Several years ago, I had the pleasure of flying what was then Gulfstream’s latest offering, the G650ER. The ultra-long-range G650 family was a step-change improvement over its very capable predecessor, the G550. While its performance advantages over the G550 are notable, I was more interested in its fly-by-wire (FBW) flight-control system.
FBW systems have matured and are now state of the art, employed in nearly every new transport category and clean-sheet business jet offering. While I found the G650 a joy to fly, I was puzzled by one seeming omission – a sidestick. Gulfstream had elected to put a yoke in the G650. It felt a passive sidestick, a current industry standard, would not provide the feel and situational awareness enhancements that would come with active sidesticks, which electronically connect both sides, so each pilot can feel the other’s inputs.
More than five years have elapsed since the successful launch of the G650 and Gulfstream has used the interval to incorporate an active sidestick, developed by BAE Systems, into its FBW flight-control scheme. So, the world’s first civil aircraft to fly with active sidesticks is the new G500/G600.
Clear underside makes G500 wing 'a marvel to behold in its simplicity'
Gulfstream
Other than its Gulfstream moniker and accommodation for up to 19 passengers, the G500 has little in common with the G450 it replaces. The clean-sheet G500 cruises further and faster, with a range of 5,200nm (9,630km) at Mach 0.85, besting the G450’s 4,350nm at M0.80. Besides that, it does so while burning less fuel and providing a more spacious cabin; the G500’s cabin size is slotted between the G450/550 and the G650.
While its cabin may be slightly smaller than the G650’s, the G500 does share a lot of characteristics with the type. It features the same signature oval-shaped windows, as well as the same empennage. Both aircraft have FBW control systems, sharing the same basic architecture and similar control law logic.
The G500 comes equipped with Gulfstream’s new Symmetry flightdeck, based on Honeywell’s Primus Epic. What sets Symmetry apart are its displays and pilot interface. It features four large (13 x 10in) LCD displays, two wide-format standby displays, as well as four sizeable (10in) touchscreen controllers which allow the pilot to set up the avionics-related systems for flight management, communications and navigation. The G500 features two Lockheed Martin F-16 fighter-style cursor control devices (CCD) similar to those in the G650, but mounted on the central pedestal rather than outboard on the sidewalls.
With nearly 40 years of flying under my belt I can say that an overhead panel rarely warrants a shout-out, but the G500’s does. As with other aircraft, the panel hosts system control panels, as well as light switches. What is different is how it is done. There are only four traditional panels: engine start, electrical power control, bleed air and cabin pressure control. Added to these are three overhead panel touchscreens, the innovative element of the panel. Each can control any one of 13 aircraft and cabin systems, as well as six test and maintenance functions.
These flexible displays can also show information more commonly presented on instrument panel multifunction display synoptic pages. Other standout features are a LCD head-up display (HUD), enhanced vision system and synthetic vision primary flight display (SV-PFD). Like the G650, the G500’s cockpit is paperless, with JeppView charts and an electronic checklist.
MOTIVE FORCE
Another first for Gulfstream is the choice of Pratt & Whitney PW800-series turbofans: PW814GAs on the G500 and PW815GAs for the G600. The engine features a high-efficiency single-piece fan, as well as stainless steel fan case with a Kevlar wrap.
That the engine is controlled by a dual-channel FADEC is unremarkable. What is remarkable is that thrust-reverser control is an integral function of the FADEC, not a standalone system like on other engines. Like the earlier G650, the G500’s quadruplex digital flight-control system controls all three axes, with three flight-control computers (two primary and one back-up). Each primary flight-control surface has two hydraulic actuators, powered by the aircraft’s left and right hydraulic systems.
For the G650, Gulfstream had developed a set of bespoke flight control laws (FCL), which were the basis for those deployed on the G500. In the pitch axis, the G650’s control scheme changed, dependent on whether it was in a take-off and landing or up and away clean configuration. The G500 employs a single pitch-axis control scheme irrespective of configuration. Like the G650’s up and away laws, it is g-command with apparent speed stability.
