When area navigation reduced dependence on ground-based navigation beacons and provided a means to shorten flightpaths, carriers pointed out that fuel savings made en route could easily be undone by a prolonged landing approach.
Satellite-based positioning has enabled precise control over the three dimensions of space. But the Single European Sky air traffic management master plan, detailed in April, centres on harnessing the predictability of four-dimensional trajectories by tying together an aircraft's spatial and time co-ordinates, making possible a means to eliminate fuel-wasting extended approaches to airports.
Eurocontrol's latest effort on this front is being carried out at Stockholm Arlanda airport, where a series of flight trials - which began on 5 June with five Scandinavian Airlines services - will enable researchers to assess how airborne controlled time-of-arrival can support future arrival management operational concepts for airports.
The programme has been designated Cassis (controlled time-of-arrival air traffic management system integration studies).
Technical director Volker Huck says its aim is to apply time constraints to aircraft at an early stage of arrival and to explore the extent to which current flight-management systems can manage the process.
Delays to arrival at London Heathrow result in aircraft having to serve out the time before landing in a holding pattern, says Huck, while at Amsterdam Schiphol aircraft are typically put on vectors.
"In this case the flight management system is completely out of the loop. Management through the FMS is useless. And if you're flying level, it's not an optimal profile," he says.
"If you take the idea of a time constraint, the FMS can calculate how to fly it with the minimum amount of kerosene.
"The key challenge Europe is facing in the medium and long term is to decrease the time aircraft have to spend in orbital holding and on extended level segments during arrival."
Four years ago Qantas demonstrated the principle of "tailored" arrivals in conjunction with air navigation services provider Airservices Australia at Sydney and Melbourne.
During the trials, which involved a Qantas Airbus A330 and Boeing 747-400, arrival clearances were delivered via datalink to enable the aircraft to meet a specific time at an entry fix, while at the same time maintaining optimum arrival configuration.
As a sideline to this project Qantas looked briefly at the FMS required time-of-arrival function.
Qantas has demonstrated the principle of 'tailored' arrivals working with Airservices Australia
It performed a handful of flights to obtain a feel for how much time might reasonably be gained or lost over certain distances - between 325km (175nm) and 455km - if the inbound aircraft was instructed to cross a particular waypoint at a much earlier - or later - time than originally expected.
"Flying with a time constraint that might be different from your desired time is a very different thing," says Huck.
He points out that while an FMS is easily capable of operating an efficient continuous-descent approach in low-density traffic, the situation is increasingly complicated by potential conflict with other arrivals, and this demands introduction of time constraints.
"Tailored arrivals and Cassis share a common goal, but in medium-to-high traffic volume you need required times of arrival with considerable time-to-gain or time-to-lose, and that is what we're after."
The initial stage of Cassis will focus on using the required time-of-arrival functionality to help co-ordinate the arrival flow into Arlanda in the morning hours when the airport experiences a traffic peak.
Participating Scandinavian Airlines aircraft have been selected from flights originating from regional points such as Sundsvall, Lulea, Skelleftea and Kiruna, which approach Arlanda's terminal manoeuvring area through northern airspace sectors.
"What we want to do is move the horizon out from about 16 minutes before Stockholm," says Patrick Manzi, from Swedish air navigation service LFV. "By moving it out to 20 minutes, the workload is more evenly spread between sectors, and there's no need for last-minute dog-legs."
Cassis's three test leaders are given a list of inbound flights operating on any particular day and they choose the ones that appear to be good candidates.
In most cases the aircraft are using Arlanda's Runway 26, typically inbound to Arlanda via the HAMMAR beacon, which lies around 75km north-north-east of the airport, although the trials have also used the ELTOK waypoint, which is about 55km north-west.
Participating SAS aircraft are from regional flights that approach Arlanda's terminal manouvering area through northern airspace sectors.
These two fixes are entry points to the terminal manoeuvring area and are the markers to which the time constraints, such as a 1min delay, are initially being pinned. Crews of the candidate flights are given these constraints by voice over air-ground radio and the actual crossing time is recorded at the waypoint.
Cassis's trials are not only testing aircraft with the required time-of-arrival functionality in the FMS, such as the Boeing 737, but also those without, notably the Boeing MD-80.
Results from the 737 operations indicate entry-point accuracy of about 13s, while the MD-80 crews, who have more work to do because of the reduced FMS capability, are nevertheless achieving comparable accuracy of around 20s - figures that, says Manzi, are "more than enough" to satisfy controllers.
By 18 June the Cassis team had performed tests on about 50 flights on approach to Arlanda and this initial wave is expected to draw data from around 100 flights in total, lasting until the end of July. This will be supplemented by a second wave that will encompass up to 500 further flights and run from September to December this year.
The second wave will involve additional special trials, tying the time constraints to other points on the trajectory such as merge points, the final approach fix, and the runway threshold.
Aircraft meeting these lower-altitude points will be subject to different wind conditions than those crossing the terminal airspace boundary. During these trials air traffic controllers will be free to select participating flights. Manzi adds that the team also wants to explore the effects of reducing FMS tolerance.
"If we're able to set up arrivals in advance, this helps with sequencing and we can avoid peaks," Manzi says. "It will allow continuous-descent approaches for a higher percentage of the time."
Huck says: "Our overall purpose is to avoid everyone flying fast into a holding pattern. It's not supposed to increase the landing rate, or enable us to land 62 aircraft per hour instead of 60."
Arlanda is viewed as an "ideal" airport at which to perform the Cassis trial, says Huck, as it combines a moderately busy hub with sufficient manoeuvring airspace. But the Cassis team is hoping to extend its work to Heathrow and Schiphol which, he says, will provide a "good mix" of airport environments.
Heathrow and Schiphol present different requirements. London's high-density airspace means controllers are focused more on reducing the uncertainty of bringing aircraft out of a holding stack.
Dutch airspace is relatively small, which means that pushing the arrival horizon out to a point before top-of-descent, perhaps 370km, generates potential co-ordination problems.
"The target is to develop a roadmap for each site and to define and validate initial short-term implementation steps at each of those sites individually, with the aim of bringing direct benefits to participating airlines," says Manzi.
Aside from Eurocontrol and LFV, the consortium taking part in Cassis includes Avtech, General Electric, Rockwell Collins, and the UK and Dutch air navigation services NATS and LVNL.
By March next year the Cassis team hopes to be able to put forward a concept of operations in line with SESAR as well as an outline of the steps required to introduce controlled time-of-arrival applications.