EWR764

No question that the 739 is a handful to land, with high approach reference speeds, tailstrike risk and, in this case, a contaminated runway. Still, the 739's in-service record proves it is safe, and even when autoland is not in use, its performance is well within the capability of professional aviators. Still, gratuitous swipes at the "Uber-Guppy" are taken at any opportunity!

Further, not all "runway excursions" are created equal. Most of these events in winter weather occur on taxiways or runway turnoffs, long after the airplane has slowed to taxi speed and aerodynamic control surfaces no longer have authority. This differs from the "overrun" type of runway excursion, where an aircraft still engaged in the landing/deceleration phase of flight, and actually departs the runway surface. See, e.g., DL1086 (MD-88), WN1248 (737-700), AA2253 (757-200). Certainly, these are much more dangerous situations, and differ substantially from what we saw in connection with this most recent winter storm.

FightingIlliniUAL

Clearly these situations are all unique, but my curiosity has me wondering if these pilots will lose their jobs?

philf

Yellowjj

That's probably because 4R exits are high speed exit taxiways, slanted at a angle to allow decelerating planes to use them at higher speeds. Whether it made sense for them to do so in such icy conditions is the real issue here. From where they slid off taxiway V, it's obvious the plane wasn't "over speedy" at all otherwise they would have been much further in the grass. They probably turned with slightly more speed than they should have given the conditions and hit a patch of ice. Rubber does slide.

Hell Delta got stuck making a simple turn on the taxiway at BOS tonight (A321). Were they being over speedy as well?

raehl311

So a vehicle hit some ice and ended up in the grass.... it happens.

Blame airport operations for not dropping enough salt.

riphamilton

i could have this wrong (and i encourage corrections because i enjoy learning!), but i believe that a knock on the 737 is that it takes longer for reverse thrust to be generated.

as the 737 touches down and decelerates down the runway, it seems like SOP is that the reverse thrust is applied until about 80kts, and then the reversers are 'retracted' (that's probably not the correct term) back to their normal position until the next landing. there are FOD ingestion concerns if the reversers are left deployed beyond the runway. the brakes take over remaining deceleration duties all the way to the gate.

anyone who has driven a car on snow or ice knows that brakes and tires can be anywhere between 0-100% effective. reverse thrust slows the momentum of the aircraft, regardless of surface contamination.

so let's say the a/c has vacated the runway, is taxiing at an appropriate speed, and must 'hold short' of an intersection. the pilot tries to stop by using the brakes, but the plane instead begins to slide on the snow/ice. no problem, let's use some reverse thrust to slow the aircraft. with the 737, it takes around 10 seconds for that thrust to be generated (i'm not sure if this is because of longer 'spooling up' time for the CFM56 engines, or if the reverse thrust mechanism takes longer to engage - or both, or neither). regardless, a lot can happen in those 10 seconds, even at taxi speeds.

other aircraft (non-737) have larger landing gear. this increases the area of tire in contact with the surface, which can be significant in braking on a slippery taxiway. the 757 main landing gear, for example, has 8 tires. the 737 MLG has 4, including the stretched variants. the 757 also has larger brakes, and more of them. sure, the 752 is a heavier aircraft than the 739, but its 'braking to weight ratio' is still greater. its RR/PW engines may also be able to generate reverse thrust quicker than the 737's CFM.

the combination of slower reverse thrust and poorer braking may mean that a 739 ends up in a precarious scenario in icy conditions more readily than a 752. does that mean that the 739 is an unsafe aircraft? no! please don't interpret this post as an argument that the 737 is unsafe.

LarryJ

The flight was told that following traffic was three-miles in-trail on final so they knew that they needed to exit the runway in a timely manner and that the exit at the end of the runway was the only one that was available to them. They would have judged their speed by the braking action they had encountered during the landing roll as well as previous reports they had received. If the braking action at the end of the runway, and on the runway exit, was significantly worse than reported and the main portion of the runway, which it turned out to be, they can be caught by surprise. This can happen due to the heat from the engine exhaust of the slow-taxiing aircraft melting the snow as the aircraft exit the runway and which then refreezes turning snow into ice.

This is exactly the same thing that happened to SWA in OMA. The SWA flight even reported good braking action during their decelleration before hitting the ice at the runway exit. Both flights had slowed to taxi speed on the available runway then couldn't maintain traction as they were turning off onto the taxiway. The DAL flight in CVG was already on a taxiway when they slid off.No, the 737 reverse deploys and spools up no slower than any other airplanes with similar engines.

Reversers are most effective at higher airspeeds and less effective at lower speeds. The technique is to go to full reverse at touchdown. At 80 knots (airspeed) you move the thrust levers to reverse-idle and allow the engines to spool down while the reversers are still deployed. You don't want to stow the reversers until the engines are at idle because, once they stow, any residual thrust would be forward thrust. The reversers are stowed by the time you reach taxi speed.

onthesam

​​​​​

​​​​​This is most likely accurate. All aircraft were being instructed to "roll out to the end of the runway" and exit due to other other intermediate taxiways being snow covered/untreated.

goodeats21

I don't think airports use salt on runways or taxiways, do they?
Seems that would have corrosive and FOD concerns...

JimInOhio

You are absolutely correct... no salt. I believe urea used to be used but has been replaced by potassium acetate (?).

LarryJ

You are correct. They can chemically treat the surfaces with some type of deicing fluid but it isn't salt. Whatever they use, it's usually very effective.

wrp96

Just wanted to say thank you to you (and other airline employees) that are kind enough to answer questions like this and provide factual information.

EWR764

threeoh

Sodium formate is a salt. It's not table salt (sodium chloride), but it is a salt. A lot of salts spread on sidewalks, etc. aren't sodium chloride either -- there are many salts that are more effective at lower temperatures and less corrosive (though usually more expensive)

EWR764