FlightOPS

he U.S. National Transportation Safety uncontaminated ; stall speed increases Board (NTSB) is pointing to a recent ice- as ice accumulates. related accident involving a Cessna Citation Beverly Municipal Airport was reporting sur- as yet another product of what NTSB face winds from 310 degrees at 8 kt, 1 mi (1,600 TChairman Mark Rosenker calls “the ongoing m) visibility in mist and a 500-ft overcast. A cir- disconnect” between traditional guidance about cling approach to Runway 34 was in use, but the cycling pneumatic deice boots and research pilots told air traffic control that their operations that has shown the guidance to be baseless and manual prohibited circling approaches when the dangerous. The board has campaigned for more ceiling is lower than 1,500 ft. They requested and than 10 years to change both the outdated guid- received clearance to conduct the global position- ance and the habits it has fostered. The apparent ing system approach straight in to Runway 16. problem for NTSB and others seeking change The pilots activated the anti-ice systems is that a substantial number of people in the when the airplane entered clouds at 3,500 ft aviation industry have not been convinced that (Figure 1). “Moments later, the copilot noticed change is necessary. that they were picking up a trace amount of Generations of pilots have been taught to rime ice on the windscreen,” the report said. wait until a specific amount of ice accumulates “However, since neither pilot saw any ice on the on the wing leading edges before cycling pneu- wings, the deice boots were never activated.” matic deice boots. Traditional training warns The pilots acquired visual contact with the air- pilots that premature activation of the boots port as the airplane neared the minimum descent could make them prey to a hazardous phenome- altitude, 600 ft, and continued the descent at 107 kt non called ice bridging, which renders the boots — 10 kt above the reference landing speed. useless beneath a bridge, or sheath, of ice. The pilot told investigators that shortly after The report on the March 17, 2007, Citation crossing a treeline, the right wing suddenly 500 accident, published in August, is brief, a dropped. “There was no buffet and no warning, product of what NTSB calls a “limited” investiga- just a sudden loss of lift,” he said. “I attempted tion, but highlights the board’s decade-long effort to roll the wings level and added power to arrest to change the way deice boots are operated. the sink but was unable to before the right wing struck the runway.” He said that the airplane ‘No Buffet, No Warning’ then “tracked straight down the runway and The Citation pilots were conducting an air was taxied to the ramp without further incident.” ambulance flight from their base in Punta None of the people aboard was hurt. Gorda, Florida, to Beverly, Massachusetts, with a paramedic, an emergency medical technician, ‘Hollowed-Out Area’ a patient and the patient’s husband aboard. Both pilots believed that the upset had been The pilot, 45, had 4,950 flight hours, in- caused by wind shear. However, no turbulence cluding 3,200 hours in type. The copilot, 60, had been reported, and the flight crew of a Can- held a Boeing 737 type rating and had 25,982 adair Challenger that was landed on Runway 16 flight hours, including 25 hours in the Citation. shortly after the Citation said that they had not The airplane was built in 1974 and had ac- encountered wind shear on approach. cumulated more than 22,000 hours of opera- “After taxiing to the ramp, the [Citation] tion. It was not equipped with an ice-detection flight crew conducted a post-flight inspection system or a stall-warning device such as a of the airplane,” the report said. “They noted . Stall warning is provided aero- that the right wing was bent upward about 10 dynamically with inboard wing degrees and light rime ice was present on the strips that cause buffeting 5 kt above stall speed leading edges of the wings [and] horizontal sta-

in the landing configuration. This assumes an bilizer.” The pilots said that the ice was less than Deice Disconnect

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1/4 in (6 mm) thick. A customer service agent Citation Ice Protection Systems on the ramp estimated that the strip of rime ice

Windshield alcohol anti-ice on the wing leading edges was 1/16 to 1/8 in (2 to 3 mm) thick and 2 in (5 cm) wide. Pitot probes (2), Windshield static ports (4) rain removal/anti-ice An examination of the airplane by a U.S. electrically anti-iced Federal Aviation Administration (FAA) inspec- tor revealed substantial damage. “The upper Inboard wing Outboard wing leading edge leading edge pneumatic wing skin on the right wing/ had been electrically anti-iced deice boots breached, exposing the main ,” the report said. “The spar was broken, and the outboard portion of the right wing and had been bent in an upward direction.” Investigators found that the pilots had operated the Citation’s ice-protection systems as Engine inlet leading edge bleed air pneumatic deice boots required by the airplane flight manual (AFM). The anti-iced

© Chris Sorensen Photography

Source: U.S. National Transportation Safety Board

Figure 1

BY MARK LACAGNINA

Myth and habit are hindering efforts to encourage pilots to cycle boots early and often in .

