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Southwest 737 Overrun At Chicago  
User currently offlineNeilking From United Kingdom, joined Aug 2005, 101 posts, RR: 0
Posted (8 years 11 months 3 weeks 2 days 23 hours ago) and read 3699 times:

OK, I know it's axiomatic that one shouldn't speculate before the final report but a bit of semi-informed speculation on a web-site like this is OK, isn't it?

There's a lot of un-informed speculation about this incident going on under various threads on the Civ Av board so dare I ask if any of the pro's over here on the "less feverish" T/O board have a view on what might have have happened.

Although I'm not an aviation pro, I'm going to kick off with the speculation that the snowy weather had NOTHING to do with it - much as the media are bound to make a connection between snow and "skidding off a runway". (I also recall reading here something about SWA jets not having autobrakes - may be wrong - but that's figuring highly in some of the comments.)

Rgds, Neil

[Edited 2005-12-11 20:06:33]

21 replies: All unread, jump to last
 
User currently offlineGrbld From Netherlands, joined Dec 2005, 353 posts, RR: 3
Reply 1, posted (8 years 11 months 3 weeks 2 days 23 hours ago) and read 3686 times:

Neil, do you have any reason to speculate that there's no connection at all?

I think it is fairly obvious, had there been no snow, the plane wouldn't have gone off the far end. Don't underestimate the enormous impact snow (especially heavy snow) can have on airport ops. I have seen instances where braking action is less than 20% than on a dry runway during heavy snow. This means that you'd require more than 5 times as much runway in order to stop as on a dry runway!

Furthermore, the SWA planes do have autobrakes and apparently the captain has stated that there were problems with the thrust reversers. We'll see how the story develops.

Grbld


User currently offlineMiles_mechanic From Canada, joined Sep 2001, 137 posts, RR: 0
Reply 2, posted (8 years 11 months 3 weeks 2 days 23 hours ago) and read 3673 times:

A question I have about the southwest overrun, is I haven't heard, was this airplane the first one cleared to land after they had plowed the runways, since he was in a holding pattern, or were other airplanes landing before he was cleared to land, which would put more of the cause of the problem, on either the pilots actions, or anything wrong with the aircraft and not so much the runway conditions. From watching CNN, sounds like the winds were fairly light, so that wouldn't have helped keeping the actual approach speed down.

Regards

Miles


User currently offlineNeilking From United Kingdom, joined Aug 2005, 101 posts, RR: 0
Reply 3, posted (8 years 11 months 3 weeks 2 days 21 hours ago) and read 3637 times:

Quoting Grbld (Reply 1):
Neil, do you have any reason to speculate that there's no connection at all?

No! I was just "speculating" on the age old probability that one should never jump to what seems superficially to be the most obvious explanation.

Quoting Grbld (Reply 1):
I think it is fairly obvious, had there been no snow, the plane wouldn't have gone off the far end. Don't underestimate the enormous impact snow (especially heavy snow) can have on airport ops.

I believe you are a commercial airline pilot Grbld so I will respect your opinions on this far more than anything I would dream up.

This is why I posted this question on Tech/Ops - thanks for your insight.

Rgds, Neil


User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 4, posted (8 years 11 months 3 weeks 2 days 20 hours ago) and read 3620 times:
AIRLINERS.NET CREW
DATABASE EDITOR




Quoting Grbld (Reply 1):
I think it is fairly obvious, had there been no snow, the plane wouldn't have gone off the far end.



There wasn't any snow on the ground at BUR....




2H4





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User currently offlineGrbld From Netherlands, joined Dec 2005, 353 posts, RR: 3
Reply 5, posted (8 years 11 months 3 weeks 2 days 20 hours ago) and read 3611 times:

Hehe, Neil, thanks, no problem!

Quoting 2H4 (Reply 4):
There wasn't any snow on the ground at BUR....

