This is the first in a series. Special to Leeham News.
By Peter W. Lemme
Feb. 2, 2021, © Leeham News: The Boeing 707 entered service in 1958. 60 years later, a Boeing 737 MAX, the model’s fourth generation, crashed into the sea with all lives tragically lost. After, the 737 entered service in 1968, every subsequent 737 model was carefully crafted with the minimum changes necessary to deliver performance and reliability improvements. Differences were discouraged in order to reduce pilot training requirements, holding dear to visible family traits pioneered by the 707.
Changes that jeopardized an amended Federal Aviation Administration Type Certificate were culled. A new type certificate is a costly and lengthy process, with considerable risk.
The 757 should have saved Boeing from endless 737 generations, but the will of the customer changed its trajectory from the start, in the end leaving no mark at all.
The 737-100 was a marvel by fitting the engine tight under the wing, and every generation since has marveled at how to fit the latest engine under the wing.
Every swept wing air transport suffers growing pains, working out issues with flying qualities.
The 737 seems to have diverged from the other Boeing airplanes progressively. Should the 737 MAX’s now-infamous Maneuvering Augmentation Characteristics System (MCAS) implementation have been applied to the stabilizer? Did Boeing have a better choice?
Flying qualities approved “to the letter”, quantitatively, are balanced by qualitative measures, with latitude in judging acceptable performance.
The 737 family was fundamentally refreshed three times: Classic, NG, and MAX. In each case, customers with earlier 737 models expected that the newest model would require minimal differences training. Airlines prefer to minimize training, especially full-flight regime simulator time, to reduce cost and to speed up the approvals.
FAA criteria (AC 120-53B) defines a process for addressing differences between aircraft models for pilot qualification required for flying different variants of transport aircraft with the same type certificate (TC) and between aircraft with different TCs, that have been “designated” by the Administrator as related (Common).
To address aircraft differences pertinent to pilots safely flying those assigned aircraft, the FAA’s Flight Standardization Board (FSB) issues a report to specify the basis for the approval of airline programs for pilot qualification, as necessary for the operation of any airline aircraft under 14 CFR Parts 61, 91, 121. Airline pilot qualification to fly these aircraft types and variants with differences include necessary provisions for training, checking, and recency.
The FAA FSB Report for the B737 addresses the specifics needed for each variant of B737 flown. These are included in the FAA FSB Report for the B737, in the criteria listed as the “Master Difference Requirements” (MDR) Tables.
Master Difference Requirements are applied to each specific airline flying B737s, tailored for their particular set of B737 models or variants flown, in their airline specific “Operator Difference Requirements (ODR) tables”. Each airline’s pilot qualification program is designed to fully address any specific training, checking, or recency requirements needed, to fly each B737 variant applicable to that airline.
US regulatory requirements for the pilot in command of a jet transport aircraft flown in airline service specify that the pilot in command must both hold a suitable type-rating for that type aircraft flown, and also have completed any required qualification under FAR 121 as needed, before that pilot can operate that airplane in airline service. This typically involves training, checking, and recency of experience, but can also involve various aspects such as supervised flying with a Check Airman termed “Initial Operating Experience” (more recently just termed “Operating Experience”). Other aspects of qualification include Route Qualification, International Qualification, special Airport Qualification, and potentially other provisions.
In some instances, a pilot serving as a “Second-In-Command” pilot may also need to be “type rated”, to meet ICAO requirement as well as meeting additional qualifications.
One of the key elements of Initial or Upgrade Pilot training is “Differences” training applicable to flying different models or variants of a model. As an example, consider the B757 and B767. Originally, a pilot could be type-rated in the Boeing 757, or the Boeing 767, or both. Later (in 1983) FAA made a determination that a Pilot holding either a B757 Pilot Type Rating, or a B767 Pilot Type rating would be considered to have both type ratings (designated as a “B757 B767 Common Type Rating”) listed on that pilot’s Airman Certificate. However, to fly either type in airline service, suitable differences training still would be needed in each specific model actually flown. While both the Boeing 757 and 767 may in general be flown by a pilot with a “Boeing 757 767” Common Type Rating, all the other FAR 121 specific qualification requirements, such as “Differences” training, must still be met.
For the Boeing 737 series, one pilot Type Rating currently is assigned by the FAA (B737), and that Type Rating on the Airman Certificate covers flying any Boeing 737. However, in airline service, a B737 pilot still must also meet all the necessary qualification provisions for each specific model (variant) of the B737 flown.
Both a 737 PIC and SIC must satisfy any necessary difference training, checking, and recency requirements, as specified by the FAA’s Flight Standardization Board (FAA FSB), to operate each different Boeing 737 model (variant).
A new type certificate (14 CFR Part 25) application sets the regulatory basis in effect at the time of application, as the certification basis for the type. Whereas an amended type certificate may or may not necessarily have to re-establish parts or much of the certification basis, at the time an amended Type Certificate is initially sought. An amended type certificate can prolong applicability of criteria otherwise no longer used, based on acceptable safe in-service history of the basic type to be modified.
Usually, some elements of an updated regulatory basis are agreed upon, with some subsequent regulatory provisions not required. In other instances, an applicant may be encouraged to voluntarily apply an updated provision, or an equivalent provision.
Boeing applied for an amended type certificate with each new 737 model. Significant regulatory basis changes were also agreed between FAA and Boeing, and subsequently made, for each successive B737 derivative generation.
The Boeing 737 was the most advanced airplane Boeing had built. Pushing the envelope for their first two-crew flight deck meant more automation, reliability and simplicity. The issue was not simply putting all the flight engineer instruments and controls in front of the pilots. The Boeing 737 was the first airplane certificated in production with Category II automatic approach capability.