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manual says that the anti-ice equipment should be the pneumatic deice boots. Also causal [were] activated when operating in visible moisture with the FAA’s inadequate directives, which failed an indicated outside air temperature between 4 to require manufacturers to direct flight crews degrees C and minus 30 degrees C (40 degrees F to immediately operate pneumatic deice boots and minus 22 degrees F). The pilots had done so upon entering icing conditions.” before entering the clouds on descent. The report noted that many other AFMs di- The AFM also says, “Surface deice [the rect pilots to delay operation of their deice boots boots] should be used when ice buildup is esti- until 1/4 to 1 in [25 mm] of ice has accumulated. mated between 1/4 and 1/2 in [13 mm] thick- “This guidance was included to prevent the oc- ness. Early activation of the boots may result in currence of ice bridging, though the FAA and ice bridging on the wing.” Both pilots said that manufacturers have been unable to substantiate its existence,” it said. In theory, ice bridging begins with a thin, malleable layer of ice that deforms, rather than shatters, when a deicing boot is inflated. The layer is molded into the shape of the inflated boot, then hardens, accretes more ice and cre- ates a shell (bridge) that is impervious to further inflation and deflation of the boot. Concern about this phenomenon was found to have been involved in the Jan. 9, 1997, crash of Comair Flight 3272 in Monroe, Michigan. The crew of the Embraer 120 Brasilia was being vectored for an approach to Detroit Metropoli- tan Wayne County Airport when the U.S. Federal Aviation Administration disconnected during a turn at 4,000 ft. The twin-turboprop airplane rolled nearly inverted Residual ice remains they had visually checked the wings after enter- and descended rapidly to the ground, killing all on a Mitsubishi MU-2 ing the clouds and saw no ice; therefore, they 29 people aboard. deice boot after the did not cycle the boots. The cause of the accident, according to NTSB, first cycle during The pilot told investigators he had been was a small amount of rough ice that accumulat- flight tests conducted taught that if you “blow [the boots] too soon, ed and triggered a stall as the airplane was slowed behind a tanker you can get a hollowed-out area.” The co- for the approach. Following company guidance, airplane dispensing pilot said that he had little experience with the pilots had not cycled the deice boots. colored water. boot-equipped airplanes but had learned that The investigation revealed that about a year “boots have some adverse features” and should before the accident, Embraer had revised the not be activated “unless you have 1/4 to 1/2 in AFM to advise pilots to activate the boots at of ice.” the first sign of ice accumulation. NTSB found, however, that because of concern about ice Ice Bridging bridging, Comair and six of the nine other op- Finding little to fault in the pilots’ performance, erators of Brasilias in the United States had not NTSB laid the blame for the Citation accident incorporated the revision into their procedural squarely at the feet of the FAA and Cessna guidance. Comair’s flight standards manual Co. The report said that the probable (FSM) said that pilots should not activate the causes of the accident were “the inadequate boots until 1/4 to 1/2 in of ice accumulates guidance and procedures provided by the because premature activation could “result in airplane manufacturer regarding operation of the ice forming the shape of an inflated deice

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boot, making further attempts to deice in flight remains on the boot after an inflation/deflation impossible.” cycle and ice that accumulates between cycles. The investigation of the Comair accident “The general consensus of the workshop par- generated several recommendations, includ- ticipants was that ice bridging is not a problem ing a call for an industrywide effort “to educate for modern pneumatic deice boot designs,” the manufacturers, operators and pilots of [turbo- FAA said. prop airplanes] regarding the hazards of thin, Deice boots essentially are fabric-reinforced possibly imperceptible, rough ice accumulations, rubber sheets cemented to the leading edges of the importance of activating the leading edge the wing and tail. A pressure source is used to deice boots as soon as the airplane enters icing inflate tubes within the boots and to create a conditions … and the importance of maintain- vacuum that deflates the boots and holds them ing minimum airspeeds in icing conditions.” flat against the leading edges. Subsequent ice-related accidents prompted Modern boots have short, segmented, small- NTSB to include operators of boot-equipped diameter tubes that are operated by relatively jets in similar recommendations. high-pressure engine bleed air. Older boot designs, which date back to the 1930s, have long, Mass Revisions unsegmented, large diameter tubes typically In July 1999, the FAA cited the Comair ac- operated by engine-driven pneumatic pumps at cident and several other ice-related accidents pressures that are relatively low and vary accord- in proposing rule making to revise the AFMs ing to engine speed. “This low pressure, coupled The results of of 43 airplanes to “include requirements for with long and large-diameter tubes caused early extensive icing wind activation of the airframe pneumatic deice deice systems to have very lengthy inflation and tunnel tests in 2006 boots … at the first sign of ice accumulation deflation cycles and dwell times [i.e., the period supported activation [anywhere on the airplane or upon annun- in which the boot remains completely inflated],” of deice boots at the ciation by an ice-detection system] to pre- the FAA said. first sign of ice. vent reduced controllability due to adverse U.S. Federal Aviation Administration aerodynamic effects of ice adhering to the Doubting Thomases airplane prior to the first deice cycle.” Among Several people who the proposed requirements was continued commented on the operation of the boots until the airplane exits rule-making proposal the icing conditions. pointed to airplanes In the proposal, the FAA discussed a on the list that have workshop that was held in Cleveland in No- no history of ice- vember 1997 to explore the phenomenon of related accidents. One ice bridging. The workshop was attended by said that the FAA 67 representatives of aircraft and deice boot merely was speculat- manufacturers, the airlines, pilot groups, the ing that the proposed National Aeronautics and Space Administra- AFM revisions will tion, NTSB, and civil aviation authorities. The improve safety and participants shared and discussed the results of challenged the agency icing wind tunnel and flight tests. to test the proposed The boot manufacturers, for example, said procedure on each that they had been unable to reproduce ice of the airplanes. The bridging under any wind tunnel or laboratory FAA rejected these conditions and that reports of ice bridging they comments, saying had investigated turned out to actually have that “the potential involved residual or intercycle ice — ice that still exists for reduced www.flightsafety.org | AeroSafetyWorld | December 2008 | 23 FlightOPS

