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Comment by oakmad | Hacker News Reader

For everyone commenting about different hardware failure modes and possible solutions. Take a look at the airbus « flight control laws » such as https://apstraining.com/wp-content/uploads/FCS-Airbus-Quick-... it governs what happens to flight controls and what protections are available in different scenarios. In this case the system worked, it crashed and reverted to direct law providing manual braking control.

Off topic but I found this very interesting « The deceleration performance of the occurrence flight between 6,600 feet and 7,300 feet from the threshold of runway 10 deteriorated. It may be due to paint marking and rubber deposit on the touchdown zone of runway 28. »

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Build-up of rubber from the tires of landing airplanes is a routine problem with runway maintenance. Typically the airport authority will use a friction-measurement device at regular intervals to determine when the surface friction of the runway has fallen below required parameters, and resurface. But rain makes things much worse and might have combined with rubber build-up to produce a particularly slick section.

Flying light aircraft, I was taught to avoid landing right in the touchdown zone just to have better traction. I'm not sure if this is actually useful or just Old Pilot Superstition but it has some logic to it, and putting a Skyhawk down on a 14k foot runway you have a lot of room for eccentric opinions (similar to landing just a touch off centerline so the nosewheel isn't on the painted stripes, another one I've heard people mention as a "best bad practice").

My dad was a commercial pilot, and taught me to fly at a young age. One of the bits of sage advice was something along the lines of "if your nosegear has two or more wheels, try to get it bang on the centre line, every time. Otherwise, miss a little tiny bit. The lights go THUMP-THUMP-THUMP and eventually drive you mad or, combined with a little gust of wind, cause you to hop once more".

I think there's a lot of Old Pilot Superstition with more than a dollop of truth in it.

I don't think anything too bad could happen when landing light aircraft on slippery but long runway. I was doing some winter flying and was landing pa28 on slippery runway (compressed snow) and I didn't noticed any difference until i started pressing brakes which had virtually no effect on deceleration. On touchdown you have a lot of aerodynamic authority on your controls so runway friction doesn't really matter unless you have super strong crosswind that could blow you out the runway i guess.

If I recall rightly Concorde pilots often took control from autoland and landed it slightly off the center line.

This was to avoid upsetting the passengers and to ensure the champagne didn't get shaken too much.

I don't think you were allowed any service items during landing, so this can't have been true.

Could be referring to the champagne still in bottles. How long does it take for a "shaken" bottle to return to "won't explode everywhere when you open it" state, anyway? Does the low pressure in the cabin make it more prone to erupt?

It's very odd they don't do anything to speed up the wheels or apply a coating of water to them on dry days to avoid extreme wear and excess rubber for future planes landing.

Even a passive design with the shape of the tread could help substantially reduce the amount of rubber scraped off the wheel on each landing.

Apparatus to spin up wheels was analyzed exhaustively, early on. They determined that the added weight and complexity did not pay.

That is not to say that some clever physical design could not help. But the idea of spinning up wheels is well-known to gear engineers. Tire wear is a substantial expense, so any bright ideas would be welcomed, and eventually tried out.

Seems like one could design the tread (or an extra rubber tab) to catch the wind and self spin.

And, as I said, these ideas have been explored in depth, and abandoned. Nothing has changed in tires or landing gear, since, to merit going back over everything that was tried.

Suppose your stuff got the tire spinning at the circumferential rate of 20 knots. The plane touches down at 130+ knots. Is the decrease in rubber deposited noticeable? How fast would you need the tire to be spinning to make a noticeable difference? How much does your extra apparatus weigh? How much extra space does it take in the wheel well?

The extra weight has to be carried throughout the flight. Space in the wheel well is tightly constrained, because it is near the center of gravity of the aircraft, where cargo space is most valuable, and where the wing spar crosses through the fuselage.

Everybody thinks of spinning up wheels. Thus far nobody has had answers to the questions that led to acting on the idea.

Well using sprinklers to wet the initial part of the tarmac would work. But the safety folks don't have an understanding of nuance that a runway where just the first 300 feet is wet is different from the runway being fully wet.

Thanks, more or less the reality check I was hoping for :)

So you are suggesting hydroplaning?

> The deceleration performance of the occurrence flight between 6,600 feet and 7,300 feet from the threshold of runway 10 deteriorated. It may be due to paint marking and rubber deposit on the touchdown zone of runway 28

Just to clarify for others, this is the same runway but taken in two different directions (west to east and east to west, multiply the number by 10 and you get [rounded] angle in degree, so 100ish vs 280ish). It's 8500 ft. long and it looks like there was a loss of performance around the "touchdown" point of going the other way on that runway, where you had accumulated rubber from those landings (which is pretty normal and monitored, as pointed by leecb below), if that makes any sense.

