At 4:01 p.m. on January 13, 1982, Air Florida Flight 90 crashed into the ice-filled Potomac River just 30 seconds after takeoff from National Airport in Arlington, Virginia. Seventy-eight individuals diedin the crash, including four people who were in cars on the 14th Street Bridge spanning the Potomac. Five passengers from the plane survived the crash, due largely to the efforts of passersby and emergency personnel who plucked people from the frigid waters. The story of what happened on that January day is one of tragic human error in the face of extreme weather conditions; following the crash, the National Transportation Safety Board determined that the cause of the accident was icing on the aircraft and the failure of the pilots to abort the takeoff or use all of their anti-icing equipment.
In the years since the crash of Air Florida Flight 90, the industry's approach to de-icing aircraft has changed considerably. What once could have been characterized as a "laissez-faire" system of plane de-icing has morphed into a strictly regimented program with new regulations that have eliminated any room for doubt. No place better illustrates the new era of plane de-icing than the Central De-icing Facility (CDF) at Toronto's Lester B. Pearson International Airport. As oneof the most northerly countries in the world, Canada must take its plane de-icing seriously, and the CDF's massive complex illustrates just how committed the country's airline industry is to safety.
Tragedies Prompt a Change in Regulations
Unfortunately, the first history of commercial air travel is dotted with tragedies much like that of Air Florida Flight 90. On Dec. 12,1985, a large DC-8 aircraft loaded with American soldiers rolled offthe end of the airport runway in Gander, Newfoundland, in freezing drizzle, killing 248 U.S. soldiers and 8 crewmembers. For years the telltale scar it gouged in the terrain acted as a vivid reminder of theproblems that ice on wings can cause. Meanwhile, the crash of a commuter jet in Dryden, Ontario, Canada, in 1989 further brought to lightthe perils of airframe icing. The Fokker 28 aircraft crashed 15 seconds after takeoff, unable to achieve enough altitude to clear the trees beyond the end of the runway due to ice and snow on the wings. Thecrash resulted in the deaths of 21 of the 65 passengers and 3 of the4 crew members.
In the early years of commercial air travel, the decision to de-ice a plane was made by the captain or the airline. Throughout the industry, there was a tendency to resist de-icing as much as possible because of time constraints, low operating budgets, and a general lack of knowledge about the perils of ice on an aircraft. Use of technologywas limited, particularly for smaller cargo or charter companies whose airplanes sometimes did not have amenities such as heated windshields. In one case, a pilot was equipped with a car windshield scraper to scrape the ice off the plane's windscreen from a side window whileon approach.
Meanwhile, although it was technically illegal for an airplane to take off with ice-contaminated wings, a gray area existed because thedecision was generally left to the captain's discretion. For example, if a light snow was falling, some pilots would elect not to de-ice,thinking that the snow would blow off. In most cases, it probably would, but as the history books can attest, there are always exceptions. In the case of the Air Florida flight that crashed into the Potomac, the aircraft's crew attempted to de-ice the aircraft by intentionally positioning it near the exhaust of the aircraft ahead in line, against the regulations in their flight manual. This may have contributed to the adherence of ice on the wing leading edges and to the blocking of the engine's probes.
In both the United States and Canada, it took a horrific crash related to airframe icing to instigate a change in de-icing regulations.In Canada, it was the crash of the Fokker 28 commuter jet in 1989 that proved to be the impetus for changing de-icing regulations. In theUnited States, the crash of USAir flight 405 from LaGuardia on March22, 1992, instigated changes by the Federal Aviation Administration (FAA). In the aftermath of the crash, which resulted in 27 fatalities, the NTSB found that although the plane had been de-iced twice before leaving the gate, the time between the second de-icing and take-off(35 minutes) exceeded the "de-icing fluid safe holdover time" for that particular type of fluid. The result was a buildup of ice on the wings that resulted in aerodynamic stall shortly after lift-off. According to the post-accident report by issued by the NTSB, "the entire airline industry had been lax in training crews to detect hazards caused by ice and to compensate for such conditions."
These days, any second-guessing is removed from the equation, and the old gray area no longer exists. Both Canadian and American regulations now prohibit take-off when ice, snow, and frost is adhering to any critical surface of the aircraft, including lifting and control surfaces, wings and tail, and upper fuselage surfaces on aircraft withrear-mounted engines. The rule is known as the "clean aircraft concept."
