{"id":2372,"date":"2019-10-31T16:01:11","date_gmt":"2019-10-31T23:01:11","guid":{"rendered":"https:\/\/dennisholeman.com\/?page_id=2372"},"modified":"2020-01-13T19:59:01","modified_gmt":"2020-01-14T03:59:01","slug":"the-boeing-737-max-a-case-study-of-systems-decisions-and-their-consequences","status":"publish","type":"page","link":"https:\/\/dennisholeman.com\/the-boeing-737-max-a-case-study-of-systems-decisions-and-their-consequences\/","title":{"rendered":"The Boeing 737 MAX: A Case Study of Systems Decisions and Their Consequences"},"content":{"rendered":"

\n\t\tThe Boeing 737 MAX: A Case Study of Systems Decisions and Their Consequences\n\t<\/h2>\n\t

Revised 3 July 2019<\/p>\n

The Boeing 737 MAX program provides an illustrative example of how incentives can shape the behavior of complex technical and economic systems decisions that result in serious problems.\u00a0 After more than a century as one of the most respected companies in the entire field of aviation, Boeing\u2019s credibility as the builder of safe commercial aircraft has come into doubt. How the 737 MAX will regain the confidence of regulatory agencies, airline customers, and the flying public is an open question at this time.\u00a0 This article describes the path that brought things to this point\u00a0Read Less<\/a><\/strong><\/p>\n

The Boeing 737 Family<\/b><\/p>\n

Boeing designs the 737 starting in 1964 as a short-to-medium range, smaller-capacity member of the Boeing single-aisle jetliner family. It uses the same fuselage cross section, cockpit, and controls technology as the predecessor 707 and 727 aircraft.\u00a0 The initial seating design (on the 737-100) is for 85 passengers. However, airlines find this aircraft is too small and the first major production version is the lengthened 737-200 with 102 seats in two classes.<\/p>\n

Because the 737 is intended to serve secondary airports with less-developed infrastructure (boarding stairs instead of jetways, limited baggage loading and engine servicing equipment, etc.), it is designed with short landing gear to allow the fuselage to sit as low as possible.\u00a0 The initial engines are small-diameter JT8D low-bypass turbofans, mounted directly under the wings without pylons.<\/p>\n

After a somewhat slow sales start, the 737 becomes very popular, eventually becoming the most-produced airliner family in the world (over 10,500 produced).\u00a0 It supplants virtually all of its early competitors in the smaller jetliner segment (e.g., Douglas DC-9, BAC 1-11, Fokker F28\/F50\/F100, Sud Caravelle, BAe 146, etc.).<\/p>\n

The design is extended to a number of growth versions with different length fuselage stretches.\u00a0 These include the 737-300, 737-400, and 737-500. The engines selected for the new versions are more powerful CFM-56 high bypass turbofan engines.\u00a0 To accommodate the larger diameter high-bypass engines while retaining adequate ground clearance, the nacelles are mounted higher and further forward and the nacelles are flattened on the bottom<\/p>\n

The biggest change occurs with the 737 Next Generation (737NG) series that features enlarged and redesigned wings, larger fuel tanks for more range, new cockpits, and uprated CFM-56 engines. The 737NG series is first produced in 1996 and includes the 737-600, 737-700, 737-800, and 737-900.<\/p>\n

All the new versions are certified by the FAA under the original 737 type certificate, even though the largest 737NG versions of them have nearly three times the original passenger capacity (230 in maximum density), twice the engine power, twice the range, all-digital \u201cglass\u201d cockpits, and serve different market segments than the original 737 design.<\/p>\n

Boeing\u2019s largest customer for the 737 family is Southwest Airlines, whose fleet is exclusively made up of 737 variants.\u00a0 Like a number of Boeing\u2019s other customers, Southwest wants its pilots to be able to fly any 737 version in its fleet with one pilot type certificate and common training.<\/p>\n

