Pilot Report :Flying the 737-8

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Pilot Report: Flying the 737-8, Boeing’s New Narrowbody Breadwinner

There is more to Boeing’s MAX than new engines, but it is still a 737, our pilot finds

May 12, 2017 Fred George | Aviation Week & Space Technology 

 

The 737-8, marketed as the MAX 8, is set to enter service in early June with the first delivery this month of more than 3,700 orders for this third generation of the world’s bestselling jetliner. Fifty years after the 737’s first flight, Boeing will pass the 9,500-deliveries mark in May. And that number could easily be eclipsed if the aircraft stays in production until 2030, as planned.

The Guppy, a popular nickname among pilots of Boeing’s smallest jetliner, has been in continuous production since 1967, the longest manufacturing run of any airliner. Significant advances in engine technology over the last half century—up to the CFM Leap 1B turbofans that power the MAX—are a prime reason for its ongoing success.

Aviation Week had the opportunity to fly the 162-passenger 737-8 from Roswell, New Mexico, with Keith Otsuka, Boeing’s test and evaluation chief pilot, to experience the differences between it and the 737 Next Generation we flew nearly two decades ago. Most important for operators, the -8 can fly 14% farther on virtually the same fuel quantity as the latest 737-800.

Giving a Best-Seller a New Lease on Life

Improvements in 737 MAX reduce fuel consumption 14% over 737NG

Reengining with CFM Leap 1Bs contributes the biggest fuel saving

Rest comes from new winglets, recountered tail cone and other refinements

Upgrades include large-format cockpit displays and fly-by-wire spoilers

The Leap 1B turbofans account for most of the range gain. Dual-surface, laminar-flow winglets contribute 2% over the blended winglets fitted to NG-series 737s. A recontoured tail cone, removal of aft-body vortex generators, revised auxiliary power unit (APU) inlet and exhaust, a new bleed air system computer, and numerous small aerodynamic improvements account for most of the rest.

With a seats-full, tanks-full range of more than 3,500 nm, the 737-8 opens up more long, thin routes between new city-pairs such as Washington-Anchorage, New York-Panama City, Buenos Aires-Panama City, Singapore-Tokyo, Dubai-Kuala Lumpur, and Mumbai-Moscow.

 

The MAX 8 can fly 162 passengers between city pairs such as Washington-Anchorage and Singapore-Tokyo. Credit: John D. Parker/Boeing

The aircraft has a 40% smaller noise footprint than its predecessor, despite its 7,000-lb. heavier maximum takeoff weight. The Leap 1B engines also produce 20% lower carbon emissions and 50% lower nitrogen-oxide emissions compared to the CFM56-7BE turbofans that power the 737NG.

The single-aisle jetliner market is more competitive than ever, with several new or reengined aircraft being offered by different manufacturers. However, the contest for the 150-200-seat, 3,000-4,000-nm segment boils down to a battle between Airbus’s A320neo and Boeing’s 737 MAX families. Airbus has a head start. The 737-8 is the first of the MAX family, and despite a last-minute hold on flight tests in early May caused by inspections for turbine-disk defects in some engines (see page 22), deliveries remain on track to begin 17 months after the A320neo entered service in January 2016.

Boeing asserts that the -8 can carry more passengers a longer distance than the A320neo, that the 737’s dispatch reliability at 99.7% is second to none and that its seat-mile operating costs are the lowest in class. The 737-8 has an 8-ft. longer cabin than the A320neo and can accommodate two additional seat rows for 12 more passengers. And it has a lighter empty weight, which combined with the higher maximum takeoff weight provides a greater useful load.

Similar to the NEO, most of the MAX’s systems are carried over from the previous-generation aircraft, making it possible for flight crews to upgrade to the new model with a comparatively short differences-training course.

Walking around the aircraft, it is apparent that the -8’s nose sits slightly higher than the 737-800’s because the nose-gear strut is 8 in. taller. This assures the same 17 in. of ground clearance below the larger Leap 1B engine nacelles as with the smaller CFM56-7s of the NGs.

