The warning on Aerad's North Sea low-altitude chart is stark: "Extensive helicopter operations in this area in support of North Sea oil and gas industry. Aircraft operating below FL100 are advised to maintain vigilant lookout."

For the pilots and rig personnel who spend their lives flying through the region, however, such cautions can never be adequate. This uncontrolled airspace hosts some 40 helicopters flying around 40,000h a year and transporting about 900,000 passengers. Extremely near misses have occurred, which is the reason that the civil-aviation authority of Norway (NCAA) intends on 1 January, 1999, to impose order on the situation in the shape of positive air-traffic control (ATC).

That is not so simple however, because much of the region is outside radar and VHF radio range, and the result is the satellite-communication (satcom)-based surveillance trial which has been running for two years (Flight International, 12-18 April, 1995). It constitutes a system christened Modified Automatic Dependent Surveillance (M-ADS), and resembles the classic ADS concept in all but its compact lightweight avionics, specially developed by Racal for helicopters. The encouraging news is that the trials have shown that the concept works. Indeed, the results have prompted the Netherlands CAA also to begin studying a similar plan for its North Sea sector.

Today, the M-ADS equipment is on a single Sikorsky S-61N and a single Eurocopter AS332L1 Super Puma of Helikopter Service (HS). Avionics programme manager of prime contractor Kongsberg Aerospace (formerly Norsk Forsvarsteknologi), Per Usland, says that, by about June, three more aircraft will be equipped. Those will be an HS Super Puma MkII with a "glass" cockpit; a Norsk Helikopter Sikorsky S-76C+; and another Super Puma MkI for trials in the far north. The additional work is designed to ensure that the equipment can be satisfactorily installed and operated in different helicopter types, and to examine problems with satcoms in northern latitudes. Ten machines could be equipped by 1998.

Usland explains that the equipment specifications were tough, resulting in a 9.5kg satcom unit, consisting of just four modular concept units which can operate in the avionics-unfriendly environment of a helicopter tailboom.


Confidence in the system

He is optimistic that the system will work acceptably in the north, too, despite the challenge of trying to contact a satellite sometimes located only 6 degrees above the horizon, through the rotor disc from extreme low level, with a non-optimum satcom installation. Helicopter operators are interested in the area up to about 72-73 degrees N, and Usland notes that Norwegian fishing vessels have made contact as far as 80 degress N.

The other difficulty which Usland has observed is tied to the parallel development of the advanced ground network used with M-ADS. Norway's M-ADS data is compatible with the Inmarsat Data 3 format used to link efficiently with terrestrial X.25 telecommunications networks - effectively forming part of the evolving aeronautical telecommunications network (ATN).

The single most important element of the ATN is the so-called routers, which ensure that all the data are passed to their correct destinations. Routers, while no longer quite in their infancy, are a distinctly immature technical breed, however, and the Norwegians have had to wait for generic router problems to be solved. The Norwegian programme has been using the same routers as those in the successful ADS-Europe project - the world's only substantial airborne ATN programme. At one point, they could contact only one aircraft at a time, but the issues now seem to have been ironed out.

The NCAA's task in implementing its positive ATC plan also appears to be less of a challenge than it previously did. The difficulty is not so much with the operational arrangements - likely to involve "M-ADS corridors" between 1,500ft (460m) and flight level 85 open only to M-ADS-equipped aircraft - but with the legalities of introducing positive ATC in international airspace. Doubts have been expressed as to whether that is permissible, but NCAA programme manager Tor Helgesen says that his latest legal advice is encouraging and he is reasonably confident that it can now be done. HS project pilot Capt Knut Lande notes that positive ATC has already been implemented west of Bergen - although it is not clear that the precedent is valid. Helgesen says that the latest proposed solution relies on domestic Norwegian aviation law for its justification.

The energy-exploration companies funding the work are also delighted. Jan Taarland, aviation advisor at Statoil, is "very pleased", anticipating not just the reduced collision-risk, because of improved ATC, but also the advantage of knowing helicopters' positions offshore should they be ditched.

Kongsberg, meanwhile, is looking at possible enhancements to the system, including differential-global-positioning-system techniques for navigation, and even using a VHF datalink to implement an ADS-broadcast function giving pilots displays of each other's positions. Even with the basic M-ADS concept, however, the airspace over the North Sea should become a safer place.

Source: Flight International