Hot answers tagged

55

MCAS doesn't have its own on/off switch It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation ...


21

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 ...


20

You can disable the stabiliser trim which will prevent MCAS from making inputs to the aircraft.


12

On the 737 NG, at high angles of attack, the nose of the plane would naturally pitch down, helping to recover from a stall and to increase airspeed. The larger engine nacelles on the 737 MAX are located forward of the center of gravity, which means that at high angles of attack they are pushing the nose up. MCAS helps to push the nose down in this situation,...


11

The new 737 MAX has a more advanced pitch control system called maneuvering characteristics augmentation system (MCAS), which automatically adjusts the stabilator trim in case of high thrust and high angle of attack (AOA). This makes it very hard to overcome the down force with elevator input alone (pulling up). In case of a wrong trim adjustment (e.g. due ...


10

From what I have read, the other answers explain the cutout procedure correctly. Edit: The ongoing investigation appears to reveal that the aerodynamic forces on the stabilizer may become so high that it becomes hard or impossible to manually correct a nose-down trim. (This is not a B373 MAX issue but concerns earlier models as well.) The forces increase, ...


10

Noting that the details of the MCAS update are yet to be publicly confirmed by Boeing - no I don't believe you are missing anything. MCAS was meant to be a system that only kicked in when the pilots were letting the situation get out of hand. It was to aid in stall prevention, but does not do anything the pilots can't (as long as their situational ...


9

No, MCAS was required for the 737 MAX to be certified by the FAA. It cannot be deactivated by any airline, including airlines in the U.S. Source: "M.C.A.S. was necessary then for the airplane [737 MAX] to be certified by the F.A.A. to have met all of the regulatory design requirements for stability and control." https://www.nytimes.com/2019/02/...


9

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 forwrd 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 (eg due to airflow). Thus ...


8

The pitot tube has nothing to do with the angle of attack. It is used for measuring pressure which in turn gives you the airspeed. Accelerometers are used for the Inertial Navigation System. The angle of attack is another sensor (shown below): the principle is that it rotates and reads the angle between the current position and the reference position. ...


8

We don't know - until we hear about such a case! A positive answer would require a trace of MCAS action sufficient evidence that the pilot could not have handled the (near) stall without MCAS. This information could come from a pilot who notices the trim interference, and perceives the situation as dangerous, and reports the event or a flight ...


8

The Boeing 737 MAX MCAS system is there ONLY to meet the FAA longitudinal stability requirements as specified in FAR Section 25.173, and in particular part (c) which mandates "stick force vs speed curve:, and also FAR Section 25.203 — "Stall characteristics". FEDERAL AVIATION REGULATIONS Sec. 25.173 — Static longitudinal stability. Sec. 25.173 — Static ...


7

MCAS uses stabiliser input to retain full elevator authority in both directions for pilot input. MCAS is set up as an Inner Loop autopilot: it controls behaviour around the CoG of the aeroplane without displacing the cockpit flying controls. For aeroplane pitch control there are indeed two options: the stabiliser and the elevator. If the elevator is chosen ...


7

No, the 737 NG stabilizer trim system does not feedback from AOA. My source of information comes from this LinkedIn Slide Share and corroborated by this online FCOM. There are several ways through which the stabilizer can be commanded: Manually via the trim switches. Manually via the trim wheels. Automatically via the Speed Trim System (STS). A helpful ...


6

No, we don’t know for sure. We do know, however, that it wouldn’t make any sense to install it. It’s a bit like asking „are we sure the electric VW Golf doesn’t run the Diesel cheating software?“ - We don’t know, as the VW electric motor control code is not public (as far as I know), but it wouldn’t make much sense to fit it, right?


6

A change in the pitch feel system wouldn't solve the problem. It's the MAX's natural behaviour separate from the flight control system (that is, behaviour when you aren't touching the controls). As Fooot says, the MAX's engines have the effect of moving the overall center of lift forward somewhat, which is more or less the same thing as moving the center of ...