Haha, that's true! I've actually been based at BUR for a while flying turboprops but this was before the SWA overrun there. Don't remember the specifics. In this case, though, I think it is safe to say that if it were a dry runway, they probably wouldn't have gone off the other end. Of course, there may be other factors, such as other system failures, a long landing etc. but it is indeed too early to speculate. They did come to a stop not too far off the end, leading me to believe that on a dry runway with normal brakes operating, they would've easily stopped on the runway.


User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 6, posted (8 years 11 months 3 weeks 2 days 19 hours ago) and read 3598 times:
AIRLINERS.NET CREW
DATABASE EDITOR




Quoting Grbld (Reply 5):
They did come to a stop not too far off the end, leading me to believe that on a dry runway with normal brakes operating, they would've easily stopped on the runway.



That sounds logical. Certain overruns are really frustrating in that, had they been travelling a mere 20 or so mph slower, the entire accident would have been avoided.




2H4




[Edited 2005-12-12 00:20:33]


Intentionally Left Blank
User currently offlineDColeMAN From United Kingdom, joined Nov 2004, 274 posts, RR: 10
Reply 7, posted (8 years 11 months 3 weeks 2 days 4 hours ago) and read 3465 times:

Quoting Neilking (Thread starter):
I also recall reading here something about SWA jets not having autobrakes - may be wrong - but that's figuring highly in some of the comments.

Yeah I've also hear this rumour. The camera doesn't lie though....

You can clearly see the autobrake in the top left corner of this photo, positioned to the left of the three green landing gear lights -

View Large View Medium
Click here for bigger photo!

Photo © Bruce Leibowitz


Dale



Topless Women Drink 4 Free
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 8, posted (8 years 11 months 3 weeks 2 days 4 hours ago) and read 3451 times:

Without seeing any DFDR data, I don't really want to speculate at all. However, using and not using autobrakes really shouldn't make too much difference. Just like the reverse thrust really shouldn't make any difference at all, since reverse thrust is not factored into stopping distance.

Upon touchdown, if the PF judiciously applied the brakes, then you would have the same braking as autobrakes. So, that really leaves several options as possibilities. Among them, RCR reading worse than reported, "unstabilized" approach resulting in long, fast touchdown.

Now, I am of the school that if there things on the aircraft to reduce my workload, such as autobrakes, I will use them. However, Southwest's SOP don't allow for that. The bottom line is there SOP was approved by the FAA and Boeing.


User currently offlineGrbld From Netherlands, joined Dec 2005, 353 posts, RR: 3
Reply 9, posted (8 years 11 months 3 weeks 2 days 3 hours ago) and read 3419 times:

Phil, it's true that the autobrakes use a fixed deceleration rate and just apply less brakes when you use reverse thrust. But when using, for instance, autobrakes 2 and due to the medium or poor braking action, the autobrakes are braking at the limit (ie. the anti-skid is working all the time), and if the required deceleration rate is still not achieved, then reverse thrust WILL be beneficial.

Same goes for manual braking: You can only brake so hard on a slippery runway until you're at the anti-skid limit, so adding reverse thrust, would obviously contribute to the deceleration.

When you're not at the (anti-skid operating) limit, it is the way you say it is.

The only thing in this case is, (I see you have ample flying experience) there might be a difference between the time of brake application between the autobrakes and manual braking. Obviously, the autobrakes start upon wheel spinup, but it's hard to time manual braking, even if it is SOP. You can't land with your feet on the brakes, cause you could blow the tires that way. And if you apply the brakes a second later, that may be a big difference as well (since you're going over 200 feet per second!).

It is indeed speculation as to the rest of the events (the approach, conditions etc).

Grbld


User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 10, posted (8 years 11 months 3 weeks 2 days 2 hours ago) and read 3403 times:

Quoting Grbld (Reply 9):
But when using, for instance, autobrakes 2 and due to the medium or poor braking action, the autobrakes are braking at the limit (ie. the anti-skid is working all the time), and if the required deceleration rate is still not achieved, then reverse thrust WILL be beneficial.