FBW systems offer an opportunity to enhance safety in several ways. One is when control laws ensure that the aircraft’s response to control input is normalised throughout the flight envelope, yielding consistent and predictable handling qualities. Another is automated protection from flight-envelope exceedances. Finally, they are also used to mitigate the adverse effects of asymmetric thrust caused by an engine failure.
The G650 flight test showed that Gulfstream had fielded FCL that, on a spectrum from “highly protective” to totally “hands off”, leaned towards the “hands-off” end. In the G650 I found their proportional control scheme (in all three axes) provided crisp and predictable handling qualities, a major boon to flight safety.
As to envelope protections, they were limited to the pitch axis with limited overspeed and robust stall protections. In the engine failure case, Gulfstream opted for a total hands-off approach. An engine loss requires timely and correct pilot intervention to ensure a safe outcome. As I would find out during my preview flight from Savannah/Hilton Head International airport, near Gulfstream’s headquarters in Georgia, this control scheme philosophy has been carried over to the G500.
ENHANCED CABIN AND COCKPIT
Our aircraft for the preview flight was serial number 72005 (registration N505GD), the first production G500. I accompanied Gulfstream experimental test pilot Kevin Claffy as he performed the pre-flight walk-around inspection. The G500’s large supercritical wing is based on the G650’s and was a marvel to behold in its simplicity, with fixed leading edge and large single-panel barn-door trailing-edge flaps.
Besides the flap size, the clean underside of the wing was notable. All of the flap tracks, actuators and hinges are internal, with nothing protruding below the lower wing skin.
Supercritical wing features large single-panel barn-door trailing-edge flaps
Gulfstream
Entry into the aircraft was via the large electrically actuated main cabin entry door, which, like the wing, is based on the G650’s. Once on board I turned right to survey the large passenger cabin. The 12 oval cabin windows let in a large amount of ambient light and gave the cabin an airy feel as I walked its length. In response to customer feedback, Gulfstream has elected to add an additional cabin window on each side of the G500 for aircraft number 7 onward.
Helping me get settled in the left seat was another Gulfstream test pilot, Todd Abler, who would act as an additional safety pilot during the flight. I slid the manual seat forward so that my left hand reached the sidestick and raised it to the design eye position by referencing the centre pillar alignment balls.
The spring-loaded rudder pedals were easily adjusted to comfortably allow for full displacement. As a final check of my seating position, I lowered the standard Rockwell Collins HUD to ensure I could see its entire presentation. Finally, I raised the console-mounted arm support so that my hand rested comfortably on the sidestick.
Meanwhile, Claffy had completed most of the pre-start setup. Once the auxiliary power unit was started, its generator automatically came online. Claffy next pointed out that the bottom row of overhead panel switches support initial power-up/engine start. This was done, he says, to create a simple left-to-right flow for the pilots. A simple push of the APU bleed pushbutton was the final step needed to prepare for engine start.
Once cleared for engine start I placed the right engine’s quadrant-mounted fuel control lever to the run position. That single switch action also turned on the rotating beacons as well as starting the respective fuel boost pump. A push to the overhead panel-mounted engine button allowed the FADEC to start the engine, which reached idle in less than 30sec. Start of the left engine mirrored the right with no pilot action required. With both engines running, the APU bleed was turned off to complete the start sequence. One design goal was that a cold/power-off G500 could be ready for taxi in less than 10min; the overhead panel layout certainly helps.
While the G500 was still in the chocks, Claffy showed me some of the unique features of the Symmetry flightdeck. The touchscreen control interface is a pressure-based one with no audible feedback. Pushing a virtual switch causes it to “bloom”, indicating it has been selected. Lifting off the glass completes the switch action. If your finger moves off the target switch before lifting from the glass, the switch action is not executed.