Figure 2: 737-200 Advanced Flight Deck. Source: Wikimedia Commons
Southwest Airlines led the way on the 737, yet held it back at the same time, for the Classic, the NG and for the MAX. Is there such a thing as too much LUV? Is the most successful airplane Boeing ever built handicapped by the dynamic that makes it so attractive?
T.M. Sell, Valley Daily News, 17 June 1993: For Boeing worker, 737 is No. 1
In early 1981, then Renton Division chief Frank Shrontz (Boeing Chairman in 1993) pursued the idea of an upgraded 737. Boeing’s own marketing experts predicted only 140 sales of the new version. Ron Woodard (Boeing Commercial EVP) said Shrontz had to talk them into raising the estimate to 180 to convince Boeing’s board to launch the new version.
By 1993, customers had ordered 3,100 737s.
“No one has ever built so many commercial airplanes of any one type,” Woodard said, “In fact, no one has come close.”
Sales for the 737-300 did not budge the year after it was launched, while the Original models continued unabated, reaching the 1,000-airplane milestone.
Figure 3: Chuck Yeager with Herb Kelleher. Source: Southwest Community Forum
In Southwest’s own recount, the airline takes credit for the 737 “classic.”
Southwest Community Forum: If not for Southwest, the 737 might have stopped with the -200 Advanced
Deregulation in 1978 allowed Southwest to expand beyond the borders of Texas, but it needed an aircraft that would allow it to compete on longer routes from San Antonio and Houston to the West Coast.
The larger 737-300, with its more powerful engines, greater range, and greater performance was the perfect airplane.
Southwest and US Airways were the launch Customers.
Ironically, none of the other airlines showed initial interest as they were happy with the 737-200 for shorter flights and had 727s serving the same markets that would be flown by the 737-300.
Without the launch customers’ support, the 737 line might well have ended with the -200.
The brand new 737-300 made its worldwide airline debut on December 17, 1984. Because this was the anniversary of the Wright Brothers first flight, the aircraft, N300SW, was named The Spirit of Kitty Hawk. The Spirit of Kitty Hawk is on permanent display at the Frontiers of Flight museum in Dallas.
The funny thing about calling a product “next generation” is that the name makes no sense once the product is in service plus it boxes you in for naming the inevitable “next” product afterwards.
The 737 Next Generation, or NG, represented almost a whole new airplane. It was all about performance, cost, commonality. Nothing more.
T.M. Sell, Valley Daily News, 17 June 1993: 737 on a Roll
Ron Woodard, EVP Boeing Commercial, and Gordon Bethune, vice president and general manager of the Renton Division, said Boeing is pursuing development of the 737-X, a jet that will have greater wing-span and improved engines to provide greater performance at lower cost.
He said the new jet will include only improvements that customers definitely want. “One very important thing we’ve learned is you don’t mess with success,” said Woodard.
“Simplicity, reliability and family commonality,” Woodard said. “That’s what our customers are telling us.”
“One airline executive has said ‘Stop right there. Put a lock on the cockpit.’” Bethune said. “They’re not interested in technology for technology’s sake.”
On Dec. 17, 1997, Southwest became the first operator of the 737 NG.
Boeing Mediaroom: First Boeing 737-700 Goes to Southwest Airlines
“Boeing has been fortunate to have Southwest as a successful business partner over the past 26 years, and we wish them continued success with their Next-Generation 737s,” said Ron Woodard, president of Boeing Commercial Airplanes. Southwest’s fleet of 258 jetliners consists entirely of Boeing 737s.
Modifications to the Next-Generation 737’s wing and engine provide improved fuel capacity, fuel efficiency, speed and range. The total wing area increased by 25 percent to 1,340 square feet (125 square meters), providing 30 percent more fuel capacity for a total of 6,878 U.S. gallons (26,136 liters).
The airplane’s range is approximately 3,200 nautical miles (3,682 statute miles or 5,926 kilometers), an increase of up to 900 nautical miles over current production 737s. This will allow U.S. transcontinental flights and increased 737 route capability throughout the world.
The 737-700 is powered by new CFM56-7 engines produced by CFMI, a joint venture of General Electric of the United States and Snecma of France. The CFM56-7 will have a 10-percent higher thrust capability than the CFM56-3C engines that power today’s 737s.
Commonality does not advance like engines, wings, and materials.
Boeing pilot Ray Craig promoted commonality.
Dan Dornseif: Boeing 737, The World’s Jetliner
Ray Craig said, “The goal from the Classic to the NG was one day of differences training for pilots, with no simulator required. We were striving to keep, as much as we could, similar handling qualities and minimize the training differences for the airlines.”
Aside from the addition of EFIS flight instrumentation to the flight deck (which could be configured with software to be visually similar to the Classics), most of the other changes made to the 737NG series were not any major consequence to the flight crew.
Flightblogger, Feb. 10, 2011
Speaking at the Cowen and Company Aerospace and Defense Conference in New York City, McNerney says: “We’re gonna do a new airplane. We’re not done evaluating this whole situation yet, but our current bias is to not re-engine, is to move to an all-new airplane at the end of the decade, or the beginning of the next decade.”
McNerney suggests that the A320neo has put pressure on the smaller Bombardier C-Series, which shares a common power plant in the Pratt & Whitney PW1000G geared turbofan platform. While Airbus has focused on its existing customer base McNerney sees a looming threat to the 737: “That doesn’t mean that as [Airbus gets] deeper in the development they’re not going to approach our customer base. I think they will.”