Origin of the 737 MAX<\/b><\/p>\n

The Airbus A320 family (A320, A319, A321, and A318) becomes the primary competitor to the 737 family.\u00a0 First flying in 1987, the A320 is a clean-sheet-of-paper design not tied to previous Airbus aircraft. Because it is not optimized for serving secondary airports, it has a higher stance with plenty of ground clearance to accommodate large diameter engines.\u00a0 The A320 family sells well.<\/p>\n

Airbus introduces the A320 Neo family incorporating new technology ultra-fuel-efficient engines with larger engine diameters.\u00a0 The A320 configuration accommodates the new engines easily. The first flight is in 2014. With its improved economics, the A320Neo family is very attractive to customers.<\/p>\n

In order to avoid losing next generation single-aisle jetliner sales to Airbus, Boeing decides it needs to create a new 737 family with comparable ultra-fuel efficient engines.\u00a0 This becomes the 737 MAX series. It is closely based on the 737NG family (737-600, 737-700, 737-800, and 737-900).<\/p>\n

To be able to install the larger diameter engines on the 737 MAX design, the engine nacelles are moved even further forward and higher than the previous CFM 56 engines.<\/p>\n

Selling the 737 MAX Family<\/b><\/p>\n

Boeing aggressively markets the 737 MAX as being just like the previous 737 variants but much more economical to operate.\u00a0 It claims essentially no additional training is required for a 737 pilot to transition to flying the 737 MAX. Only an hour or two\u2019s study of instructional material on an iPad is sufficient.\u00a0 Boeing takes orders for almost 5,000 737 MAX aircraft and sets up to produce about 60 units per month.<\/p>\n

737 MAX Flight Characteristics\u00a0<\/b><\/p>\n

Previous 737 models had the center of gravity well forward of the center of lift. In a stall, with neutral control inputs, the plane will nose down and recover on its own.\u00a0 The natural nose-down force is counteracted by downward lift generated by the horizontal stabilizer. This lift creates drag and increases aircraft fuel burn. It appears that Boeing changed the 737 MAX\u2019s relationship between the center of gravity and center of lift to minimize this trim drag effect and optimize efficiency.<\/p>\n

In flight test, the 737 MAX variants are found to have flight characteristics that differ significantly from previous 737s.\u00a0 This is particularly true at a high angle of attack where body lift from the large engine nacelles mounted ahead of the wings creates a strong nose-up force.\u00a0 The center of lift shifts forward. The thrust from the low-mounted engines acting below the center of gravity also provides a nose-up force. This latter effect is especially pronounced at high power levels.<\/p>\n

Without corrective input, at a high angle of attack a 737 MAX will continue to pitch up further, leading to a stall.\u00a0 As a result, Boeing finds the 737 MAX design does not satisfy Federal Aviation Authority (FAA) airworthiness criteria for stability, particularly Federal Aviation Regulation (FAR) 25-173 [see appendix].\u00a0 If the angle of attack of the aircraft exceeds 14 degrees, the nose will rise on its own until the aircraft stalls, unless a corrective action is taken.<\/p>\n

Creating a Fix for the Stability Problems\u00a0<\/b><\/p>\n

Rather than doing an aerodynamic redesign of the airplane, Boeing decides the quickest and least expensive fix for the flight characteristics of the 737 MAX is to provide a new software system. Called the Maneuver Characteristics Augmentation System (MCAS), it endeavors to make the 737 MAX aircraft behave like previous 737NG versions through flight control software algorithms.<\/p>\n

As a priority, MCAS is intended to prevent the aircraft from getting into a hazardous unstable flight regime.\u00a0 In addition to traditional pilot warning mechanisms (e.g., a \u201cstick shaker\u201d stall warning system), the MCAS will automatically drive the stabilizer trim to force the nose down when sensor data indicate a dangerously high angle of attack.<\/p>\n

Characteristics of the initial design of the MCAS software include the following:<\/p>\n