One thing I did notice about the flightdeck was that it was almost devoid of “guarded” switches. Guarded virtual switch functions on the touch-screens are indicated by a box around their label. Traditional guarded switch action is replicated on the overhead panel touchscreen via a two-step process. First, the switch is pressed and released. Then, a second dialogue box appears asking to confirm the switch action. Overall, I found this mechanisation a good digital substitute for legacy, guarded switches.
SIDESTICK HIGHLIGHTS
Prior to taxi, I familiarised myself with the sidestick. Both Gulfstream and Embraer sourced their active sidesticks from BAE, and I had been fortunate to sample Embraer’s during an earlier KC-390 flight (Flight International, 14-20 November 2017). Unlike Embraer’s sidestick installation, where the pitch axis parallels the aircraft longitudinal axis, Gulfstream’s rotates the pitch axis outboard by about 3° to more closely reflect actual arm movement.
While stick forces in the KC-390 were symmetrical in both pitch and roll, Gulfstream has opted to tailor them to mimic real-world exertion requirements. During my initial full-range control sweep I told Claffy that control forces seemed excessive, but he recommended we reserve judgement until airborne.
With the G500 still in the chocks, Claffy demonstrated the interconnection feature of the sidesticks. He had me displace my stick, with him putting a minor input in the opposite and then same directions. I could feel even the most minor of his inputs, making an “audible dual input” warning redundant.
During our taxi to runway 28 for take-off, I had a chance to evaluate Honeywell’s SmartView synthetic vision system. It combines synthetic vision and moving maps to enhance situational awareness. Moving maps can be presented in a typical 2D or a newer 3D format. While on the ground and at speeds below 60kt (111km/h), the 3D presentation can be viewed from either an egocentric (out of the windscreen) or exocentric (above and behind the aircraft) perspective. At faster speeds and while airborne, only the egocentric view is available. During the taxi, I found airport signage displayed by the synthetic vision system helped maintain positional awareness on what was for me an unfamiliar airfield.
PATH-BASED GUIDANCE
Once on runway 28 and cleared for take-off, I advanced the thrust levers and engaged the auto-throttle. For our 20° flaps configuration, computed V speeds (V1/VR/V2) were 115/120/136kt. As this was my first time in the G500, Claffy would call out speeds 10kt faster than book to provide a little margin should I be overly aggressive in rotation. The engines advanced and stabilised at 91.2%N1.
Acceleration was brisk for the light aircraft, our fuel load of 6,710kg (14,800lb) less than half the maximum of 13,720kg. When Claffy called “rotate” at 130kt indicated air speed (KIAS), my initial pull was indeed a bit too much, an action primed by what I had perceived to be high forces needed during the post-start control sweep. I immediately relaxed the back pressure with the G500 lifting of the runway in an 8° nose-high attitude.
Once airborne with the gear and flaps retracted, I followed the flight-director guidance in the HUD for the 200KIAS climb. The flight-director guidance cue is “winged diamond” in both the HUD and on the panel-mounted PFD. Flight-director guidance is intuitive where one places the “winged circle” flightpath marker over the flight-director cue. Once above 3,000ft, I lowered the nose and accelerated the G500 to 250KIAS in a left-hand turn towards the Atlantic Ocean, where we would work in one of the charted warning areas.
In the climb to flight level 400 (40,000ft), I hand-flew the aircraft and would periodically execute bank-to-bank turns at speeds from 250 to 300KIAS, with my feet on the floor and at bank angles of 30° to 45°. Aircraft response in roll was crisp and predictable, with desired bank angles easily captured. During the higher bank-angle turns I found I needed to add back pressure to keep the nose from dropping; the G500’s flight-control scheme does not include pitch compensation for banked turns, a feature found in other FBW control schemes.
HIGH-SPEED CRUISER
After levelling at flight level 400 the G500 accelerated to and stabilised at M0.90, its high-speed cruise condition. A total fuel flow of 2,920lb/h held M0.90 and an indicated airspeed of 274kt. Static air temperature was -61°C, about 5°C colder than standard, with a resultant true airspeed of 510kt. Gulfstream’s published NBAA IFR range is 4,400nm at this speed.