McNerney adds: “It’s our judgment that our customers will wait for us, rather than move to an airplane that will obsolete itself when [Airbus develops] a new airplane. I understand why they’re doing [the neo], we haven’t seen the need for it yet. I feel pretty comfortable we can defend our customer base, both because they’re not going ahead of us, they’re catching up to us, and because we’re going to be doing a new airplane that will go beyond the capability of what the neo can do.”
Boeing had struggled with downsizing the 787 in Project Yellowstone, or Y-1. In the end, the flight instruments were the biggest contribution, yet without managed crew alerting.
David Parker Brown, AirlineReporter, 16 Feb 2011: Could this be what the next Boeing 737 will look like?
At a recent Boeing media event Scott Fancher, the Vice President and General Manager of the 787 Dreamliner Program was asked what new technologies from the 787 could be scaled down for the new Boeing 737. When asked about composite materials being found in the new 737, he stated, “some composites scale down nicely, but others, they don’t.” He explained that some of the new systems technology and engine efficiencies could also be scaled down. Boeing feels that all the investments made on the 787 Dreamliner will serve as a basis for future new aircraft.
Southwest was the launch customer for the 737-300 Classic and, in November 1993, for the 737 NG. This history and the growing size over the decades of Southwest’s orders for 737s, became a crucial factor in the development of the MAX.
Scott Hamilton, Leeham News, “When the MAX was launched in July 2011 by American Airlines, Southwest’s CEO Gary Kelly was reportedly miffed. Southwest had urged Boeing to launch a re-engined 737, but the company dithered between this and an entirely new design. Airbus forced Boeing’s hand with a huge order from American for the A320 family. Backed into a corner, Boeing launched what became the MAX.”
Kelly tried to take the sting out of American’s being the first customer for the new 737 model.
There’s more to being a launch customer than bragging rights — although that’s a large part of the appeal of being the first to market with a shiny new jet. Aircraft makers also promise that their newest models boast lower fuel and maintenance costs than prior airplanes, providing an immediate bottom-line boost to airlines.
“We’re the launch customer, regardless of when we take the first delivery,” Southwest CEO Gary Kelly said of the MAX.
His chief operating officer, Mike Van De Ven, said Thursday that “we’re the ones that have done the service-ready operational validation for Boeing. We’re the ones working very closely with Boeing to make sure it’s operating as everyone intended.”
Boeing’s MAX development is on-budget and ahead of schedule. All this good news, as far as production is concerned, is a rarity in an industry renowned for being late to everything.
In the end, the MAX ran significantly over budget. And schedule and budget would haunt Boeing in the aftermath of two fatal accidents.
The first Boeing 737 was the last new airplane to be built at Plant 2 on Boeing Field in Seattle, with a production run that included the legendary B-17 Flying Fortress, B-52 Stratofortress and the world’s first large swept-wing jet — the XB-47 Stratojet.
The 737 evolution was stalled by commonality. Airlines want to minimize differences to make it easy for pilots to transition between the various models. Once cast, the 737 was forever anchored in the past.
Airbus A320 entered the market in 1989 with full-flight regime fly-by-wire. The choice of non-moving throttles and uncoupled sidesticks was a significant departure from any other air transport. Boeing remained faithful to the control column and moving throttles with the 777 and 787 entrants. The Airbus A220 stands in contrast to the rest of the Airbus fleet, with moving throttles and uncoupled side sticks. Airbus has replicated the A320 flight deck design to the A330, A340, A380, and their latest A350. Gulfstream G500/G600/G650 have moving throttles and coupled sidesticks or active sidesticks.
While most examples of commonality hold an airplane back, the 747-400 was pushed ahead, instead.
Philip Bartles, Boeing 747-400
Boeing consulted 14 major international airlines. Boeing wanted to go for a minimum-change aircraft, particularly on the flight deck, where the analog instruments were to be retained. This was supported by Cathay, one of the launch customers, in order to minimize training.
However, there was strong pressure from the other potential launch customers to update the flight deck, since they were experiencing the advantages of the 757/767 entry into service and the advanced flight decks of the Airbus family.
Each generation of the 737 accommodated new engines, new displays, and new features. The 747, when faced with the same proposition, broke with the 747-400, an advancement from the 757/767. The 777 and 787 each followed from the 747-400. Yet the 757 influence is lost to the 737.
Figure 4: 737 development compared to its competitors. Source: Peter Lemme
Boeing never perceived that technology itself holds the 737 back, for it has been behind since 1989. This position was re-affirmed in the most recent two generations, the NG and MAX. Design changes focused on fuel savings, growth, common parts, and minimum difference training for the pilots.
Airline emphasis on commonality discourages change or favors older designs as a winning proposition.
The Boeing 757 set the stage for the Boeing 737 to persist for four generations. Had the 757 been built as planned, with a two-member 757-100/200 family (the -300 came much later) to fill the 150-160 seat capacity that the 727-200 pioneered, everything would have been different. The 737-500 would have been the last 737, there would have been no NG.
Peter Rinearson offers some very keen insights that relate well to the 737 story.
Seattle Times: Making it Fly
Then, on that August (1978) morning in Miami, (Frank) Borman surprised (Tex) Boullioun and other Boeing executives who had gathered at Eastern’s headquarters at the edge of the airport. Borman asked to see data on a hypothetical plane about 15 seats bigger than they’d talked about earlier.
Boeing officials, though not eager to increase the size of the plane, obliged and laid out the material for Borman, who was noncommittal.
Boullioun had to leave early, and Borman said he’d see him to the airport terminal. Once together in the back seat of the car, Borman told Boullioun he liked the bigger airplane design and was willing to gamble on it.
