NICK HOWARTH / LONDON
Thanks to advances in navigational technology, helicopters, long operationally restricted, are about to take a fuller role in air transport
Imagine an air transport-licensed, instrument-rated pilot flying a certificated twin-engine aircraft allowed to enter terminal airspace only under special visual flight-rules clearance. Imagine an aircraft which is capable of flying a 10¼ approach to a zero-zero landing on a terminal rooftop, but which will be ordered to fly a full instrument landing pattern to the runway threshold. From there it will be directed to hover-taxi along the taxiways to a landing point which is sufficiently remote to make passenger transfer to a terminal less than smooth.
This is the helicopter - arguably, both the most versatile of aircraft and the least able to do its job, at least in the public transport role, where it has long been the poor relation of commercial fixed-wing aircraft. Consigned to grubbing along below the cloudbase since the first type was certificated in the 1940s, the helicopter's relatively low speed and non-pressurised cruising altitudes have kept it from the airways, even after the first twin was instrument flight rules (IFR)-certificated. Even now, the helicopter is only grudgingly admitted into instrument landing system (ILS) patterns.
The problem is that, although the helicopter has been capable of flying itself down to a pre-determined hover above the surface for years, civil machines have not had the navigation equipment to do it accurately enough at the low speeds required by a steep descent angle, and during the transition to the hover.
Technology is now available to enable the helicopter to overcome these limitations. And it comes at a time when, as runway slots become scarcer and perhaps dearer, helicopters may be in a position to make a positive contribution. In the next five years rotorcraft, including the tiltrotor, may finally be able to play a greater role in the wider transport infrastructure.
Eurocopter is integrating avionics and a four-axis autopilot to enable helicopters to carry out precision approaches into airports and city-centre heliports. Installation work on a prototype is under way at the company's Marignane facility near Marseille, France. First flight with the new suite is scheduled for the second quarter, says programme manager Daniel Bouheret.
The "hélicoptère tout-temps" (24h helicopter) is a Eurocopter EC155 with standard autopilot augmented by a new differential global positioning system (DGPS) card for the approach and landing phase. Controlled by a powerful PC workstation, the system is not intended for production, but will generate the three-dimensional information and symbology necessary to prove the concept.
Third dimension
The third dimension is needed because a pilot carrying out a 10¼ approach needs information on hazards, such as wires and masts, as well as the helicopter's flightpath position. Most of this work will be carried out in Phase 2, due to start after the initial flight tests. It will include the integration of an obstacle-detecting laser radar - such as Dornier's Hellas or Amphitech's Oasys - with a head-up display, so the pilot can see the ground as it comes into view. Further flights are due next year, with partial DGAC (French civil aviation authority)certification expected by 2005. The DGAC is partnering Eurocopter, and co-funding the programme as part of the European Commission environmental initiative. Respect 2000.
A 10¼ approach demands a low airspeed and rate of descent to be comfortable for passengers and crew, and Eurocopter is working on a 30kt (55km/h) minimum - around the limit for accurate pitot static readings. Washing that speed off to zero quickly will place demands on autopilot authority that only military fly-by-wire systems, such as that in the NH Industries NH90, can provide. "We expect the pilot to take control during the final 200ft [60m] of the approach, and modify the approach angle to around 6.5¼ before, finally, transitioning to the hover," says Bouheret. "The next generation of helicopters will incorporate some of these military functions."
A ground station will be an important part of the system, both to add accuracy and to ensure there is equipment independent of the aircraft's avionics that the airport/heliport can "own", and which can be inspected, calibrated and approved by the relevant authorities. Eurocopter trials are using an old Airbus ground station which is unsuitable for production - but Bouheret expects an alternative to be available by the time the project matures. "DGPS is suitable and available, but we are looking at achieving even greater accuracy through EGNOS [European Geostationary Navigation Overlay System]," he says.
Sikorsky's pioneering work on precision, hands-off approaches to the hover during the 1990s has slowed, but is expected to pick up again once the S-92 achieves civil certification, due at the end of this year.
S-92 programme director Nick Lappos, who piloted the modified S-76B during the earlier trials, says Sikorsky will soon be applying for US Federal Aviation Administration approval in principle for both wide- and local-area augmentation system (WAAS/LAAS) technologies to be used for hands-off approaches. "Systems such as these offer better accuracy than DGPS can," he says. "Once we have a WAAS or LAAS that works for all situations, we have a product we can present for certification."
Sikorsky trials
The Sikorsky trials used a Special Category I ground station, but Lappos says any DGPS/WAAS/LAAS technologies will work. The automatic flight control system, adapted from that in the search-and-rescue version of the S-76, can be set up for a range of approach angles - from 3º to 9¼ - and can fly the machine to a hover over any point on the ground. The system will even yaw the nose 10¼ to one side, so the pilot can see the landing point below.
Only multi-engine helicopters are being considered for precision approach procedures. In the event of an engine failure, the autopilot would either convert to a go-around (which might involve a 180¼ turn) or, if already committed, lower the nose and take up a single-engine profile until the pilot can take over for a visual landing.
Lappos says no one is trying to make the helicopter do anything it does not do every day of its working life. "All we're doing is substituting instrument cues for visual ones. Given the right weight and conditions, an S-76 can do a single-engine landing on an oil-rig platform. Given an acceptable breakout height, why shouldn't it at an airport?" he says.
Lappos does see the low approach airspeeds as causing problems for the flight instruments and autopilot. "When the airspeed indicator reading drops off, the GPS can take over - it already does in some autopilots. It's just the same as those anti-submarine helicopters. But instead of Doppler [radar] taking over when your airspeed drops off you get GPS. We're just introducing a lot more accuracy," he says. "The approach trajectory is really only limited by the flight envelope - and by crew and passenger comfort requirements."
Specialist training
Lappos acknowledges that the techniques involved will require special pilot training: "Precise flightpath control at lower airspeeds requires techniques not covered during helicopter IFR training," he says.
Among manufacturers, Bell/Agusta Aerospace has carried out coupled runway approaches with both its new AB139 medium helicopter and the old Bell XV-15 tiltrotor prototype.
As a precursor to development of the BA609 civil tiltrotor, XV-15 crews have flown coupled glideslopes of up to 15¼ at 60kt, before transitioning to the hover over a runway threshold. The BA609's Rockwell Collins Pro Line 21 suite will allow coupled GPS non-precision approaches, and can be upgraded to support precision approaches, using both wide-area and local-area augmented GPS services.
Bell/Agusta's AB139 is a more likely public transport contender, in the short term at least - and its Honeywell Primus Epic avionics suite will be available with three- and four-axis automatic flight control system IFR packages including GPS-based flight management system. The flight-test programme is under way, but has not yet progressed towards hands-off approaches.
Honeywell, however, has already obtained civil certifications for fully automatic instrument approaches to the hover over water for SAR applications, and has been involved in DGPS-based approaches to a 20ft hover over land.
There is still a way to go before 19-seat rotorcraft replace 19-seat turboprops on hub-and-spoke operations, freeing up runway slots for Airbus A380s and Boeing Sonic Cruisers. The technology is now in place, however, and regulators and airport operators are, for the first time, being let in on the act as it is being tested.
But no one in the industry is holding their breath - they will believe it when they see it.
Source: Flight International