The 777-300 handles like a smaller, lighter aircraft. We test the latest version of Boeing's fly-by-wire airliner
Peter Henley/EVERETT, WASHINGTON
An earthquake measuring 6.8 on the Richter scale hit the Puget Sound region on the day Flight International was set to fly 777-300, registration N50281, from Boeing's Everett plant. Although some services, such as local air traffic control, were still out of action and Boeing facilities had been damaged, 777 senior project pilot Andy Messer and I took off 24h later in the Rolls-Royce Trent 892-powered aircraft soon set to join Emirates Airlines' fleet, thanks to the Boeing 777 team's concerted efforts.
Inside, the 777 was more tranquil. Placed relatively high above the ground, the cockpit is an impressively modern, light, computer room, dominated by six large liquid crystal displays (LCDs) and three control display units with colour screens. Interactive control is via a cursor control device (CCD), of which there is one for each pilot, on either side of the power lever quadrant. Each CCD has display select keys, a select button and a touch pad, while a palm rest makes it easier to use when manoeuvring or in turbulence. Each CCD controls a cursor with a different appearance so that each pilot can identify his own cursor on a common display. The six LCDs are logically laid out. The instrument panels provide clear mimic diagrams for the overhead system control panels. The flight management mode and function selectors are ranged neatly across the glareshield and can be reached by both pilots.
The 777 cockpit is, however, less of a cybernetic centre than the equivalent Airbus cockpit is. Whereas Airbus has delegated pitch and roll control to a pistol grip sidestick, Boeing has kept a conventional looking control wheel, offering no overt clues that this is a fly-by-wire (FBW) aircraft. Computer-control of the aircraft means the wheel can be smaller than is used with hydro-mechanical systems. Computer technology has also been exploited to reduce the size of the undercarriage and flap levers. They retain their conventional handles shaped like a miniature wheel and an aerofoil section respectively, but the lever for the undercarriage is now little more than a switch.
Logical layout
The 777's large rudder pedals are adjustable for reach via a neat, small fold-away, crank handle between the pilot's legs. A flight bag will fit easily outboard of each seat and several useful panels with clips are on hand to take notepads or approach plates.
The 777-300's 73.8m (242ft) length raises significant considerations for the pilot while ground manoeuvring, taking off and landing. The cockpit is a long way forward of the main undercarriage and must be swung in arcs that are exorbitant distances from the pivot point at the main wheels during tight turns. To help the pilot keep the main wheels on or near taxiway centrelines, video cameras mounted in the tail-plane and fuselage belly provide a Ground Manoeuvre Camera System (GMCS) display. GMCS is a popular customer option and, when selected, gives a three-segment picture of the nosewheels and both sets of mainwheels on the multifunction cockpit display. It helps the pilot to judge tight turns accurately.
Tandem wheels
Each main undercarriage leg has three pairs of wheels in tandem, a configuration prone to resist tight turns. The aft pair of wheels have axle steering to reduce turning radii, minimise thrust required and reduce tyre scrubbing. Because of the aircraft's weight and the engines' high thrust, conservative use of power during taxiing is essential to avoid environmental damage from the jet efflux.
The tiller worked well for a 90° turn as the cockpit crossed the centreline of the taxiway we were turning on to, and the tiller was light and easy to use. The main landing wheel's aft axle steering, which is activated automatically whenever the nosewheel deflection exceeds 13°, was transparent in operation. The mainwheel carbon brakes were smooth, powerful and progressive, but a little fierce when selected to standby for a check.
Bringing this large and complex aircraft to life in preparation for the flight has been impressively simple. Messer switched on the flight instrument and flight management systems using ground power. He then started the auxiliary power unit (APU). The start cycle was automatic, but he selected synoptic displays from the systems menu to show clearly on the multifunction display (MFD) how the APU generator had automatically assumed the electrical load from the ground power supply and provided cabin and cockpit conditioning. Both the engines were started simultaneously. To illustrate the automatic sequence of the change-over from APU to engine generators and hydraulic pumps, plus the subsequent automatic check of systems, all were again monitored on the MFD synoptic pages.
