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Aviation: 737-MAX - a story as strange as fiction

It’s beginning to sound more and more like the plot of a blockbuster by Dale Brown – or more possibly Michael Crichton’s thriller Airframe.

Airbus drives the agenda

The story goes back to the first decade of the millennium when Boeing was working on an all-new narrow-bodied airliner to replace its best-selling short-haul 737 twin jet. Its replacement was widely expected to have a light-weight fuselage with an elliptical cross section, advanced systems, greater capacity than the then current 737s, and space under the wings to install the latest fuel-efficient turbofan engines.

Meanwhile, Airbus was cracking on with its own project to update the A320 family, a type that was two decades younger than the original 737. The European manufacturer launched the A320neo in December 2010 with a timeline that was planned to lead to service entry four years later. The updates were relatively modest, primarily offering customers a choice of two new engines – the Pratt & Whitney PW1000G or the CFM International Leap-1A – each of which were somewhat more efficient than those they replaced. Crucially as it turns out, the original A320 stood on fairly tall undercarriage which left Airbus plenty of room to hang the new, larger engines below wing level.

Enter the MAX

Boeing was backed into a corner. It needed to do something to compete with the updated Airbus, which burned less fuel than the equivalent 737s. It was in the midst of its pre-production problems with the 787, which were delaying the program and costing the firm a great deal of money. The advanced new widebody was taking much longer to bring to market than expected, and Boeing already knew from the 787 roll-out fiasco that ground-up programs sometimes run into problems that delay delivery of early airframes.

An all-new aircraft was preferred by Boeing, but finding a cheaper solution was a potential way out for the firm when its more loyal customers were pressing for a counter to the updated Airbus. Boeing’s third generation 737, the NG (next generation), was selling well – around 370 delivered in each of 2009, 2010 and 2011 – and the firm was anxious not to cede the market to its rival. Short-haul narrow-body jets are the lifeblood of aircraft manufacturers and Boeing also had to figure out how to work through an inevitable dip in orders and output of the outgoing aircraft, if it transferred production to an all-new airframe. There was another issue Boeing had to consider too. All new aircraft have to undergo extensive testing – often taking as long as two years – before they can be awarded a type certificate of airworthiness. But under certain circumstances, updates to older designs can be ‘grandfathered’ approval – the testing is still rigorous, but not as quite as extensive or time consuming as an all-new aircraft. Grandfathering also means that certification can be a little less rigorous than it would be for an all-new aircraft, as some aspects of aderivative design are measured against the original rather than current and potentially more stringent standards.

The 737-MAX was a cheaper and more expedient solution than an all-new design that enabled Boeing to compete against the Airbus A320neo (Clemens Vasters)

While Boeing wanted to design an all new aircraft, an update to the venerable 737 would be cheaper and could be brought to market more quickly. Boeing’s customers also liked the idea of a 737 update because pilots could easily be switched between the 2nd generation classics and the newer generations. Differences training is required for new models, but it isn’t as time consuming or expensive as transferring crew to a completely new aircraft type. More to the point, pilots could be scheduled to fly either the 2nd generation classics or the NG.

  • 1st generation (classic) – 737-100 / -200

  • 2nd generation (classic) – 737-300 / -400 / -500

  • 3rd generation (NG) – 737-600 / -700 / -800

  • 4th generation (MAX) – 737-7 / -8 / -9 / -10

Short legs

So some of Boeing’s major customers pressed for yet another update – the third – to the venerable 737 as a cheaper, quicker and more easily adopted solution to their need for better economy and greater capacity. However, there was a problem – the 737 was designed when jet engines were long and thin. That meant the powerplants of the original version could be slung under its wings. And that allowed short undercarriage legs which enabled ground crew to access the baggage holds without the need for complex and expensive ground equipment.

When the original 737 classic was upgraded with CFM-56 engines in the 1980s, the nacelles had to be flattened at the bottom to provide sufficient ground clearance. By the time the fourth generation 737 came along, the latest engines were larger still - as well as being much more efficient again. Boeing selected a derivative of the CFM International Leap engine for its updated 737, dubbed the 737-MAX. The Leap-1B it used had to have a smaller diameter front fan than the -1A hung on the competing Airbus to enable it to fit on the 737. Reducing the diameter of the fan wasn’t enough though - it had to make other changes too.

Why didn’t Boeing just increase the height of the 737’s undercarriage? Well, the 737 main wheel legs fold inwards into bays in the lower fuselage.

The larger diameter CFM Leap-1B engines on the 737-MAX were installed further forward and canted upwards slightly to ensure they would not hit the ground. (Anna Zvereva)

There wasn’t much lateral space between the main undercarriage mounting points on the original 737. And if longer legs had been installed, essential structure in the lower centre fuselage would have obstructed the wheels before the gear was completely stowed in the under-fuselage bays. Boeing could have moved the legs further outward on the wings, but that would have interfered with the placement of the engine mounts. That in turn would have necessitated moving the engines further out too, and would have led to a major wing redesign. And that would have jeopardised Boeing’s ability to certify the 737-MAX as a derivative of the original 737, potentially throwing away all the benefits of grandfathering certification and pilot commonality.

As a result the Leap-1B engines on the 737-MAX were instead moved forward, raised a little, and canted upwards slightly at the front to provide sufficient ground clearance. That approach worked. Boeing was able to offer its customers a cheap and timely competitor to the Airbus A320neo, and the airlines got an aircraft that could be flown by current 737 pilots following minimal retraining. The 737-MAX is actually a family of four aircraft (737-MAX7 to -MAX10), each with successively longer fuselages and greater passenger capacity. Airlines signed up in droves – there were commitments for almost 5,000 by the end of 2018, making the MAX Boeing’s fastest-selling aircraft in the firm’s history.

In to service

The A320neo flew for the first time on September 25, 2014 and received joint certification from the European Aviation Safety Agency and the US Federal Aviation Administration on November 24 the following year. The new Airbus derivative entered service with Lufthansa on January 25, 2016. Four days later – 49 years after the original 737-100 took to the air - the first 737-MAX8 flew from Renton near Seattle.

Launch customer Southwest Airlines received its first MAX at the end of August 2017, and put it into service around a month later. Its pilots, all of whom were familiar with the 737 already, received differences training. As more airlines took delivery of MAX aircraft they mirrored this approach. In most cases the conversion comprised a one hour session with an iPad or computer based training package which explained the differences between the 737NG and the 737-MAX. It was a very simple process – the content mainly concerned changes to the aircraft systems, as Boeing had strived to ensure the 737-MAX handled as much like an earlier NG as possible. In fact, the differences between the 737NG and the MAX were so insignificant that some pilots went through the course then didn’t fly a MAX for quite some time, only to suddenly find they were scheduled to fly one at a later date. It didn't seem to be a problem because the controls and the flying characteristics of each were so similar.

Everyone was happy – Boeing, its customers and the regulators – although the Federal Aviation Authority had baulked at the idea of pilots flying 737 Classics, NGs and MAXs concurrently, and instead limited aircrew interoperability to only two of the 737 sub-types at a time. The airlines that operated all three versions keenly started to retire their older 737 Classics to maintain operational flexibility.