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

Boeing 787 Suffers Rare Dual Engine Failure on Landing

thepointsguy.com

Post reply on HN
Print view View original

231–240 of 365 posts

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#231

Post by a1369209993 »

gugagore wrote: post #208
AshleyGrant wrote: post #197

Earlier quoted context omitted.

>The cruising airspeed of an airplane is roughly speaking an order of magnitude faster than its landing speed. No. No it is not. 787 approach speed is 145-153 knots. It does not cruise at 1400+ knots.

Thanks for bringing more precision to the conversation (really). What would you like to call the factor? 3? Call me a weasel, but I was hoping "roughly speaking an order of magnitude" would go down to 3. I suspected it was comfortably above 2. Am I right about that?

> I was hoping "roughly speaking an order of magnitude" would go down to 3.

I'd call that half a order of magnitude, considering log₁₀(3) is around 0.477. So you're off by more than a factor of two.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#232

Post by phire »

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'm willing to bet that the plane does prevent the pilot from activating reverse thrust when not on the ground.

But what if a thrust reverser self-activated, without being commanded? The previous safety mechanism wouldn't help because it's being bypassed.

So there is a secondary safety system that detects such situations. Something along the lines of "I think the trust reversers shouldn't be activated right now, yet they appear to be activated. The engine is clearly malfunctioning, lets shut it down"

It appears that this secondary safety system has been activated. There was probably a bug, or a sensor malfunction that triggered it.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#233

Post by vpribish »

xadhominemx wrote: post #174
vpribish wrote: post #151

Earlier quoted context omitted.

physics says no. There was something wrong with that demo if this was the conclusion.

Nah the demo was right. The impulse from the sucking (ie air molecules bouncing off the inside of the fan blade) is countered by the impulse from the air molecules bouncing off the inside off the back T-pipe. Different at a low Reynolds number with a reversible flow.

Reynolds number, compressibility, reversibility don't really matter for this - the principle under discussion is a simple momentum-balance. Find the force on the pipe by analyzing what is happening at the boundaries, you can ignore what happens inside. (in engineering this is called control-volume analysis).

The exiting flows out the sides neatly cancel. So what's happening from front-to-back? There is air flowing in at some velocity x cross-sectional area x air density. this momentum has to be balanced completely for the pipe to stay still - but there is no source of momentum in the other direction so the pipe will feel this force and move.

if the pipe was open at the back, there would be momentum exiting the pipe balancing the incoming momentum - or even over-balancing (as in a jet engine)

here's a video going through the math on a similar problem which is a little more complex in detail, but the same in principle: https://www.youtube.com/watch?v=hXApWf1r0Eo

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#234

Post by isostatic »

frogpelt wrote: post #219
NAHWheatCracker wrote: post #209

Earlier quoted context omitted.

A bit like opening your car door while still driving 60 mph... Nobody does that in normal operation. You don't open and shut a door when you notice it didn't fully close because you can hear the wind?

A better comparison might be putting the car in reverse at 60 MPH.

You never missed a gear, thinking you were driving a 6 speed rather than a 5 speed?

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#235

Post by billforsternz »

gugagore wrote: post #208
AshleyGrant wrote: post #197

Earlier quoted context omitted.

>The cruising airspeed of an airplane is roughly speaking an order of magnitude faster than its landing speed. No. No it is not. 787 approach speed is 145-153 knots. It does not cruise at 1400+ knots.

Thanks for bringing more precision to the conversation (really). What would you like to call the factor? 3? Call me a weasel, but I was hoping "roughly speaking an order of magnitude" would go down to 3. I suspected it was comfortably above 2. Am I right about that?

3 is half an order of magnitude (because it is a good approximation to 10^0.5)

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#236

Post by [deleted] »

weaksauce wrote: post #230
bunderbunder wrote: post #223

Earlier quoted context omitted.

Cutting power to all engines in response to an event that is most likely to happen while the aircraft is operating close to, but not actually on, the ground is not failing safe.

I am not sure there is a perfect solution here. you have competing issues that you need to resolve. If it gets pulled in normal flight you would lose both engines but that's fine because you can glide and restart the engines. if the altitude sensor/ground sensor was broken and you want to override the reverse thrusters you have a conflicting goal there. If you accidentally pull it during the approach you lose your en…

[deleted]

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#237

Post by Tushon »

gugagore wrote: post #208
AshleyGrant wrote: post #197

Earlier quoted context omitted.

>The cruising airspeed of an airplane is roughly speaking an order of magnitude faster than its landing speed. No. No it is not. 787 approach speed is 145-153 knots. It does not cruise at 1400+ knots.

Thanks for bringing more precision to the conversation (really). What would you like to call the factor? 3? Call me a weasel, but I was hoping "roughly speaking an order of magnitude" would go down to 3. I suspected it was comfortably above 2. Am I right about that?

re go down to 3: Use different words and see if you think it was reasonable

roughly 10x going down to 3x would also mean it would go up to 17x. That's a pretty wide range, so I don't think that order of magnitude is going to ever really be similar to 3x of something on the basis of what it means.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#238

Post by Animats »

This is really strange. Not only did they have a dual engine failure, they couldn't restart while stuck on the runway. After 40 minutes, the aircraft had to be towed in. Then maintenance couldn't find a fault.

This is an ANA aircraft, which means it's probably maintained well and flown by competent pilots. Looking forward to a good analysis on this.

Most large aircraft have a "weight on wheels" switch, activated when the aircraft is on the ground. There might be some bad interaction between the "weight on wheels" sensing and the thrust reverser protection. Pure speculation: initial touchdown, pilots deploy reversers, small bounce into the air, weight on wheels switches open, engine controller detects flight condition with thrust reversers deployed and shuts down engines.

The other side of this kind of failure is dealing with unintended thrust reverser deployment in flight. Here's an overview.[1] Historically, it wasn't a big deal at altitude, but on some newer aircraft, it is, and so stronger steps have been taken to avoid it.

[1] https://lessonslearned.faa.gov/ll_main.cfm?TabID=3&LLID=34&L...

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#239

Post by gugagore »

Tushon wrote: post #237
gugagore wrote: post #208

Earlier quoted context omitted.

Thanks for bringing more precision to the conversation (really). What would you like to call the factor? 3? Call me a weasel, but I was hoping "roughly speaking an order of magnitude" would go down to 3. I suspected it was comfortably above 2. Am I right about that?

re go down to 3: Use different words and see if you think it was reasonable roughly 10x going down to 3x would also mean it would go up to 17x. That's a pretty wide range, so I don't think that order of magnitude is going to ever really be similar to 3x of something on the basis of what it means.

but what about a rough order of magnitude? There is definitely some qualifiers you can put on "order of magnitude" to make it include 3.

Re: Boeing 787 Suffers Rare Dual Engine Failure on Landing

#240

Post by deathanatos »

NAHWheatCracker wrote: post #209
t0mas88 wrote: post #142

Earlier quoted context omitted.

It's not very typical for pilots to push buttons quickly. Especially not the thrustreversers and engine and flight controls. But it is entirely possible that nobody pulled the reversers while still airborne in 9 years, because it's a very strange thing to do. A bit like opening your car door while still driving 60 mph... Nobody does that in normal operation.

A bit like opening your car door while still driving 60 mph... Nobody does that in normal operation. You don't open and shut a door when you notice it didn't fully close because you can hear the wind?

At 60 mph? No. I listen to the wind until I get a safe opportunity to properly close it.

(I might try to pull it more shut while moving. But not open it.)

Post reply on HN