All at once Borman had a flash that a 175-passenger airplane was what Eastern wanted,”
Boullioun recalled. “He said, ‘If you’ll build that, we’ll go.'”
Boullioun replied: “You’ve got it.”
And so, with a handshake-just as the car jiggled over some railroad tracks-the Boeing 757 was born.
Boeing was anxious to get a launch customer. Once the airplane was launched, things did not improve.
Seattle Times: Making it Fly
For a long time, the 757 didn’t have any orders — and by some measures, it still doesn’t. The modest number of orders fed rumors of cancellation, as did nagging concerns that Boeing had blundered into making the airplane too large — at 178 seats, too close in size to the 210- seat 767.
“Some people felt the 757 wasn’t the right size for the market, this sort of thing. That the 150-passenger plane was the one,” said Benjamin Gay, who oversaw construction of the 757 for Eastern Airlines
The 757 broke away from its 727 roots solely by the synergies resulting from building the premier, new 767 at the same time.
Seattle Times: Making it Fly
The transformation effectively linked the 757 technologically with the new 767 rather than with the tried-and-true 727, the Boeing airplane the 757 was intended to replace. The changes were improvements, but they raised eyebrows.
The 757 sits in the shadow of the 767. The 737 was even further in the shadows, and would never emerge, effectively a branch in the Boeing development team.
Seattle Times: Making it Fly
“The 767 got the best of everything,” said a rank-and-file engineer on the 757 program, emphasizing the final word of the sentence. “They got the money. They got the appropriations. They got the management. Corporate gave them whatever they wanted. And we got what was left over.”
The 757 was to be a derivative of the 727, making it a comparatively low-cost program. The 767, which got an earlier start, was to be the all-new showpiece. It was to be, in the words of the 757 engineer, “Boeing’s baby.”
It seemed to some of those working on the 757 that their achievements were shrouded in obscurity and they were relegated to “me-too” status: Boeing had this great new airplane it was building, the 767. It was the first new Boeing since the 747 in the mid-1960s. It would be ultra-modern. It would be more fuel-efficient. It would be a great seller. And, oh yes, Boeing was also making this other airplane, the 757.
Human family traits are visible, the color of hair, tall, short. Animals physical traits evolve as they domesticate. The Boeing 757 changed its shape to accommodate the 767 flight deck layout to promote commonality. The narrow 727 nose section was replaced with a wider, more rounded section.
The ME262 had one jet engine installed tight under each wing. Yet Joe Sutter, the legendary Boeing engineer, was probably not thinking of that when he proposed to install engines under the wings on the 737-100. Jack Steiner, 737 chief engineer, expected a T-tail derivative with two tail-mounted engines; it was just a smaller 727, after all.
Steiner’s decision to go with the 707/727 fuselage as a competitive advantage made the airplane a bit shorter than its competition, simply because at least one less seat row was needed. The proximity of the wing’s trailing edge and the resulting tail-mounted engine inlet created challenges with reliable operation and required structural compromises.
Putting the engines inside the wing, as had been done by others, was rejected. Damage tolerance was a large deterrent.
Sutter considered pod-mounted engines. Boeing had discovered pod-mounted engine mounts could be positioned favorably for the B-47, using the “broomstick in the wind-tunnel” technique. The B-52, 367-80, KC-135, 707, and 720 had all followed suit with pod-mounted engines. Pod mounted engines also damped out flutter, or instability in the wing structure itself.
Sutter considered how the smaller jet, operating more frequent cycles and in less-improved areas, would benefit by being low to the ground. A mechanic could service the airplane without always needing a ladder. A last-minute bag could be loaded by just running out to the airplane.
Sutter proposed to install the engines tight up against the underside of the wing, because traditional pods needed too much ground clearance and created challenges positioning exits. The resulting conventional empennage opened up the cabin to more seat rows. Sutter was named inventor on a patent for the concept.
The brilliance from Joe Sutter would ironically dog the airplane every step along the way.
The 737-100 Pratt and Whitney JT8D performed well during flight test, with only one engine needing to be changed out. The 747 Pratt and Whitney JT9D fared far worse; 55 were changed during the 747 flight test. The 747 does carry twice as many as the 737.
The CFMF56-3 had to be installed forward on the 737-300 and ended up with a flattened inlet.
Figure 6: 737-300 with CFM56-3 non-circular inlet cowl. Source: Wikimedia Commons.
The 737 NG used a new wing entirely, but still was pressed to fit the latest CFM56-7 engine.
Figure 7: 737-100 Vs 737-800. Source: Wikipedia
Top officials in Boeing Commercial Airplanes were cool to the idea of re-engining the 737 NG even as the engineers proceeding with a design.
Leeham News reported that at an employee meeting Jan. 14, 2011, Jim Albaugh, president of Boeing Commercial Airplanes, dismissed the neo and its potential competitiveness vs the 737NG.
“I think Airbus will find re-engining the A320 more challenging than they think it will be,” he told employees during one of his periodic “Excellence” meetings. “When they get done, they will have an airplane that might be as good as the Next Generation 737. We think we can continue to make incremental improvements to the 737 to make sure that it is a more capable airplane than even the re-engined A320.
“At the same time, while we haven’t made a firm decision, I don’t think we will re-engine the 737. It’s really hard to come up with a compelling business case to do that. We think the right answer to probably do a new small airplane that might come out toward the end of this decade. We’ll make that decision probably sometime in the middle of this year,” he said.
“Every customer I talk to has a real hard time understanding why a re-engined airplane makes sense,” Albaugh said
In a March 2011 interview with Scott Hamilton, Mike Bair, 737 product development head, said:
Boeing was focused on what a new airplane might be that would enter service in the “back half of the  decade,” around 2019 or 2020.
