By Ralph Vartabedian / Los Angeles TimesA set of stairs may have never caused so much trouble in an aircraft.First introduced in West Germany as a short-hop commuter jet in the early Cold War, the Boeing 737-100 had folding metal stairs attached to the fuselage that passengers climbed to board before airports had jetways. Ground crews hand-lifted heavy luggage into the cargo holds in those days, long before motorized belt loaders were widely available.That low-to-the-ground design was a plus in 1968, but it has proved to be a constraint that engineers modernizing the 737 have had to work around ever since. The compromises required to push forward a more fuel-efficient version of the plane — with larger engines and altered aerodynamics — led to the complex flight control software system that is now under investigation in over the last five months.Boeing’s problems deepened Thursday, when the company announced it was stopping delivery of the aircraft after the Federal Aviation Administration’s decision Wednesday to ground the aircraft. Associated press fileStewardesses from the 15 airlines with orders for Boeing 737 jets pose in front of the first aircraft in a hanger in Seattle on Jan. At the christening ceremony Boeing announced sales to two other airlines and 17 flight attendants all smashed bottles of champagne over the wing of the new aircraft.“Boeing has to sit down and ask itself how long they can keep updating this airplane,” said Douglas Moss, an instructor at the University of Southern California’s Viterbi Aviation Safety and Security Program, a former United Airlines captain, an attorney and a former Air Force test pilot. “We are getting to the point where legacy features are such a drag on the airplane that we have to go to a clean-sheet airplane.”Products go their wayFew, if any, complex products designed in the 1960s are still manufactured today.

Boeing 737-300/400/500 Aircraft Maintenance Manual. 2 methodology also was used to update maintenance programs for the MD-80, DC-9, DC-10, and DC-8 during the late 1990s. Updated maintenance programs for the 727 and 737-200/-300/-400/-500 are scheduled for completion by year-end 2002 and third-quarter 2003, respectively.

The IBM 360 mainframe computer was put out to pasture decades ago. The Apollo spacecraft is revered history. The Buick Electra 225 is long gone. And Western Electric dial telephones are seen only in classic movies.Today’s 737 is a substantially different system from the original.

Boeing strengthened its wings, developed new assembly technologies and put in modern cockpit electronics. The changes allowed the 737 to outlive both the Boeing 757 and 767, which were developed decades later and then retired. A Boeing 737-300, part of the small Air One fleet, is seen in this Nov. 7, 1995 photo taken at Rome’s Ciampino Airport during the presentation of the new company. (AP Photo/H.O., file)Over the years, the FAA has implemented new and tougher design requirements, but a derivative gets many of the designs grandfathered in, Moss said.“It is cheaper and easier to do a derivative than a new aircraft,” said Robert Ditchey, an engineer, aviation safety consultant and founder of America West Airlines, which purchased some of the early 737 models. “It is easier to certificate it.”But some aspects of the legacy 737 design are vintage headaches, such as the ground clearance designed to allow a staircase that’s now obsolete.

“They wanted it close to the ground for boarding,” Ditchey said.Andrew Skow, founder of Tiger Century Aircraft, which develops cockpit safety systems, and a former Northrop Grumman chief engineer, said Boeing has had a good record modernizing the 737. But he said, “They may have pushed it too far.”To handle a longer fuselage and more passengers, Boeing added larger, more powerful engines, but that required it to reposition them to maintain ground clearance.

As a result, the 737 can pitch up under certain circumstances. Employees view the first of three new Boeing 737-700 during rollout ceremonies on Dec. 8, 1996, at the Renton plant. This version, which included a 737-600 and 800 series, was longer and had better fuel efficency, speed and range.

(AP Photo/Robert Sorbo, file)What’s at fault?Airline crashes seldom are caused by a single factor, and the two 737 accidents may yet involve poor maintenance, pilot errors and inadequate training. But it appears increasingly likely that Boeing’s software system and the company’s lack of recommendations for pilot training on it may have played an important role in the mishaps.Low-slung aircraftThe entire need for the software system is fundamental to the jet’s history.The bottom of the 737’s engines are a minimum of 17 inches above the runway.

By comparison, the Boeing 757 has a minimum clearance of 29 inches, according to Boeing specification books. The newer 787 Dreamliner has 28 inches or 29 inches, depending on the engine.The 737 originally was equipped with the Pratt & Whitney JT-8 series jets, which had an inner fan diameter of 49.2 inches. “They looked like cigars, long and skinny,” Moss said.By comparison, the LEAP-1b engines on the Max 8 have a diameter of 69 inches, nearly 20 inches more than the original. There wouldn’t be enough clearance without some kind of modification.In the 737-300, which came after the original planes sold in West Germany, Boeing came up with an unusual fix: It created a flat bottom on the nacelle (the shroud around the fan), creating what pilots came to call the “hamster pouch.”“They made it work,” said Ditchey, whose America West was one of the original customers of the 737-300.But the LEAP engines required an even bigger change. Boeing redesigned the pylons, the structure that holds the engine to the wing, extending them farther forward and higher up. It gave the needed 17 inches of clearance.

The company also put in a higher nose landing gear. Boeing employees take a first look at Boeing’s next-generation 737-800 airplane at its debut in Renton on June 30, 1997. (AP Photo/Loren Callahan, file)The solution was MCAS, which ordered the stabilizer to push down the nose if the “angle of attack” — or angle that air flows over the wings — got too high. The MCAS depends on data from two sensors. But on the Lion Air flight, the MCAS relied on a sensor that was erroneously reporting a high angle of attack when the plane was nowhere near a stall.The pilots tried to counteract the nose-down movements by pulling back on the yoke. But even pulling with all their might they could not counteract the forces, according to data in a preliminary accident investigation report.Skow criticized MCAS, saying it acted only on the basis of angle of attack. The Lion Air jet was traveling so fast that when MCAS ordered the stabilizer to pitch the nose down it was a violent reaction.

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The software should have factored in air speed, he said, which would have better calibrated the pilots’ reaction.Skow’s firm has developed a cockpit display system that he says would have identified the failure of the angle of attack sensor and allowed the crew to abort the takeoff. “We believe we could have prevented the accident,” he said.If the results of the investigation do not undermine the fundamental design of the aircraft, then the 737 Max’s future may not be in peril, aviation experts said. It may turn out all that’s needed is a software fix or additional pilot training.The 737 has survived other crises. In a 1988 accident on a flight between Honolulu and Hilo, the entire top of the plane came off in an explosive decompression. A flight attendant was sucked out and 65 passengers and crew were injured. It was blamed on faulty lap joints in the aluminum skin of the fuselage, which Boeing reengineered.“The 737 is the most successful commercial jet ever produced,” said John Cox, an air safety expert and veteran pilot, adding that commonality among its models helps airlines with pilot training. “It is nearing the end of its production life.

The technology will eventually drive Boeing to a replacement.”.