Boeing 787 Suffers Rare Dual Engine Failure on Landing • Hacker News Board

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#131

Post by breischl » Wed, Jan 30, 2019, 6:03 PM UTC

maxxxxx wrote: post #112

nck4222 wrote: post #110

Earlier quoted context omitted.

Do you have experience with aircraft engine design, or landing protocols, or insight in to the meetings when the engines were designed? If so, I'll take back my words but considering the manufacturer doesn't know exactly why this happened, and the engineers on the ground said it seems like a "software bug", it seems a bit presumptuous and frankly a bit comical to start saying things like: "Surely a better solution wo…

I always love it when you have worked on something for months and someone new comes by and says "Why don't you just X?".

Had someone do that this morning. They ran across a problem that has been the focus of a multi-year effort involving dozens of people. Fixing the problem is a big part of the literal #1 priority for the company.

Anyway, he hit an example of the issue, and wanted to just put in a bug ticket to one team, and didn't understand why that was not useful or necessary.

My analogy is that it's like showing up at NASA in 1965, and wanting to submit a ticket that says "Your rockets can't actually go to the moon. Fix rocket so that it can go to the moon."

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#132

Post by ChuckMcM » Wed, Jan 30, 2019, 6:03 PM UTC

nck4222 wrote: post #110

code4tee wrote: post #19

Boeing’s answer seems to say if you accidentally pull the thrust reverser lever before the aircraft is on the ground it just force shuts down the engines. That sounds like an odd safety override. Surely a better solution would be to just not activate reverse thrust. Unlike car engines a jet engine can’t just be quickly restarted if it accidentally shuts down. It typically takes 30-60 seconds to get going. This and ea…

Do you have experience with aircraft engine design, or landing protocols, or insight in to the meetings when the engines were designed? If so, I'll take back my words but considering the manufacturer doesn't know exactly why this happened, and the engineers on the ground said it seems like a "software bug", it seems a bit presumptuous and frankly a bit comical to start saying things like: "Surely a better solution wo…

Reminds me of the quote,

"For every complex problem there is an answer that is clear, simple, and wrong." -- H.L. Mencken

One of the things I have experienced in my career is the mind boggling complexity of the software in systems that are capable of killing people. The more people it can kill the more complex the software.

As a result when I read articles like the one posted I find it tantalizing to speculate about the requirements behind the story. Taking the story at face value that Boeing has thought a lot about it and that is how the software has to be, what prevents the other solution of disabling the reverser? My guess is that you want the reverser to work in the event of software failure so it has to always work, but if the pilots pull it while you are flying that would probably rip the engines off the plane. Perhaps the compromise is to take the reverser off during flight and relight the engines using aerial pressure to spin up the engines. (which has me wondering if airlines still have a turbine they can drop down to start an APU which can then be used to start the engines). Which leads me to wondering if the pilots turn off the reverser right away do they have enough residual engine power to relight? Versus when they got to the end of the runway and had nothing? Clearly the 777 doesn't have an APU running when it lands since they would have used that to do an engine start at the end of the runway or if it does, it isn't running.

It is one of those things that my like minded systems friends could sit around lunch and make a good hour and a half discussion out of.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#133

Post by t0mas88 » Wed, Jan 30, 2019, 6:05 PM UTC

cm2187 wrote: post #114

metalliqaz wrote: post #70

Earlier quoted context omitted.

It's not an odd safety override. An overthrust situation is always dangerous for various reasons. One of the more spectacular is the possibility of liberating airfoils from the engine and shooting them into the cabin. It's not a self-destruct button, it's anti-destruct limit. As for the thrust reverser, there are many integrated systems on the aircraft. It's possible that the cockpit detected weight-on-wheels, but th…

By self destruct, I meant switching off the engines in flight seems to me to be a terribly dangerous behavior.

Switching off is much safer than a turbine overspeed past certain limits. Because an engine failure (one engine) is a situation all pilots are trained for and it has an almost certain safe outcome. Airplanes fly and land just fine on a single engine. We all train for engine failures all the time.

On the other hand, a runaway engine fire or uncontained turbine failure is much much more likely to cause a crash.

