Flight test: Gulfstream’s G600 long ranger

Launched in October 2014, Gulfstream’s G500/G600 family was designed to supplant its older G450/G550 models. The smaller and shorter-range G500 led the way, making its flight debut in May 2015, followed by the stretched and longer-range G600 on 17 December 2016: the 113th anniversary of the Wright brother’s historic first flight.

The G600’s initially promised range of 6,200nm (11,470km) would have been incomprehensible to those aviation pioneers, whose longest flight that day covered only 260m.

While they are aimed at different segments of the business jet market, the G500 and G600 have a lot in common. Both have a great ramp presence, with

14 large oval cabin windows that instantly mark them out as Gulfstreams. Like the G650, they also have shorter landing gear than their predecessors, giving them a more solid appearance on the tarmac. Previous, non-fly-by-wire (FBW) Gulfstreams have a unique nose-down attitude in their three-point stance.

While range performance remained roughly equivalent between the families, the G500/G600 offer notable improvements over the G450/G550. Their cabins are slightly longer, and fully 18cm wider. They also promise shorter trip times due to cruising Mach 0.05 faster, at the same time burning less fuel.

But from a design standpoint, the clean-sheet models have little in common with their predecessors, instead sharing more with the top-of-the-range G650. Structurally they share a common empennage, and their FBW flight control system (FCS) architecture is nearly identical to the G650’s. Additionally, the flight control laws (FCL) are basically the same. While their flight controls have many commonalities, the addition of an active control sidestick (ACS) marked a welcome advancement.

RESTFUL CABIN

Although they are similar visually, Gulfstream has made numerous improvements versus the G500 when fielding the more capable G600. The company fitted the latter with a bespoke wing that is 22.2% (19.6sq m) larger: a super critical design with a fixed leading edge and large span, single piece trailing edge flaps that also allows for the tankage of an additional 5,000kg (11,000lb) of fuel: a 37% increase.

During flight testing, the larger aircraft performed better than expected, enabling a 300nm bump in range, to 6,500nm. The larger wing also gives the G600 the lightest wing loading of any aircraft in its class, at 397kg/sq m, helping to improve field performance. Befitting a longer-range aircraft, the G600 is 1.5m (4ft 9in) longer than the G500, thanks to fuselage plugs before and aft of the wing.

The G600’s cabin ordinarily features three living areas, with seating for up to 19 passengers. Alternatively, its extra 1.12m of cabin space allows for the addition of a crew rest facility aft of the flightdeck – almost a must-have given its range. An additional passenger sleeping berth is also available, and both new models can be equipped with a forward or aft galley.

A lower cabin pressure altitude is proven to cause less fatigue, so the G500/G600 share the G650’s pressurisation schedule. With a maximum 10.7psi, this yields a remarkably low cabin pressure altitude of just 4,850ft at their 51,000ft operating ceiling: on most airliners this is in the region of 8,000ft.

GREATER SYMMETRY

Over the years I have been fortunate to have flown nearly the entire Gulfstream range, from the G150 to the G650. I was looking forward with great anticipation to flying the G600 from the company’s facility in Savannah, Georgia, for several reasons, mostly focused on the flightdeck.

When I flew the G500 in 2018, I was impressed with its Symmetry flightdeck and the advance it represented over the G650’s capable PlaneView II. The G600’s flightdeck features the same hardware as the G500, but with some additional features and capabilities, to be discussed later in this report.

The cockpit features four 33 x 25.4cm LCD displays, as well as four 25.4cm touch-screen controllers (TSCs). As I detailed in my G500 report, the overhead panel is innovative. There is a row of push-button switches along the forward edge of the panel, used to power up the aircraft. On the overhead itself there are only four traditional panels: engine start; electrical power control; bleed air; and cabin pressurisation control.

Adding to these are three overhead panel touchscreens (OHPTS), each of which can control any one of 13 aircraft and cabin systems, along with six test and maintenance functions. These flexible displays can also show information more commonly presented on instrument panel MFD synoptic pages. Other standout features are a LCD head-up display (HUD), enhanced vision system (EVS) and synthetic vision primary flight display (PFD). Like the G650, the G600’s cockpit is paperless, with JeppView charts and an electronic checklist.

