Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

seattletimes.com

Post reply on HN
Print view View original

111–120 of 189 posts

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#111

Post by puffoflogic »

jki275 wrote: post #79
hn_throwaway_99 wrote: post #26

Earlier quoted context omitted.

> but being wrong under load sounds like a horrible internal architecture problem. Tons of software can be "wrong under load" - things like race conditions and memory leaks are common problems, and they don't necessarily point to a huge architectural defect. E.g. Cloudflare had a bug years ago that caused highly sensitive data to leak, but only for a teeny (relatively) portion of their site. Similar issue happened to…

That's still an architecture defect. Software's accuracy being affected by load is completely unacceptable.

For what it's worth, I had the same reaction upon reading this paragraph. It seems there are HN commenters who understand software design and those who do not.

To be fair to the other commenters, it's conceivable that the practical difference between the current unsound architecture and a sound replacement might come down to to some small defect. But an attempt to fix the problem by fixing just the defect, even some kind of RCA process on it, will fail if the architecture itself is not fixed, and that requires a change in attitudes and understanding by the devs responsible for the system, not just a software change. That is where the real flaw lies. But some people don't believe that it's possible for the flaw to be in these places, outside the code. All the RCAs in the world can't help them.

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#112

Post by bsder »

Merad wrote: post #53

I was under the impression that SOP for most aircraft was to take off at full power, and if you happen to be lightly loaded you just get into the air faster. Was I wrong, or is this a recent change? If so, it seems like a concerning way for airlines to cut costs. Having excess power available at take off means there's a greater margin for error to handle emergencies (engine failure, bird strike, etc.).

Airlines are reducing safety margins in order to do cost cutting.

As long as you save more than $1.5 billion ($5 million x 300 passengers) before a plane crashes, no problem.

Until we completely bankrupt a company for killing people under its jurisdiction, this will all continue.

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#113

Post by dilyevsky »

sudhirj wrote: post #10

> Peyton added that even though the update to the DynamicSource software had been tested over an extended period, the bug was missed because it only presented when many aircraft at the same time were using the system. That seems horribly wrong to me. I can understand software being slow under load, but being wrong under load sounds like a horrible internal architecture problem.

From the description it sounds like they tried to give a non-technical explanation of a concurrency bug

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#114

Post by AceyMan »

fwlr wrote: post #17

“the bug was missed because it only presented when many aircraft at the same time were using the system.” The system reports data on number of passengers, weight of cargo, plane balance, etc., to the pilots. The calculation is done by the plane’s flight computer. How can it be off by 20,000 pounds, but only under heavy server load? The explanation that comes to mind is that DynamicSource has a subservice for each sou…

>The calculation is done by the plane’s flight computer.

Not exactly true.

The dispatch release has a flight & performance plan based on the load manifest. It's calculated out of band (here, by the software vendor) and punched into the airplane configuration for departure.

If the plan is wrong, the plane flies wrong. GIGO at its finest.

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#116

Post by mnw21cam »

basch wrote: post #4
Aloha wrote: post #2

This seems like mostly how it ought to work - * There were two unusual events in very short order * Someone quickly noticed and gave the order to go to ground stop * The problem was figured out quickly and a work-around was developed * Flights Resumed after successfully deploying the work around to the 'production process' * A patch was quickly developed and deployed once the underlying bug was uncovered. I think eve…

The purpose of telling pilots thrust was to save money and fuel. It’s a “least viable” optimization. >“The goal is to lower the power used on takeoff,” he said. “That reduces engine wear and saves money” on fuel and maintenance. >Flights to Hawaii are typically full, with lots of baggage and a full load of fuel for the trip across the ocean. The planes are heavy. > That morning, a software bug in an update to the Dyn…

> The purpose of telling pilots thrust was to save money and fuel.

No, it's more that using lower thrust during take off saves on engine wear and noise levels around the airport. It doesn't really have that big an impact on fuel use, at least that's not the primary purpose.