 

Airport neighbors can expect to experience a 40% smaller noise footprint around runways; engines emit half the nitrogen-oxides of the 737-800. Credit: Boeing/Leo Dejillas

The 18-blade, woven carbon-fiber fan on the Leap 1B is 69.4-in. in diameter compared to 61 in. for the 24-blade titanium fan of the CFM56-7. The larger fan provides a 9:1 bypass ratio versus 5.1 for the older engine. Leap 1B is a two-shaft design. The N₁ low-pressure section has the fan, three booster and five turbine stages. The N₂ high-pressure section has a 10-stage axial flow compressor powered by a two-stage turbine. Overall pressure ratio is 41:1, compared to 28 for the CFM56-7. The increased pressure ratio and higher operating temperatures made possible by advanced hot-section materials are the main reasons for the 15% reduction in specific fuel consumption.

Each Leap 1B weighs 6,129 lb., 849 lb. more than a CFM56-7BE. The added heft has a ripple effect on aircraft empty weight. New struts and nacelles for the heavier engines add bulk, the main landing gear and supporting structure are beefier, and fuselage skins are thicker in certain areas. The changes add 6,500 lb. to green aircraft weight, but operating weights are boosted by 7,000 lb. to preserve full-tanks, full-seats loading flexibility.

From the outset, the 737 was designed to sit low on the ground to make it easy to load baggage, service the engines and replenish fuel. The sills of the baggage doors, for instance, still are about 5.5 ft. above the ramp, even with the taller nose gear. But the short landing gear limits the size of engines that can be used. With such a low undercarriage and long fuselage, the -8 has a tail skid under the aft fuselage to prevent airframe damage in the event of over-rotation during takeoff or the landing flare.

Strapping into the left seat, there is no mistaking this cockpit for anything other than a Boeing 737. There is ample room for two pilots, but the cockpit gets crowded with third and fourth crewmembers on jump seats. Up front, the NG’s six 8 X 8-in. Arinc D-size screens have been replaced by four 9 X 12-in. landscape-configuration displays from the Boeing 787. The integrated standby instrument is relocated to the center of the panel along with the landing-gear handle and indicator lights and nose-wheel steering switch.

The primary flight displays (PFD) have edge-to-edge sky/earth backgrounds with vertical insets for air data. The magenta cross-pointer flight director is carried over from the 737NG. A single-cue flight director is optional. Spare area on the outer edges of the display may be used for clock and flight ID data.

The larger-format navigation displays (ND) allow vertical profile windows to be displayed below the maps, on demand. The engine gauges, fuel- quantity indicators, slat and flap position indicators also share ND screen area on either the left or right display, on command.

 

Leap 1B engines, having a 41:1 pressure ratio and higher operating temperatures, account for most of 737-8’s range increase. Credit: Boeing/Leo Dejillas

Removing the sixth D-size display from the center console frees up area at the front to relocate some controls and indicators displaced from the instrument panel by the large-format displays. Some other instrument panel and display dimmers have been relocated, so pilots upgrading from the NG to the MAX will need a little time to get accustomed to the differences in cockpit layout.

Most of the rest of the cockpit design is carried over from the 737 Classic and NG models, including the left and right recall/annunciator light arrays, overhead panels and control-stand layout. Preserving cockpit commonality ruled out upgrading the flight deck with an engine-indicating and crew-alerting system, electronic checklist function and full integration of the flight management and display systems. Notably, Boeing does not offer an approach capability using satellite-based augmented GPS or WAAS LPV (localizer performance with vertical guidance using the GPS wide-area augmentation system). However, a ground-based augmented GPS approach system is available.

Cockpit flows and checklists for the crew are nearly identical to those for the 737NG. There is the well-rehearsed litany of dozens of switch throws, button pushes and knob turns that dates back to the original 737 designed in the Apollo era.

However, there is a new twist in starting the Leap 1B engines. The rotor shafts that support the compressors and turbines are susceptible to thermal warping, or rotor bowing, caused by heat retention after shutdown. As a result, when initially starting the engine, a “motoring” message may appear on the N₂ tachometer display if a bowed rotor condition is detected by the electronic engine computer (EEC). This can delay start for 12-60 sec. while the EEC allows the air-turbine starter to turn the engine at 18-24% N₂ speed to true the rotors prior to start. Operators will have to adjust pushback to taxi times to account for the rotor-bow motoring delay.

 

Aviation Week’s Fred George (pictured) flew the MAX 8 with Boeing’s chief test and evaluation pilot, Keith Otsuka. Credit: Boeing/Leo Dejillas

Aircraft Serial No. 1A003, the 737-8 I flew, is chock-full of flight-test equipment, water ballast tanks and other gear. With 10 people aboard, zero-fuel weight was 123,059 lb., at least 23,000 lb. heavier than a production aircraft with a typical interior including 12 business-class and 150 economy-class seats, forward and aft lavatories, plus galley. With 20,840 lb. of fuel, taxi weight was 143,899 lb.

