[ysangkok]

> hit the brakes very hard in the last few seconds

In direct law, is it possible to brake too hard and have the tires skid on the runway? Is it possible that the pilots were afraid of this?

I ask because people are characterizing direct law as "less smart". So how smart is it? Smart enough to include ABS?

[noduerme]

I was a passenger on a JetBlue flight in the winter of 2000/1, where air traffic was being diverted from JFK due to a snowstorm, but for whatever reason (fuel?) we had to land there anyway. The runway was basically unplowed at that point. As soon as we touched down there was a sensation we were heading into a skid. I wouldn't really call it a braking skid... it the plane just started turning away from its angle of motion. The pilot somehow managed to slow down before we went off the side of the runway. Ended up stuck in deep snow, partially in a ditch. After what seemed to be some attempts at getting us out if the snow, we were evacuated off the slide. A couple people movers were brought out which we got into, amd which themselves became stuck in the snow on the runway for an hour ... fun times.

[oakmad]

Oh 100%! Anti skid is only available in alternate and normal laws. Section 6.5 braking system of this document.

https://www.smartcockpit.com/docs/A330_Flight_Deck_and_Syste...

[danielschonfeld]

Similar naming scheme, but actually its normal and alternate break systems. Powered by different hydraulic systems.

Not to be confused with flight control laws, normal, alternate and direct. One isn't necessarily related to the other.

[doctor_eval]

I don’t know for sure but I think the article or the comments said that ABS was still operative.

However, I’d add that the pilots would not have had time to fully appreciate the situation, probably didn’t know that all three systems had failed, hadn’t had time to run checklists, and so they might have been worried about the possibility that ABS had also failed. All the other stopping systems had failed after all.

Remember this happened as they touched down, which is a pretty high workload environment at the best of times.

[iso8859-1]

But that page does not mention direct law. Wouldn't anti-skid protection depend on the switch shown on the landing gear panel shown in 6.9?

[FatalLogic]

I don't know, but the computer had failed, the runway was wet, and they were confused, so, yes, it's not unlikely they were also afraid of skidding

[kayodelycaon]

Aircraft anti-skid is more complicated than a car. There are a lot of hydraulics and sensors working with extremely high loads.

To simplify how things work, there are two sets of systems in the plane.

There are the computers controls that move all of the surfaces. They have no connection to the various other sensors, because they only move things as they are ordered to. These form the base flight-by-wire system.

On top layer, you have the three flight computers handling all of the automation. A triply redundant system with all of the plane's sensors available to them, not just the sensors in the braking system.

So, the ABS is handled by the flight computers and the flight computers are their own backups. The automation will attempt to degrade gracefully into alternate law. If they all go, the plane reverts to direct law and the pilots interact with the base fly-by-wire system which has little to no automation.

Failure of the flight computers are rare. Every other part of the braking system is less reliable than the flight computers.

From another angle, if a failure caused the flight computers to crash, is there also a problem with the braking system? By having a clear, defined failure mode, a pilot only needs to be trained to land the plane with manual braking.

When the flight computers drop out, the fly-by-wire system becomes directly connected with the flight controls. This includes the anti-skid functionality.

With the above design, it doesn't make a lot of sense to add a layer of complexity between the flight computers and the fly-by-wire system. The redundant flight computers are the backup systems. They can run with partial functionality. If they all fail, the plane reverts to a known, extremely well tested, working systems.

There are multiple overlapping failure modes already. Adding another failure mode is a bad idea.

[iso8859-1]

> To simplify how things work, there are two sets of systems in the plane.

If the plane was not simplified, how many systems would there be? It seems to me that the goal is not simplicity, but automation and safety. Otherwise, there would still be flight engineers.

> This includes the anti-skid functionality.

Fly-by-wire includes anti-skid? Direct law includes anti-skid? What is 'this' referring to?

> If they all fail, the plane reverts to a known, extremely well tested, working systems.

Well tested compared to what? Well tested compared to flying with flight computers?

> Adding another failure mode is a bad idea.

Why is the anti-skid toggle a physical switch if it is only available in some systems of laws? It's physical presence means pilots will think it is always available. Isn't a misunderstanding a failure mode in itself?

[mertd]

How hard is it to make airliner lose grip? There is a lot of weight above the wheels and a lot of downforce from the wings.

[DC-3]

> a lot of downforce from the wings

Not unless the plane is upside down, surely...