PARIS AIR SHOW: Perception by wire

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Despite the fact that no new large airliner will ever again be designed with a flight control system that is purely mechanical or hydraulic, control by digital "fly-by-wire" remains an issue with which some of the media still struggle when reporting on aviation issues. In its early days in the 1990s that used to be true also of pilots' perceptions of FBW, but that has reduced now to a peripheral level among those with no direct experience of it.

The same could be said of one of the most obvious FBW-enabled components: the sidestick or mini-stick flight control. Airbus has embraced it completely but Boeing has chosen not to. That visible symbol has always tended to be the lightning conductor for disagreement where it existed. Now that FBW is mature, however, and the industry has refined and deployed it to best advantage, it is easier to argue that the controversy was mainly in people's heads and never had any real substance.

So what of the future for aircraft control systems? None of the manufacturers now disputes the principle that digital FBW systems are optimal in almost every respect for all but the smallest and simplest of aeroplanes. Meanwhile two major airframers have chosen different - but equally valid - FBW system architecture. They also show varying approaches to pilot/aircraft interface and to what aircraft responses the system's normal flight law should deliver to the pilots. Finally, they differ a little over where flight envelope protection intervenes, and a lot over whether it can be overridden.

But the outcome of all of them is that the aeroplanes are much the same to fly manually as they would have been with conventional controls, despite the behind-the-scenes electronic trickery that the pilot is effectively employing to control the aircraft.

 airbus
 ©Airbus

The bottom line is that, when the pilots of modern airliners are managing them using the autopilot/flight management system - which is what happens for about 99% of every trip - there is absolutely no difference, from the flightcrew point of view, between an FBW aircraft and one which is conventionally controlled.

In 1993 Flight International wrote: "Before the dramatic air transport avionics changes in all the major manufacturers' types between 1981 and 1991, there had been no conceptual change in aircraft man/machine-interface design since the 1940s and, arguably, since well before that. Improvement in pre-1980s cockpits consisted of the gradual application of ergonomics - which is important, but not fundamental - and increasing sophistication in electro-mechanical instruments."

We said: "Formerly, pilots had faced no conceptual change in flying or flight management technique at any stage between primary trainer and airliner cockpit." It was these conceptual changes, rather than the actual flying characteristics of the aircraft, which were - and remain - the main issue, because the differences in line pilots' day-to-day tasks in the new machines (compared with the traditional) was wrought far more by the advance of systems automation, advanced avionics and sophisticated flight management systems than by FBW or flight envelope protection.

Today most new business jets and some of the latest regional airliners are - or will soon be - rolling off the production lines with FBW flight control systems, so it is clear that the concept of having the pilots' manual control inputs vetted and - under certain circumstances - modified by a flight control computer system is not at issue. Those issues that are still debated all surround how much control should remain with the pilot, at what point the computers should intervene, and what the manual control interface should be like.

When the debate comes down simply to sidestick versus control yoke in a digitally controlled aircraft, it is difficult to see whether the eventual manufacturer choice is cultural, a matter of conviction, or simply a decision to maintain an aircraft family link that makes cross-crew qualification easier.

If the decision is to go with a control yoke, the cultural theory assumes the manufacturer's choice of a control yoke is driven by the knowledge that their core pilot market has a tradition-related concept of the way flightdecks should look and feel. This affinity for "retro" design, it is argued, is analogous to the preferences of US truckers or bikers who continue to buy new Mack trucks and Harley Davidsons despite - or even because of - their old-fashioned design.

Bombardier's comments to Flight International about the projected Learjet 85 recently are revealing in this respect. On that programme Flight International recently reported: "Bombardier had considered moving to fly-by-wire design and sidestick controller for the Learjet 85, but prompted by customer input, selected a traditional control yoke and control cables, although brake-by-wire will be included."

SIDESTICK/FBW ROUTE

Bombardier's Ralph Acs, vice-president for the Learjet 85 programme, says: "If you're a Learjet owner, you really like the rock-and-roll ride." This is of particular note because Bombardier has decided to take the sidestick/FBW route with its CSeries regional jet. Perhaps Bombardier recognises Learjet pilots as different from airline pilots.