Next, I slowed the G500 to M0.85 for a long-range cruise condition. At an indicated airspeed of 258kt, total fuel flow dropped to 2,400lb/h. On the cool test day, resultant true airspeed was 476kt with a book range of 5,200nm. These are impressive figures; the G450 had a maximum range of only 4,350nm at M0.80. While this performance is a marked improvement, it does just bring the G500 in line with the capabilities of the Bombardier Global 5000, one of its major competitors.
Once level at flight level 400, I noted a differential pressure of 0.719bar (10.4psi) with a cabin altitude of only 3,020ft. It is well-proven that lower cabin altitudes are less fatiguing. The G500 shares the G650’s 0.74bar maximum differential-pressure cabin pressurisation schedule, which yields a remarkably low cabin altitude of 4,850ft at 51,000ft.
For the flight test, I borrowed a lightweight active noise-reduction headset from Gulfstream. The headset is the same one I use flying the Boeing 737NG. While it provides some relief from the Boeing’s loud cockpit it is by no means as effective as more expensive ear-cupping models I have worn on other business jet test flights.
What I noticed while setting up for upcoming cruise performance points was how quiet the cockpit was. I could have a conversation with Claffy without using the aircraft’s inter-phone.
PITCH ENVELOPE PROTECTIONS
With the high-altitude work complete, we started a descent towards a medium-altitude block for some low-speed handling evaluations. In the descent, I accelerated the G500 to its maximum operating speed (MMO) of M0.925 and, once lower, its 340KIAS VMO.
Ample warnings are provided of the high-speed condition both on the PFD and calibrate air speed (CAS). At the limit speed/Mach the FCL raise the nose to slow the aircraft, as well as prohibiting further pitch trim. At MMO a “high speed protect active” (advisory) CAS message alerted me that the FBW protections had kicked in. Additional forward sidestick pressure had no effect, the G500 could not be oversped. While not active on our aircraft, the latest software loads will also use the auto-throttle to reduce thrust and further slow the aircraft. At both MMO and VMO I put in a number of sharp small-amplitude control inputs in each axis. As expected, the aircraft’s response to these was deadbeat, with no residual oscillations. Speed brake extension at these high speeds caused a minor nose-up pitching moment, easily countered by forward pressure on the sidestick.
After our speedy descent from altitude, we levelled at 15,000ft, where I again used the speed brakes to slow the aircraft, retracting them before extending the landing gear at just below its limit speed of 225KIAS. The flaps were then extended to 20°, a typical configuration for just after take-off. With the power set to 81%N1, the G500 slowed in level flight. For test day conditions the top of the slow speed amber band in the PFD was 132KIAS. Slowing further sounded an audible “airspeed low” warning.
These cautions were purposely ignored as I applied full aft sidestick, slowing the G500 to its alpha limit of 110KIAS for our conditions. At the limit, a “FCS AOA limiting” (advisory) CAS message was displayed. In the G650 this slow speed would have triggered the slow-speed stick-shaker. For the G500, Gulfstream has made significant improvements to the slow-speed protection scheme, fielding a robust system that allows the stick-shaker to trigger at speeds below the red band.
Satisfied with the G500’s slow-speed handling and warnings, I advanced the thrust levers to power out of the slow-speed condition in level flight. As the G500 has apparent speed stability built into it, I used the sidestick’s trim button to relieve stick forces as we cleaned up and accelerated to 250KIAS for our transit back. During the transit, I lowered the HUD and turned on the enhanced vision system. After a short cooling period I found the dual infrared-band cameras presented a crisp view of the low-country marshes and sloughs as we manoeuvred towards Savannah/Hilton Head International.
While at altitude I had familiarised myself with Symmetry’s flight management system (FMS), using the CCD to graphically select and define a number of waypoints in our working area. I had found its operation to be straightforward and quite powerful. Returning to Savannah/Hilton Head International, Claffy helped me install our planned approach, the ILS or LOC runway 1.
