The 737 MAX is a re-engined and enhanced version of Boeing’s proven aeroplane – the 737 Next Generation (NG) including the popular 700 / 800 / 900 models. Amongst the new features, the MAX boasts of 14 percent fuel economy than its NG version. The MAX was rolled out to rival the Airbus 320 NEO which has been swiftly gaining recognition as the future of medium haul planes.

The crash of two 737 MAX of Lion Air and Ethopian Airlines, followed by the worldwide grounding of the MAX fleet, has brought the aircraft model under heavy scrutiny. As on date of this article (14th Apr 19), we know that both planes went down for similar reasons. Working backwards, the investigators have learnt that the mishaps on MAX were waiting to happen. As evident, the design and development trail of the MAX does not make a great narrative and may even smells of mischief. Lets figure out how.

To make the enhanced features possible, the MAX variant is fitted with new and more powerful engines which are bigger in size. Unlike the NG series, the bigger sized engines could not be accommodated under the wing at its present location as it would not have been possible to maintain the mandatory ground clearance required in case of the collapse or a malfunction of the Nose Gear.

To overcome the design issue, Boeing saw two options – firstly, to make a new aeroplane altogether. The second option was to carry out major modification of the 737 NG in terms of airframe and the wings which would also amount to designing a new plane. Boeing probably saw both of these options as a commercial challenge especially when its rival, the Airbus Industry, was already in lead with the release of A320 NEOs months ahead of them. The quicker alternative that Boeing actively considered was to keep the aeroplane design unchanged except to bring the engines further forward of the wings to enable raising it for necessary clearance from ground. This method has its own pitfalls.

To understand the underlying problem, it is important to know that all the aeroplanes powered by underslung Jet engines ( aircraft with engines under the wing) have an inherent tendency to produce a nose up force at full power rating and speed – as is typical during take off and climbout. This is a known aerodynamic issue which is usually kept within limits during the design of the aeroplane by correct placement of engines within the aerodynamic loading regime. During the flight, further adjustments are done using the ‘Trim’ mechanism ( a system in aeroplanes to counter the pitching forces on aeroplane by moving the Horizontal Stabiliser manually or electrically from the Flight Deck). The required amount of trimming is done automatically when the aircraft is flown by the ‘autopilot’.

The sad and significant fact is that Boeing adopted the quicker alternative, leading to the problems of controlability of the plane that followed and resulted in two crashes. While conceptualising the MAX, Boeing possibly realised that the forward positioning of engines was producing a nose up force during takeoff and climb which was far beyond the controllability of the pilot. The ideal thing to do was to redesign the aircraft and get the issue sorted at fundamental level. It appears that Boeing knew the pros and cons of this issue but decided to handle it differently!!

Instead of revising the aircraft geometry, Boeing introduced a software control called Maneuvering Characteristics Augmentation System (MCAS). The MCAS system uses the combination of Angle of Attack (AOA), Air Speed and few more parameters to senses the increasing nose up attitude of the aircraft. The MCAS accordingly pitches the aircraft nose downwards (using the Trim mechanism) before the aircraft can stall (and fall). However, in both the accidents, the MCAS has erroneously pitched the aircraft nose downwards into a steep dive even when the aircraft was in normal attitude. Boeing learnt about this abnormal behaviour of the plane from the Lion Air crash and prescribed a procedure for pilots to gain control over the plane. Unfortunately, the mechanism prescribed by Boeing to counter this abnormality somehow didn’t work in the case of the Ethopian crash. In short, the MCAS has not served the intended purpose, instead it has been instrumental in both Lion Air and Ethopian crash. A step further, the aviation community now learnt that MCAS is a cover up for the basic aerodynamic issues and it is neither an automation feature nor a safety enhancement that are typical of the developement cycle of a new aeroplane.

If the above explanation was too intimidating for a layman, let’s review it with an example from our life experiences. Assume that while building a house, the wall of the living room developed a major crack due to defective design. Now, you had two options to deal with the situation – to bring down the wall and remake it correctly. The other approach is to let the crack remain and put a nice ‘Wall Paper’ over the wall to cover the defect. Considering that it is a house in making, which one would you choose to address the issue? Evidently, the quick fix option doesn’t even work for our homes, leave alone the aeroplanes where safety is paramount. It is the authors understanding that the MCAS route adopted by Boeing is a coverup for the aircraft manufacturer’s reluctance to take the logical and the ethical route to aviation safety. The MCAS is essentially a ‘Wall Paper’ for the pitch up ‘attitude’ of the plane due to defective design of taking the engine forward from the wing instead of reworking the plane’s fuselage and the wing.