“We think there is a fairly compelling bundle of technologies we could put into a new airplane,” he said.
Engine technologies for the new, future airplane (which we’ll call NFA) could come from maturing the GTF, LEAP and Trent designs from P&W, CFM and Rolls-Royce, he said.
Boeing, in 2011, was looking at airplanes above and below 150 seats, which at the time was what officials liked to call “the heart of the market.” (Seven years later, this had moved to 162 seats.)
“I try to avoid saying ‘replace the 737,’ because what we’re doing is something a little bit different. We’re trying to figure out what does the world want in 2030, in 2035?” Bair said. “That’s when this airplane will be kind of at its prime. On the surface, it’s hard to image what worked well in 2000 is going to work well in 2030.
Boeing was indecisive about which direction to choose, Leeham News reported.
The ambiguity was driven in no small part during this era by the continued 787 debacle, which at the time of this interview, had not entered service.
By July 2011, when its hand was finally forced, the 787 was more than three years late, billions of dollars over budget and it hadn’t been delivered to any customer yet. Word on the street was that there was no way the Board of Directors would approve a new, clean-sheet airplane program under these circumstances. It had lost confidence in Boeing Commercial Airplanes and the Longacres executives to meet their promises, to keep a schedule and within budget.
The MAX was hastily launched in July 2011, when American Airlines informed Boeing it was about to place a record-setting order for nearly 500 airplanes. Airbus was lined up to snare it all with the A320ceo and neo families unless Boeing could make a credible offer.
Within 48 hours, Jim McNerney, then-CEO of Boeing, made the decision to launch the re-engined design of the 737. This later was branded the MAX.
It was a plane Boeing designed but didn’t want to build.
Once launched, Boeing had to play catch up to Airbus, which had a seven-month lead with its neo.
The MAX used the same wing as the NG, but the larger fan moved the engine forward and upward again.
Figure 8: The profile of the engines on the 737 NG (top) and 737 MAX (bottom). Source: Leeham News
The low standing profile surely is advantageous. The nose gear was lengthened by eight inches, but the main gear was unchanged. Longer landing gear have to go somewhere. It is a credit to the many engineering disciplines that Boeing has made each 737 model with the latest engines while retaining the stance. (The 737-10 MAX, still years away, has articulating main gear to accommodate the longer fuselage.) Of course, again, the 757 would much more easily accommodated the new engines.
Commonality acts as a moat barring the way into the kingdom. The toll to cross the moat is to pay back the cost of change in direct operating benefits. Airlines want to avoid pilot difference training, whatever the ultimate cost. Yet, almost all airlines have taken on more than one manufacturer of airplanes. There is no common type across suppliers. Regulators push the envelope, but suppliers work around them. What is the answer?
The 737-100 was designed for two-crew operation. A number of operators persisted with a third crew member after the airplane entered service. The use of fail-safe to prevent hazard from malfunction and dual systems to increase availability were designed to simplify pilot workload.
Consider crew alerting as a showcase of how, step by step, a focus on commonality, cost, and schedule, lead to a 737 decades out of step with every single competing airplane.
The original 737 models, now called the Jurassic by some, utilized a Caution Annunciator Panel to direct the flight crew to the appropriate control panel to diagnose system issues. Each system manages their own indications. The flight crew must prioritize their actions by assessing the situation and choosing which systems to attend to first.
The 757/767 spotlighted the quiet and dark cockpit. Normally, all lights extinguished. A central display with system synoptics and message priorities.
The 757 was a two-crew airplane. Boeing pioneered Engine Indication and Crew Alerting System (EICAS) for the 757 to simplify the pilot interface when addressing non-normal situations. EICAS was a great success. The 767 adopted EICAS in its decision to go two-crew as well. In this case, the 757 led the 767. The 757 entered service in 1983. Every Boeing airplane built since then uses EICAS, except the 737.
The 737 Classic was built after the 757.
For reasons of commonality, the 737 Classic “idiot light” caution annunciator panels were retained. The Classic included EFIS as an option. Many airlines also retained the “steam gauges,” of mechanical instruments, for commonality.
The original 737 was already holding back the Classic. Both followed the same panel installed on the Boeing 367-80, which first flew in 1954.
Figure 10: Boeing 367-80 (precursor to KC-135 and 707) flight instruments. Source: Boeing 367-80 Jet Transport Panorama.
Figure 11: Non-EFIS 737-300. Source: b737.org.uk. Note the annunciator panel for automatic flight control mode on the upper left.
The concepts that led to the Electronic Flight Instrument System (EFIS) were initially developed on the very first 737, NASA 515. The 757/767 were first to market with EFIS. A prominent difference with EFIS Vs. non-EFIS is the integrated mode display along the top of the upper display, a huge improvement in automatic mode awareness.
Figure 12: 737-300 EFIS: Source: b737.org.uk.
The MD-95 become the Boeing 717, entering service in 1999 with an advanced alerting system.
Boeing Aero: 717 Advanced Flight Deck
A highly integrated, advanced alerting system replaces most conventional indicator lights, warning lights, annunciator panels, and other dedicated indications.
The alerting system consists of the Engine and Alert Display (EAD); system displays; the master warning, master caution lights; and the central aural warning system.
When a condition requiring an alert is detected, the VIA displays the alert, illuminates the master warning or master caution light if required, and sounds an appropriate aural warning.
The flight crew acknowledges the alert by pressing the lighted cue switch on the systems control panel (SCP), which displays the appropriate synoptic display.
If a checklist procedure is required, the flight crew refers to the abnormal checklist to accomplish the required corrective action.
