Coincidental to the early taste of winter weather the Northeast received in mid-October, NBAA–in conjunction with local airport user groups and state business aviation associations–sponsored a pair of cold-weather operational seminars aimed at exploring the challenges and threats presented by in-flight and ground icing, as well as runway contamination.
In-flight icing has long occupied a slot on the NTSB’s most wanted list. According
to the National Center for Atmospheric Research, the Great Lakes and the Pacific Northwest are the parts of the country where pilots have the highest probability of encountering in-flight icing, but the phenomenon is by no means limited to those areas.
Dr. Judith Van Zante, an icing specialist who works with NASA’s Glenn Research Center, warned that even those pilots who have flown in icing before should not become complacent. She pointed out the variability in icing conditions (temperature, liquid water content and droplet size) as well as changing flight conditions (airspeed, exposure time, angle of attack and leading-edge radius) that make each encounter different from the last.
Researchers have found that ice will accrete first on objects with a small leading-edge radius, as roughness elements form beyond the leading edge and act as a dam, preventing the sheeting action of water and causing the build-up of ice. “What we are seeing more recently in accidents with more high-performance aircraft is handling anomalies,” said Van Zante. “You might see a wing stall due to icing but you might also see a tail stall in some aircraft. The wing stall will manifest itself as either a roll or pitch upset; a tail stall will always be a pitch upset.”
Van Zante recommended that pilots and operators who believe they might encounter icing conditions obtain as much weather information as possible on the ceilings, cloud tops, freezing level, and active fronts as well as any pireps that might have been filed along their flight path. Pilots are advised to depart areas of icing as soon as possible.
She also warned pilots to be alert for engine problems that can occur when a turbofan engine ingests ice crystals.
Recently, power loss events such as stall, surge or flameout have been attributed to the ingestion of very high concentrations of small ice particles. Such conditions can occur during flight over deep convective activity with heavy rain below. Post-event interviews with pilots generally indicate that they thought they were in fairly benign conditions: in cloud but black or green painting on radar, with light to moderate turbulence and usually warmer than normal temperatures. NASA has icing resources and courses available on the Web at http://aircrafticing.grc.nasa.gov/.
In terms of ground de-icing, wing contamination of just one ice particle the size of a grain of table salt per square centimeter can reduce lift by up to 30 percent and increase drag by up to 40 percent, according to Walter Randa of Leading Edge Deicing Specialists. He suggests that pilots closely observe the de-icing operations that are carried out on their aircraft, and pointed out that some companies are issuing their pilots with refractometers so they can monitor the quality and consistency of the de-ice and anti-ice fluids being applied to their aircraft.
Before takeoff operators should perform a visual/tactile inspection, as the presence of slush on top of the fluid signifies it has failed and will require removal and reapplication. Randa emphasized the importance of symmetrical fluid application due to the dangers of uneven aerodynamic drag on the airframe.
Winter weather can also have a significant effect on runway surfaces despite the best efforts of airport workers to maintain black pavement. In slushy snow conditions, tire displacement drag as the nosewheels and mainwheels plow through contamination and spray impingement drag as the tires throw spray against the wings and fuselage can alter takeoff lengths, according to Patrick Connor, an engineer with Gulfstream. He noted how a jet such as a G550 normally requires a balanced field length of 5,770 feet in dry conditions but emphasized how that distance could rise to as much as 8,250 feet in the presence of icy conditions.
Pilots should also be aware of how conditions might change at their destination. The crew of an aircraft that is dispatched to an airport with sufficient field length might find that its runways can no longer safely accommodate them upon landing. As temperatures decrease, an aircraft that was cleared for an expected 2,820-foot landing might find that it would require 3,490 feet in less than half an inch of slush and 4,600 feet for icy runway surfaces.