaft horsepower) and drives to operate the deice boots in the continuous mode at the first a four-blade Dowty Rotol propeller. sign of ice accumulation.) Maximum rate of climb at sea level is 1,800 feet per minute (fpm). Maximum single-engine rate of climb at sea level is “[At the time of the accident], the information contained in 550 fpm. Best-range cruising speed at the service ceiling, the [AOM] and [AFM] on activation of the deice boots 25,000 feet, is 250 knots. Maximum cruise speed at 15,000 feet and 11,793 kilograms (26,000 pounds) is 272 knots. supported the ‘myth’ of ice bridging [in modern turboprop Stall speed with flaps retracted is 104 knots. Stall speed aircraft],” the report said. with flaps in landing configuration is 82 knots. Source: Jane’s All the World’s Aircraft Ice bridging occurs when ice accumulates on an inflated deice boot and then remains in place after the boot deflates. A cavity 2 FLIGHT SAFETY FOUNDATION • ACCIDENT PREVENTION • OCTOBER 2001 is created between the ice accumulation and the deflated boot, percent to a torque setting of 47 percent to reduce airspeed and subsequent boot activation has no effect on the ice. from 170 knots to the published holding speed of 154 knots. (The Saab 340 AOM recommends holding torque settings of “Ice bridging is a phenomenon that has existed in folklore since 30 percent to 40 percent.) About one minute later, the captain the early days of aircraft operation, when deicing boots were observed that the indicated airspeed was 149 knots and told first introduced to aircraft design,” the report said. “These early the first officer to check the airspeed. The first officer increased boots were characterized by long, uninterrupted spanwise, power to 62 percent torque. Airspeed stabilized at 144 knots. large-diameter tubes, which were inflated by low-pressure The first officer increased power to 73 percent torque. engine-driven pneumatic pumps. “The resultant torque setting of 73 percent was greater than that “This combination of low-pressure pump and long-and-large- required during cruise,” the report said. “Neither crewmember diameter tube for the deice boot resulted in a long inflation appears to have been concerned at this apparent anomaly. It time. The subsequent deflation time was also lengthy, resulting appears likely that they both lacked situational awareness about in a long ‘dwell time.’ Dwell time is that time that the boot what was causing the adverse effect on the aircraft performance.” remains inflated between inflation and deflation. … The autopilot was engaged in full-bank mode, which limited “Modern [turboprop] aircraft are equipped with a different form bank angle to 28 degrees. The AFM recommended operation of deice boots [consisting of] short lengths of inflatable tubes of the autopilot in half-bank mode in icing conditions; half- that are segmented across the span and are of a much smaller bank mode limited bank angle to 13.5 degrees. diameter. They are inflated at much higher pressures by engine bleed air. “The manufacturer advised [that half-bank mode] would provide extra margins above the stall [and should be used] “This combination of high-pressure air and shorter tube length whenever possible, particularly in icing conditions,” the report and diameter results in very short dwell times, often less than said. “The operator had previously used half-bank mode in all two seconds in some configurations.