A LEADING air safety expert is warning of a serious flaw in the risk analysis undertaken before rules were changed to halve the vertical distance between planes flying over Europe. He says this means the risks of flying in European airspace are not as low as the public is being led to believe.
The warning comes from Peter Ladkin, a computer scientist specialising in dependable systems at the University of Bielefeld in Germany. He says that Eurocontrol, the pan-European air traffic control organisation, failed to take into account a clutch of air incidents and accidents involving sharp changes of altitude. This, he says, led to an unduly optimistic estimate of the risk when aircraft fly closer together.
The new flight rules, called the Reduced Vertical Separation Minima (RVSM), were introduced on 1 January 2002. By cutting the vertical separation between cruising airliners from 2000 feet to 1000 feet, the change doubled the number of aircraft able to fly in the busy cruising air lanes between 29,000 and 41,000 feet.
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The ability of modern aircraft to maintain their height precisely means the new rules are safe, says Joe Sultana, who heads the RVSM project in Brussels.
But Ladkin says the safety argument failed to take into account other factors that influence an aircraft鈥檚 altitude. In particular, he points to the unpredictability of the response by pilots and air traffic controllers to warnings from ACAS, the airborne collision avoidance systems.
ACAS uses radar on board the plane to look for approaching aircraft, and advises pilots to dive or climb out of trouble. But the reduced vertical distance between air lanes means that this could put the plane on collision course with a third aircraft much sooner than in the past.
Eurocontrol鈥檚 safety case claims the reduced separation will lead to only one collision every 150 years. 鈥淏ut some recent air incidents show that might not be the case,鈥� Ladkin says.
Ladkin points to three cases 鈥� one mid-air collision in which 71 people died and two near misses 鈥� which expose the potential risks associated with RVSM, two of which involved ACAS (see 鈥淔lying into danger鈥�). Ladkin believes that more work is needed to study the way pilots respond to emergency instructions from ACAS. 鈥淚 don鈥檛 think we yet understand the behavioural side of ACAS interactions well enough to guarantee safety under RVSM,鈥� he says. Eurocontrol strongly disputes Ladkin鈥檚 analysis, insisting that ACAS has no bearing on the safety of RVSM.
Using the first 18 months of flight data under the new system, Eurocontrol is updating its analysis of RVSM safety. It expects to publish the result in the next three months, but seems unlikely to change its assessment of the new system鈥檚 safety. 鈥淲e have found that ACAS in RVSM has not caused any significant difficulties,鈥� Sultana says.
Ladkin is unconvinced by Eurocontrol鈥檚 claim that the data will justify its claim that RVSM will lead to no more than 2.5 脳 10鈭�9 collisions per aircraft flight-hour 鈥� or one collision every 150 years. To attain that level of confidence, he says, would take at least decades of data.
Flying into danger
One fatal mid-air collision and two near misses were enough to convince Peter Ladkin that packing aircraft more tightly into the skies is not as hazard-free as Eurocontrol claims.
In October 2000, an Airbus A340 crossing the North Atlantic hit turbulence that upset the plane鈥檚 avionics, turning off the autopilot and forcing the plane into a fast 鈥渂alloon鈥� climb of some 6000 feet 鈥� equivalent to six RVSM flight levels 鈥� before control was regained. The Airbus missed another airliner by just 200 feet.
And in September 2000, four aircraft were involved in a 鈥渟tacked鈥� near-miss over Trasadingen in Switzerland. The planes came dangerously close when erroneous advice from one plane鈥檚 collision avoidance system led to a chain of events that caused the planes to bunch up.
And in July 2002, with the new rules already in force, 71 people died when a Russian Tupolev passenger jet collided with a Boeing 757 cargo plane over 脺berlingen, southern Germany. While the precise cause of that disaster has yet to be established, confusion over ACAS anti-collision advice between pilots and air traffic control is known to have been a factor.
While Ladkin is not suggesting that the new separation rules were to blame in any of these cases, he believes the factors that did cause the accidents should be taken into account when assessing the safety of the reduced vertical separations.