The main exception to the new regulations allows a coating of frost up to one-eighth of an inch thick on wing lower surfaces in areas cold-soaked by fuel, between the forward and aft spars. De-icing also is not mandatory if the captain expects dry snow lying on top of a cold, dry, and otherwise clean wing to blow off during take-off. For aircraft types where the upper fuselage is a critical surface, a thin coating of frost is permitted in the area provided the deposit is thinenough that underlying surface features such as paint lines, markings, or lettering can be distinguished. Although pilots are in charge of deciding whether de-icing is needed, the "lead" ramp attendant can overrule a decision not to de-ice. Even flight attendants and passengers can voice concerns about the plane's de-icing efforts, although the final decision rests with the pilot.
Why a Clean Wing?
Many believe ice on the wings of an airplane is dangerous solely because of the additional weight on the aircraft. However, it is actually loss of lift and the resulting drag on the body of the aircraft that causes problems. Airplanes achieve lift when air flows smoothly over the contoured surface of the wing. If this streamlined flow is disrupted because of ice buildup, decreased lift occurs. A wing can lose 30 percent of lift with just a small accumulation of ice. The stallspeed, or the speed at which the wing ceases to be able to keep the aircraft aloft, can decrease by 15 percent with drag potentially increasing by 200 to 500 percent.
For example, a unique ice formation composed of clear ice that builds up into a single or double horn on critical surfaces can severelydisrupt airflow and increase drag 300 to 500 percent. Meanwhile, ice, frost, and snow that accumulate to the thickness of medium or coarse sandpaper on the leading edge and upper surface of a wing can reduce wing lift by as much as 30 percent and increase drag by 40 percent.
Toronto's Central De-Icing Facility
In Canada and similar locales, icing conditions can lurk nearly nine months of the year, so the de-icing checklist is always within reach because it's part of doing business. The old aviation adage, "If you think safety is expensive, try having an accident," is a rule to live by.
The CDF at Toronto's Airport is the largest de-icing facility in the world. Fully operational since the 1999-2000 cold season, this 65-acre "drive through airplane wash" consists of 6 huge bays capable ofhandling hundreds of aircraft daily. It has an official de-icing season of October 1-April 30. Many pilots jokingly refer to the CDF as the "central delay facility," but the fact that most pilots are paid by the minute takes the sting out of any wait. In addition, the short time it takes to spray a plane with de-icing fluid is insignificant compared with the potential for disaster if a pilot did not take the time to de-ice his or her aircraft.
Moreover, the CDF has actually reduced time between de-icing and takeoff because it was built closer to the runways and has increased overall throughput and improved turnaround times.
On the way to the CDF, after passengers have boarded the plane, pilots radio "pad control," which assigns the aircraft to a de-icing bay. Because this is a "live" or "engines running" operation, precise terminology and electronic signboards are used to eliminate any potential for accidents. Pilots then contact the "Iceman" in the de-icing control center, appropriately nicknamed the Icehouse.
Once the aircraft is in position to receive the de-icing spray, a machine called the Denmark Vester-gaard Elephant Beta springs into action. Smaller planes might need only one Beta for de-icing, while larger jumbo jets might need as many as four. The CDF has 27 Beta machines, each of which costs about one million Canadian dollars, or about $876,000 U.S. The iceman tells the pilot the exact time de-icing started, the type of fluid used, and when the vehicles have retreated to their safety zones. A safety zone is an area ensuring a safe distancebetween the aircraft and de-icing vehicle. The de-icing vehicles must be behind these lines before an aircraft can exit the de-icing area.
While many airports still employ manually operated "cherry pickers" staffed by ground crew who must brave the bitter winds and back spray, the CDF machines are operated remotely by the Iceman from a heated enclosed cab. They are armed with de-icing fluid, nozzles, whisker-like probes to prevent aircraft contact, and a telescopic boom to reach distant spots and critical flight surfaces.
The de-icing procedure involves spraying fluids that remove or prevent ice build-up all over the aircraft. Strictly speaking, de-icing refers to the removal of existing ice, while anti-icing prevents new ice from forming. Made up of combinations of glycol and water, de-icing and anti-icing fluids come in different varieties that each serve a specific function. The difference between the types of fluid is the"holdover time," or the time from when de-icing commences to the time the airplane must be airborne, based on temperature, precipitation rate, and type. For example, with Type I fluid at -3[degrees]C in light snow, the holdover time is about 40 minutes. For most operations, the de-icing Type I fluid is used to remove the snow and ice, and Type IV is used to prevent further adhering of ice.
