By now, the aircraft that operate in Europe’s upper airspace should be flying in an environment that promises 20 percent greater air traffic capacity, a reduction in commercial flight delays and enhanced fuel economy. On Jan. 24, 2002, reduced vertical separation minima (RVSM) procedures were to be implemented, creating six additional flight levels between FL290 and FL410 over 40 European states. Eurocontrol’s policymaking body, the Provincial Council, gave the green light last July to the agency’s first European-wide project, a critical first step toward the continent’s "single sky" concept.
Since then, commercial aviation suffered an international shock in the aftermath of Sept. 11, 2001. During its November 2001 meeting in Brussels, the Provincial Council took initial steps to repair the damage 9/11 has caused to European aviation.
Although the meeting’s main focus shifted to air travel safety and security, the council still deemed the airspace capacity issue to be pertinent. Its members said that while the flight delay problem was showing improvement over 2000, even before 9/11, there is no room for complacency. Thus the council and member states agreed to proceed with RVSM on Jan. 24.
Reducing vertical separation minima from 2,000 feet to 1,000 feet followed considerable planning and preparation, as Avionics Magazine discovered from an interview with Eurocontrol’s RVSM program manager, Joe Sultana. He coordinated the RVSM work within the states engaged in the project. Each state also has an RVSM manager.
RVSM was introduced over the North Atlantic in 1997 and subsequently over the North Pacific. But Europe’s transition to RVSM will be the first implementation over a heavily populated land mass and in one of the world’s busiest airspace segments. And unlike oceanic traffic, which largely follows parallel east/west routes, Europe’s crowded skies are highly dynamic. Other regions planning RVSM, therefore, can learn a lot from Eurocontrol’s experience. The agency had to:
Assure that aircraft and ground facilities were properly equipped,
Train air traffic controllers,
Consult with the pilots’ union,
Study the impact of turbulence, and
Establish an interface with other airspace.
To ensure the safety case for RVSM, Eurocontrol had to demonstrate that the technical target level of safety (TLS) of no more than 2.5 x 10-9 fatal accidents per aircraft flight hour could be met and maintained. So Eurocontrol established a height-monitoring infrastructure, based on International Civil Aviation Organization (ICAO) requirements and North Atlantic RVSM experience.
The Monitoring Program
The agency’s monitoring program commenced on May 25, 2000, with the first operational height monitoring unit (HMU) established at Linz, Austria. The Nattenheim (Germany) and Geneva HMUs became operational on Nov. 9, 2000. An HMU set up for North Atlantic RVSM exists in Strumble, UK, as well.
The HMU is a passive, ground-based system that gauges the altitude of aircraft flying within its coverage area. It consists of a set of ground stations arranged as a central site, with four additional receivers arranged in a square. Each site receives aircraft secondary surveillance radar (SSR) transmissions, from which the aircraft’s three-dimensional position is derived. The HMU calculates altimetry system error using meteorological information and the Mode-C/S height data.
Aircraft had to be equipped and approved for the new minima by March 2001. Eurocontrol needed to monitor the height-keeping performance of aircraft prior to RVSM implementation to produce a safety case that was the basis of the Provincial Council’s "go" decision.
Each aircraft has to be approved and fitted with the following: two independent, cross-coupled altitude measurement systems; one automatic altitude control system, to within ï¿½65 feet; one altitude alert system; one SSR altitude reporting transponder; and RVSM-compliant avionics. Other requirements include physical checking of static port and pitot-static probe installations.
Monitoring the properly equipped aircraft didn’t necessarily go as planned. "We should have monitored 90 percent of the flights made by operator/aircraft type combinations," says Sultana. However, some aircraft were not properly equipped and some equipped aircraft did not fly over the HMUs often enough. So, Sultana admits, "We went for a ‘go’ decision with not enough aircraft having been monitored."
However, Eurocontrol supplemented its data from HMUs by targeting the non-equipped operators/aircraft types and using a portable GPS monitoring unit (GMU). The GMU is a carry-on system brought onboard the aircraft; it functions independently of aircraft systems.
Was the pace to implement RVSM too fast? "No", replies Sultana, though he concedes that operators pressured Eurocontrol to expedite RVSM implementation to relieve the problem of flight delays.
Lufthansa’s perspective is typical of the operators’ view. Jurgen Raps, the carrier’s senior vice president, flight operations manager and chief pilot, tells Avionics Magazine that his company welcomes RVSM’s introduction.
"Our fleet was certificated for [RVSM] flight operations when airspace restructuring commenced a monitoring and trial phase in Europe in April 2001," he says. Lufthansa has "persistently advocated early introduction of RVSM…Its introduction will bring a high cost/benefit effect."
Jumping the Gun
In fact, the UK introduced RVSM in its domestic airspace in April 2001 to avoid conflict with the opening of its new Swanwick Area Control Center. And Germany and Austria introduced tactical RVSM prior to Jan. 24, 2002. This limited implementation allowed controllers to give clearances for 1,000-foot separation but did not alter the flight planning process.
Though the operators sought quick RVSM implementation, pilots were wary of yet another new technology. So RVSM program officials kept "close contact’ with IFALPA [the International Federation of Air Line Pilots Associations], says Sultana.