Roswell’s field elevation is 3,617 ft. The outside air temperature was 64F (18C) and barometer was 30.17inHG. Based upon a 143,600-lb. takeoff weight, slats/flaps 5-deg. configuration and full thrust, takeoff V speeds were 133 kt. for V₁ takeoff decision speed, 135 kt. for rotation and 145 kt. for V₂ one-engine-inoperative climb speed. Computed takeoff field length was 5,991 ft. for Runway 21.

If we had been departing at the 181,200-lb. maximum takeoff weight, the numbers would have been 153 kt. for V₁, 154 kt. for V_R and 160 kt. for V₂. Takeoff field length would have been 9,250 ft.

 

The MAX 8 nose gear is 8 in. taller, to preserve a 17-in. nacelle clearance with the substantially bulkier Leap 1B engines. Credit: Boeing/Leo Dejillas

At such comparatively light taxi weight, it took little increase from idle thrust to start the aircraft rolling on the ramp. Carbon brakes now are standard, saving 693 lb. I used them frequently to check taxi speed, as a result of the relatively high idle thrust.

The aircraft had the optional Rockwell Collins head-up display (HUD), a significant safety and situational awareness upgrade, in my opinion. I used it almost continuously during the evaluation flight.

Our flight plan would take us from Roswell about 100 nm northeast to Texico VOR (VHF omnirange navigation aid) and then back via airway V-280 to Chisum VOR. This would give us enough time to climb to altitude to spot-check cruise performance.

Rolling onto Runway 21, I advanced the thrust levers to stabilize the engines at 40% N₁ fan rpm, then engaged the autothrottles. As the engines reached full thrust, it appeared that the -8 was noticeably quieter than the 737NG on takeoff.

 

Advanced technology, laminar flow and split winglets reduce drag by 2% compared to the 737NG’s blended winglets. Credit: Boeing/Leo Dejillas

Initial pitch feel at rotation was moderate, with nicely proportionate roll feel. At 1,000 ft., I reduced pitch attitude and accelerated. Once speed had increased above V₂ + 15 kt., I retracted flaps to 1 deg. At 1,500 ft., I retracted all high-lift devices at about 190 kt.

The aircraft has sufficient natural speed stability through much of its flight envelope. But with as much as 58,000 lb. of thrust available from engines mounted well below the center of gravity, there is pronounced thrust-versus-pitch coupling at low speeds, especially with aft center of gravity (CG) and at light gross weights. Boeing equips the aircraft with a speed-stability augmentation function that helps to compensate for the coupling by automatically trimming the horizontal stabilizer according to indicated speed, thrust lever position and CG. Pilots still must be aware of the effect of thrust changes on pitching moment and make purposeful control-wheel and pitch-trim inputs to counter it.

We settled into a 250-kt. climb through 10,000 ft., then accelerated to 280 kt. until reaching Mach 0.78 for the remainder of the climb to flight level (FL) 350. Once level, the aircraft cruised at 449 kt. true airspace while burning 4,460 lb./hr. in ISA-3C conditions at a weight of 140,500 lb.

Admittedly, this brief snapshot of cruise performance was subject to many variables, including the test aircraft’s unusually far forward CG, at 10% mean aerodynamic chord, and atmospheric disturbances over land, among other factors. Otsuka also noted the optimum cruise altitude at this weight would have been FL390.

 

A recontoured tail cone, removal of aft body vortex generators and revised APU inlet and exhaust all help reduce drag. Credit: Boeing/Leo Dejillas

Near Texico, Albuquerque Center vectored us back toward Roswell to avoid other air traffic. With the autopilot engaged, the aircraft smartly rolled to a 30-deg. angle of bank. There is no automatic half-bank mode built into the flight-guidance system for high-altitude cruise, as the aircraft is buffet-free up to a 40-45-deg. bank angle at optimum cruise altitude up to ISA+15C. But for passenger comfort, the crew manually may dial in 10-, 15-, 20-, 25- or 30-deg. bank angles as appropriate for the phase of flight.