Our preview flight aircraft was serial #73009 (registration N600GS). I accompanied Gulfstream experimental test pilot Howard Judd as he performed the pre-flight walkaround inspection. The large wing was a sight to behold, with its smooth upper and lower surfaces unbroken by control hinges or flap tracks. While rounding the empennage I took note of the size of the T-tail-configured horizontal stabiliser and elevator. They were fairly small, which is no doubt a drag-reducing benefit of the G600’s FBW control scheme.

After boarding the aircraft, I slid into the left seat and adjusted it to attain the design eye position, and dropped the Collins Aerospace HUD to ensure I could see its entire presentation area. The console-mounted forearm rest had height and tilt adjustments, so the ACS fell comfortably to hand. Judd then started the auxiliary power unit, its generator automatically coming online once up to speed. He then talked me through flight management system (FMS) initialisation and other pre-flight steps. The primary FMS interface is through any of the four TSCs.

The TSCs and OHPTS are elevated above their mounting panels, making their frames convenient hand holds in the event of turbulent airborne conditions. FMS initialisation was intuitive, facilitated by its “Phase of Flight” tab process. Gulfstream set a goal of 10min or fewer to prepare a cold aircraft for taxi, which is certainly an attainable mark based even on my unfamiliar hands.

After an easy left to right flow on the forward edge of the overhead panel and FMS initialisation we were ready for engine start. The FADEC-controlled Pratt & Whitney Canada PW815GAs were a snap to start: each reached IDLE approximately 60s after hitting the large overhead panel START switch. Like the commercial geared turbofan engines they share a core with, the high bypass PW815GAs are susceptible to rotor bowing from asymmetric heating after shutdown. But, had cooling been needed, the FADECs would have motored the engines until it was safe to initiate light off.

To ensure checklist completion before leaving the chocks we used Symmetry’s electronic checklist, its auto-sensing feature checking off most items. During the taxi to runway 01 for departure, I re-familiarised myself with the ACS. This would be my third flight with an active sidestick, after the G500 and also Embraer’s KC-390 military transport/tanker.

As I noted in my G500 report, Gulfstream rotates the ACS’s pitch axis outboard by about 3°, to more closely reflect actual arm movement. While stick forces in the KC-390 were symmetric in both pitch and roll, Gulfstream opted to tailor its system to mimic real world arm strength. There is more forward ACS motion than aft, and wrist pronation is harder than outboard supination.

One of the benefits an ACS offers is electronic connection between the left and right sidesticks, so each pilot can feel the inputs made by the other; much like mechanically connected yokes/sticks. Before taking the runway I felt the ACS’s interconnection feature as Judd conducted a flight control sweep.

RATED THRUST TAKE-OFF

Cleared by the tower, I advanced the thrust levers (TLs) and engaged the auto throttle (AT) with the thumb-actuated TL-mounted switch. Pedal controlled nose wheel steering allowed me to smoothly track the runway centreline as the AT set our “Rated” take-off thrust of 93.6% N1. The G600 can perform a “Flex” power take-off, with reduced thrust to lower noise and increase engine longevity, but I appreciated the brisk acceleration experienced using full thrust.

I referenced the HUD as airspeed increased, before Judd called “Rotate” at 119kt (220km/h). I pulled the ACS aft and rotated the flight path marker (FPM) up to the flight director’s (FD’s) guidance cue, which set a rotation angle of about 9°.

Once airborne with the gear and flaps retracted, I followed the FD guidance in the HUD for the 200kt climb. This was intuitive, placing the “winged circle” FPM over the FD cue. Once above 3,000ft I lowered the nose and accelerated the G600 to 250kt in a right-hand-turn towards the Atlantic, where our working area was located.

Passing 10,000ft, we accelerated to 300kt for a climb to 45,0000ft. With only three occupants and 5,400kg of fuel, our gross weight was just over 28,800kg – much lighter than the G600’s maximum take-off weight (MTOW) of 42,900kg. At MTOW and standard conditions the aircraft can climb directly to 41,000ft.

During the climb I performed a series of bank-to-bank turns at up to 45° to gain a feel for the aircraft’s handling qualities. The G600 has a turn co-ordinator, so I was able to smoothly execute these with my feet on the floor.

While some FBW schemes provide pitch compensation for banked flight, Gulfstream FCLs seek to replicate a conventional aircraft, so I needed to increase ACS back pressure to stop the nose from dropping as the angle of bank (AoB) increased. The FMS allows for user-selected climb speeds, and Judd suggested we transition to M0.87 rather than the default of M0.85. According to our safety pilot, Stefan Eling, the higher speed makes for a smoother ride with only a 4% reduction in fuel efficiency. During the climb I also exercised the FMS’s graphical waypoint capability, which would be particularly handy if trying to manoeuver around thunderstorms.