> accumulated rubber from those landings

Rubber accumulation on runways is a normal and expected situation. Part of runway maintenance at airports serving larger aircraft includes regular removal of the rubber using high pressure water or chemicals.

https://en.wikipedia.org/wiki/Airfield_rubber_removal

I'm still always surprised just how much rubber is allowed to accumulate.

Here's a snapshot of one of the Zurich runways: https://www.google.com/maps/@47.4792249,8.5411849,660m/data=...

Scroll a little NW or SE to see what the actual color of the runway is underneath all the rubber.

i'm not surprised, look how invasive the cleaning techniques are.

Not going to lie, I never thought this was a thing that'd need doing, but on further reflection it seems so obvious I'm kinda mad it never occurred to me in retrospect.

Its unfortunate they don’t calculate how big a difference it made. They’d calculated a 170ft margin and stopped with 30ft margin and I’m curious what their braking delta was for that 700ft of deterioration.

« With three FCPCs inoperative, actual remaining runway distance (30 feet margin) of the occurrence flight was shorter than the calculated value (172 feet margin), possibly due to tailwinds, runway conditions, and manual braking as these factors might increase the braking distance. »

I get sucked into how all the little things that are not right make such a big difference in these kind of scenarios.

Complex systems and aerospace particularly are dominated by little things. Failure or near misses are never single cause, those are all designed for. When fails to happen it is almost always a conspiracy of serval things out put together any one of which would have mostly prevented the incident.

There is a lot that practices involving life critical complex systems can teach other fields.

Agreed, and let's not lose sight of the fact that...this was a successful landing. Everyone walked away from it and the passengers likely never knew anything went wrong.

Four hopefully independent things went wrong here: three computer failures plus less-than-documented braking performance. Additionally, there were at least two aggravating circumstances: wet conditions and a tailwind landing. Part of the investigation is to find out if there was a fourth or fifth system failure (poor pilot reaction time? Rubber level on the runway unacceptable? Either is plausible, but far from indicated by the report so far).

One extra thing going wrong here, in the wrong direction (anything that impaired braking or pilot reaction time) would likely have led to loss of life. Investigating near misses like this, and not only being exercised once five things go wrong and a hundred people die, is a sign of a healthy safety culture.

Overshooting the runway isn't that bad, it's expected in emergency scenarios. In those cases, minor injuries and extensive damage to the aircraft are accepted in favor of something worse. Usually you find rubble pits and soft earth behind a runway for the nose gear to dig into, a 10-20ft drop on the nosegear will hurt but the aircraft is now breaking with it's entire front, not just a rubber wheel. The Mayday TV show had a few incidents where the aircraft overshot and that happened.

> the passengers likely never knew anything went wrong.

If the deceleration varied as much as the report said, then the passengers probably could tell. That was very different from a normal landing. If that wasn't enough, being towed to the terminal might have given them another clue.

> One extra thing going wrong here, in the wrong direction (anything that impaired braking or pilot reaction time) would likely have led to loss of life.

Well, it would have lead to the plane running off the end of the runway. What's past the end of that particular runway? A cliff? A river? Or just another 100 meters of grass leading up to a perimeter fence, then more flat ground for some distance beyond it? If it doesn't overrun the runway too far (say, 100 or 200 meters), no, that doesn't seem likely to lead to loss of life - not unless there's some specific hazard there.

According to the first comment on TFA, it's in a built up area. I checked Google maps [0] and it's pretty bad: 50m of tarmac (no EMAS I think), 50m of grass, then a minor road with commercial buildings.

Regardless, that's one to be considered another aggravating circumstance, not an additional failure. When deciding whether this was a close call, you can treat it as if every runway is wet and in a permanent tailwind and immediately followed by a wooden building full of schoolchildren.

[0] https://maps.app.goo.gl/u4eFmqDAysMVbbWC8

Oh, absolutely. You treat it as if there is no margin.

And you don't put wooden buildings full of schoolchildren immediately past the end of runways.

Unless I am mistaken, runway 10 is west to east so it would have gone through the parking lot and then into the river. That also the direction I remember most planes landing there.

I read this in an old 1950s era Readers Digest (Harper Subscribers only). Interesting (fictional) article about a jet that was doomed before takeoff by an accumulation of a number of small things each of which would not individually have caused an incident but taken together were fatal. https://harpers.org/archive/1957/09/the-jet-that-crashed-bef...

Edit: Accessible version of the article here. https://books.google.co.in/books?id=2gk-2rP34LIC&pg=RA2-PA14...

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