I don't think Boeing varies the autobrake system too much between fleets, with that in mind, the auto brakes on the 400 when 2 is selected does give you a rate of deceleration. Using reverse thrust, max that is, will give you a deceleration rate above what reverse thrust gives you. Thus your brakes are not being applied when max reverse is fully applied. However, when you select autobrakes 4 then the autobrakes give you a constant brake application regardless of reverse thrust and deceleration rate. It is quite efficient I might add.

I was trying to get two points across, the first is reverse thrust, or the lack of reverse thrust isn't a factor in this accident/incident. Reverse thrust isn't used in the calculation of stopping distance when the aircraft is certified. Secondly, I agree with your statement about autobrakes being applied on wheel spin-up and manual braking, however, if the PF "stood" on the brakes, then that would have been just about max on the autobrakes. So, the other possible culprit would have been a bad RCR. Most airlines in the US Ops Specs don't allow landing with braking action of Poor or worse, so since they were the first flight after the runway was cleared, that certainly could have been a factor.


User currently offlineOkie From United States of America, joined Jul 2003, 3155 posts, RR: 3
Reply 11, posted (8 years 11 months 3 weeks 2 days 1 hour ago) and read 3378 times:

A landing speed of 144 kts and a little tail wind component = about 250 ft/sec
it would not take much of a "floater" to eat up a lot of the runway. However, with 5,500 foot available and a 6 second floater that would still leave 4,000 ft to get a less than MTOW 37 stopped which is doable, I would suspect.
That being said, I think the brakes are controlled separately for each gear or each wheel. A brake anti skid failure would tend to swerve the aircraft profoundly towards the direction of the operating side as I suspect there is plenty of redundancy to keep all wheels/axles from failing at one time.

I am just guessing of course but something not configured or armed for landing correctly on the landing check list assuming a proper approach?

Okie


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 12, posted (8 years 11 months 3 weeks 2 days 1 hour ago) and read 3369 times:

Quoting Grbld (Reply 9):
And if you apply the brakes a second later, that may be a big difference as well (since you're going over 200 feet per second!).

This is one of those things that eventually comes to pilots. "Too soon is just right!" Two hundred feet is an inconsequential distance even on a "short" seven thousand foot runway - until you get near the end of that runway. Then you would do anything to get that two hundred feet back!

Not speculating that such a thing was a causal factor here, but just as a general statement.

Quoting PhilSquares (Reply 10):
the first is reverse thrust, or the lack of reverse thrust isn't a factor in this accident/incident.

Not sure I could agree with this statement, as written. As we've discussed many times before, in performance planning reverse thrust is usually not a factor. It may be used (FAR 25.109(b) unless that has changed) but if you do use it, then reverse thrust becomes a no-go item. So normally it is not factored into performance questions. I'd bet that it was not used in this case for planning purposes.

Though I'm not qualified on the NG and not knowledgeable of WN's flight ops procedures I'd still bet that it was the intent of this crew to use it. MDW is a no-nonsense airport, and the weather conditions were the type that make all experienced pilots sit up, pay attention and do things right the first time. So, assuming a touchdown in the proper place, on-speed, followed by application of aggressive manual braking and use of reverse thrust, it seems that the plane should have stopped in the distance available.

It is often repeated now that there was a tailwind. I don't know where this "fact" came from as the METAR applicable at the time called for a very light left crosswind, no factor, really.

There are a couple of factors that I very much want to know more about in this event. One is the actual braking action available for this crew. How recently tested, how soon after the accident was it tested again? Also, the reported problem with thrust reversers: Mechanical problem or simply a delay getting weight on the wheels so it could be deployed?

There is another. This one is new to me, but apparently there is a "crossover speed" additive being widely used on VREF for 737s. Was it used on this approach? Is it pilot's discretion to use it?