Engine control
The Trent engines have full authority electronic engine control (EEC). Normally the EEC sets thrust by controlling the engine pressure ratio (EPR) in accordance with power lever position as determined either manually or by the auto-throttle. The 'Autostart' selection provides start-abort if a malfunction occurs. In flight, the EEC provides engine overspeed and thrust protection and determines idle speed to compensate for various engine configurations, such as when anti-ice is operating.
For the initial take off from Everett's Paine Field, a flap setting of 20° was used. The all-up weight (AUW) was 220,420kg (485,500lb) against a maximum of 299,370kg and included 63,100kg of fuel. Paine Field's elevation is 500ft, temperature 5°C and surface wind 110° at 12kt (22km/h). The standard airline take-off technique of full power and auto-throttle was used.
Messer advanced the power levers a little above idle to 1.05 EPR and momentarily allowed the power to stabilise. I then pressed a take-off go-around switch on the front face of the power levers and the auto-throttle moved the levers smoothly and uniformly to full power. Once the engines had spooled up and the aircraft had begun to roll, the acceleration was impressive for such a large aircraft. I kept straight using rudder pedal nose wheel steering (the tiller is for taxiing only). The rudder became effective at about 60kt. The scheduled take-off speeds were V1 (decision) 135kt, V2 (rotate) 142kt and V2 (safety) 152kt.
Because the 777-300 is so long, there is obvious potential for striking the tail on the runway during rotation. A retractable tail skid, similar to that used on the 767-300 and -400, is fitted. It has an electrical detector which sends a 'tailstrike' message to the engine instrument crew alert system if a tail contact occurs. Messer says Boeing has been notified of only two inadvertent strikes. Both, ironically, were with the shorter -200.
This good record stems from careful observation of recommended techniques for take-off - and also from the fact that only hamfistedness will result in clouting the tail. Rotation exceeding an 8° attitude is not safe because tailskid contact will then occur with all main wheels still on the runway and the main legs extended. Safe rotation results from a progressive, smooth control movement, aiming for a pitch change rate of about two to 2°/sec. Using this technique, the aircraft became airborne readily and cleanly. Once in the air, the climb pitch angle of 15° demanded by the flight director was easy to establish.
Boeing is about to harness the FBW to deter over-rotation simply by changing the digital flight-control system's software to prevent it. The software can be modified to limit aircraft rotation during take-off and this programme will be underway soon with the 777.
Increased ground clearance
Boeing is working on the design of a "smart gear" modification to artificially increase ground clearance during rotation by about 18in (0.46m), and thus allow shorter take-off runs. Now, as the aircraft lifts off the runway, the six wheel main gear bogies trail under gravity. If bogey angle is sustained by a mechanical link, the aircraft would effectively be "jacked up" as the wing progressively gained lift. This would incur weight and cost penalties, but these would be outweighed by the improved range and payload from currently limiting runways.
Primary flight control of the 777 is via a complex blend of two elevators, a moveable tailplane, two ailerons, two flaperons, 14 spoilers and a single rudder with a large tab. Flaps and slats augment lift for take-off and landing; spoilers are deployed symmetrically as airbrakes and ailerons are drooped for increased lift whenever take-off flap positions of 5°, 15° or 20° are set. The flaperons are located between the inboard and outboard flaps. In normal mode they are used for roll control, but for increased lift the flaperons are moved down and aft in proportion to trailing edge flap extension. Fortunately these complicated co-ordinations are automatic; the pilot uses his primary controls and flap lever as he would in a less sophisticated aircraft, and FBW sorts out the rest.
Well harmonised
The result is light and well-harmonised control forces in pitch, roll and yaw, with good artificial feel. The handling is consistent at sea level or 40,000ft (12,200m). Pitch-trim changes resulting from configuration alterations are smoothed away by automatic trim compensation. The fine tuning of the control gains has clearly been pursued by Boeing to the point of unusually good handling.
Two slight intrusions mar the otherwise smooth handling characteristics, but these fall outside the normal flight envelope of airline operations. Firstly, the 777-300 has sprightly roll performance - roll control being powerful and effective - but at the cost of distinct burble when high rates of roll were demanded. The progressive combining of flaperons and spoilers in response to roll demand was nevertheless seamless and the burble is presumably triggered at higher angles of spoiler deflection. Secondly, a roll from 45° bank to 45° of opposite bank had to be accelerated and decelerated cautiously by the pilot to avoid a noticeable couple with the tail surfaces, presumably because of the long moment arm between tail and wing.
