10 June, 2009

What ever happened to LIFO logic? Or... are Southwest Airlines and US Airways run by crazy people?

LIFO = Last In, First Out

It's an acronym describing the logic of a particular type of computer storage system. There were two basic designs that we learned in school: LIFO, and FIFO (you can figure that one out, right?)

The idea of each is as follows. Think of LIFO as a storage tube where you insert elements from only one end. Therefore, the first one in will be the last one out and vice-versa. Thus, to get at that last one, you would need to get all the other ones out. A FIFO "stack" is different. Think of that one as a storage tube that has openings at both ends. So things just progress through it. The first one in the tube at one end will be the first one out of the tube at the other end. You do not need to move all items to get at the last one (necessarily, presuming you can access both ends for both entry and exit).

The human digestive system is a good example of a FIFO stack. An airplane, you'd think, is a good example of a LIFO stack.

Until the airlines decided to change the way that works, of course, in their effort to screw up not only the things that are easy to screw up, but also those that seem impossible to screw up.

Throughout all of history, airplanes have boarded how? (all together now)


If you were seated at the back of the plane, you would be the first one to board. If you were seated at the front of the plane, you'd be last to board, but first to exit (i.e. LIFO).

Then at some point in time, they mucked this up in FOUR ways.

1. Those passengers needing assistance (anywhere on plane)
2. Those passengers traveling in first class (front of plane)
3. Those passengers traveling with small children (anywhere on plane)
4. Those passengers who belong to some asinine make-believe special club (who knows)

This, of course, is designed to show some type of respect or preference to certain demographics. But what it really does is demolish all semblance of an orderly boarding process. Because if you then begin your LIFO loading process AFTER this bullshit "preboarding", all you're doing is throwing more crap into a clogged toilet.

The logic is so ass-backwards that it is mind-boggling. But then, so is the idea of allowing private, for-profit organizations to compete with one another in the non-trivial process of rocketing large numbers of the populace through the sky, while keeping their costs as low as possible. But I digress.

The first class people usually board pretty quickly. This might be because these are people who travel a lot, and they don't futz around. And perhaps the Mickey Mouse flying club partners are also okay fast. It's the other two groups that cause the problem. Infants and fossils. These people should not be allowed on airplanes at all, never mind before the rest of us.

Of course, all of this has been going on for years. So why am I complaining now? Well, because none of this nonsense is the stuff that really makes me mad! What really makes me mad is that some airlines (Southwest and US Air, to name two) have decided to completely abandon the LIFO process, and switch to their own special, bizarre, inefficient methods of boarding.

US Airways uses the "Zone" method of boarding. People are given zone assignments: 1, 2, 3, 4. And you board by zone. Of course! Makes perfect sense! Not sure why we don't just go by row, but zone is okay. Zone 1 is the back quarter of the plane... Zone 2 is the next one... Zone 3 after that... and Zone 4 is the front of the plane.


Um... I'm so sorry. You've mistaken us for an airline that knows anything about anything about anything. Let me explain to you how it works. Zone 1 is a random smattering of seats throughout the aircraft. Zone 2 is... guess? A random smattering of seats throughout the aircraft. Zone 3? You got it. That's the idea. So when I call Zone 1, people will be trying to get to seats in all parts of the aircraft, and you can be sure that the first person to board in Zone 1 will just happen to be assigned to seat 4F, and they're going to adjust their adult diaper, try to fit their oversize bag in the bin above, and then take 11 minutes to pull out their Fishing magazine before sitting down, while the rest of the "smattering" that is Zone 1 waits in "The Jetway" to board the airplane.

This goes on forever. And then they have to include extremely complex rules, due to this smattering concept. "If you are traveling in a group, your entire group may board the plane when the lowest zone number is called". Yes, because if Mommy, Daddy, and Little Suzie bought tickets to Fort Lauderdale, it's entirely possible that, although they're seated together in seats 22A, B, and C, they'll have been designated in Zone 1, Zone 3, and Zone Twilight. Excellent plan. I really love it.

So how did Southwest Airlines top this one? It's a good one. I think they had to engage a team of creative directors to come up with their boarding plan. Here's what we do. Regardless of when you bought your ticket, you are assigned a boarding Letter/Number when you check in (presumably based on when you arrive at the airport). For example, A22. Or C35. When you arrive at the gate, there are numbered spots in the queue at the gate. When they call your LETTER, you all are supposed to line up, in sequential order, to board. So they call group A, and everyone with the letter A lines up.

