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Boeing 737MAX/Airbus Differences

Most of you are aware of the current issues that Boeing is having with the Boeing 737MAX. There is a lot of information on the media about this issue, but in this article I will focus on two things. This article is very technical, so if you are more interested in my personal experiences with flying, you might want to skip this one, but if you like technical details this could prove to be a good read.

First I want to talk about the fundamental differences in Boeings and Airbus design philosophy and in the second part I will explain the technical reasons behind the issues that the Boeing 737MAX is facing at the moment. I will try to start with the very basics and will try to simplify it as much as possible (so a few details may be “inaccurate” but I wanted to avoid going too much into detail), so everybody can follow this explanation.

Every airplane uses flight control surfaces on the wing and on the stabilizer (the “tail” of the airplane) for control. The surfaces on the wing are called ailerons and are used to roll/turn the airplane. The surface on the tail is called elevator and is used to control the airplane in pitch. These deflect the oncoming airstream and thereby generate a force that is then used to control the airplane. When the speed of the airplane is increased the stabilizer has to generate more upward force (called lift). This would require to pilot to constantly pull on the cockpit control which would be very tiring, therefore airplanes are equipped with a stabilizer trim which, depending on the design, either shifts the neutral point of the elevator (f.e. by the use of piece of ballast that is attached to the cockpit control and can be moved by trimming) or, on bigger airplanes, moves the whole stabilizer to generate more lift. The opposite is the case in the case of a speed reduction. In the very beginnings of aviation there was a direct connection from the control in the cockpit (control stick/wheel and rudder pedals) to the respective control surface, so moving the control in the cockpit mechanically moves the associated surface. This system is still being used on small airplanes that operate at low speeds. As aviation advanced and airplanes got heavier and faster the forces required to operate these surfaces increased as well and at some point exceeded human strength. Therefore airplane designers started to use hydraulic systems to assist the pilot in moving the flight control surfaces. The input on the cockpit control is sent to an hydraulic actuator on the corresponding surface to move it. This is the system that is in use on the Boeing 737 (and most, but not all, other aircraft of that era) since the first 737 was build in 1967. The Boeing 737 went through several major upgrades throughout its lifetime, and most systems are totally different nowadays compared to the original version, but this basic principle never changed.

Some military fighter jets designed in the mid-70s (for example the F-16) already had (very basic) computers that were processing the pilots input on the control stick, optimized these inputs and then send these modified inputs to the hydraulic actuators (this is called “fly-by-wire”). This concept proved very successful and Airbus decided to implement this (for the time) radically new approach on their Airbus A320 which was designed in the mid 80s. The Airbus system eliminates the need for pilots to manually trim the airplane as the computer automatically positions the stabilizer in the optimum position for the current speed, so with the cockpit control (Airbus calls the cockpit control “side stick”) released the airplane will maintain its orientation. If you release the control on a “conventional” airplane, for example on a Boeing 737 without trimming the airplane pitch will change until it reaches and eventually stabilizes at the speed for which the trim is set. An other feature that Airbus introduced were so called “hard” protections. These systems were designed to protect the airplane from over stress and critical conditions by limiting pilot authority at the edge of the flight envelope. Many accidents at this time were caused by pilot error which lost control of their airplanes for various reasons and this new approach was introduced to reduce the amount of these accidents. If you keep pulling back on the control column of a Boeing 737 the aircraft will (after giving you some warnings) eventually stall, which basically means that the wings are not able to generate enough lift to support the weight of the airplane and the airplane will start to loose altitude at a high rate and also loose a lot of control authority. At a low altitude this can be deadly and has caused many accidents. If you do the same on an Airbus A320 the computer will modify and reduce the pilots input, so that the airplane will be on the verge of stalling, but actually never enters a real stall, so altitude and control can be maintained. There are also several other protections in place in the Airbus system. This concept was initially met with great skepticism after all a bunch of computers (which was still a new invention in the 80’s) was limiting the authority of the pilot in a potentially critical condition. However the systems layout was very redundant from the start and became the standard over the years. For example, the aircraft has a total of five flight control computers, 2 “ELAC”s, short for Elevator-Aileron Computer (these are the “main” computers in charge of most control surfaces in normal conditions and each ELAC actually consists of two independent parts, that check the other parts computations and inhibits the computer in case of a discrepancy in computations) and 3 “SEC”s, short for Spoiler-Elevator computer. Even if four of these computers failed the pilots would still be able to maintain control through the remaining computer. Boeing eventually also developed a fly-by-wire system in the 90’s that is in use now on the Boeing 777 and Boeing 787, however the Boeing 737 is still conventional. The philosophy behind Boeings implementation of the Fly-by-wire system is different. I won’t go too much into detail, but Boeing features a “soft” protection system. They provide the pilots with clues that he is about the exceed the safe limits of the airplane (by indications on the displays, but also by tactile clues on the control column, which becomes “heavier” when limits are approached). The underlying philosophy is the Boeing is giving the pilot ultimate authority in any situation (for the good or the bad, as a disoriented or badly trained pilot might make a situation worse by exceeding safe limits) while Airbus is limiting the pilots authority to keep the airplane within a safe envelope.

