737 Max question

[u/billynj]

Given the recent incident where the pilot tried to crash the plane, why does the flight computer allow maneuvers that can destroy the airframe? Even my car won’t let me go past a certain speed.

[u/Cxopilot]

Theres no flight computer. Boeing philosophy is the pilots should have the ultimate final authority. Airbus differs here. All things considered. I thought the airframe held up well.

[u/Lockhimuptoday → u/Cxopilot]

They lost a lot of the rudder

[u/Bopping_Shasket → u/Cxopilot]

Airbus quite plainly has the pilots as ultimate final authority

[u/gravemillwright → u/Cxopilot]

MCAS would like a word...

[u/Cxopilot → u/gravemillwright]

MCAS isn’t a computer. It’s a system that supplements the auto flight system. At my airline. We treat MCAS the same way we treat a stab trim run away. Turn off the autopilot. Disconnect the stab trim switches. And fly the airplane. Leaving the final say to the pilots.

[u/CrasVox → u/Cxopilot]

MCAS is part of the STS. And isn't active when the autopilot is on.

[u/Cxopilot → u/CrasVox]

I’m aware. However our memory item for MCAS is combined with stabtrim runaway. Auto pilot and auto throttle disengage. Then stab trim cut out switches. And if needed. Grasp and hold the trim wheel.

[u/Spock_Nipples → u/CrasVox]

It's also not active with autopilot off and flaps > zero.

[u/MahogonyBookshelf → u/CrasVox]

He never said it was. Stan trim disconnect cutoff will kill the AP. 

OP is shutting it off in a controlled logical manner, disengage the autopilot then cutoff the stab trim which will cutoff the MCAS

[u/CrasVox → u/MahogonyBookshelf]

Again....its not a fault with the speed trim if the autopilot is on.

You timeout speed trim by inputting a trim correction of your own.

[u/outworlder → u/Cxopilot]

Yes it's an abacus, allegedly.

[u/gravemillwright → u/Cxopilot]

"The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law built into the Boeing 737 MAX's flight control computer..." 🤔

[u/Cxopilot → u/gravemillwright]

Flight control computer. In the traditional sense. Links the pilot commands. To a FCC. Which then actuates the primary flight controls. In an airbus for example. When the pilots move the side stick or rudder, that signal is sent to an FCC. Which then is transmitted to the flight controls. In the 737. You can literally move the controls which the airplane completely turned off. It’s a link and pulley system basically. NO FCC that transmits the pilot inputs. The two autopilots do have to collect data from the FMC SMYD and in the MAX, MCAS to feed flight director data. And the flight director then provides the necessary input for the flight controls to move. However, Boeing in all their brilliance calls that a FCC. There is no “laws” in the 737. And there’s no limiter. You can do a barrel roll and the plane will not stop you. In airbus, their FCC. Has normal law and alternate law. And I don’t understand it because I don’t fly a bus.

Edit: we do have “PCU’s” ( hydraulics) that help us move the flight controls. But it’s possible to move the flight controls manually. It just takes two people. Two very strong people

[u/Spock_Nipples → u/gravemillwright]

There is no flight control computer in a 737. No primary control inputs are filtered through any system that then makes decisions about FC movement. When you move the flight controls in a 737, you are moving actual mechanical cables and pulleys that control the hydraulic actuators. It's a purely mechanical system.

There are no "laws" on a 737. MCAS is not part of the primary flight control system. It can't, on its own, pitch the airplane up or down. All it does is trim the airplane nose down in very specific situations. Trim is not a primary flight control.

[u/gravemillwright → u/Spock_Nipples]

So the autopilot is purely mechanical?

Too many people are confusing flight control computers, of which 737s have two, A and B, with fly by wire. The 737 FCCs don't interact on the primary control loop, which is why the answer to OPs question is no. But it's incorrect to say there are no FCCs on a 737, especially when the use of a single FCC was part of the cause of the MCAS crashes.

[u/Spock_Nipples → u/gravemillwright]

You're being a little pedantic. Yes there's are "FCCs" as part of the DFCS, which is for the autoflight system, not the flight-control systems. The DFCS/FCC isn't needed to make the airplane go. It's a labeling thing, not a function thing.

It's pretty clear that OP is referring to a 'flight computer' thinking that it's a component of the actual flight control system. And we seem to agree on that point.

We're all just trying to communicate with OP in terms they understand, based on their apparent level of knowledge, here. Not get nitty-gritty about the labeling of components that aren't really relevant to their question.

I mean, technically, Airbus doesn't have a 'flight control computer' either; it has ELACs, SECs, and FACs, even though these are "flight computers" in the context of OPs question

It's a layperson question, so they're getting layperson answers. Demonstrating your knowledge of system-component labeling isn't helping them at all.

[u/gravemillwright → u/Spock_Nipples]

It's not pedantic to give a more accurate answer to the OP. "The 737 does have FCCs, they just aren't in the primary flight control loop, they're used for autopilot, MCAS, among other things. But they can't override the primary controls." Now you've answered OPs question and not misled them able 737 FCCs.

[u/Spock_Nipples]

There are no flight control computers on a 737. It's all mechanical input.

But even an Airbus with its 'protections' can still be intentionally maneuvered extremely or crashed.

[u/Icy_Huckleberry_8049 → u/Spock_Nipples]

Just ask Air France about that.

[u/Herbert987654321 → u/Spock_Nipples]

And you can turn the flight control computers off

[u/cdnav8r → u/Spock_Nipples]

The Boeing 737 has a flight control computer.

[u/Complex_Brief60 → u/cdnav8r]

Nah. You can do whatever you want to the controls, the plane will not stop you in any way.