The localiser frequency and final approach course were automatically loaded by the FMS: a handy feature. I engaged the autopilot and used its HDG mode to follow air-traffic control vectors to intercept the localiser. Once tracking the LOC inbound, I disengaged the autopilot and hand-flew the approach. While slowing and configuring, rather than using the sidestick trim switch to null forces, I used a neat feature resident in the FCL architecture. The sidestick autopilot disconnect button has another function: it is also a “trim speed sync” button. Pushing the button, regardless of speed or configuration, sets pitch trim to the current condition.
This feature is similar to the trim control switch found on Embraer’s Legacy 450/500 and KC-390. It should be noted that the Embraers only exhibit speed stability in landing configurations, while the Gulfstream does so in any configuration. In the G500 the trim reference speed is displayed to the pilot on the “FLT CTRLS” 1/6 synoptic display to the left of the PFD. In the Embraers, it is displayed directly on the PFD speed tape, a location I preferred. During the approach I found flight-director guidance and precise aircraft response to sidestick inputs allowed me to accurately track both the LOC and ground speed. Engine response to thrust lever inputs allowed me to easily maintain our approach speed of 135KIAS with flaps set to 39° (full).
V2 CUT
At minimums, Claffy directed a go-around. I advanced both thrust levers and selected take-off/go-around for the flight director mode. As I raised the nose and called for the landing gear to be retracted, Claffy retarded the right thrust lever to idle to simulate an engine failure. With the flaps set to 20° and 92%N1 on the left engine, approximately 25kg of pedal force was required to maintain co-ordinated flight in the 160KIAS climb to pattern altitude.
I used rudder trim to reduce pedal forces during the climb-out. Forces were just about nulled out when I ran out of trim authority. Once level on a downwind, with a reduced power setting on the good engine, I centred rudder trim for the approach. As I hand-flew the pattern, Claffy loaded the RNAV (GPS) Z runway 28 approach. I planned on using its guidance for the final segment of the simulated single engine approach. Target speed was again 135KIAS as the approach was flown with flaps 39°. As with the prior approach, the G500 was stable and responsive on final. At 100ft Claffy called for a go-around, both engines used for the climb-out to enter a visual left-hand circuit.
My last approach in the G500 was a visual to a full stop on runway 28. As with the prior two approaches it was flown at flaps 39°. I flew the entire circuit without reference to any charted approaches, the precision approach path indicators providing a good reference for a 3° glidepath. Had path indicators not been available, the pitch ladder in the HUD would have provided a handy reference for a 3° glidepath.
At about 40ft radio altitude I retarded the thrust levers to idle, and started the flare manoeuvre passing through 30ft radio altitude. I said to myself: “Swing and a miss,” as we were several feet too high. I then milked the G500 to the runway. Had I dropped it in, the trailing link main gear most likely would have absorbed any untold sink rate and made me look like a seasoned Gulfstream pilot. After flying the nose to the runway, I deployed the thrust reversers and applied moderate toe braking, which rapidly slowed the G500 to a safe taxi speed. The taxi back to Gulfstream’s ramp was uneventful with the shutdown and post-flight checks easily accomplished.
When I had approached my G650 flight several years ago I was interested in how Gulfstream would implement its first full FBW control system. As I detailed in that test, I found its proportional control schemes and limited envelope protections made it a joy to fly.
With the G500 they have taken the flight controls a step further. The active sidestick allows for accurate and precise control inputs, while freeing up valuable real estate on the flightdeck. Electronically tied together, the sidesticks enhance situational awareness by keeping the other pilot in the loop.
I found the Symmetry flightdeck a marked improvement over the very capable PlaneView II in the G650. The passenger experience has also been upgraded with a larger cabin. After what can be a 5,200nm flight, passengers will no doubt arrive more refreshed owing to the G500’s quiet cabin, low cabin altitude and M0.85 cruise speed.
With US Federal Aviation Administration certification on the horizon, it looks certain that the G500 will be pleasing passengers and pilots alike, Gulfstream having raised the bar for the super-large segment.
Source: FlightGlobal.com