"Their concerns were mainly over wake vortex, transition between non-RVSM and RVSM areas, and the adequate performance of airborne collision avoidance systems [ACAS]," he adds.
Concern over vortex problems led to the formation in August 2000 of a wake vortex data collection program, using aircraft flying above FL245. By July 2001, 24 reports of wake vortices had been filed, roughly one every 15 days. Eurocontrol made an initial assessment of the reports and then sent them to a university for analysis. The results to date suggest no cause for concern.
Eurocontrol also considered the difficult problem of wake turbulence and of mountain wave effects over the Alps. The agency sited an HMU in Geneva to identify mountain wave effects and found that, under normal conditions, they pose no problem.
But not so for clear air turbulence–which is why Eurocontrol established contingency procedures for controllers and supervisors to suspend RVSM upon learning of turbulence reports. This would not mean reverting to the old system of flight levels but would entail 2,000 feet of separation or more. Suspending RVSM would create a capacity restriction, calling for flow confinements to reduce the number of aircraft in the sector, says Sultana. Eurocontrol, therefore, has asked each state to ensure that supervisors do not execute this procedure ad hoc but in concert with other states.
Eurocontrol had to account for two types of interfaces. One interface is between European RVSM and the North Atlantic RVSM–because there is one significant operational procedure difference between the two adjacent regions. Over the North Atlantic, non-RVSM aircraft can climb through FL290 to FL410, but this is not permitted in European RVSM, except in the transition airspace. The other interface is between the European RVSM and Russian Federation airspace, which uses its own set of measurements for vertical separation minima. (The Russians have indicated that they intend to accept the ICAO metric standard when they change to RVSM in 2004.) Eurocontrol addressed the transition out of European RVSM by having flight information regions (FIRs) designate the airspace in which the transition is to be performed.
Pilots also wanted assurance that their ACAS systems would help avert any midair mishaps in RVSM airspace. ACAS version 6.04 was of particular concern; therefore, Eurocontrol proposed a 5 percent reduction in capacity, should ACAS become a problem. However, many aircraft are fitted with the more capable ACAS version 7.0, and feedback from early introduction of RVSM in UK airspace confirmed that it works effectively. Eurocontrol concedes that pilot awareness of the RVSM program should have been raised earlier. "Perhaps it is Eurocontrol’s fault or maybe it wasn’t," says Sultana. "We were planning an awareness campaign for January 2002, [but] when the UK, Germany and Austria began to introduce the technique, we were left slightly at the starting block."
The agency tried to correct this delay by producing a CD-ROM and video for training captains on how to carry out the checks and flight planning. It also offers workshops for flight planning organizations and flight operations departments. A special European RVSM Web site (www.eur-rvsm.com) is another source of continually updated material.
Inevitably, however, RVSM has a bigger impact on ATC. UK authorities, for example, worried that controllers might revert to past procedures when under pressure. So, to ensure an acceptable workload, the authorities reduced flow rates in some sectors by up to 20 percent during the first two weeks of RVSM operation. Subsequently, regular reviews have taken place and flow rate reductions of up to 15 percent were maintained.
There also remains the concern that air traffic controllers cannot train for the new procedures more than three months in advance and still retain currency. A training program for controllers was launched in October 2001.
The controllers must become comfortable with the ATC system software modification, which ensures that they can check the approved or non-approved status of an aircraft. After being granted RVSM approval, operators must file a "W" in field 10 of their flight plans to facilitate this identification.
ATC simulations have been conducted to make controllers aware of government-owned aircraft, which are not required to be RVSM-equipped and thus must maintain 2,000-foot vertical separation. Indication of these airplanes’ status appears beside the radar label on the controller’s screen.
Factoring in 1 Percent
Eurocontrol carefully considered the inclusion of government-owned aircraft. "We could have overreacted," says Sultana, "by saying [government-owned aircraft that are not properly equipped] are going to be excluded [from RVSM airspace]."
Instead Eurocontrol conducted an extensive review of government aircraft activity. The agency found that government aircraft account for 1 percent of the flights above FL290. The U.S. Air Force is a major operator in that airspace; however, most of its transport aircraft already are approved for RVSM, and the USAF plans to have the remainder of the fleet approved by 2002. Other air forces–French, British, Dutch and German–are not so far advanced, but have plans to modify their transport aircraft. The 1 percent would be problematical only if all were to operate in RVSM airspace at the same time in one sector. Eurocontrol research indicates that only about two air force planes are usually in a sector at one time, and this does not significantly impact controller workload or sector capacity.
Eurocontrol has learned plenty from its RVSM experience. Mindful that reduced vertical separation minima "will eventually be global," Sultana emphasizes the agency’s capacity to assist other regions that plan to convert to RVSM.
Would reducing lateral separation be less troublesome than RVSM? In fact, it is much more complex, according to Eurocontrol’s Joe Sultana.
Lateral separation is a navigation accuracy issue, with a direct bearing on the route network, he explains. Eurocontrol has conducted studies to look at closely spaced routes. It seems that you cannot reduce the lateral separation of aircraft enough to create another route, which would be the primary benefit, says Sultana.