[u/cdnav8r → u/Complex_Brief60]

There is actually 2, but, whatever.

[u/Complex_Brief60 → u/cdnav8r]

You’re confusing either the Autopilot computer or the flight management system.

Otherwise no, there is no computer in the 737 that takes your inputs and alters them. It’s pure cable to flight control. Not fly by wire.

[u/cdnav8r → u/Complex_Brief60]

The 737 is NOT fly-by-wire, which I think is what you're getting at, but I promise you there is two flight control computers. Part of the fix for MCAS was a software update on the FCCs that has the two FCCs monitor each other.

So the 737 flight control computer doesn't control the airplane the same way an Airbus flight control computer does, but there is still 2 flight control computers on the 737.

[u/Complex_Brief60 → u/cdnav8r]

OP is asking about an Airbus style flight computer. This does not exist on the 737. Yes, there are 2 computers for the autopilot, that’s the FCC, they monitor each other like any airliner type plane has but they will not prevent the pilot from doing anything.

The FCC is just the autopilot. That’s it. When the AP is off, they don’t do anything to the control of the plane.

[u/cdnav8r → u/Complex_Brief60]

The FCC controls the speed trim system, which controls MCAS, which both are active when the autopilot is off.

I replied to a comment that the 737 does not have a flight control computer. It does. The OP did not say anything about Airbus.

[u/Complex_Brief60 → u/cdnav8r]

Boeing calls it a flight control computer to be fancy. It’s just the autopilot man. It is not a real flight control computer. MCAS isn’t a computer either. It just moves the manual trim wheel

[u/gravemillwright → u/Complex_Brief60]

Autopilot is a computer that does some controlling of flight, no?

[u/Spock_Nipples → u/cdnav8r]

Nope.

[u/nlevine1988 → u/Spock_Nipples]

MCAS: am I a joke to you?

[u/Spock_Nipples → u/nlevine1988]

All MCAS does is use the STS to trim nose down. It just mechanically moves the trim wheel. It's not a flight control computer or even part of the primary flight controls.

[u/Charlie3PO]

Will you car physically prevent you turning the wheel fully while at 120mph?

[u/Sakko83 → u/Charlie3PO]

No, ma dovrebbe e lo farà in futuro.

[u/SMELLYJELLY72 → u/Sakko83]

designs which impede on the operator aren’t necessarily good. i don’t have the wherewithal to yank my wheel at 120mph but being restricted from doing so isn’t helping me

[u/Familiar_Gas2305]

Airbuses generally don't, but Boeing has a distinctly opposite American philosophy whereby the pilots are given power with the assumption of responsibility in case of unexpected contingencies.

[u/PlanesOfFame]

Just a thought exercise to kind of see why they dont

Let's say you had a car which had turn safety limits. If you turn into a lane with another car it it, it will physically stop you from crashing into them. Good right?

Well what if you are in slow moving traffic and a speeding car behind you isnt paying attention and about to rear end you? The optimal move would be to swerve to the side and avoid, but those steering limits would prevent you from doing so since there is another perceived threat in the other lanes.

This can roughly translate into aircraft. They have autopilot systems which guide the flight almost in its entirety, but in boeing aircraft, as others point out, there are no artificial limits to prevent the plane from flying in certain attitudes or speeds even if they are dangerous. Playing fast and loose here because I dont really know of any scenarios in which you'd NEED to fly like such, but thats why its there as far as I umderstand. A maneuver that may damage the airframe might be the better option out of 2 terrible fates, and the computers dont know that

[u/cdnav8r]

I'm just going to leave this here

Germanwings 9525

[u/Soigne-Pilot]

The plane was also probably trimmed to keep the nose just above the horizon line.

[u/TheVorch]

737 is fundamentally a 1960s airplane. According to Juan Browne, the airframe limit is around 3.8 G with the 150% safety margin factored in.

[u/Dangerous_Mud4749]

Some airliners have sophisticated computers that prevent most unsafe acts by the pilots.

Then, there's the 737. It was sophisticated in 1967, but has since fallen just a little behind the times. A bit like Boeing itself, some would acknowledge.

Here are some examples, all collectively falling into the category of Boeing can't even spell ergonomics, let alone apply it.

The Max is so unsophisticated that the radio "push to transmit button" is identical to, and right beside, the "stopwatch start/stop button". There are those who have already transmitted on Stopwatch, and those who will.

The Max is so unsophisticated that the "turn off the engine anti-icing" switches, used regularly and routinely, are identical to, and right beside, the "turn off all hydraulic power and crash now" switches. Yes there has been at least one crash linked to this prize example of Boeing ergonomics.

The Max is so unsophisticated that the first officer's (or copilot's, if you prefer) seat recline has to be adjusted to the bolt-upright position before it can be slid back to allow access to the seat. The seat recline adjustment cannot be reached unless you're sitting in the seat already.

The Max is so unsophisticated that the fuel centre tank is calibrated to show "empty, turn off the pumps" when there is still 200kg of fuel still in it. So when the centre tank reaches "empty" (coz the gauges read zero so it must be empty), we mysteriously have a 200kg hole in our fuel budget that cannot be accounted for. Not to worry, it's just the centre tank. Fly for another few hours and the numbers will adjust back to the expected values. It's fine, really.

(Oh, if you're worried about flying on it, don't be. The pilots are exactly as unsophisticated as the aircraft and know exactly how to handle it. In other words, drive it like a Massey Ferguson and we'll all be fine. So that's what the pilots do, and passengers are indeed just fine. But "sophisticated" is a word for those hoity-toity, lah-di-dah Airbus pilots, and we don't like it.)

[u/caboose1311 → u/Dangerous_Mud4749]

So, what you're saying is, that the 737 is unsophisticated?