So almost all jet engines are designed to have a shutdown (sometimes helped by the built-in fire extinguishers) as a worst case outcome. The quick response drill for an engine overspeed or temperature past certain limits is to shut it down immediately and pull the fire extinguisher handle.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#134

Post by alexdumitru » Wed, Jan 30, 2019, 6:08 PM UTC

cm2187 wrote: post #50

code4tee wrote: post #19

Boeing’s answer seems to say if you accidentally pull the thrust reverser lever before the aircraft is on the ground it just force shuts down the engines. That sounds like an odd safety override. Surely a better solution would be to just not activate reverse thrust. Unlike car engines a jet engine can’t just be quickly restarted if it accidentally shuts down. It typically takes 30-60 seconds to get going. This and ea…

I thought that most modern aircrafts were not allowing the reverse to be deployed if the wheels didn't touch the runway. Talking about an odd safety override. It's kind of a self-destruct button.

I was watching Air Crash Investigation yesterday, where a reverser deployed right after take off and caused the plane to crash. They said now reversers can't be activated unless all wheels are on the ground, as you've said.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#135

Post by phkahler » Wed, Jan 30, 2019, 6:08 PM UTC

exabrial wrote: post #12

Sort of off topic, but reverse thrust sort of boggles my mind. If you're sucking air in from the front of the aircraft and ejecting it out the front, don't those forces counterbalance each other?

If the suck force and the blow force were equal, you wouldn't need to burn fuel inside the engine during normal operation. Combustion produces a lot higher volume of gasses at higher energy than the air that goes in the front.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#136

Post by hirowan » Wed, Jan 30, 2019, 6:08 PM UTC

code4tee wrote: post #19

Boeing’s answer seems to say if you accidentally pull the thrust reverser lever before the aircraft is on the ground it just force shuts down the engines. That sounds like an odd safety override. Surely a better solution would be to just not activate reverse thrust. Unlike car engines a jet engine can’t just be quickly restarted if it accidentally shuts down. It typically takes 30-60 seconds to get going. This and ea…

I think you've misunderstood. The system you're describing kicks in when (a) the aircraft is on the ground, (b) commanded thrust is idle and (c) the engines continue to produce thrust above idle (when there is an overthrust condition). Boeing does not cut power to the engines to prevent reverse thrust application in air - that would be absurd.

On this particular occasion the system is thought to have malfunctioned in some way (e.g. it might have not registered the application of reverse thrust) triggering a shutdown.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#137

Post by hopler » Wed, Jan 30, 2019, 6:09 PM UTC

code4tee wrote: post #19

Boeing’s answer seems to say if you accidentally pull the thrust reverser lever before the aircraft is on the ground it just force shuts down the engines. That sounds like an odd safety override. Surely a better solution would be to just not activate reverse thrust. Unlike car engines a jet engine can’t just be quickly restarted if it accidentally shuts down. It typically takes 30-60 seconds to get going. This and ea…

It's weird to call it a "danger" when the automation is making a suboptimal response to override a human error that could crash the plane.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#138

Post by Nokinside » Wed, Jan 30, 2019, 6:09 PM UTC

deugtniet wrote: post #10

Is there something special about the software in new Boeings? Dual engine failure is nothing to sneeze at, and should never happen. Airbus also had its fair share of software errors in history, but if we count the recent Lion air crash as software related, Boeing has had 2 serious software related incidents in a year and I haven't heard anything serious from airbus. Is it because Airbus started its fly-by-wire develo…

It's not the aircraft, it's the engine. Boeing 787s have been grounded recently because problems with Trent 1000-C have been increasing. Mainly compressor blade durability.

Modern jet engines like Trent 1000 from Rolls-Royce have Full Authority Digital Engine Control (FADEC). It comes with the engine and there are no manual overrides. The computer system runs the engines and if it fails the engine fails almost instantly.

Before the flight pilot sets flight parameters to flight management system (FMS) and FMS communicates them to FADEC in the engines. FADEC runs the engines and 'micromanages' the system in different phases of the flight. There is large number of parameters and engine health monitoring and diagnostics going on. Most of it is not the responsibility of the aircraft manufacturer. Engine sends data to Rolls-Royce.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#139

Post by cbhl » Wed, Jan 30, 2019, 6:10 PM UTC

The Boeing 787 is now about nine years old. I wonder how much of this is operator proficiency triggering latent bugs (aka, "pilots got too good at pushing buttons quickly").

The bugs in the Therac 25 were only discovered after the machine had been in production for a while (you had to be typing "too quickly" in order to get the software to trigger the bug with the safety interlock).

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#140

Post by Rooster61 » Wed, Jan 30, 2019, 6:11 PM UTC

The second high-profile potentially deadly software caused failure related to automated safety features in as many years (if that ends up being the cause, of course).

Very glad the touchdown went nominally. Had they pushed the point of touchdown just a bit further down the tarmac, they'd have skittered right off the pavement.