6

According to the article, the KC-46 uses the dual FCC/AoA sensor for computing both FCC channels and comparing the command signals. With two disagreeing signals there is no way to determine which one is correct and the usual remedy is to disengage both, warn the pilot, and engage in the appropriate crew procedure. And that requires training. The B737MAX was ...


5

It does not appear that MCAS was in any of the maintenance documents. This was part of the issue with the Lion Air crash. On the previous flight with the accident aircraft, MCAS acted on the faulty sensor data, but the pilots used the trim cutout and completed the flight. They had no reason to suspect that a trim issue could be related to a faulty AOA sensor,...


5

What do you know, I found the answer to my exact question in the Seattle Times: First of all, even after two fatal crashes, Boeing declined to detail the specific functionality of the two switches. Nevertheless, The Seattle Times found that the left switch on the 737 NG model is capable of deactivating the buttons on the yoke that pilots regularly ...


4

Most airlines have a Flight Data Analysis section which uses the routine download of the FDR to automatically flag deviations from the normal flight profile for investigation. The kind of maneuvering that would activate MCAS would vastly exceed the typical limits and be subject to a detailed investigation. Most of the 737 Max FDR do have provision for ...


4

All large swept-wing aircraft have a yaw damper, which is always active. It takes an input from a yaw gyro and applies rudder inputs in order to counter Dutch roll.


4

Are there other automated systems providing direct control surface actuator inputs when autopilot is off on the Boeing 737s? One such system on the 737 Classic and NG is the Speed Trim System: As you can see above, when certain conditions are met, of which 5 seconds has passed since a pilot trim input is made, the STS trims the plane if required. The ...


4

The MAX 10 fuselage is also lengthened behind the wing: In October 2016, Boeing's board of directors granted authority to offer the stretched variant with two extra fuselage sections forward and aft with a 3,100 nautical miles (3,600 mi; 5,700 km) range reduced from 3,300 nautical miles (3,800 mi; 6,100 km) of the -9. Wikipedia: Boeing 737 MAX 10 ...


4

The fuselage length is immaterial. The problem of the Max series relates to the positions of the engines versus the wing, and their aerodynamic interference near the at high alpha. The amount of fuselage ahead of the wing or behind the wing is not a major factor here.


3

The problem appears to be mostly the effect of a larger cowling, moved more forward, that generates significant lift when operating at higher AOA and has an effect similar to moving the entire wing forward, or moving the center of gravity aft, partially unloading the horizontal tail in the flaps up low speed regime and making the pitch stability a bit ...


3

Mike's answer is right. The yoke controls the elevator not the stabilizer. MCAS trims the stabilizer, which can be overriden through the thumb switch, trim cutout switches (disables MCAS and the thumb switch), or by manually turning the Trim Wheel. (To see what that looks like, refer to the end of this video.) To expand on Mike's answer, the preliminary ...


3

So in the case of this flight, would pulling up have not recovered the flight? "Pulling up" (pulling back on the yoke) would NOT have recovered the flight. In fact, the Lion Air pilots were pulling back on the yoke for most of the flight (see the flight data recorder's Control Column Force graph). Can computer imposed inputs be overridden on the Boeing ...


3

From publicly available sources, the first motive of the MCAS was to satisfy the stick force per G requirements, or maneuvering stability (not static longitudinal stability, which deals with speed stability). This is confusingly captured in 14 CFR 25.255(b) and (c) under Out-of-Trim Characteristics, but also applies broadly to buffet characterization at mid/...


2

The new system will not be a single point of failure. Normally, the AOA sensors should not disagree. But then again, normally pilots should not be flying the aircraft near stall margins. However, if the sensors do disagree -- it will tell the pilots with a cockpit indication: effectively "MCAS will not rescue you today, watch your trim". It should ...


2

Every automated system has a possibility of a false positive and a possibility of a false negative. In the system design you have to consider ( Probability of a false positive * consequence of false positive ) versus (probability of a false negative * consequence of false negative). A team of engineers at Boeing certainly looked at the tradeoff above in ...


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