Aircraft take off safely in wind and rain every day. But aviation experts say there are times when conditions are so severe or unpredictable they can foil even the most experienced pilots and sophisticated aircraft.
It remains unclear what may have caused a Singapore Airlines 747 jetliner to slam down on a runway during take-off from an airport in Taipei, Taiwan today. But experts agree if weather conditions did cause the crash, the conditions most likely appeared very suddenly.
“If you’re already on the ground there isn’t much incentive to proceed under marginal conditions,” says George Hamlin, an aviation consultant at Global Aviation Associates in Washington, D.C. “And usually a pilot is aware of marginal conditions, so accidents during take-off are rare.”
Reports suggest that heavy rains were soaking the city of Taipei on the day of the jetliner’s crash. But Jim McKenna of the U.S. Aviation Safety Alliance points out that such conditions usually don’t deter pilots.
All airliner companies maintain their own guidelines regarding acceptable weather conditions for take-off and landing, but it’s the pilot at the controls who makes the final decision whether or not to proceed.
“It’s not unusual for aircraft, particularly the 747, to take off in high winds and rain,” McKenna says. “It usually poses little problems so this sort of incident is unusual.”
One possibility is that a deluge of intense rains could have interfered with the pilot’s visibility once the jetliner began speeding down the runway. Reports say the plane’s pilot recalled hitting an object on the runway while trying to take off from the airport.
Paul Czysc, an aerospace engineer at Parks College of Engineering and Aviation in St. Louis, Mo., says that a 747 jetliner must reach a speed of about 150 miles per hour before lifting from the runway. While on the ground, Czysc says, a pilot relies almost exclusively on his or her own vision to keep the craft’s nose aimed straight. If rains are intense, the water striking the aircraft’s window can seriously restrict visibility.
“You know how much a downpour can limit your vision while driving a car,” says Czysc. “On this aircraft it’s much worse.”
He says either striking an object or suddenly aborting the take-off could have lead to the jetliner’s reported damage of charring and a gaping hole in the roof of its forward section.
Engines Flooded by Rain?
There is also a remote chance that heavy rains entering the jetliner’s turbo jet engine could have extinguished the engine’s fire. Czysc says that once rainfall exceeds about two inches per hour, it can douse the engines and halt their firing.
“A turbo jet engine swallows about 200 pounds of air per second,” says Czysc. “If more than 10 percent of that is water then it can extinguish the combustion chamber.”
McKenna believes this scenario is unlikely since most 747 aircraft are able to take off with one engine out since there are at least three other engines in place.
Vertical Wind Shifts
Another possibility is the pilots flying the aircraft encountered a vertical wind stream called a microburst.
Czysc explains that microbursts commonly flow out of nearby hurricanes or typhoons, as the weather systems are called in the Pacific. Typhoon Xangsane was reportedly approaching the island early Wednesday and had already begun dousing Taipei with heavy rains.
“Most radar can’t predict a microburst,” says Czysc. “They can occur for 30 seconds and then disappear and jump someplace else.”
Microbursts are a form of windshear — or a sudden change in wind direction. When these quick vertical gusts are directed downwards, McKenna explains, they can disrupt the lift of an aircraft and force it down.
Finally, it is also possible the pilots may have encountered a malfunction in the aircraft. But Czysc, who has inspected Singapore Airlines facilities in the past, considers that scenario highly unlikely.
“Statistically their engines have been extremely reliable,” he says. “So I suspect the aircraft encountered an external problem.”