The 737 NG started development in 1993. That follows the 767, 757, A320, 747-400, MD-11, and A330/A340. The 777 and MD-95/717 were in development. The Caution Annunciator Panel has filled out since the 737-200 but is identical otherwise.
The 737 NG lack of managed alerting is remarkable. Once again, airline concerns over commonality, and the inherent simplicity of idiot lights was compelling. Since steam gauges were not an option, Boeing simulated steam gauges on the glass to satisfy airlines wanting to minimize differences with pilots still flying the 737 Jurassic series.
Figure 14. Southwest 737 NG with steam gauges on glass. Source: airliners.net
Boeing Common Display System offered the complete flexibility to take on the features of old or new. Boeing described this in a company periodical.
The next-generation 737 airplane (737-600/-700/-800/-900) offers more advanced, even more reliable airframe systems and digital avionics for increased altitude, speed, and range capability.
Key to the next-generation 737 is the computer-generated graphic representation of flight instruments on the common display system (CDS).
Simply by loading flight-deck software, operators can use the CDS to replicate either the electronic flight instrument system with map (EFIS/MAP) display of classic 737 models (737-300/-400/-500) or the primary flight display/ navigational display (PFD/ND) used in the 777 and 747-400 models.
As a result, operators can continue to fly their earlier 737s while introducing next-generation 737 airplanes into their fleets with minimal training.
Operators who fly the latest 737 exclusively will find the more advanced PFD/ND display formats are aligned with those on the advanced 747-400 and 777.
In order to duplicate the functionality of the 737-300 flight deck, any change to its integral presentations had to be minimal in the following displays on the next-generation 737 flight deck:
The FAA’s Flight Standardization Board ruled that simply “transitioning from CDS EFIS/Map to PFD/ND in the -600 through -900ER series group the minimum training requirement has been determined through testing to meet C level training.”
Yellowstone 1, or Y-1, was to replace the 737 family. The study ran from about 2004 to 2008. Issues with downsizing 787 components and composites chilled the project.
Airbus announced the A320neo series in 2010 and Boeing was faced without a new airplane to match.
Realizing that the fuel gains were the main motivation, Boeing launched the MAX in 2011 as “Min to the MAX” to restrict changes from the NG to only those mandated to meet the performance or to install the engines.
Figure 15: The year after the 2011 program launch, Boeing promoted the MAX as the “same” as the NG, including simulators. Source: Boeing
Boeing considered the certification risk of continuing with idiot lights and chose to retain them instead of the effort and risk to revise the alerting system altogether which would then create significant differences with all other 737s.
Figure 16: 737 MAX Flight Deck, Caution Annunciator Panels are shown on the upper left and right, on the glareshield, with master warning and caution illuminated. Source: wraltechwire.com
The 737 MAX has the same Caution Annunciator Panel design from the 737-100. The website Aviation Today wrote:
Since winning the Max display contract from Boeing in 2012, Rockwell Collins has worked with the Original Equipment Manufacturer (OEM) to effectively mimic the cockpit package featured on the 787. Engineering teams at Boeing and Rockwell Collins worked to replace, revise and relocate 23 displays and controls, and the Max design also reduces the six-display configuration of the 737 NG to four 15.1-inch Liquid Crystal Displays (LCDs).
“The clocks, which were standalone pieces of equipment in the cockpit before, have now been incorporated into the displays. There are separate switches and functions in knobs that have either been relocated or incorporated into the displays, as well. Those changes were made to accommodate for the four large screen displays in that smaller cockpit. Generally speaking, regarding what is in the Max versus what was in the previous Next Generation version of the airplane, everything is the same, it is essentially in the same location, which was all part of Boeing’s plan to simplify the transition between the NG model airplane to the Max,” Keith Stover, principal systems engineer at Rockwell Collins told Avionics Magazine.
From submittals gathered by the US House, Boeing recognized the best solution and the most practical solution.
Figure 17: 737 MAX EICAS Risk Review. Source: US House-Boeing Submission
Figure 18: 737 MAX EICAS Risk Review. Source: US House-Boeing Submission
Figure 18: Exemptions sought by Boeing from the FAA. Source: Seattle Times.
Figure 20: 737 models compared to other airplanes, noting fly-by-wire and centralized crew alerting. Source: Peter Lemme.
The 737-100 went through quite a bit of tweaking as it evolved to the 737-200 ADV. The 737-100 started with much higher drag and stall speeds than expected. It had a pitch-up at stall. It had “stick lightening” at high Mach numbers. The changes to correct the deficiencies from the start of flight test to delivery (737-100) included:
The aerodynamic center of the wing shifts backwards when transonic, in part due to shock waves that form along the leading edge. The lift shifting backwards causes the airplane nose to drop, which if unchecked, will cause the airplane to accelerate further, worsening the tendency. This is called Mach Tuck. The pilot must pull back on the column to keep the nose from dropping as the airplane accelerates. Mach Trim applies an offsetting control input to compensate for Mach Tuck.
Movable Stabilizer restores X-1 pitch control. Source: chuckyeager.com.
Then, during his eighth flight on 10 Oct., he lost pitch control altogether, as a shock wave formed along the hinge line of the X-1 elevator. He reached Mach 0.997 but without pitch control it would have been foolhardy to proceed. The X-1 had been designed with a moving horizontal tail and Capt. Jack Ridley convinced Yeager that by changing its angle of incidence in small increments, he could control the craft without having to rely on the elevator. This had never been attempted at extremely high speeds but Yeager was game to give it a try on the next flight.
The 737-100 discovered the need for a Mach Trim function during flight test. An actuator was added to the elevator feel-centering unit to apply a small elevator bias as a function of Mach number.
