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How much do you really know about the 737-MAX?
John Quayle 573

How much do you really know about the 737-MAX?

There is a lot of technical stuff here but it makes one wonder how these decisions are made

This article continues our series about aviation and some of the issues of the day.


Both Airbus and Boeing have recently posted eye watering first quarter losses of €481m and $641m respectively. The CEO of Airbus, Guillaume Faury, warned earlier this month that it may take up to five years for passenger figures to be back at their pre-Covid numbers. But at Boeing, they have more than the pandemic to deal with. The latest MAX-8 and MAX-9 models of company’s best selling jet, the 737, have been grounded for over a year following two similar accidents involving MAX-8s, with no suggestion of an imminent return to service. A report coming out of Reuters in recent days suggests that the model will not return to service until August, at the earliest. That the model remains grounded after such a period of time demonstrates, if any demonstration was needed, that something has gone seriously wrong with the new version of this popular jet, which its designers are having trouble putting right!


The grounding arises out of two losses of 737 MAX-8s. A Lion Air 737 MAX-8 crashed shortly after take off from Jakarta on 28th October 2018, killing all 189 souls on board. This was followed, on 10th March 2019, by the loss departing Addis Ababa of another Max-8, this time belonging to Ethiopian Airlines, with 157 people losing their lives. Investigators soon realised that there were striking similarities between these events, and the finger was soon pointed at a piece of software called MCAS (Maneuvering Characteristics Augmentation System). It was not fitted to previous 737 models. Basically its job is to produce artificially the pitching (nose up/down) characteristics of earlier 737 models, because without it, the new MAX models had a nasty and potentially catastrophic vice.


The story really begins in the mid 60s, when Boeing designed a short/medium range jet airliner, to be flown with just two pilots (no flight engineer), to compete with Britain’s One-Eleven, and the American Douglas DC-9, which were already in service. Assigned the title 737, it was powered by a pair of jet engines with a low by-pass ratio, which were tube-shaped in appearance. They were attached to the bottom of the wings. The aircraft had short undercarriage as these engines did not require a lot of ground clearance. That eased baggage loading and boarding, as well as routine servicing of the power plant. The initial models were the 737-100 and -200, and entered service in 1968. It was hugely successful.


In the mid-1980s high by-pass ratio jet engines, developed originally to power long haul wide bodies, became available for single aisle short haul aircraft. They basically work by having a huge fan at the front of the engine, which has a significantly larger cross section than the compressor, combustion, and turbine stages. A high proportion of air drawn in by the fan is ducted around the other stages. This joins the combusted air as it exits the engine at great speed, drawing it along, thereby increasing the thrust available without using additional fuel. This makes fan engines much more economic and much quieter. The fan stage is what you see when you look into the front of a jet engine. The core, containing the other stages, is much smaller.


This created a problem for Boeing when designing its new series of 737, to be fitted with the new engines. The fan stage barely fitted under the wing. As a result the forward cowling of the engine pods was flattened off at the bottom, rather than being exactly circular, a feature of the 737-300 to -800 models. The new engines were not without initial problems; heavy precipitation could cause a flame-out, and on the more powerful versions, catastrophic fan failure occurred (e.g. the Kegworth Disaster, 1989). But Boeing and their engine partners (Franco-American CFM International) were soon able to modify the CFM56 engines to eliminate both of these issues. The Boeing/CFM partnership on the 737 went on to produce a massively successful product.


Also in the mid-80s, a new kid on the block arrived, and Boeing’s baby jet now had a European competitor when Airbus introduced their first single aisle aircraft, the A320 family. Unlike the 737 it was designed from the outset to accommodate the new generation of high by-pass ratio engines. Thus whilst similar in configuration and size to a 737, its appearance differed in one very obvious way. It sat much higher off the ground on longer undercarriage than its Seattle built counterpart.


In 2016 a new engine, the CFM LEAP, designed to power both the new Airbus A320neo (LEAP-A) and the even newer 737 MAX-8/9 (LEAP-B), entered service. It offered a 16% reduced fuel burn against its predecessor the CFM56, significant emission reductions, and a reduced noise output of 15dB. The LEAP had an even larger diameter fan stage than the CFM56. The new Airbus product could easily accept its new engines, but the new 737, with its 1960s vintage sub-wing height, could not, and CFM designed a slightly different version of the engine, the LEAP –B, for the 737, with a reduced fan diameter, but higher rotational speed.


Even so, to accommodate the LEAP-B on the 737, the top of the fan sat slightly higher than the top of the wing, and the engine also had to be mounted further forward than the CFM56.


These changes to engine height relative to the wing caused the lift distribution over the wing to move forward, which along with the power plant’s more forward mounting, resulted in a tendency for the aircraft to pitch up, especially at high power settings (e.g. initial climb – both accidents occurred immediately after take off).


Hence the need for MCAS. The idea was to make the MAX series mimic the handling qualities of the earlier models. If the aeroplane started to pitch up, MCAS was intended to gently feed in forward stabilizer trim to counteract this tendency. The problem was, they did not tell the pilots operating the MAX of its existence. It did not appear in their operations manuals, nor was it covered during training. It was meant to work quietly in the background, so Boeing did not think it necessary. To the crews, the MAX felt like any other 737 they had flown, but they did not know that this was being artificially achieved.


The sole source of data for the MCAS is a single AOA (Angle of Attack Indicator) on the forward fuselage. If this is damaged or becomes unserviceable, the MCAS can think that the nose is pitching violently up, and aggressively winds on forward trim. It does not first check parameters from other sources.


Following the Lion Air crash, Boeing issued an Airworthiness Directive to MAX operators setting out how crews could disable MCAS in the event of a runaway trim. The Ethiopian crew did just that. Unfortunately with MCAS disabled, the stabilizer must be manually trimmed back to neutral from a chronic nose down position. Because of its gearing (requiring 40 turns of the manual trim wheel on the flight deck) and the sheer aerodynamic loads (in excess of 50lbs), the crew were physically unable to effect a recovery.


In respect of their top selling single aisle product, Boeing had chosen to modify a design from over 50-year ago, rather than start with a clean sheet of paper. With twenty twenty hindsight was this in fact the best policy? This hugely expensive and let us not forget deadly design flaw has at its roots the length of the undercarriage legs on the classic 737s designed and built in the 60s. It might further be justified to ask whether a single source of data is acceptable for what is a hugely important piece of equipment, and finally, is an aircraft design which requires the intervention of software to give it acceptable handling qualities a good and acceptable design? Whilst all Airbus products are entirely flown via software, that was the manufacturer’s policy decision from the outset, and not an attempt to offset the consequence of a further up-dating of a long-standing and hitherto hugely successful product.


Logically, if the modified MAX comes back into service, it will be the safest jet in the sky, having been the subject of so much scrutiny from not just Boeing and CFM, but also the FAA and the world’s certifying agencies. Nonetheless, one can but wonder what the passenger reaction will be when they see a MAX waiting for them at the gate, if it is by then still called the MAX, and indeed if it ever does get back to flying passengers.


(Photo: John Quayle on the Boeing 737-300/400 production line, Seattle, 1989).

Capt J W H Quayle 19-05-20

The views expressed above are the views of the author and accept no responsibility for them.

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