We read that MCAS on the Boeing 737 MAX was needed because of the new, bigger, heavier engines of the MAX compared to the previous 737 versions.

The closest equivalent to MCAS on the Airbus A320 NEO is the alpha protection even if they are not the same.

But what have been the adaptations to the A320 due to heavier, larger engines? Can we be sure that A320 does not suffer from issues comparable to those the 737 MAX has, due to their similarities (bigger engines on same airplane)?

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    $\begingroup$ You're going to get shot down here due to having 6 questions in one request. $\endgroup$
    – CrossRoads
    Apr 2, 2019 at 13:02
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    $\begingroup$ And your premise is not correct. It wasn't just "new, bigger, heavier engines", it is their change in location "The new Maneuvering Characteristics Augmentation System mitigates the pitch-up tendency of the new flight geometry due to the heavier engines being located further forward and higher than the previous ones." $\endgroup$
    – CrossRoads
    Apr 2, 2019 at 13:06
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    $\begingroup$ Given that MCAS was a hidden feature, I don't think the question(s) can be answered factually. More so because the A320/neo is fly-by-wire, the 737/Max is not. $\endgroup$
    – user14897
    Apr 2, 2019 at 14:16
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    $\begingroup$ "Can we be sure that A320 does not suffer from issues comparable to those the 737 MAX has, due to their similarities" looks awfully close to accident speculation, which is off topic here. $\endgroup$
    – user
    Apr 2, 2019 at 15:34

3 Answers 3


The A320 and 737 have very different flight control architectures.

The 737 has physical cables that transmit pilot (or autopilot) input directly to the hydraulic actuators. This was common in the 1960's when the airplane was first designed. This means that the airplane handling comes down to the aerodynamics and the pilot input.

The 737 MAX presented an issue when it was found that certain situations approaching stall could cause the airplane to pitch up, exacerbating the stall, rather than naturally pitching down and recovering as expected. This was due to the larger engine nacelles, which are located ahead of the center of gravity. An aerodynamic change would have taken lots of time and money and had its own drawbacks. Instead, Boeing created MCAS. This applies trim to help to pitch the airplane down when it reaches high angles of attack.

The A320, on the other hand, was designed as a fly-by-wire aircraft. This means that a computer is using sensor inputs to control the airplane all the time, and pilot or autopilot inputs only provide a guide to that computer control.

Fly-by-wire allows some helpful safety features, such as not allowing the airplane to exceed the design envelope, including stall. This means that any A320 in normal conditions will allow the airplane to reach a maximum angle of attack and no higher. Any aerodynamic differences such as a different fuselage length, larger engines, etc, are countered by the flight computer. Other than some possible fine tuning of the control laws of the fly-by-wire system, the A320 neo would not need any special new system to control angle of attack.

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    $\begingroup$ [1] Neither this, nor the answer posted by @Barath are not actual answers. There are several issues: (1) Is the 320 ceo inherently less aerodynamically stable than the 737 NG? (2) Do bigger engines introduce similar pitch-up issues on the Neo - aerodynamically, not in piloting - as they did on the Max? (3) Does neo mitigate these issues using existing fly-by-wire system without affecting piloting? (4) How many computers the neo has? (I believe the ceo has 3 independent 286-based computers. The Max has two computers). (5) Can the Neo switch between computers in-flight? (The Max cannot). $\endgroup$
    – Rusty Core
    May 1, 2019 at 17:33
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    $\begingroup$ [2] (6) How many sensors does each Neo computer reads from? (The Max reads from a single AOA and a single airspeed sensor on one side of the aircraft; sensors on the other side are connected to the second computer, that is turned off in flight). (7) What is the design and positioning of the sensors on the Neo compared to the Max, are they better protected from impact or more durable? (8) When the fly-by-wire system activates on the 320, how much freedom does it give to a human pilot? (The MCAS is explicitly designed to overpower the pilot). $\endgroup$
    – Rusty Core
    May 1, 2019 at 17:33
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    $\begingroup$ @RustyCore Welcome to Aviation.SE! I would suggest you ask new questions, at least splitting the architecture (how many computers, how many sensors, location of sensors) and the functionality (FBW effect on handling, pilot input) into separate questions. We also have some related questions already, such as this and this, feel free to look around and reference existing questions if there's something more you'd like to know. $\endgroup$
    – fooot
    May 1, 2019 at 18:03

The 737 Max had short landing gear. Since the engines need to have a minimum ground clearance, this meant that the new larger engines had to be repositioned further forward and higher on the wing.

