In Depth | Weather
By Katia Moskvitch13th March 2014
Flight delays are tedious at the best of times. But it’s even worse when it’s cold, and the plane you’re in poised for take-off suddenly lumbers down a taxi way to join a queue of aircraft waiting to be de-iced – as many people in the US can testify during the recent bad weather.
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Ice and aircraft do not mix well. The weight of ice on the wings and fuselage of a fully-fuelled aircraft can prevent it taking off. During the Arctic conditions the US this January, when temperatures dropped as low as -51C (-60F) thousands of flights were cancelled or delayed. Some airports further south, where sub-zero temperatures are rare, didn’t even have de-icing equipment. At O'Hare International Airport in Chicago, Illinois, the jet fuel and de-icing fluids froze altogether, according to American Airlines.
This leads to more than just delays and ill-tempered passengers – it can have deadly consequences. One such tragedy occurred in October 1994 when American Eagle flight 4184 crashed in Roslawn, Indiana, killing all 68 people on board.
Frozen surfaces can sieze - and when a pilot loses control, the results can be deadly (AOPA)
A study several years ago by researchers from the US National Transportation Safety Board found there were 583 accidents and more than 800 fatalities between 1982 to 2000 that could be blamed on ice. And icing is thought to be a factor in up to 10% of fatal airline accidents, according to Nasa’s Glenn Research Center.
So how do airlines make sure that their aircraft don’t ice up – not just during take-off, but the whole flight? The answers range from a kind of “bootie” for wings to a freezing wind tunnel where Nasa engineers blast model planes with icy air.
‘Airline’s responsibility’
Ice usually accumulates in flight when small cloud droplets impact and freeze on the front surfaces – the leading edges – of the aircraft. The ice changes the shape and texture of their wings and flaps, and can also interfere with the flow of air over the plane. It increases drag and reduces the lift forces needed to keep the aircraft in the air.
In these conditions, the plane is likely to come to a complete stall, leading to total loss of control, as happened with American Eagle flight 4184. It had been kept in a holding pattern near the airport, because of bad weather. Suddenly, it found itself going through a patch of freezing rain. The super-cooled droplets hit the aircraft’s fuselage and immediately turned to ice. The rapid and intense build-up of ice quickly resulted in a malfunction of moving wing parts. The pilots stood little chance.
“It’s the obligation of the commander to make sure before a flight that the aircraft is free of ice and is safe to fly,” says Richard Taylor of the Civil Aviation Authority (CAA) in the UK, which is responsible for air safety. Planes should also have the appropriate technology to knock the ice off when in the air, says Taylor. “And it's the airline’s responsibility to make sure that the personnel that removes ice from the aircraft is appropriately trained.”
De-icing is usually the responsibility of engineers, says Taylor, who are trained to recognise which chemicals and other methods to use. The techniques are certainly different from the kind you might use to remove ice from slippery pavements, such as applying grit and salt. Granted, at the right temperature salt will turn the glittery surface into slush and prevent a fall. But for a plane’s aluminum surface, salt doesn’t really work – it’s corrosive. So other approaches are needed.
Planes are often sprayed with a water and alcohol mix on the ground (AFP/Getty Images)
De-icing procedures have been in place pretty much since the beginning of air travel. Back in 1923, American aerospace manufacturer BF Goodrich Corporation came up with so-called de-icing boots – pneumatically-operated covers installed on the leading edges of wings and control surfaces which inflate and crack ice that’s formed in flight.They are still used, mainly on medium-sized passenger jets and transport planes.
While the boots have been in service for almost a century, de-icing technology has not stood still. Many other methods have been tested.
Larger aircraft and military jets tend to use heating systems, which are installed below the wings. These keep them warm and prevent the build-up of ice.
Electrical ice protection systems such as ThermaWing, for example, are used on small general aviation aircraft. They consist of a flexible and electrically conductive graphite foil attached to the wings' leading edge. Electric heaters heat the foil, melting the ice.