And then they let you on the plane to go to your assigned seat? So? No big deal.

Um... no... there's just one catch.


Um... don't laugh. I'm serious. There are no assigned seats. Yes. It's a mad free-for-all, of people trying to get windows. Trying to avoid windows. Trying to avoid the restrooms. Trying to avoid the wing. Trying to avoid the front of the plane. Trying to avoid the exit rows. Trying by all means to avoid middle seats. Trying to avoid sitting next to a really fat person, or a baby, or a dirty person. People trying to play out every paranoid fantasy of what could go wrong on the flight. Personally, I don't like to sit just behind the wing, because sometimes when an engine blows out, the person sitting right behind the wing is killed by shrapnel, even though the rest of the passengers are fine. That would be unfortunate.

So, you've got your Boeing 737s, because that's all that Southwest flies. They have no first class. In single-class configuration, the Boeing 737-800 holds 180 passengers. So that is not a trivial free-for-all. How this saves time is beyond me. And I love when they let all of group A board, and then pause a bit, and then do group B, then pause a bit, etc. You expect that they've waited for the group A people to find their seats. But you discover that after groups A, B, C, and D are allowed to "board", that what you really have is 180 people lined up in "The Jetway" waiting for the old guy with the adult diapers to find a few pillows to use for hemorrhoid cushions before beginning his fart-filled nap in Row 4.

Seriously. I could design a better airline boarding process than this. I think it would involve machine guns pointing down the aisles and evaporating anybody who remains in the aisle for more than 3 seconds per row of travel to their assigned seat. That would really help eliminate the need for several of the special categories. And it would also help save money on discounted fares, because the concept of a "frequent flyer" would pretty much cease to exist.

And I'm kidding.


  1. Hey, Mr. Feeble Paula Berg here from Southwest. I read your post and immediately reached out to my colleague to see what he thought of your assessment. Doug knows a thing or two about boarding an aircraft, and he was kind enough to share his thoughts…hope you enjoy!

    Dear Mr. Feeble: LIFO is definitely the best way to fill and empty a tube with one opening and a diameter equal to that of the objects inserted. There are 47 such "tubes" on the Boeing 737-700 aircraft as configured for SWA. So the problem is to get Customers to board such an aircraft so that LIFO occurs for each of the tubes. Boarding window, middle, and aisle seat Customers (in that order W-M-A) by alternating half rows (46 tubes) from back to front (the main aisle is the 47th tube) achieves LIFO for a SWA aircraft. If you run a movie of people deplaning in reverse and then forward, you will see LIFO for an aircraft (except everybody is walking backwards as they get "on" the aircraft ;-). (Incidentally, movie theaters and ball parks avoid a "stack" and the consequent "row churning" by making the rows (tubes) effectively two people wide).

    WilMA - You would think you could achieve W-M-A boarding with assigned seating. And you could, if it weren't for the fact that many people don't travel alone. If you rigidly enforce the boarding groups by window-middle-aisle, these Customers will not be in the same boarding group. These people, who want to wait in line and board together and not wishing to offend the boarding priority, rearrange themselves when they get in line. This actually destroys the row LIFO pattern since, for example, the window person of a window/aisle pair will board with the aisle person. They will, of course, arrive at their row after any middle seat person. This problem can be alleviated by acknowledging this tendency and adjusting the boarding group assignments. In the case mentioned, assign the aisle person to the window boarding group with their companion. Although this arrangement (W-A-M) is not perfect LIFO its better than the arrangement (M-W-A) the Customers were creating considering the number of and how long the people will be in the aisle while filling their side of the row. This modified W-M-A boarding is the fastest that can be achieved with only three boarding groups (about 7 and 1/2 minutes for 100 people to board with no through passengers already on board). However, all airlines that use WilMA introduce additional non-WilMA (row) boarding groups which slows the boarding process considerably.