The problems with the Boeing 737 MAX that Boeing is facing at the moment as caused by a system called MCAS (Maneuvering Characteristics Augmentation System). Most modern airplanes (both the Airbus A320 and well as the Boeing 737) have their engines mounted beneath the wings. Because this location places them below the airplanes center of gravity the thrust output of the airplane has an effect on the pitch axis. If thrust if increased pitch tends to increase, if thrust is reduced pitch is reduced. This effect is particularly noticeable at low speeds, as seen during takeoff and landing. Airbus with its fly-by-wire system automatically compensates for this effect, however on the Boeing 737 there is a need to trim the stabilizer if there is a change in thrust. To help the pilots Boeing introduced a system called “Speed trim System” already during the early versions of the type. Under certain conditions this system would move the trim to reduce the need for the pilot to trim during takeoff and landing. When Boeing developed the Boeing 737 MAX the biggest and most noticeable change were the engines. The airplane was designed in 1967 for the engines in use at this time which were rather small so not a lot of emphasis was placed on ground clearance, actually it is an advantage to have to airplane closer to the ground, for example for baggage loading. The newest engines would not fit under the wing and lengthening the landing gear to archive a better ground clearance would have required Boeing to redesign many parts of the airplane and the re-certify them which would have been uneconomical. Therefore the engines were placed further forward and closer to the wing, so that they could still be mounted onto the airframe. One undesired side effect of this was that the effect of thrust changes was more pronounced than on earlier versions of the Boeing 737. This effect was so strong in certain situations that it could not be certified without some additional system in place, so Boeing developed the MCAS system which would enhance the speed trim system. However for some reason Boeing based this system on a single sensor which, if giving an erroneous high reading, would result in the stabilizer trim running in the nose down direction until the mechanical stop of the system. With the stabilizer trim in the full nose down position there is not enough authority by the elevator to maintain control of the aircraft and the aircraft will nose-dive into the ground. The system used by Airbus is also based on sensor readings, however Airbus uses three sensors and has introduced a system that is able to detect and inhibit a single false sensor reading. If multiple sensors are unreliable on an A320 the system will revert to a more direct mode of operation and stop the automatic trimming. It is impossible to archive absolute safety, no matter what system is used but common sense would dictate that multiple sensors should be used for such a flight critical system, unlike the system that was in use on the Boeing 737MAX until now.

Boeing has updated the software by now and is working with the authorities to find a solution to re-certify the airplane. One of the changes is that the system now uses two sensors for MCAS activation and that generally the authority of the system is reduced. Obviously after two accidents it will take a while to satisfy the authorities and the public that the airplane will be flying safely after the upgrades. If you are interested in the details of the changes that are being introduced by Boeing, you may want to have a look at their website: https://www.boeing.com/commercial/737max/737-max-update.page

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