Time constraints and, ultimately, deteriorating weather meant I could only sample the -300 FBW in the normal mode. (There are secondary and direct modes by which the aircraft can be safely flown if the normal mode becomes degraded, but with reduced FBW capabilities).
There are a couple of subtleties when flying the -300 in normal mode. Firstly, if the pilot initiates a turn by applying up to 30° of bank, the FBW applies pitch-trim compensation so that the pilot does not have to apply the usual stick back-pressure to maintain a level turn. For angles of bank over 30°, the pilot does have to apply stick pressure. Secondly, although the control wheel has conventional dual pitch-trim switches, they do not change the angle of incidence of the tail-plane directly in flight; instead they change the trim reference speed. This is the speed at which the aircraft would eventually stabilise assuming no control column inputs. Climbs or descents can thus be made at constant airspeeds, embracing power changes. There is no need to retrim unless airspeed is changed.
FBW provides stall, bank and overspeed protection. "Load relief" prevents flap limiting speeds being exceeded by automatically retracting the flaps. As speed is decreased or increased beyond the normal flight envelope, pitch trim is automatically inhibited and the pilot has to apply a distinct and abnormal stick force to decelerate or accelerate further.
A similar philosophy applies to bank angle protection. When a bank angle of about 35° is exceeded, control wheel force tends to roll the aircraft back to 30° bank or less. This roll command can be overridden, but only by applying force to a distinct and abnormal extent.
The Airbus system, where the rate of roll is limited to 15°/s, differs from the Boeing system which always provides maximum control surface deflection for the most control wheel application. Another fundamental difference is that the Airbus pilot cannot, ultimately, override the envelope protection; in the 777 the envelope protection can be overridden - but only against opposing control forces.
Next, an approach to the stall was made with flaps at 5° and the auto-throttle off. Again stick force markedly increased as speed decreased. I persisted with this back pressure and provoked the stick shaker at 128kt, and a natural buffet at 124kt. Strong cues, therefore, warn the pilot that the aircraft was flying slow. If speed was further reduced, good artificial stall warning from the stick shaker and natural buffet at the brink of the stall itself would provide excellent situational awareness that no crew could conceivably fail to notice.
Differing philosophies
Again, a fundamental difference between the Airbus and Boeing FBW philosophies lies in the engine failure case. With the 777-300, there is a thrust asymmetry compensation (TAC) system, while the A319, for instance, lacks any form of rudder boost. TAC monitors engine thrust and if there is a difference between engines of 10% or more, rudder is applied to minimise yaw. A fertile area for debate is whether the aircraft handling should reveal obvious cues of an engine failure, or should it relieve the pilot of the extra handling during a potentially demanding occurrence?
The 777 was then flown manually on one engine during which its handling was as thoroughbred as in any other phase of flight. The complete electronic checklist for an engine failure (automatically displayed on the MFD) was completed using one of the CCDs. This is much clearer than a paper checklist at points where there is a choice of action, such as 'Is the engine to be restarted? Yes or no'. This interaction clearly shows which choice has been made.
A coupled (autopilot) instrument approach to Moses Lake airport was prepared to culminate in a single engine, autoland touch and go. A 10kt crosswind blew from the left at Moses Lake but the coupled approach and single-engine autoland was flown faultlessly, and there were clear displays of mode (ie localiser and glidepath) and autoland status via legends on the PFD.
Visual circuits
I then flew two visual circuits at Moses Lake, and one simulated asymmetric pattern. Admittedly the aircraft was not anywhere near maximum AUW, but its nimble performance and ease of handling belied its considerable size. The 777-300 is, of course, sensitive to attitude in the flare and at touch down. The recommended technique is to fly at the threshold speed (VAT/VREF) until the audio alert of "20ft", then close the power levers to idle and check the sink rate by raising the nose by only a couple of degrees. I found the landings easy to fly well. The fuselage length leads to the nosewheel being high above the runway at mainwheel touchdown and it must be lowered using elevator before reverse thrust and brakes can be applied.
The 777-300's performance and flying qualities belie its size and the fly-by-wire flight control delivers excellent handling. The cockpit is a comfortable working environment with flight management systems and displays which promote excellent situational awareness and, consequently, the aircraft's safe and efficient operation.
Source: Flight International