Less than 20min after brake release the G600 leveled at our cruise altitude of 45,000ft, where I made another series of 45° AoB turns. There was ample thrust available to maintain M0.87 during the banking and 1.4g pulls. Remarkably, no airframe buffet was felt during these high altitude steep turns, where I found the G600’s control forces to be low and well harmonised.

Next I engaged the AT and allowed the G600 to accelerate to and stabilise at M0.90, its high speed cruise condition. Total fuel flow was 2,760lb/h at an indicated airspeed of 246kt. Our resultant true airspeed was a very respectable 505kt for the ISA +9°C conditions. Next I slowed the aircraft to its M0.87 intermediate cruise speed, with 236kt indicated. Total fuel flow was 2,580lb/h as we clipped along at a true 487kt. The final cruise point was at the published long-range cruise speed, M0.85, where a 2,480lb/h total fuel flow held an indicated 229kt for a true air-speed of 475kt.

QUIET, AIRY CABIN

Pleased with the cruise performance, I left the flightdeck to sample the cabin environment. The forward section was comprised of an optional crew rest facility, forward lavatory, galley and several closets. Passenger living areas – of which our demonstrator had three – are separated from the forward area by an acoustical door. Gulfstream also offers a four-living-area configuration, which dispenses with the crew rest provision.

The large oval windows give the cabin an open and airy feel and, as could be expected, the furnishings were luxurious. The environment is controlled by a bespoke cabin management system with obligatory app. Ambient noise levels were astoundingly low – while Gulfstream does not publish numbers, it says the G600 has “industry-leading cabin sound levels”.

AUTO-THROTTLE INTERVENTION

After my brief respite in the lap of luxury, I returned to the flightdeck. With the high altitude work complete, we started a descent in preparation for low-speed handling evaluations at medium altitude. In the descent I accelerated the G600 to its maximum Mach operating speed (MMO) of M0.925 at part power (AT off). Ample warnings were provided for the high-speed condition, both on the PFD and HUD.

Approaching the limit, further aggravating pitch trim is prohibited. And on exceeding MMO, a “high speed protect active” (advisory) crew-alerting system (CAS) message alerted me that the FBW protections had kicked in. Although not available when I flew the G500 two years earlier, both new models now have an AT “wake-up” feature. An imminent or actual MMO excursion will cause this to activate and retard the TLs to IDLE.

I clicked off the AT and continued to apply forward ACS pressure. At the limit speed, the FCLs raise the nose to slow the aircraft, and no amount of forward ACS pressure can exacerbate the overspeed condition: a safety-enhancing addition. Should a cabin depressurisation trigger the G600’s emergency descent mode, the AT will retard the TLs to IDLE and extend speed brakes (SBs) to facilitate a rapid descent. It will level the aircraft at 15,000ft, retract the SBs and advance the TLs.

We continued the descent into a 12,000-17,000ft altitude block to set up for three approaches to stall. The first was flown in a clean configuration. Decelerating in level flight at IDLE power, we entered the PFD’s amber speed tape band at 131kt indicated airspeed. Slowing further sounded an audible “airspeed low”. These cautions were purposely ignored as I applied full aft ACS. At the limit, the AT woke up and advanced power to keep us above the normal angle of attack (AoA) limit of 0.94. As with the high-speed condition, I disengaged the AT and held full aft ACS. The aircraft stabilised in a wings level descent at 116kt. Had the pilots been totally unaware that the envelope protection schemes had kicked in, an “FCS AoA limiting” (advisory) CAS message is displayed.

The next approach to stall was in a departure configuration, with gear up and flaps set to T/O APP (20°). Using the same procedures, the amber band was entered at 110kt, with a limit speed of 100kt. The final manoeuvre was flown in an approach configuration, with gear and flaps down (39°). Here, the amber band was entered at 106kt, with the G600 stabilised again in a wings level descent at an indicated 97kt. At the slow speed limit in all three stall approaches, the jet was predictable and responsive to small control inputs in all three axes.

