A US probe into propeller-driven planes flying in certain cold conditions may cause concern for Irish air passengers, writes Gerry Byrne.
Are propeller-driven aircraft safe to fly in icy weather? Two leading US air safety agencies are this month squabbling over that question. One, the National Transportation Safety Board (NTSB), which investigates crashes and recommends better air safety rules, wants the other, the Federal Aviation Administration (FAA), which governs US aviation, to produce tougher rules to safeguard passengers and crew in icy weather. It's an issue of compelling interest to Irish travellers because 10 similar airliners, also called turboprops, operate from this country, and, equally, because the last Irish airliner crash was blamed on icing.
On January 31st, 1986, an Aer Lingus Shorts 360, a turboprop commuter airliner, was approaching East Midlands Airport, near Nottingham, when the aircraft unexpectedly started to rock from side to side and lost height rapidly. Two miles from the end of the runway, it crashed. Miraculously, nobody was killed.
An official UK government investigation blamed icing and rebuked the pilots for not using their wing de-icing equipment. It also suggested that over- enthusiastic handling of the controls by the captain might have contributed to the wild rolling, which it could not otherwise explain.
However, the captain said he saw no ice on the wings. An inflatable black rubber gadget called a "boot" along the wing's leading edge de-ices by expanding and cracking off the ice. Unlike larger jets, turboprops lack the power to heat wing surfaces.
Five years later, a Ryanair pilot narrowly averted disaster over Wales by diving his French-built ATR aircraft until he reached warmer air that melted the ice which had formed on the wings.
In October, 1994, at Roselawn, Indiana, another ATR turboprop, this time a larger 70-seat ATR 72, was less fortunate. The pilots spotted ice building up on their wings, and immediately operated the de-icing equipment. It made no difference. After rocking violently from side to side in a manner eerily similar to the Aer Lingus Shorts at East Midland, it plummeted upside down into a soyabean field, killing all 68 aboard.
NTSB experts probing the Roselawn accident were puzzled at how an airliner certified to fly in almost all weather conditions could still crash. They had a surprise coming. When they checked with meteorologists they learned that there were weather conditions on that day with which the ATR, or almost any similar turboprop aircraft, could not cope. More alarming, such conditions were far from unique.
Circling and waiting to land in Chicago, it was flying through rain in temperatures below freezing. Instead of turning into hail or snow, raindrops can cool below zero and still remain liquid. However, they will freeze on contact with a cold surface, like an aircraft wing. When aircraft such as the ATR and the Shorts were designed, it was assumed that the ice would mostly form on the front of the wings where it could be broken off by the de-icing boots. But at Roselawn, the raindrops on that day, called supercooled drizzle drops by weathermen, did not freeze on the wing's edge. Instead, investigators were horrified to learn that they flowed a considerable distance across the top of the wing before freezing in the form of a dangerous ridge of ice which could stretch from the fuselage to the wing-tips. Out of reach of the de-icers, this ridge of ice formed a dangerous interruption to the smooth flow of air over the wing. Bizarrely, the "new" form of icing was well known to meteorologists since the 1980s, yet specifications for anti-icing technology are based on the sketchy knowledge of ice formation from the 1940s.
By interfering with the airflow, the ice ridge could cause the characteristic rocking movement also noted in the Aer Lingus Shorts crash. Similar ice-generated disturbances at the tail affected the elevators, the movable panels used to make the aircraft climb, or descend. And ultimately, the ice destroyed the wing's ability to keep the aircraft airborne, leading to a stall, then a crash.
In the wake of the Roselawn crash, the NTSB urged the FAA to restrict ATR operations in cold, wet weather. Many aircraft remained grounded for almost a month and, when the FAA allowed them fly again, limits were imposed on their use in icy weather.
But the NTSB was also worried about the assumptions about ice employed by aircraft designers. Its worries appeared justified in January 1997, when another iced-up turboprop, this time a Brazil-built Embraer 120, crashed at Monroe, in Michigan, killing 29. Again the pilots appeared to have done things by the book. But was the book wrong? Following an offer by ATR to enlarge the size of the de-icing boots on all ATRs, the FAA relented somewhat and expanded the cold- weather conditions under which ATRs and similar aircraft could fly. They could, for example, once again take off in freezing weather, although they were still forbidden to fly in supercooled drizzle drops (the sort of rain encountered at Roselawn).
Although several airlines fly ATR and similar aircraft on routes to this country, Aer Arann remains the only Irish-registered airline to use the type. It operates a fleet of five ATR 42s which can seat up to 50, and five ATR 72s, which can seat about 70. Aer Arran has never had an accident with its ATR aircraft. Spokeswoman Jennifer Mooney said the airline has satisfactorily completed all the icing boot modifications on its aircraft and flies them only in full compliance with the latest safety standards of the Irish Aviation Authority, and their French and other European equivalents, standards which mirror all the new FAA regulations in the wake of Roselawn and Monroe.
Yet, a decade since the Roselawn disaster, the NTSB remains unconvinced the FAA is moving far enough, or fast enough, to deal with the icing threat to turboprops. For example, the new FAA limits on the ATR and other turboprops specify the size of raindrop to avoid in terms of fractions of a millimetre, but, from inside the cockpit, pilots have difficulties in gauging size simply by watching splashes on the windscreen. Neither can they tell if the water in the drops is above, or below freezing.
There are other indications that the post-Roselawn and post-Monroe restrictions on turboprop flying may not be enough. In 2002 an Embraer 120 got into difficulties after icing up over Florida, while icing is suspected of causing an ATR 72 cargo freighter to dive into the Taiwan Strait in December 2002, killing both pilots.
After missing its own July deadline, the FAA has now promised new regulations for next April, but the NTSB said last month it believes the process will take a minimum of at least a year. Although it has been accused of dragging its feet by the NTSB, the FAA has not been idle. It drafted new rules, although it has so far failed to enact them, saying that the draft was too wide-ranging and penalised aircraft types which already coped well with icing.
"I suppose the fact we are working on a revision of the icing code means that we believe that it can be improved," says Les Dorr, an FAA spokesman. "But don't take that to mean that there are aircraft out there certified for flight in icing conditions which should not be flying." At the University of Illinois at Urbana- Champaign, research by Prof Michael Bragg showed how ice ridges can seriously affect turboprop safety. Now he worries that the resumption of the Space Race (President Bush wants to put a man on Mars) is robbing NASA of funds for further icing research.
"Supercooled drizzle drops still pose a safety problem but we don't yet have the tools to deal with it," said Bragg this week. "I don't think we're moving research along as fast as we might because funding has dried up." Bragg says lives may be at risk, not just in turboprops, but in a new generation of small regional jet airliners he suspects may also be vulnerable to icing.
Gerry Byrne is theauthor of Flight 427: Anatomy of an Air Disaster, published by Copernicus Books, New York
