Europe and the USA are working separately towards one goal - enabling helicopters to make all-weather approaches in public transport operations
A new air of co-operation between international regulators and manufacturers is leading to progress on improving helicopter approach capability to 100ft (30m) minima at Category I airports. The ultimate aim is to enable pilots to make perfect hover touchdowns in instrument meteorological conditions (IMC).
Ken Knopp, of the US Federal Aviation Administration Technical Center, says low-visibility instrument landing system (ILS) approach capability is merely an intermediate milestone on the agency's helicopter-technology research and development roadmap. "The ultimate goal is zero-ceiling, zero-visibility operations," he says.
On the other side of the Atlantic, French aviation authority DGAC is part-funding Eurocopter's "hélicoptère tout-temps" (HTT or 24h helicopter) research programme as part of a European Commission environmental initiative aimed at freeing up airport slots.
Knopp says the argument over integrity of ILS signals at low altitudes will eventually go away, as the airspace system moves away from dependence on ground-based navigation aids. He believes the wide-area augmentation system (WAAS), which uses ground reference stations to correct GPS signals and provide positioning to within 1m, will help the transition while the long-delayed local area augmentation system (LAAS) will be the ultimate solution. WAAS, he predicts, will be the near-term basis for a helicopter-specific low-altitude route structure that will lessen pressure on the air traffic control system. And, despite its recent problems, he also expects progress on LAAS will eventually resume.
WAAS uses 25 ground-based reference stations located across the USA to monitor signals from GPS satellites. These sites send data to two master stations where GPS corrections for specific geographic areas are computed. Correction messages are uplinked to geostationary satellites for broadcast on the standard GPS frequency to GPS/WAAS receivers aboard aircraft flying within the coverage area.
Precision approaches
Waco, Texas-based FreeFlight Systems builds the GPS/WAAS receiver selected for the FAA's Capstone technology demonstration in Alaska. President Steve Williams says 750 WAAS approaches have already been FAA-approved across the USA, but the potential is "at least 4,500". By removing the requirement for ILS hardware at individual airport sites, he says, there are savings to be made anywhere where aircraft need to make instrument approaches. "The technology also removes the final technical obstacle to the goal of achieving precision helicopter approaches to the hover over a given point in space," he says.
The FAA says WAAS is a "critical component" of its objective to create a "seamless" satellite navigation system for civil aviation. The system is designed to improve the accuracy, availability and integrity of GPS, thereby improving the capacity and safety of the airspace system. Ultimately, WAAS will allow GPS to be used as the primary means of navigation, from take-off through to Cat I equivalent precision approach and landing. The only missing piece of the jigsaw is some sort of surveillance service for aircraft ploughing their lonely en-route furrows, and automatic dependent surveillance - broadcast (ADS-B) does that job for Capstone.
While the infrastructure is taking shape, the development of helicopter avionics and flight-control laws that can take advantage of it is not proceeding at the same rate - at least not in the USA. Sikorsky did some work in this area during the 1990s but that is now stalled.
Eurocopter, on the other hand, continues to make strides toward its aim of certificating a helicopter for IFR approaches to the hover, over a given point on the ground and at a defined height, by 2007. At the end of June, the first part of a new phase of flight tests involving a specially modified EC155 came to a successful conclusion near the manufacturer's headquarters in southern France.
With the final flight-test phase taking place as Flight International went to press, ahead of the team lies the tasks of compiling the data and entering certification talks with the DGAC, although co-operation between manufacturer and regulator suggests there will be no surprises for either.
The HTT programme was launched in 1997, leading to a first flight at the beginning of 2002 (Flight International, 5-11 February 2002). The EC155 test aircraft is equipped with a four-axis autopilot using new differential GPS-driven control laws that enable it to carry out steep approaches - up to 10° - to a pre-determined hover height. A computer, essentially a powerful PC workstation, generates the three-dimensional information and symbology necessary to both prove the concept and help the crew safely monitor the aircraft's approach. The third dimension gives the pilot information on any obstacles (such as wires and masts) in the vicinity as well as the helicopter's position on the flightpath.
The bulk of the work under this part of the project was completed during the earlier flight-test phase and involved the integration of a Hellas laser radar, loaned to Eurocopter by parent company EADS, the images from which were projected on to a head-up display (HUD). This allowed the pilot to keep watch outside the cockpit as the ground loomed into view.
Vortex-ring avoidance
A comfortable 10° approach demands a low airspeed and relatively high rate of descent. To eliminate the risk of the aircraft entering vortex-ring state, in which the rotor essentially stalls, the autopilot software needed to be carefully written. The trials were based on a minimum speed of 30kt (55km/h) - about the practical lower limit for accurate pitot-static readings.