With the area work complete I turned the G600 towards Savannah and accepted air traffic control (ATC) vector to the final approach course for the ILS runway 10. En-route to the field Judd watched as I loaded and activated the approach. Using the phase of flight feature of the FMS we calculated an approach reference speed of 111kt for the 26,300kg G600. With the prevailing winds our target speed was 116kt. I hand-flew the approach, with the AT off. During the approach I found the FD guidance and precise aircraft response to ACS inputs allowed me to accurately track both the localiser and glideslope.

As with my previous G500 experience, the G600 displays speed stability in all configurations. I found the yoke-mounted trim switch easily allowed me to null out pitch forces as we slowed for approach. On several occasions I used the autopilot disconnect switch in its trim speed sync mode, which immediately set the pitch trim to the current airspeed. Engine response to TL inputs allowed me to easily maintain our approach speed of 116kt with flaps set to down.

INDUSTRY FIRST

The light workload on final allowed me to evaluate the G600’s EVS display in the HUD. The HUD has a nominal 42° x 30° field of view, with the EVS’s infrared (IR) view displayed in a large rectangular portion. As detailed by the US Federal Aviation Administration, aircraft properly equipped with an enhanced flight vision system (EFVS) and trained pilots can continue an approach below minimum decision altitude to 100ft above runway touchdown zone elevation if certain runway or lighting elements are visible without the EFVS.

In an industry-first, properly trained G500/G600 crews can fly all the way to touchdown solely with reference to the EVS. While using the feature during approach I was able to make out various topographical features, with land/water contrast particularly prominent. At 300ft, visual approach slope indicators were clearly visible in the display, as were the actual runway edges. The clarity of the IR display continued to improve as we descended, before Judd called for a go-around at 100ft radar altitude (RA).

Our next approach was to RNAV (GPS) runway 01. I hand flew the aircraft as ATC provided vectors to the final approach segment. I found that the large map display on the MFD greatly enhanced situational awareness as we navigated what was, traffic wise, an unexpectedly busy time. Once established on final, Judd retarded the right TL to IDLE to simulate an inoperative engine. Initially I used rudder trim to compensate for the adverse yaw, but once on the final descent path I zeroed it out. At 116kt, very little left pedal pressure was needed to maintain co-ordinated flight, with the left engine at around 65% N1.

At 100ft RA Judd called for a go-around. I smoothly rotated the nose towards a 10° attitude and called for gear up as I advanced the left TL. I fed in left rudder as the thrust was increased to a TOGA setting of 93.6%. Approximately 45kg of pressure and nearly full pedal was needed to counter the asymmetric thrust as we climbed to a downwind for our final circuit and approach.

While I found the G600’s flight controls allowed me to properly manage the simulated engine failure, I think Gulfstream has missed an opportunity. By fully leveraging the capabilities that a FBW control scheme provides, an engine failure at even the most inopportune time could be turned into a non-event.

Once on downwind, Judd used the FMS interface on the TSC to set the auto-brakes to MED. Auto-brake status was displayed as a white message on the CAS. The last approach was a visual one to runway 01, with flaps again set to down. In the turn to final I used the HUD’s FPM and pitch ladder lines to ensure I rolled out on a 3° glide path. About 55% N1 on both engines was needed to hold our target speed of 116kt. Approaching 50ft RA I retarded the TLs to IDLE, with a shallow flare initiated at about 20ft RA. After a small balloon, I relaxed aft ACS back pressure and the G600 settled gently onto the runway. I felt the auto-brakes engage as I lowered the nose gear onto the surface. The aircraft was slowed for runway turn off with auto-brakes and IDLE thrust reverse. The taxi back to Gulfstream’s ramp and shutdown were a snap.

ACCOMPLISHED WITH APLOMB

Having already flown the G500 I was predisposed to be impressed with the G600, and our test flight confirmed my expectations. A 25% range increase from the G500’s 5,200nm to 6,500nm is no small feat, but Gulfstream has accomplished it with aplomb.

The G600’s bespoke wing and tweaking of the FBW flight controls ensure it handles just as well as the smaller and lighter model. It can cruise as fast as M0.9, shortening even the longest legs, while its passengers are coddled with a remarkably quiet and low-pressure altitude cabin.

A dedicated crew rest compartment ensures that even the pilots will arrive rested after a globe-spanning flight. The new auto-throttle wake-up feature of the envelope protection schemes and emergency descent mode are both laudable safety enhancements.

Oval cabin windows alone may be heraldry enough to show the G600 is a Gulfstream, and my preview flight revealed that there is much for passengers and pilots to like.