The 707 had a Mach Trim function that applied a small amount of stabilizer trim.
The 757, 767 and 747-400 use stabilizer for Mach Trim function.
The 757, 767 and 747-400 models adapted the same Speed Trim System used on the 737 to include a Mach Trim schedule as well. These models implement the Mach/Speed Trim System in a two channel, dual lane (ARM and COMMAND) with dual inputs manner. In contrast, the 737 Speed Trim System (later with MCAS) was originally implemented as an active/standby two channel, single lane, single input manner.
The 707 also had a stick-nudger installed in some cases, which effectively pushes the control column forward with a modest force, less than 10 lbs. The stick nudger is operating on the elevator to offset pitch-up.
The 747-100/200/300, the 767, and the 747-8 all have some form of stick nudger or pitch augmentation that operates on the elevator.
The following figures show the 767 stick nudger, which is attached to the elevator feel and centering unit.
The 737 Mach trim is attached in almost exactly the same manner as the 767 stick nudger.
The original 757/767 design included a pitch augmentation system that operated on the elevator but was removed prior to delivery.
Every Boeing model, other than 737, has used augmentation on elevator controls to counter pitch-up.
The 737 MAX uses elevator for Mach Trim and stabilizer for Pitch Augmentation.
Every other Boeing airplane uses elevator for Pitch Augmentation (if required) and stabilizer for Mach Trim.
It is not known if Boeing considered moving the 737 MAX Mach Trim function to the Speed Trim System (as done on the 757, 767, 747-400) and redesign the Mach Trim actuator as a stick nudger. Adding Mach Trim to Speed Trim System would have driven some form of Fail-Safe design.
There were redacted sections mentioning other methods to manage pitch-up which remain unknown. The 737 MAX final configuration included:
Malfunction is inevitable. The hazard from malfunction can be managed. The objective is to minimize the crew workload in response. Fail-safe systems trade protecting the pilot from the effect of malfunction to the effect of loss of function. Redundancy can allow for Fail-Op, if continued functionality is required.
Logical limits are the result of executable software and computing. The reliability of software is associated to the level of hazard it exposes. If the hazard is greater, the software has to be better; as in more documents, more analysis, and tested much more rigorously. Much credit is taken for software to protect from malfunction, but when software itself malfunctions, anything is possible.
Electrical limits are the result of component failure. The hardware can be designed with tandem components, or gates, such that any one gate stuck open won’t cause a malfunction. Removing power should disarm all functional outputs.
Mechanical limits afford the next backstop, where the actuator has only a limited “throw”. The 767 stick nudger and the 737 Mach trim actuator have limited mechanical authority, no matter how they fail. The 737 Yaw Damper was once thought to be the source of catastrophic lateral upsets, but analysis showed its authority was too limited (the issue was in the rudder).
The last backstop for malfunction is the pilot. The pilot must recognize the situation, formulate a response, and take action. In the case of stabilizer runaway, the pilot has no more than four seconds to stop the runaway from its unexpected onset.
Elevator augmentations have mechanical limits that restrict malfunction without any pilot action.
Autopilot stabilizer trim malfunction:
The risk from logical limits is based on software Design Assurance
The risk from electrical limits is based on adding tandem components in a Fail-Safe manner
The risk from reaching mechanical limits is entirely up to the flight crew.
Stall Identification is an augmentation to encourage a nose down pitching moment at the stall break point. The 737 is the only Boeing model that lists stall identification as a function in its Flight Crew Operations Manual.
The high alpha mode drives the stabilizer continuously Airplane Nose Down (AND) until a washout gain (-2.5 units) or by the aft column cutout.
Source: Unofficial 737 Airplane Maintenance Manual
The stall detection circuit monitors the flap position and the angle of airflow. Near stall, the speed trim function trims the stabilizer to a nose down condition to allow for trim above the stick shaker AOA and idle thrust. The trim continues until the stabilizer gets to its limits or the aft column cutout position is exceeded.
Once the high alpha mode is released (by AoA going below the reset threshold), speed trim will trim Airplane Nose Up (ANU) as the airplane accelerates.
The 737 NG High Alpha mode, as part of Stall Identification, operates even more aggressively than the 737 MAX MCAS. It appears to be a single sensor application. With no service issues to draw from, there does not appear to have been any significant issues with a single AoA sensor triggering an Airplane Nose Down (AND) stabilizer runaway. There are some interlocks.
Source: Unofficial 737 Airplane Maintenance Manual
The gain is 100 percent for an average N1 more than 80 percent or zero for an average N1 less than 60 percent.
The gain is 100 percent for a Mach less than 0.5 or zero for a Mach more than 0.68.
The FCC controlled neutral shift region (FCNSE) appears to be a unique feature for the 737 taking advantage of the Mach trim actuator. The primary function of the flight control computer (FCC) controlled neutral shift is to reduce the pilot column force necessary to trim the airplane during initial climb out after takeoff with a forward CG and both engines operating.
The Mach trim actuator can trim the elevator 4.7 deg Airplane Nose Up and -1.65 deg Airplane Nose Down.
The 737 MAX incorporate other sophisticated functions. A fly-by-wire spoiler system was programmed to improve handling. Load alleviation relates to distributing the load across the wing. LAM = Landing Attitude Monitor is designed to prevent tail scrapes. Elevator Jam Landing Assist uses the spoilers to enable a flare.
Figure 30: New 737 MAX Features. Source: 737 Flight Standardization Board Report.
The following figure traces significant features as they evolved from the 737 Jurassic to the 737 MAX. Those features that required changes for return to service are highlighted in red (airworthiness directive).