One other purely speculative thought. You NG pilots out there - landing a 700, say, at 300 weights, does it tend to float more due to the greatly increased wing area or do the approach speeds used cancel that out? Seems like the latter should be true, but if the crossover additive was used, wouldn't that put the wing at a higher speed in ground effect than the engineers had planned for it?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 13, posted (8 years 11 months 3 weeks 2 days ago) and read 3358 times:
AIRLINERS.NET CREW
DATABASE EDITOR




Quoting PhilSquares (Reply 10):
I was trying to get two points across, the first is reverse thrust, or the lack of reverse thrust isn't a factor in this accident/incident. Reverse thrust isn't used in the calculation of stopping distance when the aircraft is certified.



Here's what Barney Captain (a current WN captain) had to say about that in another thread:


I've been avoiding comment on this topic (for obvious reasons) but I would like to clarify one thing: the -700 was certified using reverse thrust for landing. We use an on board performance computer (opc) to calculate T.O. and landing data. From our F.O.M.; "The opc on the 700 takes into account the thrust reverser's for landing,certificated that way, unlike most other aircraft."

It's true the classic fleet (and every other a/c I'm aware of) considers reverse thrust for landing simply a bonus. Not the -700.





2H4





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User currently offlineDerik737 From United States of America, joined Dec 2004, 333 posts, RR: 3
Reply 14, posted (8 years 11 months 3 weeks 1 day 20 hours ago) and read 3277 times:

And here's the reply I posted to Barney's comment from our 737NG performance engineer:

Interesting post. I do not agree with the “-700 was certified using reverse thrust for landing”. That is incorrect.

Here’s the deal:

The FAA type certification requires landing distance certification of maximum performance stopping distances only. This flight testing specifically does not allow credit for any reverse thrust, don’t care what type of large turbojet aircraft and has been this way for a long, long time. This certification is required on dry runways and they recently added wet runways to the certification requirements. At this time, the FAA does not require certification on slippery or contaminated runways.

Going back to SWA’s note regarding their OPC, I suspect that landing distances provided by their OPC, does take credit for reverse thrust, idle detent reverse thrust only (Reference Boeing QRH, Performance Inflight, Advisory Information section). These distances are based upon autobrake deceleration rates, not the AFM certified data. In addition, it is very important to note that these landing distances are advisory only (in other words, not certified by the FAA) and should be used as guidance when determining which autobrake setting to use. Reference AFM section 4.0, page 16 for more information regarding the use of autobrakes.

The last sentence on this page in the AFM “Extremely wet and slippery conditions can result in longer distances than indicated”. This will be the case when the autobrake system cannot obtain the scheduled deceleration rate due to poor runway friction. When this occurs, it is imperative to use reverse thrust to reduce landing distance.

In summary, this person appears to be confusing AFM certified landing distance with advisory/guidance landing distances also provided by Boeing.


Also, keep in mind that the NG can deploy the thrust reversers when the aircraft is under 10 ft of radio altitude and the forward thrust levers are at idle.


User currently offlineGrbld From Netherlands, joined Dec 2005, 353 posts, RR: 3
Reply 15, posted (8 years 11 months 3 weeks 1 day 20 hours ago) and read 3267 times:

Quoting Derik737 (Reply 14):
keep in mind that the NG can deploy the thrust reversers when the aircraft is under 10 ft of radio altitude and the forward thrust levers are at idle.

Yes, but if you do this, you'll likely be fired.

Quoting SlamClick (Reply 12):
There is another. This one is new to me, but apparently there is a "crossover speed" additive being widely used on VREF for 737s. Was it used on this approach? Is it pilot's discretion to use it?

One other purely speculative thought. You NG pilots out there - landing a 700, say, at 300 weights, does it tend to float more due to the greatly increased wing area or do the approach speeds used cancel that out? Seems like the latter should be true, but if the crossover additive was used, wouldn't that put the wing at a higher speed in ground effect than the engineers had planned for it?