As an airplane is being de-iced, all of the extraneous fluid that falls off the aircraft is collected in holding tanks to ensure compliance with environmental regulations, as de-icing fluid can be a hazard to nearby bodies of water. The tanks can hold up to 3,434,237 gallons of reclaimed fluid. Some of the spent fluid is used to make car windshield wash and engine coolant, but it cannot be re-used for airplane de-icing because possible degradation of the fluid means that its effectiveness cannot be guaranteed. Air Canada prohibits the use of recycled fluid.
According to Joe Forbes, Senior Manager of De-icing Operations at the Greater Toronto Airports Authority, a typical Airbus A320 that holds about 150 passengers in light snow conditions requires 80 gallonsof Type I fluid and 69 gallons of Type IV fluid, with actual de-icing time taking just over 4 minutes. The throughput time at CDF for an Airbus is an amazing 12 minutes.
At more than four dollars per gallon for Type I and double that for Type IV fluid, de-icing an airplane is an expensive proposition. During one 3-day ice storm in April 2003, the CDF used 396,258 gallons of de-icer in a single day, the highest amount in the facility's history. At one point the CDF actually ran out of de-icing fluid and scrambled to get more from Chicago, Denver, Forth Worth, and Montreal, Forbes said. One truckload of 4,497 gallons that was brought in from Chicago was dispensed on a single jumbo aircraft. Because de-icing fluid has a limited shelf life once it has been sprayed on an aircraft, pilots consult onboard charts and consider current temperatures and types of precipitation to determine how long they have before they mustget airborne. If the take-off is delayed for any reason, they may need to head back for a re-spray.
In-Flight Ice Formation
Airframe ice does not occur only on the ground. Although there exist some 30 variables when it comes to the formation of ice on an aircraft in flight, the two primary factors are visible moisture (clouds)and freezing temperatures. Clouds contain supercooled water droplets, which are composed of water in a liquid state, even though temperatures are below freezing. When a super-cooled droplet strikes an aircraft, it freezes upon impact. To prevent such freezing, airliners are outfitted with heated leading edge wings that are warmed by the hot air bled from engines. Heated windscreens, instruments, and engine probes and intakes, as well as continuous use of engine igniters, all aid in the battle against ice accumulation.
In turboprop aircraft, electric heaters de-ice the large rotating propellers. Turboprops also have a rubber cover called a "boot" alongthe leading edge of the wing. The boot can be expanded during the flight to break off any ice that has attached itself to the aircraft. In 1994, an American Eagle ATR-72 turboprop plane succumbed to airframe icing while stuck in a holding pattern near Chicago. The plane wentdown in an Indiana bean field, killing all 68 people aboard, after ice on its wings forced it to spin violently out of control. The culprit was a design flaw allowing ice to form aft of the boot.
In the United States alone there are an average of 50 aviation accidents each year involving airframe icing. However, the number of such accidents has decreased in recent years, in part because of the stricter regulations and the construction of more effective anti-icing facilities like the CDF. Most air-frame icing accidents now pertain tolower-tier general aviation operations and private aircraft. In fact, thunderstorms are responsible for more crashes and deaths in the airline industry than icing; in 2004, thunderstorms caused 14 crashes and 28 deaths, compared with 12 crashes and 25 deaths for airframe icing. As improvements continue to be made in airlines' de-icing systemsand engineers continue to find new ways to address airframe icing onaircraft, perhaps one day the risk of aviation accidents caused by ice will be eliminated altogether.
Just the facts about the CDF
Most aircraft de-iced/anti-iced in a day: 513 (February 3, 2000).
Most fluid dispensed in a season: Just over 7.5 million liters (1,981,290 U.S. gallons) in the 2002-2003 winter season.
Most aircraft de-iced/anti-iced in a year: 14,299 in the 2004-2005season.
Most aircraft de-iced/anti-iced in one month: 4,200 in January 2004.
DOUG MORRIS flies the Airbus 340 and is a certified meteorologist.He is the author of From the Flight Deck: Plane Talk and Sky Science, to be published by ECW Press in May 2007. Special thanks to Joe Forbes, Senior Manager of De-icing Operations at the Greater Toronto Airports Authority.
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