Old-design deice boots, like those on the Gulfstream I, have longer and larger tubes that cycle much more slowly than those in modern boots.

test data provided by British . The Douglas DC-3 and the Gulfstream I were among other airplanes withdrawn from the proposed rule making because they have old-design boots and may be prone to ice bridging.

‘Accidents Could Still Occur’ © Chris Sorensen Photography The FAA and several other civil avia- tion authorities have published guid- controllability of all airplanes equipped associated with wearing out the boots. ance based on what has been learned with pneumatic deice boots due to The FAA rejected these comments, also. about icing from research and recent the adverse aerodynamic effects of ice accident investigations. For example, adhering to the airplane.” Citations Withdrawn in Advisory Circular (AC) 91-74A, the Other comments reflected the NTSB’s conclusion that the FAA’s “inad- FAA says that “even a thin layer of ice at reasons, beyond concern about ice equate directives” were a causal factor the leading edge of a wing, especially if bridging, that many operators prefer in the Citation air ambulance accident it is rough, can have a significant effect to wait until some ice accumulates referred, in part, to the withdrawal of in increasing stall speeds” and recom- before cycling the boots. Chief among Citation 500-series airplanes — about mends that deice systems be activated them is the perception that boots work 1,400 total — from the proposed rule at the first indication of icing. best, shedding ice more cleanly, if making. The agency currently is considering cycling is delayed. The FAA acknowl- The FAA’s decision not to pursue whether to make this recommendation edged that residual ice and intercycle Citation AFM revisions was based on a requirement for newly manufactured ice can cause adverse aerodynamic flight tests conducted by the manufac- transport category airplanes. NTSB effects but pointed out that persistent turer. Cessna fitted artificial ice shapes has called on the FAA to expand the ice accretions result even when boots simulating 1/2 in of clear and rime ice proposed requirement to all airplanes are cycled after 1/4 to 1/2 in of ice to a Citation 550, which has a similar with deice boots. Noting that Cessna builds up. It said that the proposed wing and tail as the original 500 and removed the reference to ice bridging procedure, which calls for continuous 501 models, and a Citation 560. Evalu- from the Citation AFM in February but cycling of the boots while flying in ation of the airplanes’ stall characteris- retained the recommendation to wait icing conditions, “will minimize the tics was performed in level flight and until 1/4 to 1/2 in of ice builds before residual and intercycle ice accretions.” steep turns. The FAA said that the activating the boots, the board said that The FAA pointed out that “the residual flight tests demonstrated “acceptable many AFMs contain similar guidance. and intercycle ice accretion thickness stall protection and maneuver margins NTSB said that since 1982, it has resulting from this procedure is less at operational speeds” and showed that investigated 43 ice-related turbine- than the ice accretion thickness typi- the airplanes “can safely operate with airplane accidents that have resulted in cally recommended prior to operation ice accretions associated with the AFM 201 fatalities and 16 serious injuries. “If of the pneumatic deice boots.” normal operations procedures of the pilots continue to adhere to guidance Among other objections were that deice boots.” about delaying deice boot activation, cycling boots early and often increases The Jetstream 41 also was withdrawn similar accidents could still occur,” the pilot workload and maintenance costs based on original ice-certification flight board said. 

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