Quickly washing that speed off to zero places great demands on autopilot authority, requiring limits that only new-technology aircraft, such as the fly-by-wire NH90 or the fly-by-light EC135 testbed now undergoing trials in Germany, can provide. So HTT project manager Daniel Bouheret admits the approach will not be an entirely hands-off affair. "For a conventional helicopter we expect the pilot to take control during the final 200ft of the approach and modify the approach angle to perhaps 6.5° before transitioning to the hover," he says.
From the start, the positioning accuracy required meant it was necessary to refine the GPS signals via a ground reference station. This hardware has progressed from an obsolescent Airbus differential GPS (DGPS) installation to exclusive use of a ground station developed for the European EGNOS space-based augmentation system at La Fare airfield, near Eurocopter's Marignane headquarters.
EGNOS, the European equivalent to WAAS, uses 34 ground-based ranging and integrity monitor stations positioned across Europe to broadcast GPS error corrections via Inmarsat satellites. The network is now in place in a test configuration: the system is expected to become operational next year and to enter service in 2007 - coinciding with the hoped-for certification of a production all-weather helicopter. The accuracy of EGNOS data is said to be similar to that of a Cat I ILS.
"With the navigation system and DGPS installed, we made the first series of flight tests between October 2002 and March 2003," Bouheret says. "We tested the flight management system functions, based around a three-dimensional route, and all automatic and manual control functions. It gave us good results all round." The flight plan and approach path were presented on the mission display and ground proximity was colour-coded relative to the aircraft's position.
3D approach
"During the second phase, we also tested the autopilot mode that allows the helicopter to make a fully automatic three-dimensional approach, from cruising altitude to a hover at about 12m [40ft] above the ground," Bouheret says.
At the end of the first flight-test phase, the team demonstrated system capabilities to organisations including the DGAC, Eurocontrol, the European Helicopter Association, European Space Agency and interested operators, German and Swiss public service operators DRF and REGA.
The trials team then began working towards a second flight-test phase, dedicated to an obstacle warning system providing both audio and visual alerts. The team developed new symbology for the Thales HUD to provide the pilot with guidance information and obstacle warnings along the flightpath. Both infrared and image-intensification cameras were used and either image could be used as the background to the HUD symbology during the landing phase.
"As far as I know, this was the first time it was fitted to a helicopter," says Bouheret. "It was not tested in flight but in a fixed-base research and development simulator in Marignane, which helps us to develop new control functions and enhance human-machine interfaces. When this was complete this March, we started flying again."
Eurocopter had hoped to evaluate the obstacle warning and infrared/intensified images during a night flight-test phase that was completed at the end of June. Unfortunately, a compatibility problem involving the co-pilot's night-vision goggles, now solved, prevented those tests being conducted.
At the end of May, Eurocopter finished work on the cockpit of its EC155 demonstrator, now equipped with 150 x 205mm (6 x 8in) portrait-format multifunction displays and HUD. It then carried out an assessment of improved 3D navigation and guidance functions and refined the map display.
"We tested HUD functionality for the first time and the OWS [obstacle warning system] based on the Dornier Hellas sensor and Eurocopter avoidance symbology. The flight trials also addressed the ground collision avoidance system based on a terrain database," says Bouheret. "We changed over the pilot displays to see how they worked in horizontal [landscape] 8 x 6in mode - as fitted in the larger EC225/725 - and installed a Goodrich Aerospace laser obstacle awareness system to compare its performance to Hellas."
All-weather capacity
During the final part of the flight-test campaign, now underway, Bouheret is hoping to test the Goodrich OWS; then the head-down symbology in portrait mode; and then assess the full HTT system, by day and night, with a new batch of test pilots. "That will end the demonstration activities of the programme," he says. "We have also started to write a specification for DGAC, to define the equipment needed to provide all-weather capacity in a production machine. We have to complete that by the end of this year."
Bouheret says: "We are very pleased with progress: our improvements have provided pilots with many new functions." To help the pilots use the new functions, a cursor control device has been installed - a trackball mounted to the rear of the centre console. "This enables either pilot to move to different points on the map display, to alter the planned track, waypoints and so on." Most of the testing has been carried out with one pilot, Eurocopter only using a safety pilot at night.
"After our report is prepared, we will work to see how best to develop the equipment for a production version, within certification constraints. We will also work with the DGAC to develop the regulations required to certificate this type of helicopter to fly IFR at low altitudes," Bouheret says. "Finally, we have to address the international aspect and there the debate goes to ICAO [the International Civil Aviation Organisation]. That's where it gets difficult - the technical part is relatively straightforward in comparison. We will start that process about this time next year. I anticipate certification and first customer deliveries before the end of 2007."
As the HTT programme reaches its 10th anniversary there is still a long way to go before a precision approach to the hover is an everyday task for a helicopter pilot but by the end of the decade the ultimate barrier to the introduction of an all-weather helicopter into public service might prove the hardest to crack - economics.
ANDREW HEALEY / LONDON
Source: Flight International