The facts became evident from the accountability demanded from Boeing after Ethopian crash and the public outcry worldwide. Since it was likely to be the doomsday for Boeing if it finds fault with its own innovation called MCAS, they conveniently shifted focus to another device, the Angle of Attack (AoA) sensor fitted on the fuselage of the plane. The job of the AoA sensor is to determine the pitch up or pitch down attitude of the plane. To give a simple description of the AoA, it is a basic device consisting of a vane which is exposed to the airflow around the plane. The position of the vane is sensed by an electrical circuit and transmitted to the aircraft computer. Usually, there are two vanes, one each on the captain and copilot side of the fuselage which feed the computers. Interestingly, the vane has no option but to remain aligned with the airflow and give the correct attitude of the plane. It is said that the starting point of the sequence leading to the catastrophic crash was the erroneous input from the AoA sensor fitted on the Captain’s side of the fuselage of the plane. For reason unknown, the AoA device on Co-pilot side was not utilised in the design of the MAX.

It is difficult to agree with the investigations so far that the AOA vane misguided the MCAS with a wrong attitude input. The AOA device is fitted on a large number of aircrafts worldwide and has been performing seamlessly without any hitch. Why should it malfunction repeatedly in the MAX? On the other hand, if the MTBF (Mean Time Between Failure) of AoA device was known to be low, why was the AoA vane on the Co-pilot side not incorporated into the MCAS system during initial design. Clearly, there is a lot to ask. In the authors opinion, it is more likely that the mess up happened at the Air Data Computer that receives the inputs or for that matter, it may be the MCAS software which misconstrued the inputs from AoA vane. Either ways, we are back to the software driven MCAS.

In view of the emerging facts, it is futile for Boeing to rework on the MCAS software as the inherent pitch up tendency of the aeroplane during takeoff and initial climb will remain a serious concern as long as the plane’s geometry and design is not altered. Understandably, this will affect the commercials and market share of Boeing in view of it’s competitor A320 NEO going strong (save the engine problems with a particular type).

History is repleat with instances of grounding of planes until a safer version was designed and put to service. More than 55 yars ago, in 1954, Comet jetliner was grounded over safety concerns after an in-flight breakup near Italy and few more crashes in quick succession. The aircraft model was re certified and allowed to fly only after a thorough design change which took almost 4 years.

It is definite that yet another accident of MAX will lead to loss of trust on Boeing brand and may be the end of the story for Boeing. It may therefore be better and necessary to take the hard decision to review the MAX project for its fundamental flaw and discontinue the beleaguered model which has anyways lost the trust of air travelers. It is still better to come up with an aircraft with safer design and a new name!! In the overall interest of Boeing and Aviation safety, these decisions must be taken urgently and made public before the travelers loose trust on the Boeing Brand.

It is natural for Boeing to defend its design to avoid the litigation and law suits over the crashed MAXes and the fate of those on order or already in service till now. But that is not sufficient reason to disregard safety concerns of the aviators and the trust of unsuspecting passengers. The Certification Agency (FAA of US in this case) had a significant role to see that aviation safety was not compromised. On the contrary, it is noted that during the certification process, the FAA made major policy changes and exceptions by delegating many evaluations to Boeing, allowing the manufacturer to review their own product. It was widely reported that Boeing pushed to expedite approval of the 737 MAX to compete on timelines with the Airbus A320 NEO. Since there are intriguing questions about how the FAA certified the MAX with the MCAS, it would not suffice to get a certificate from FAA that the plane is safe to fly once again. EASA and Aviation Authorities of ICAO member countries may need to do due deligence immediate to arrive at their independent conclusion. Meanwhile, the aviation community has a tough job ahead to coax Boeing and FAA to set aside all other considerations and offer a safe plane to fly.