Another system is called “weeping wing”: it pumps a liquid based on ethylene glycol into porous titanium panels on the leading edges of the wings. The fluid then flows continuously through these pores during the flight, coating the wings’ surface and preventing the build-up of ice. It works like antifreeze on a car windscreen, and reduces the freezing point of water to about -50C (-58).
One more recent innovation is an electro-mechanical technology called Expulsion De-icing Systems (Emeds), which uses an electrical charge to flex the metal skin of the wing, causing ice to crack and fall off.
Icing tunnels
Advances like these are being made with the help of facilities such as the Glenn Research Center and its purpose-built Icing Research Tunnel (IRT).
The research centre has been inventing and testing de-icing systems since the end of World War II. It is the oldest and largest refrigerated wind tunnel in the world. It measures 6ft-high, 9ft-wide, 20ft-long test section and today it boasts a much larger fan motor, more efficient heat exchanger and fully computerised tunnel controls. It is busy throughout the year trying to come up with new ideas to keep planes free from ice.
The tunnel can produce airspeeds up to 350 knots (648km/h) and temperatures as low as -25°C (-13F), and artificially produced ice – super-cooled water droplets between 15 and 50 microns – form an icing cloud. It is possible to test many full-sized aircraft components and large-scale models, modifying angles of attack – the angle of an aircraft wing moving through air.
The tunnel and Nasa’s Propulsion Systems Laboratory (PSL) can “test turbofan engines in a replicated in-flight ice crystal icing environment, at altitudes of up to approximately 40,000 feet (12,000m)”, says Francis Jennings of Glenn.
Elsewhere, the Chinese Academy of Sciences has developed an anti-icing coating that uses a self-lubricating layer of water between the ice and the sample surface. The coating is made of special polymers able to attract and hold water.
As the temperature decreases, these polymers – known as hygroscopic – start to swell as they suck in water. This way, a lubricating layer of water forms naturally during icing. “Thus the ice adhesion is reduced greatly and the ice formed atop could be shed off by an action of wind,” says the project’s lead author Jianjun Wang, who presented the results at a recent American Physical Society meeting in Denver, Colorado. The approach is very different from conventional anti-icing methods that are often “energy-consuming, high-cost, and environmentally harmful,” he adds.
Ice sensors
One big problem is that for pilots and crew, it can be tricky to spot whether a dangerous build up of ice is actually happening at all. If it is grainy, so-called rime ice – and according to the National Oceanic and Atmospheric Administration (NOAA) guidelines on aircraft icing, about 72% of in-flight ice build-up is just that – then it is possible to see it. But about 21% of ice is so clear it’s almost invisible.
So planes have to rely on special sensors, installed outside the aircraft.
Usually, they are acoustic or optic sensors, such as tiny spectrometers that shine an infrared beam. If there’s ice, opaque rime ice will block the beam, preventing the signal from bouncing off a reflector. And if it’s clear ice, it reduces the beam’s dissipation, making the return signal stronger.
Military aircraft, like this US Air Force EA-6B Prowler, use an electrical heating system to melt wing ice (US Air Force)
By using the same kind of man-made radioactive element found in smoke detectors, scientists might have found an even more reliable method of detecting ice build up – and not just on the sensor, but along the whole wing itself. Ezzat G Bakhoum and his team from the University of West Florida came up with a new system, using alpha particles – bundles of two protons and two neutrons emitted by certain radioactive isotopes as they decay. One of the isotopes is Americium 241 – the same one found in your household smoke detector.
The researchers have said that a 50-micrometre-thick sheet of water or ice can block the alpha particles beam.
The device consists of a thin wafer of Americium, attached to the forward surface of a test plane’s wing, and a mounted aluminium electrode just above it. If the wing is free of ice or water, the beam transfers its charges into the electrode. But even if there is just a 100 micrometres film of water or ice on the surface, the alpha stream will be blocked. It is possible to constantly monitor the charge by connecting the electrode to a transistor. And if the electrode charge is zero, the device will send a signal saying water or ice is blocking it.
It’s innovations like this that might be seen on the aircraft of the future. But in the meantime, if the thought of ice on the wings brings you out in a cold sweat, it might pay to save your flying time until summer.
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