[sfilargi]

For a car analogy, imagine Tesla creates a new model that under certain conditions tends to over-steer left. To prevent any accidents because of that, they have added a system that turns the steering wheel clockwise. To disable the system it is not enough to try to prevent by force the steering wheel from turning, but you have to press a button that disables electric steering assistance.

Notice they have never briefed you about this new system because they didn't want to retrain you.

You happily drive along the highway at 70mph when the system malfunctions and incorrectly thinks the car is over-steering left, so the steering wheel starts rotating clockwise. The car swiftly moves to the lane in your right as you use all your strength to prevent it and manage to bring the car back in the lane.

In the mean time all kind of lights and sounds go off on your cockpit. In 5 seconds, while you are trying to find out what to make out of all the lights flashing, the steering wheel starts turning the card right again. This time you are not so lucky and you crash into the truck on your right.

[rootusrootus]

I feel like every time someone insists on making a car analogy, it just piles on more evidence that car analogies never work.

At least you don't have to get a new endorsement on your license for every model of car.

[sfilargi]

Analogy implies partial similarity. Of course there will be differences.

[mschuster91]

> At least you don't have to get a new endorsement on your license for every model of car.

In Germany, if you get your drivers license on an automatic car, then you are only allowed to drive automatic cars, and have to extend your license to drive manual cars.

[iscrewyou]

Analogy implies that people could die. They have died.

[lsh123]

Incorrect analogy. Runaway trim is a common and known problem with a solution to pull the electric trim breaker. MCAS failure is very similar in appearance and have exactly the same solution.

[inferiorhuman]

MCAS failure is very similar in appearance and have exactly the same solution.

MCAS doesn't behave like a runaway stabilizer. MCAS adjusts the trim periodically and will back off if you apply opposite trim.

The steps to deal with runaway trim are[1]:

1 Control column . . . . . . . . . . . . . . . . . Hold firmly

2 Autopilot (if engaged) . . . . . . . . . . . . . Disengage

Do not re-engage the autopilot

Control airplane pitch attitude manually with control column and main electric trim as needed.

3 If the runaway stops:

  ■   ■   ■   ■ 

So someone trying to make sense of MCAS would logically stop at step 3 and not actually disable MCAS.

1: http://www.737ng.co.uk/737-800%20Quick%20Reference%20Handboo...

[oasisbob]

Why did you leave off steps 4 and 5?

4. If the runaway continues: STAB TRIM CUTOUT switches (both) . . . . . . . . . CUTOUT

If the runaway continues: Stabilizer trim wheel . . . . . . . . . . Grasp and hold

I don't see why someone would stop at step 3, given that the MCAS pause cycle is only 5 seconds long.

[inferiorhuman]

Why did you leave off steps 4 and 5?

Because that's where many pilots would stop. The "runaway" stops when the pilot inputs opposite trim.

[siwatanejo]

Can you think of a better car analogy? I'm genuinely curious.

[inferiorhuman]

All cars that drive only the front wheels experience a phenomenon called torque steer: as the engine produces more power the car tends to veer off into one direction or another. This is typically only a problem with more powerful engines.

Many manufacturers will use different length driveshafts to each front wheel, maybe they'll use fancy mounting hardware, maybe they'll reduce the maximum power at low speeds, maybe they'll apply the brake on one wheel as you start to move in a direction other than where the steering wheel is pointed.

Let's say a car company put out a little front wheel drive car so powerful that simply applying the brakes or cutting power would make the car much less fun to drive. So let's say our miracle company decided to have the computer steer the car in the opposite direction when you press the accelerator pedal instead.

Then one day the dongle that determines how far you're pushing the accelerator pedal breaks. The wiper on one of the potentiometers is dirty and instead of sending no signal it's sending an erratic and inaccurate signal. It's not a problem for the engine computer, there are two pots and it's picked the good signal and limited engine power just in case. For some reason cruise control doesn't seem to work, but other than that the car seems to drive fine.

But the torque steer doohickey is using the bad signal. The bad signal is erratic and makes the computer think you're trying to accelerate rapidly and yanks the wheel to the left for no apparent reason. You yank the wheel in the other direction and the car drives fine for a bit so you continue. As you accelerate onto the freeway onramp it happens again. You yank the wheel the other way and keep going. Then you try to change lanes to pass a slow truck in front of you. It happens again, only this time as you're trying to turn left you can't counteract the insanity. Instead your car tries to make a ninety degree turn at freeway speeds.

[lsh123]

You know that you need to break when you get close to another veichle in front. Now you have a new shiny cruise control (without latest distance keeping feature). Well, you still need to break when you get close to a veichle in front of you.

[gok]

*brake

[archon810]

An unfortunate typo.

[dmead]

That analogy doesn't really hold. My understanding is that these angle correction issues are due to the oversized engines for the 737 fuselage. The correction system is there as a solution to that which doesn't involve scrapping the line and staying with larger plane sizes .