MIAMI — Over the past several days we have watched a giant winter storm trek its way defiantly across the country, wreaking havoc on airports from Dallas to New York City. As most airplanes sit dormant, caked in ice and snow, others manage to escape after a thorough de-icing bath. Either way, the storm has left thousands stranded and wondering why, perhaps, they’re stuck at the airport. Today we’ll look at why ice and snow are aviation’s mortal enemies, and how airplanes can fight against Old Man Winter.
To understand how ice becomes a problem for normal flight, we first must understand how airplanes fly to begin with. The miracle of flight involves several forces, but the most pertinent to our discussion is lift. Lift, in tandem with gravity, keeps the airplane aloft during flight. In short, air, moving over the wing, creates a difference in pressure between the top and bottom section, causing the wing to gravitate toward the area of low pressure.
In order to work effectively, however, the flow of air needs to be smooth and consistent across the entire wing. And that’s where ice becomes a big problem. Build ups of ice and snow both disrupt and distort that airflow, particularly on the more sensitive upper side on the wing and on the crucial leading edges. The disruptions lead to portions of the wing becoming useless, as they are no longer generating lift. Once a significant portion of the wing fails to generate lift the airplane can no longer keep pace with drag and gravity, ultimately causing the airplane to stall and, potentially, crash.
The effects of ice on wings was tragically realized on January 13, 1982, when Air Florida flight 90 crashed into the Potomac River during a winter storm in Washington DC. The airplane had sat on the tarmac, waiting for some time, before being cleared to take off. Unfortunately the crew failed to turn on available on-board anti-ice systems, and attempted to take off with snow and ice on the wings. The airplane climbed up to just over 300 feet before it stalled, hit the 14th St bridge, and plunged into the icy river. Seventy out of seventy-four passengers, plus four on the bridge, were killed.
Icing of the pitot tubes, which relay critical information to the pilots on airspeed and other factors, contributed to the crash of Air France 447 in 2009. Following passing through a thunderstorm the pitot tubes on the Airbus A330 are believed to have become overwhelmed with ice, causing inaccurate and inconsistent measurements. The airplane’s autopilot disengaged, and the pilots failed to take the appropriate action to correct the problems, leading to a high altitude stall. Icing also contributed to the crash of Continental Express flight 3407, operated by a Q400, earlier in the same year.
That doesn’t mean that airplanes are defenseless in their fight against winter weather. All commercial aircraft are fitted with de-ice and/or anti-ice systems. De-ice systems are typically reserved for turbo-prop aircraft, such as the Bombardier Dash-8 and the ATR-72. Nicknamed boots, the devices are located on the leading edge of both the wings and horizontal stabilizers. The black strips inflate like a balloon via compressed air from the engines, and then literally boot the ice from the wing (see video at the bottom!).
The other systems are known as anti-ice. In smaller aircraft, particularly general aviation–think Cessna, a system called TKS is used. The system involves an alcohol like solution that is secreted out of the wings and other flight-critical surfaces. Larger aircraft, such as Airbus A320s and Boeing 787s, have heated wings to prevent build up.
Of course the most visible procedure is one we all must endure at the airport in the winter months: de-icing. The often colorful fluid is typically made of various types of glycol along with other chemicals added to increase effective duration or viscosity.
The fluid knocks any existing ice, snow, or frost from the airplane. Others, particularly heavy duty versions, can keep ice from forming on protected surfaces for up to an hour or more. The fluid gradually falls off the airplane as it departs, hopefully for somewhere warmer.
While ice and other winter weather can be extremely hazardous, a combination of common sense and the defenses outlined above have made flying safe even in the worst of weather.