The repositioned engine along with the new engine nacelle shape meant it had different flying characteristics at high angles of attack (e.g. due to airflow). Thus Boeing introduced the MCAS system to reduce chances of stall in manual mode and try to make the plane behave the same as before.

The A320 has larger "legs". It could accommodate the larger heavier engines of the A320neo without requiring re-positioning from the A320ceo. In fact, the LEAP 1A of the A320neo is marginally larger and heavier than the LEAP 1B of the 737Max. Even the PW1100G option for the A320neo is marginally larger than the LEAP1B (737MAX). [All the newer gen engines are larger and heavier than the previous gen engines of the 737NG/320ceo]

Plus Airbus just has a completely different design approach and architecture for flight control. Airbus A320 was designed as a fly by wire aircraft, from the get-go, the Boeing 737 was designed in a different era, and manual mode had no real computer control (except automatic trims & MCAS). Additionally, there may be differences in the sensor/computer architecture etc.

Bottom line, the 737Max needed a new system, while the A320neo probably needed change in configurations but no new system for control...

  • $\begingroup$ This is the correct answer. At least the first 3 paragraphs are, the B737 has Flight Control Computers. $\endgroup$
    – Koyovis
    Feb 13, 2020 at 7:08

No. The reason the 737 MAX had issues was because of the POSITION of the engines. There's nothing inherently wrong with larger engines on the same airframe. The reason the 737 MAX had issues was because the engines needed to be moved forward and up. The reason for that is that the 737 has short landing gear. Even with CFM-56 engines (737 Classic/NG), there's not much ground clearance between the nacelle and the ground. If you compare a 737 (any model) with an A320-series aircraft, you'll see that the 737 rides a LOT closer to the ground than its Airbus counterpart. You may ask yourself why this is, and to see that, we need to take a look at the history and original mission of the 737. The 737 was originally intended to perform a role similar to what we today know as a regional jet. It was intended to carry relatively small numbers of passengers to smaller regional airports. These smaller airports sometimes lacked the necessary ground service equipment to load an aircraft sitting higher off the ground. In fact, aircraft were sometimes loaded by hand. In order to accommodate this, Boeing equipped the aircraft with shorter landing gear. The original JT8D engines could fit under the airplane easily.

When Boeing decided to equip the 737 with modern engines, they chose the CFM-56. However, the CFM-56 was too big to fit under the wing with adequate ground clearance. Boeing worked with CFM to modify it, and the bottom of the nacelle was flattened. This gave the 737 Classic/NG its unique look. While there was enough ground clearance between the bottom of the engines and the ground, it wasn't much. When Boeing decided to re-engine the re-engined 737, they chose the LEAP-1B. Once again, there was not enough ground clearance with the new engines. Though all this, Boeing had kept the original short landing gear, both for weight savings and the fact that lengthening the gear would require a significant redesign of the airframe structure. There was no other place to put the engines other than moving them forward and up. The LEAP-1B also received a flat bottom. With these modifications, the aircraft had enough ground clearance. Unfortunately, the new engine placement resulted in altered high-alpha flight characteristics. This was enough of a difference that Boeing found it necessary to add a software "fix" that would give the aircraft a slow-flight feel more like that of the 737NG. The A320 came about much later than the 737, and airports by then had the necessary GSE to service taller airplanes. The A320 was then given longer landing gear, and an unmodified CFM-56 fit easily under the wing. The reason that the A320NEO does not need a system like MCAS is because the engines have the necessary clearance under the wing and are in a position that does not negatively affect flight characteristics.


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