Hi SlamClick, well there's not much extra floating because of the wing itself, especially when landing at flaps 40 on the 700. What I do notice quite well is that NGs with winglets have a much more pronounced ground effect float though, and I thought this aircraft had them installed as well.

I thought the crossover additive was added for aircraft with unmodified ruddersystems (to allow adequate control margin in case of a rudder hardover). All our NGs have modified rudders, so we don't use them anymore.


PhilSquares, actually if you stand on the brakes, you get more than autobrakes max, because even autobrakes max gives you a fixed deceleration rate. The thing is, when you set autobrakes 2 and it gives a deceleration rate of, say, 5 (-m/s^2) and due to the ice and snow isn't able to do better than 2 instead of 5, then setting even higher autobrakes won't make a difference, because the limiting factor is the icy runway and your anti-skid operating full time. So even max manual braking still won't give you more than 2. BUT, you can add some thrust reverse (because it's not related to ground friction) and get some better deceleration. It can very well be a factor on a very slippery runway.

Ever had to maneuver your airplane on a ramp with thrust reverse because it's so slippery your plane slides all over the place? Whoa, loads of fun but enormously nervewracking.

Grbld.


User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 16, posted (8 years 11 months 3 weeks 1 day 18 hours ago) and read 3224 times:

Quoting Grbld (Reply 15):
Ever had to maneuver your airplane on a ramp with thrust reverse because it's so slippery your plane slides all over the place? Whoa, loads of fun but enormously nervewracking.

Actually in HKG right now and off to ANC tomorrow morning, so Yes!

Quoting Grbld (Reply 15):
actually if you stand on the brakes, you get more than autobrakes max, because even autobrakes max gives you a fixed deceleration rate.

I agree, and my point again was perhaps a bad RCR? Even a braking action of fair will get you stopped with no problems. I have operated into MDW on a number of trips in a 727, about a million years ago, and even without autobrakes, the aircraft did a good job of stopping on a "fair" runway. However, I wouldn't want to try it on a "poor" runway.


User currently offlineGrbld From Netherlands, joined Dec 2005, 353 posts, RR: 3
Reply 17, posted (8 years 11 months 3 weeks 1 day 8 hours ago) and read 3134 times:

Quoting PhilSquares (Reply 16):
I agree, and my point again was perhaps a bad RCR?

Yes, completely agree! If they knew the actual runway condition, they may not have landed. And the condition can go from medium to poor in just a few minutes, so it's always tricky when there's snow showers.

We're not allowed to depart/land on a braking action POOR runway (I know there are airlines and aircraft that do). Ah at least they have heated runways at ANC  Smile Always fun to see the pushback car slippin and slidin around as well (as long as it doesn't mess up the nosewheel)


User currently offlineThrust From United States of America, joined Sep 2003, 2690 posts, RR: 10
Reply 18, posted (8 years 11 months 2 weeks 4 days 16 hours ago) and read 2853 times:

Is this plane a writeoff?


Fly one thing; Fly it well
User currently offlineCloudy From , joined Dec 1969, posts, RR:
Reply 19, posted (8 years 11 months 4 days 21 hours ago) and read 2550 times:

Quoting Thrust (Reply 18):
Is this plane a writeoff?

This was asked in another thread, and the consensus seemed to be:
probably not, since the plane is so new.


User currently offlineBoeing7E7 From , joined Dec 1969, posts, RR:
Reply 20, posted (8 years 11 months 4 days 17 hours ago) and read 2499 times:

In all probability it's going to have several causes ranging from bad weather, landing a little long and the lack of a proper Runway Safety Area (RSA) (which in the end, could not prevent an over run).

User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 21, posted (8 years 11 months 4 days 9 hours ago) and read 2451 times:

Quoting Cloudy (Reply 19):
Quoting Thrust (Reply 18):
Is this plane a writeoff?


This was asked in another thread, and the consensus seemed to be:
probably not, since the plane is so new.

looks a Long Job If its not a Write off.
regds
MEL



Think of the brighter side!
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