The first flight of a production standard digitally controlled airliner took place when an Airbus A320 climbed out of Toulouse on 22 February 1987. Airbus had, however, been experimenting since 1983 with an A300 testbed fitted with the fly-by-wire computer system and the sidestick pilot controls that would be used in the all-new narrowbody.

In its 30 August 1986 issue, Flight International described a widebody airliner manoeuvre never seen before at the Farnborough air show, but which still wows air show crowds today. This was the now-familiar Airbus low-speed, low-level, high-alpha air show manoeuvre, and it was being performed by the sidestick-equipped, fly-by-wire fitted A300.

The magazine's suitably impressed technical editor ended his word-picture of what he had seen with a rhetorical question inserted as if on behalf of its puzzled readers, but which may have reflected his own scepticism: "Pitched 30° nose-up, flying at 300ft and a mere 95kt, the Airbus banks 25° and begins a gentle climbing spiral. Why is this particular A300 performing a manoeuvre no other airliner could safely attempt-and how?"

Early attempts to explain the purpose and provide the justification for introducing fly-by-wire show just how aware Airbus was of the difficulty many people - including pilots - would have in fully comprehending its advantages. One early Airbus explanation reads like this: "The A320 lives in a box. It can't fly too slow; it can't fly too fast." There are many other flight envelope limitations set by the aircraft's controlling computer systems, but Airbus started by mentioning the simple ones.

In that first technical feature about the A320, the journal represented the main advantages of digital fly-by-wire as being the reduced weight and lower maintenance costs of the flight control system. Flight envelope protection (FEP) was described much later in the article, with a noticeable concentration on quantifying the limits and describing how they were technically achieved, rather than explaining the operational and safety rationales for FEP.

Airbus's senior vice-president flight test division Fernando Alonso, who was new in the company at that time, says that today the most obvious legacy of FBW's introduction is the ability to provide the manufacturer's entire fleet - from the A320 series to the A380 - with basically the same flying characteristics, and with developing advantages like load alleviation and a more comfortable ride. Of course, he points out, FBW also delivers all the other planned advantages, like FEP, plus lighter system weight and greater reliability, so it is difficult to single out one advantage as the principle benefit.

There is a case for saying that, when the early customer airlines and their pilots were introduced to the theory of the A320's FEP system, the knowledge had an unintended psychological side-effect on crews. It was analogous to the announcement of unsinkability before the Titanic's maiden voyage.

The A320 saw only three months in service before its first fatal accident, creating a public relations nightmare for the manufacturer. The second fatal accident took place at Bangalore in February 1990.

The investigators may since have found there was nothing wrong with the equipment in either case, but their reports demonstrated that the pilots had an incomplete understanding of what the A320 would do for them and - more important - what it would not. The world's press homed in on the aircraft's computer control system, aided by the French pilots' union the Syndicat National des Pilotes de Ligne, which claimed there was a conspiracy to destroy data that would have proved the system to be fundamentally flawed. History has shown the SNPL to be wrong.

Alonso admits that the message about how to work with FBW could have been much more effectively conveyed than it was when it was new. Rather than leading with explanations of the engineering concepts by which the FBW system delivered control to the pilots - like the fact that releasing the sidestick to neutral provides a 1g straight and level trajectory - he says: "We should have said: 'Fly this aeroplane the way you used to. Just forget about trimming'."

Since that time the A320 series has been involved in numerous accidents, but the flight control system was not implicated in any. The flight management interface was, however, implicated in some, with flight management system mode confusion mentioned as a possible contributory factor. To put that in context, in the late 1980s and early 1990s, mode confusion was frequently cited also in conventionally controlled types as a syndrome associated with the advent of the glass cockpit and increasing automation.

This was illustrated by an accident involving a Lufthansa A320 during landing at Warsaw in September 1993. Soon after this event a Lufthansa A320 pilot, Oliver Will of the German pilots' union Vereinigung Cockpit, commented: "[The A320] is a beautiful aeroplane. We love it. It's comfortable, it's safe - I never saw an aircraft going through windshear like an Airbus - but why build in so many traps?"

