Tony Booth/Basle
Switzerland's Crossair has completed the installation of head-up displays (HUDs) in its Saab 2000s, thus providing the schedule reliability required for its EuroCross strategy to turn Basle Airport into a major European regional hub.
With careful arrival and departure co-ordination, Crossair now guarantees more than 500 connections a day to destinations throughout Europe from Basle. The carrier's new timetable could prove hard to maintain, however, especially with unpredictable European weather conditions and the ripple effect of delays.
Reliable access at all times to heavily congested hubs such as Zurich has become essential if Crossair is to maintain a failure-free feeder service for parent airline Swissair, which has Category III capability across its fleet. The EuroCross strategy has turned the airline's Basle base into a leading interchange for European regional services, making schedule reliability vital. Crossair has taken a different route to achieving the reliable bad-weather precision approach guidance required for its hub operations.
Needing Cat III approach capability with 200m (700ft) runway visual range, the airline looked at the traditional solution of an automatic landing system linked to the ground-based instrument landing system (ILS), but this would have been costly to buy and maintain. Instead, it chose a HUD for its Saab 2000s.
The airline selected Flight Dynamics' Head-up Guidance System (HGS), not only for its versatility, safety and price, but also for its low maintenance costs and its ability to keep the pilot actively in the control loop while providing situational awareness.
Development of the HGS for the Saab 2000 began in May 1995, with Crossair's first equipped aircraft coming on line in August 1997. The final 2000 completed its fitting out in May - just in time to meet the Basle hub requirements . Six more 2000s on firm order will be fitted with HUDs by February 1999.
Crossair flight instructor Capt Marc Torrent, who was heavily involved in developing and installing the Saab 2000 HGS, says: "Automatic landing is not very smart because all you can do is monitor the aircraft coming down to land in foggy conditions. With the HGS, you can cover the whole flight catalogue, including low-visibility visual reference landing and take-off, without losing any primary flight information."
The HGS management computer integrates information from dual air data computers, inertial navigation, radio altimeter and approach guidance systems such as the ILS. This data is converted into symbology for projection on to the glass combiner, which folds down from the overhead optical unit. The combiner allows the pilot to view flight symbology while focusing on the outside world through the windscreen.
GUIDANCE CUE
The end result is a small green guidance cue displayed on the combiner. The pilot simply needs to cover the guidance cue with the flightpath indicator until touchdown. At 50ft altitude, a cross appears within the cue to direct the pilot to flare the aircraft for landing. In theory, it is accurate enough for zero visibility landings.
A choice of display capability is available: Primary, Cat I, Cat II, Cat III and visual meteorological conditions (VMC). The primary mode gives the same flight data as that of a standard head-down instrument guidance system, with two formats - in-flight and ground. In-flight is the default display and is used for take off, climb, cruise, go-around and other basic procedures normally associated with standard primary flight displays. The ground primary display has a guidance cue and a localiser line which directs the pilot to the runway centreline. Existing regulations restrict airline operations to a minimum 200m visibility for take-off; use of the HGS can reduce this to 100m.
Approach modes are similar to those of the primary display, except that the compass arc is removed and airspeed and altitude are presented as digital values. The VMC mode provides the same approach symbology, except that no cue is displayed. This mode allows the pilot to track an inertial glideslope in the absence of ground-based guidance systems such as the ILS.
Flight Dynamics' HGS has been installed on various types, including the Dassault Falcon 2000, the Bombardier Canadair Regional Jet and the Boeing 737. Equipment varies only slightly between types, but the software and control laws to drive the system must be tailored to each because of the different flight characteristics.
Software development for the Saab 2000 was more difficult than expected because of the marked differences between jet and turboprop performance characteristics. Not only that, but the Saab 2000 has a hybrid control system, with fly-by-wire elevators and conventionally controlled ailerons. This, along with airframe modifications, altered the Saab's flare characteristics and, as a result, more than eight months were spent on developing this last crucial stage of the flight guidance system.
Crossair flight operations vice-president André Dosé says: "The flare characteristics of a jet are easier to calculate because the engines do not create aerodynamic resistance when in idle power. We were also writing the software on an aircraft which was barely in service and constantly undergoing slight modifications."
Crossair's Saab 340s will soon begin to be phased out, so they will not be HGS-equipped. Dosé adds: "Our British Aerospace Avro RJs and McDonnell Douglas MD-80s already have automatic landing systems fitted and would cost too much to convert, but when we talk about new aircraft, talks to include HGS will take place."
The HGS has several safety attributes designed to assist a pilot while landing or taking off. Windshear recovery and guidance information is displayed on the combiner to alert the pilot. If windshear is imminent, a further warning appears and all landing cues are removed. An immediate flightpath out of the shear is then displayed, based on the optimum angle of attack and available energy. This drives a solid guidance cue for the pilot to follow.
'BLACK HOLE' APPROACHES
So-called "black hole" approaches, in which no distinctive horizon or landmarks are visible around an airport, can also be managed. The HGS symbology gives the pilot a horizon and all essential flight data to control the aircraft to touchdown. "White-out" and non-ILS approaches can be handled in a similar way.
Pilot training is a day and a half course followed by a theoretical examination. Accumulating 4h in Crossair's Saab 2000 simulator provides practical experience in a variety of weather conditions. After simulated approaches under supervision, the pilot is accepted on line. Since the first HUD was fitted, all of Crossair's 280 Saab 2000 pilots have been trained.
The HGS was first used by Alaska Airlines in the late 1980s. The carrier needed to improve flying reliability between Seattle-Tacoma Airport - notorious for poor weather - and its Alaskan destinations. It worked closely with Flight Dynamics to develop the HGS, and other US airlines have installed the system. Crossair was one of the first carriers outside the USA to use it and is now the largest HGS operator in Europe.
More commercial carriers - including the former Air Inter, now part of Air France - are adopting a head-up approach on both sides of the Atlantic, and some 50 business jet operators now use the HGS, says Flight Dynamics.
"It is a unique system," says Torrent. "As a pilot, it brings you back into the full manual control which is gradually being replaced in second generation glass-cockpit aircraft. With head-up guidance technology, a man is still in control - and that is the key point of this system."
Source: Flight International