UK ARB Certification Test Pilot D.P. Davies found that the FAA certificated the 727-100 with a pitch-up into a stall based on Boeing professing “equivalent safety”. He questioned his FAA counterpart on what was found equivalent and got nothing in response. Davies enforced strict compliance to a specification that no one disagreed with. What was at stake was that the Boeing/FAA agreement professed handling qualities “good enough” but not “to the letter”.
Equivalent safety is applied to an alternate means of compliance to meet the expected standard, not to argue that the standard is just a guideline.
The British ARB insisted on installing a stick pusher to protect from a “deep stall” and for stall identification.
The same issue came up with the 727-200, and Davies lost the decision. There was no stick pusher on the 727-200 and it was eventually removed from the 727-100s as well.
The 727 has powered elevator actuators and can be flown out of a deep stall, as Lew Wallick demonstrated through inadvertent encounter. Deep stall did not emerge as a common finding with 727 accidents.
A stick nudger was added to the 747-100 in yet another difference between the US FAA and the UK ARB. The pitch-up issue had been accepted by the US; the UK insisted on the stick nudger. The 747-400 replaced the stick nudger with speed trim system, the same system that was installed on the 737-300 classic in 1984.
The 757 and 767 were both initially developed with a Pitch Augmentation Control System (PACS) to offset a pitch-up. Vortex generators on both models, a stick nudger on the 767, and countless other aerodynamic tweaks, collectively, provided adequate flying qualities. PACS was removed from both airplanes before either was delivered.
TWA Skyliner Magazine, 1982-08-02, TWA Pilots Fly the Boeing 767
We went to 37,000 feet and accelerated to Mmo (maximum operating speed) and rolled the aircraft into a steep bank, pulling close to 2.5 g’s and, although the pitch augmentation had been removed, the addition of seven vortex generators on each wing seemed to correct what we understand had been an adverse stability condition. The aircraft was stable and “honest” throughout the high g/high Mach maneuver. Full stalls in all configurations were accomplished at 10,000 feet with the stick shaker turned off. This aircraft was not equipped with the proposed stick nudger but the aircraft seemed to stall with no adverse characteristics. (Ed. note The “stick shaker” is a warning device which senses a stall build-up and accentuates vibration before a pilot might feel the actual stall condition on the yoke. The “nudger” automatically presses the nose of the plane down to· forestall a stall.)
Boeing Stability and Control judged the 737 MAX flying qualities as unacceptable. They rate it a “Major” hazard if high-alpha flight is encountered while the MCAS system is not available. A Major hazard is well within the capabilities of every pilot to overcome. The vigor of the pitch-up should not be judged as a substantial threat to the safety of the airplane.
Major hazards are routinely managed successfully in isolation. Major hazards become much more challenging when multiple hazards arrive at the same time, consuming margins in workload and attention.
The 737 MAX handling qualities, with MCAS functioning as designed, were considered sufficiently similar to the 737NG. They share the same wing, which certainly is a substantial factor in their similarity.
AC 25-7C is the “Flight Test Guide for Certification of Transport Category Airplanes”, revised in 2012 and used for the 737 MAX. An Advisory Circular provides acceptable means for compliance.
A gauge for flying qualities provides three passing grades (Satisfactory, Adequate, Controllable) and one failing grade (Unsatisfactory).
Figure 32: Pilot Induced Oscillation (PIO) Rating Guidelines. Source: AC 25-7C.
AC 25-7C provides the minimum flying qualities based on atmospheric disturbances, flight envelope, and likelihood of the condition (where anything less than 1E-5 is improbable). The failure of MCAS in the Operational Flight Envelope was considered Major. The pitch-up is in the Operational Flight Envelope.
AC 25-7C teaches that the flying qualities can degrade under improbable situations by one notch but may not become unsatisfactory. MCAS failed in the operational flight envelope is considered Improbable, and for light turbulence, the handling qualities may be no worse than Adequate, which is well within the expected skill of a pilot qualified for the airplane.
Figure 33: Minimum Handling Qualities (HQ) Requirements. Source: AC 25-7C.
Aviation safety hinges on pilot response to malfunction and loss of function. The greatest challenge is to take a bad situation and not make it worse. Reducing the level of automation during malfunction should be the first step, often the most reluctant.
The 737 has embraced a reliable and straightforward alerting system but fails to manage alerting under multiple failure scenarios, where the flight crew are most stressed, shunning using capabilities evident 40 years ago.
The 737 was born in a time of analog computers and circuitry. System failures were frequent. Pilots expected the unexpected. The 737 simplicity and elegance provided a stable foundation that never wavered over four generations. While the instruments moved from dials to glass, the hardware to software, the core of the 737 became fully integrated and incredibly reliable.
The 737 should have stopped at the 737-500 and the 757 should have picked up the trail into the twenty-first century, except that the launch customer had made the 757 too big. Without a replacement, the 737 just kept getting new engines. Customers wanted commonality. What they got was the least common denominator.
The 737 is an oddball in the Boeing fleet. The choice to put MCAS on stabilizer and Mach trim on elevator, when the opposite was used everywhere else, is puzzling. Limiting malfunction on elevator can be achieved through hard, mechanical limits. Limiting malfunction on stabilizer that relies on the pilot to take action means that they can be overcome if they don’t.
A close look at the findings and analysis leading to return to service. What went wrong? What is the corrective action? What’s missing? What’s next?
Peter Lemme is a pilot, engineer, and writer with over 40 years of experience in the aviation world. He draws on his expertise in aviation history, aeronautics, automatic flight control, data link, and satellite communication; to provide insight into the performance and safety of aircraft systems, the human interface, and flying.