The "traps" to which Will refers were highlighted in the Warsaw accident. The aircraft had landed in a rainstorm with a high indicated airspeed and a tailwind. It aquaplaned and overran the runway, killing two passengers when the fuselage was broken by hitting an earthen berm in the overrun. The cocktail of unfriendly conditions goes some way toward explaining the accident, but nevertheless, according to the report, the incident probably would not have happened had the A320's control laws not delayed deployment of the lift dumpers and reverse thrust although the pilot selected them. The protection systems were designed to prevent deployment of lift-dumping spoilers and reverse thrust until the aircraft had definitely touched down, but because of the high approach speed the wings kept producing lift, so the touchdown switches were not made, and the aquaplaning wheels did not spin up, so the spoilers were inhibited.

Vereinigung Cockpit concedes that the A320's "extremely complex spoiler logic" was only one factor in a long causal chain of events, most of which were not aircraft-related. Will added: "It is quite normal with a new aircraft to have a learning process...we would never point our finger toward Airbus and claim that this is unusual."

Not long after the A320 entered service in 1988, Boeing was preparing to enter the operational ring with its digitally controlled 777, an aircraft which would digitally reproduce for the pilots the characteristics and behaviour of a traditionally controlled aeroplane. Boeing chose to fit a conventional control yoke with artificial feel, although it could have chosen a sidestick.

Unlike all the FBW Airbuses, in which trimming is automatic with the horizontal stabiliser immediately moving to the in-trim position for the attitude that the pilot has selected, in the 777 the pilots select the trim in what feels like the familiar, old-fashioned way. But when the pilot thinks he is trimming the load off, he is actually selecting the reference speed at which pitch control forces are zero.

 boeing
 ©Boeing

For a pilot new to the FBW 777, it feels almost exactly like an unusually well-balanced, conventionally controlled aircraft. In addition to its FEP functions which, by definition, are rarely invoked, the FBW system in normal law makes flying easier by compensating for pitch changes on setting flap, keeping the nose up during turns with normal bank angles, and providing engine-out rudder compensation - which Airbus does not.

The FBW Airbuses, at first impression, are suspiciously easy to fly using the unfamiliar sidestick. The consciousness of difference from a normal aircraft, however, lasts only about a minute. Within that brief period the pilot forgets there is no "feel" in the sidestick, because the inputs required to control the aircraft are exactly what they would be in a conventionally controlled aeroplane, in the sense that they are intuitive and proportionate. Also, within the normal flight envelope, the outcomes resulting from control movements are exactly what the pilot would expect, with the exception of the fact that trimming is automatic.

Like the FBW Airbuses, the 777 has flight envelope protection against overspeed, stall, excessive bank angle. On the 777-300ER/ -200LR/ freighter, it also has tail strike protection. Boeing points out: "The [777] pilots are the final authority for flying the airplane and they can override these functions by continuing to apply control inputs as they find necessary."

ADDITIONAL FORCE

The pilots would do that by applying additional force on the control yoke or rudders to overcome the artificial increase in pressure they will encounter at the flight envelope boundaries. Effectively, the 777 pilots retain the right to bend the aircraft if that is what it takes to save it, whereas Airbus pilots do not have that privilege while they are using normal control law. In the Airbuses, if the pilots put the sidestick on the stops, the system will simply deliver maximum within-envelope performance for the control axes demanded.

The more important common control philosophy in both systems, however, is that the pilots should be so effectively protected from getting into extreme attitudes, and from approaching the edge of the flight envelope, that this drastic remedy should not arise in the first place.

For the future, aircraft manufacturers do not talk about fundamental changes in control philosophy or technology, they talk about taking full advantage of the system they have to improve reactive technology like load alleviation for improved passenger comfort and lessening structural stresses.

Advances in control technology, according to Thales, will all come in the form of improved man-machine interface, with empathic, intelligent systems that work with the pilot to manage the total mission.

The pilot? Yes, one pilot. If customers will accept the concept, of course. What do you need two for when the system will be the co-pilot to the captain you have?