We began an idle descent, using the flight spoilers to hasten descent rate. Otsuka says the MAX has new fly-by-wire spoilers that add four functions to the usual flight spoiler and ground lift-dump features. In the event of an aileron control jam, the copilot’s control wheel, linked to the spoiler computer, enables lateral control through the roll spoilers. If the elevator becomes jammed, the system can provide direct lift control during landing approach by extending or retracting the spoilers in small increments in response to push or pull force inputs to the control wheels. The system also incorporates a maneuver load-alleviation function that reduces wing-bending moment and a landing attitude modifier feature that assures the main gear touch down well ahead of the nose gear.

Down at 15,000 ft., we flew both left and right 360-deg. turns at 45-deg. bank angles. The flightpath vector on the HUD and PFD makes it easy to stay on altitude. The speed-error tape and acceleration cue on the HUD make for precise speed control.

Next, we flew the aircraft with one engine at idle and the other at high thrust to simulate engine failure. Rudder pedal forces needed to keep the slip/skid indicator centered were quite moderate and, with a moderate quarter-chord wing sweep of 25 deg., there was little dihedral effect that required roll input to maintain balanced flight.

We also flew an approach to stall in the clean configuration, a maneuver I had practiced with Otsuka in the MAX 8 engineering simulator at Boeing’s Seattle campus. When the stall-warning stickshaker was triggered, I consciously pushed forward on the control wheel to reduce pitch attitude and advanced the thrust levers to climb thrust. As the engines accelerated, the thrust caused a pronounced nose-up pitching moment. I countered the effect with ample push on the wheel and plenty of nose-down pitch trim on the stabilizer.

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Returning to Roswell for pattern work, I hand-flew the first approach with flaps at 30 deg. At 140,000 lb., the V_REF30 landing approach speed was 135 kt. I set 140 kt. as the target speed. This approach would be to a touch-and-go.

As I extended flaps to 15 deg. and then 30 deg., there was a noticeable increase in lift that caused mild ballooning. Slight control-wheel inputs kept the aircraft on altitude. Guided by the instrument landing system to Runway 21, I used the HUD to control flightpath and speed, as it is a most effective tool for enhancing hand-flying precision and smoothness. As the radio altimeter called out “30” to alert me to the aircraft’s height above the runway in feet, I retarded thrust to idle, checked the rate of descent and settled onto the pavement.

On the go, Otsuka reconfigured to flaps 15 deg., reset the pitch trim for takeoff, and said, “Go!” I advanced thrust and rotated on his call. I cleaned up the airplane and flew downwind for 10 nm for our final landing.

I had flown a simulated one-engine-inoperative takeoff in the engineering simulator, along with a single-engine approach and go-around. I was impressed with the ample yaw control provided by the MAX’s comparatively large rudder and vertical fin, along with only needing moderate rudder- pedal force to maintain directional control. Dihedral effect was more pronounced in the simulator, amplified at high-thrust settings. But it was easily countered with roll and rudder inputs followed by some roll and yaw trim.

I flew the final landing approach at flaps 40 deg. and auto-brakes 2. I set the speed bug at 141 kt. for V_REF40 + 5 kt. Touchdown was uneventful. But I relaxed too much back-pressure on the wheel and the nose gear plunked down much too firmly.

It is obvious the 737-8 lacks the fly-by-wire flight controls, avionics integration and automation and other advanced technologies of newer jetliners. It is an airplane with design roots dating back to the Space Race. While its Boeing Sky Interior makes the cabin feel more spacious than its 11.6-ft. width, the A320-neo’s extra 7 in. allow carriers to install economy-class seats that are about 1 in. wider. Boeing says the MAX’s cabin windows are 20% larger in area, helping to make the cabin appear wider.

Operating economics, however, define the airlines, says John Plueger, CEO and president of Air Lease Corp.: “The 737-800 is a bread-and-butter airplane for Boeing, an 800-lb. gorilla in the market and a blockbuster winner in the Boeing single-aisle family. The MAX 8 is already successful from a leasing perspective.” It is a strong contender to replace the 737-700 and -800. While Airbus has a strong order lead with the A320neo over the 737-8, Plueger believes orderbooks will even out between those two models.

The 737 has had a record-setting production run, and the -8 will help maintain its momentum. It is a strong contender in the single-aisle market, rejuvenated with new engines, improved winglets and upgraded displays. But there are only so many ways an airplane can be stretched, strengthened, lengthened and reengined before it loses out to more modern designs. The MAX could be the swan song for the world’s best-selling jetliner.