The author, Wing Commander K Dinesh is an Aerospace Engineer by training. He is a retired officer of the Indian Air Force. Presently, he practices as an Aviation Safety Consultant with focus on Flight Anxiety removal for air travellers through his venture Cockpit Vista in Mumbai (India). For more details about him or his initiatives, please visit www.cockpitvista.com

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  1. Thanks for the easily understandable yet comprehensive analysis. The race for the bottom lines compromise even safety standards. This brings to mind Volkswagon’s fudging of their emission data and that even the most respected of companies are prone to deliberate avoidance of industry standards.

    It is quite alarming that in such a safety conscious industry the oversight agency (FAA) delegated their own vetting jobs to the designer/manufacturer!

    Will Boeing, FAA agree 737 Max 8 to be inspected by EASA? Whether they agree or not a reliable solution cannot be found without reviewing the design with respect to basic aerodynamics and not any quick fix software solution.

    Had couple of doubts though:-
    1. It’s been said that 737 Max 8 had only one of the AoA Vanes sending inputs to the MCAS. Even if two were in circuit, how does the MCAS decide which one is correct if one of the two were faulty? Or is it that if indication comes on that there is a difference between readings of the two AoA Vane inputs then the system entirely needs to be over ridden?

    2. While it has been said that the Ethiopian crew took the correct actions, did they disable (seems MCAS can be disabled only for 5 sec) or did they cut out the system completely in which case the ac should have come under their control?

    1. Dear Sir,

      You have raised two pertinent questions which i shall answer.

      (1) Regarding the choice between multiple or single AoA, it is standard when TWO or more AoA devices are invariably utilised in aeroplane design for a safety reason. Depending on the design philosophy, the two vanes can either work in parallel mode (redundancy mode), independent mode, or as standby to each other.

      In one such mode (which is the preferred design template), Inputs from two AoA vanes are compared by the onboard computers. The input is validated and approved for use by main computers only if it matches and if it is in the expected range. As stated in the article, the inputs would invariably agree as the AoA vanes are encountering the same air flow. However, in case of a disagreement a warning in form of “DISAGREEMENT LIGHT” is atleast provided in the cockpit so that the pilot can take subsequent actions to secure the flight. In some design philosophy (like Airbus and many Corporate plane makers) advance features are incorporated. In such templates, as soon as the computer senses disagreement between critical sensors, the aircraft computer switches to predetermined degraded mode / basic mode / alternate law and the pilots takes subsequent actions accordingly to fly the planes as prescribed under these conditions. Often, certain limitations apply under these conditions, the work load on pilots may increase, but the outcome is predictable and safety of aeroplane remains uncompromised.

      It seems that Boeing was aware of all of these standards but did not see this feature as a mandatory safety requirement and sold this DISAGREEMENT WARNING SYSTEM” as an optional fitment at a cost !! As reported, Boeing sold the MAX to both the Airlines (Lion & Ethopian) without the ‘optional’ system. This must be the case with many airlines who have bought or ordered MAX. As a corrective, Boeing plans to offer the DISAGREEMENT LIGHT as a standard fitment now onwards !!

      (2) As regards your point No 2, all we know at this stage from the Preliminary report released by the Ethopian CAA is that the system did not work as per procedures laid down by Boeing after the Lion Air crash. The aircraft did not ‘Trim” even after the pilot deactivated the STAB TRIM CUTOUT switches as prescribed by Boeing. Evidently, the MCAS driven design had more serious issues than the ones which became apparent from the first crash. It appears in the Ethopian crash that the pilot struggled with the MCAS and that the MCAS eventually won the fight between the pilot initiated electrical / manual trim and the MCAS initiated auto trim …. the MCAS took over the control of the plane from the pilot and nose dived the plane. Once again it boils down to design issue wrt aerodynamics. In simple words, the inability of pilot to trim the stabiliser despite manual intervention was due to the heavy aerodynamic forces on the Stabiliser in its extreme positions. It appears that it is almost impossible to trim back a plane manually or electrically if the MCAS has taken over the plane towards and beyond “Stabiliser Runaway”

  2. Thank you Wg Cdr K Dinesh for a very logical representation of the whole issue including the aviation safety and business completion.

  3. Could you throw some light on the fact that this snag was encountered by the crew of Lion Air which flew the plane before the tragic sortie. How were they able to handle the flight with the same snag.

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