With planes carefully designed for aerodynamics, a buildup of ice can affect their lift and handling. A crash blamed on ice killed 68 people in 1994 in Indiana, another killed 29 people in 1997 in Detroit.
Investigators know the Buffalo plane's deicing system was turned on and say it appeared to be working. What they don't know is when it was activated or how much time the pilot had to react.
Planes are deiced before takeoff to remove any ice that collects beforehand. Sometimes, they must be deiced in the air as they descend and encounter the necessary mix of temperature and precipitation for ice to form.
If a midair deicing system isn't working, guidelines from the Aircraft Owners and Pilots Association Air Safety Foundation say pilots can take a number of steps, including changing speed, pulling the nose up or down, or trying a 180-degree turn to rid the plane of ice.
It's not known what steps the pilot in Thursday's crash might have taken if he was experiencing ice buildup. But the plane had been approaching the airport, and investigators said Saturday that it apparently was pointing in the opposite direction when it crashed.
Pilots of turboprop planes like those in the Buffalo-area crash must turn on their deicing equipment when they notice buildup. The NTSB wants to go a step further and require them to turn the equipment on when conditions are right for icing.
Then there's talk of an automated ice-detection system like those used in jetliners - but some automated systems can cost upward of $500,000, according to The Wall Street Journal.
The commuter plane that crashed near Buffalo, a Q400 Bombardier, was equipped with rubber bladders that can be inflated by the pilot to crack ice on the nose, wings and tail; the wind then sweeps away the cracked ice.
Many procedures and devices can help ensure icing is not dangerous, and pilots need to keep up on the latest developments, Chealander said.
"You can design everything in the world, but if the human being doesn't use all those things constantly and focus on them constantly, then you can have tragic consequences," he said.
"I'm not trying to draw any conclusions about this accident, I'm just saying in general, we can never let up our focus on all these types of things," he added.
Once activated on the newer model that the Buffalo pilot flew, the bladders inflate and deflate every 24 seconds automatically, a system that NTSB Vice Chairman Steve Chealander called "very sophisticated" at a Saturday briefing.
Not mentioning the Federal Aviation Administration by name, Chealander said the NTSB's recommendations to stiffen rules on deicing have gone unheeded for years.
"We don't like the progress that's taken place right now," Chealander told The Associated Press on Saturday.
FAA spokeswoman Laura Brown said her agency has not ignored the NTSB recommendations and has issued more than 100 airworthiness directives since 1994 that address icing.
"Their concern is this is not happening quickly enough," Brown said. "As with any safety improvement that is significant, we have to go through rulemaking to get there. It takes time."
In 2007, the FAA proposed requiring better ways to detect ice buildup or let pilots know about conditions that could cause ice buildup - in future airplane designs.
It also proposed methods that could help automatically detect ice or potentially icy weather and cue the pilot to turn on deicers. The rule is in the final stages of executive review.
In big jets, crews use heat from the engines to warm the wings and prevent ice buildup. But smaller commuter planes like the 74-seater that crashed Thursday had no such option.
"The big planes are using it off their jet engines," said Justin T. Green, an aviation attorney in New York who has represented the families of victims of air disasters.
The threat of icing looms so commonly that Robert Benzon, the chief investigator of the January splashdown in the Hudson River of a US Airways jetliner, described it as a cyclical menace during an interview last month.
"You do a rash of wing icing accidents," he said. "We rattle our sword. The industry gets its act together and then as time passes, things start to slip and 10 years down the road you get another rash of this type of accident. It's a difficult thing to overcome."
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Contributing to this report were Associated Press writers Adam Goldman in New York and Slobodan Lekic in Brussels, Belgium.
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