The article concentrates far too much on the thrust setting. The important bit is the speed where the plane should be rotated to take off, which is known as V_r. That depends on the weight of the plane. On the takeoff roll, the pilots watch the airspeed indicators and pull back to lift off when that speed has been reached. If they have been told a V_r that is lower than it should be, then pulling back will rotate the aircraft without it taking off, and that's when you have this danger of the tail hitting the ground. When that happens, the plane must be inspected, because a tail strike has the potential to weaken the pressurised container that is the fuselage, and this could lead to an explosive burst and depressurisation when flying at high altitude. See https://en.wikipedia.org/wiki/China_Airlines_Flight_611 for why this is a Bad Thing.

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#117

Post by basch »

mnw21cam wrote: post #116
basch wrote: post #4

Earlier quoted context omitted.

The purpose of telling pilots thrust was to save money and fuel. It’s a “least viable” optimization. >“The goal is to lower the power used on takeoff,” he said. “That reduces engine wear and saves money” on fuel and maintenance. >Flights to Hawaii are typically full, with lots of baggage and a full load of fuel for the trip across the ocean. The planes are heavy. > That morning, a software bug in an update to the Dyn…

> The purpose of telling pilots thrust was to save money and fuel. No, it's more that using lower thrust during take off saves on engine wear and noise levels around the airport. It doesn't really have that big an impact on fuel use, at least that's not the primary purpose. The article concentrates far too much on the thrust setting. The important bit is the speed where the plane should be rotated to take off, which…

> saves on engine wear

Which is saving on money

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#118

Post by pqdbr »

drewg123 wrote: post #97

I don't understand why an airplane being heavier than what was estimated by the software would cause the tail to drag. I would assume it would cause the plane to not have sufficient thrust to take off, or to use more runway. But having the tail drag makes it seem like it had too much thrust, which translated to a steeper take off than anticipated... Can somebody explain?

Let's say your target rotation angle is 10 degrees, so at the specified speed (called VR) the pilots 'rotate' the aircraft nose up by 10 degrees.

When they start the 'rotate' maneuver, the nose of the aircraft doesn't immediately go to (for instance) 10 degrees. It's a progression.

As soon as the nose starts pitching up (by 1 degree for instance), the aircraft is already generating lift to pull the aircraft up.

If the speed is set right (so you're fast), when you get to that 10 degrees angle of attack, your tail has already cleared the runway by a very safe margin, because you've been producing X lift from 0 to 10 degrees of rotation.

However, if your speed is set too slow, you're not generating as much lift, so the aircraft will be pulled up slower (you have been producing Y% less lift from 0 to 10 degrees of rotation). This means that by the time you reach your designated 10 degrees of angle of attack, you haven't cleared the tail yet, and it will hit the runway.

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#119

Post by basch »

pxmpxm wrote: post #33
basch wrote: post #4

Earlier quoted context omitted.

The purpose of telling pilots thrust was to save money and fuel. It’s a “least viable” optimization. >“The goal is to lower the power used on takeoff,” he said. “That reduces engine wear and saves money” on fuel and maintenance. >Flights to Hawaii are typically full, with lots of baggage and a full load of fuel for the trip across the ocean. The planes are heavy. > That morning, a software bug in an update to the Dyn…

>Interesting that someone cited divorce and sleep examples as emotional pleas , reassuring us they are only human. Huh? FAA spends a huge amount of time and energy focusing on human factors, tasks saturation rates and crew resource management. Nothing to do with with argumentum ad passiones, just prudent risk mitigation.

I just don’t see how it fit the article. Unless they are surmising the pilots poorly sanityized the data going into the computer because they were having an off day.

Re: After Alaska Airlines planes bump runway, a scramble to ‘pull the plug’

#120

Post by pqdbr »

tzs wrote: post #101
ww520 wrote: post #84

If weight calculation is so crucial for take off, it seems like redundancy of the weight calculation is needed - a different weight calculation from a different company using a different method to verify against the main one.

Would it be possible to actually weigh the loaded plane to check the calculations, say with something like a bigger version of truck scales that the plane could taxi over?

Why doesn't the plane weight itself? Couldn't there be sensors in the landing gear or something?
Post reply on HN