Air traffic control (ATC) services have warned that the success of new reduced vertical-separation minima (RVSM) across the North Atlantic is being marred by safety concerns over their inability to discriminate against aircraft not approved to operate within the minima.

When the RVSM was introduced in March, the aim was to reduce minimum separations to 1,000ft (300m) within a reserved airspace from 33-37,000ft on some organised tracks, but only for aircraft with the required level of altimeter accuracy.

ATC authorities complain, however, that they have no legal authority to refuse a clearance at fuel- efficient RVSM levels, even if they suspect that an aircraft can not meet the technical requirements.

John Nordbo, general manager of the Scottish & Oceanic Area Control Centre at the UK National Air Traffic Services (NATS), told the Flight International Air Navigation '97 conference in Amsterdam on 22-24 September that the lack of policing powers was "a real safety issue".

He admits that it could take time for an aircraft, which has been approved by its national authorities, to appear on the RVSM database, but says that there have been clear breaches of the rules by non-compliant aircraft.

Types such as the Boeing 707, Lockheed L-1011 and Ilyushin Il-62 have all been observed flying at RVSM levels, although there is no known modification which would make them compliant.

Russian-built aircraft are causing particular concern, having been monitored flying at 600ft outside their assigned altitudes.

In one instance, a corporate-jet pilot flying westbound was challenged about his aircraft's ability to fly at RVSM levels while making a stop at Gander, Canada. It was established that the aircraft did not comply, yet the pilot later called for, and had to be granted, an RVSM level on the return leg.

Nordbo says that UK aviation authorities have accepted the principle of giving NATS legal powers to deny clearances to suspect aircraft, but adds that if it is to work, the effort would have to involve co-operation from all authorities in the North Atlantic region.

Despite the clearance issue, experience over the first four months in the North Atlantic has demonstrated that the RVSM is on course to achieve all of the targeted benefits, says Nordbo.

There has been a 5%increase in the number of aircraft getting their requested clearances on the Organised Track System, and 14%for those on random routeings.

Financial modelling suggests that the fuel penalty associated with cruising at fixed altitudes has been roughly halved. That translates into a cash saving of about £20 million ($32 million) in 1998, growing to £43 million by 2015.

RVSM has also allowed a 25%cut in the number of organised tracks being used, an important step towards the next goal of freeing airspace for more-flexible and efficient random routeings.

Source: Flight International