    What about zoned boarding? There are many zone boarding schemes. Zoning does not address the row arrival pattern so it is always slower than a W-M-A boarding scheme. Zoning simply deals with the fact that if people are allowed to freely walk on to the aircraft, a line quickly forms in the aisle at the first row that is seating. If you let the rear third of a full load of passengers board, they will fill the aisle all the way to the forward galley, blocking access to the other two thirds of the seats. So if you let the rear quarter of the passengers board first, they will not block access to some of the front rows. This means you can board the front quarter of a full load immediately. Customers are arranged "randomly" as to row, but the "churning" is concentrated in the aisle of the zone. Furthermore, the zone sizes must closely match the number of passengers boarding. Dynamic zoning is perhaps never done, so the maximum efficiency of zoned boarding is rarely achieved. Boarding 100 people back to front in five row groups takes about 11 and 1/2 minutes (this is the slowest scheme used by any airline).

    CONTINUED in next comment…

  2. So what about "open seating"? - First off, SWACustomers prefer the freedom of choice -- no assigned seating. Fortunately for us, many of our Customers don't travel alone (after all it's the LUV airline); most of our Customers prefer a window or aisle seat (who doesn't). The aircraft are generally 80% full. This means that rarely does anyone have to step out into the aisle to let someone sit down; but the "churning" is concentrated in the front. So it takes about 9 minutes to board 100 people. This is faster than the assigned-seating boarding schemes of other airlines especially with their additional row boarding groups. Plus it gives those Customers who are traveling alone a choice of who to sit next to (or far away from) rather than sitting in an assigned seat. The lettered-numbered boarding scheme has nothing to do with boarding speed and does not markedly affect the speed of open-seating boarding.

    - Doug Lawson

    Not sure if that answers your question as to whether Southwest Airlines is run by crazy people. I think we would say yes, it is...and, that's a good thing :)

    Paula Berg
    Southwest Airlines

  3. Paula & Doug:

    Thank you very much for the response. Of course, I figured that there had to be reasons for making the change to boarding method. A few follow-up questions for you:

    1. Why don't all airlines use one of the newer boarding schemes, if they're demonstrated to be quicker?

    2. How does baggage storage factor into this process, in terms of distribution? The discussion focused primarily on putting people in their places - but the baggage compartment loading can also affect efficiency.

    Finally, I hope and suspect that everyone understood that I do a lot of ranting on here, and that it's often for dramatic effect. I am always very pleased when my thoughts evoke a response from the subjects of my rant. So thank you for the information!

  4. Like most people in Dallas, I'm sure Doug is taking shelter from the horrendous weather we're experiencing right now, but I will see if he can answer your follow up questions tomorrow.

    And, who doesn't love a good rant?!? I just happened to know someone who is an expert on the subject (what are the chances?), so I couldn't pass up the opportunity for him to share his expertise!

    Paula Berg
    Southwest Airlines

  5. Thanks again, Paula. If you click on the "Airlines" tag on the right column, there are a bunch of other air travel related entries. Some of them are probably rants, and some are just reflecting on the experience of air travel, or thoughts that occurred while in the air. Best regards to you.

  6. I can't believe Southweat actually replied to this in person. In Corporation, I guess would be more accurate... but it's still kinda neat. Personally, I really like Southwest's boarding because it strikes me as fairer than other systems, but when I've flown them it doesn't seem random, it seems clumped. Instead of FIFO or LIFO it's CIHO (clumps in, horde out). I like how people have to queue up outside the plane by number, rather than in the giant cluster of people that forms in front of conventional boarding podia, where someone is always cutting in front of you at the last moment.

  7. Okay...sorry for the delay...just received the following information from Doug.

    Why don't assigned seating airlines use the boarding scheme?

    First off, to improve stowing and seating speed, you need to be able to see the details of what's going on inside the aircraft and be able to experiment with different boarding schemes in a controlled environment. Real boardings are far from controlled (no kidding) and our Customers are not guinea pigs. So we have to simulate the boarding. This isn't easy and it's costly. But over the last few years some airlines tried.

    Some airlines used numerical models. But these are not realistic enough since they "summarize" the processes occurring in the aircraft. The results aren't realistic enough. United and US Airways hired some physicists to develop a specialized numerical simulator. They did, and came up with a zoned W-M-A boarding scheme.

    To actually simulate the details of what goes on inside an aircraft requires a special kind of simulator --- an agent-based simulator. You can't get these of the shelf; they are custom built. Southwest built an aircraft-interior, agent-based simulation. So we could see the details of what was happening. And you're right; stowing can be a killer.

    It definitely must be included in the simulation. This is where numerical models fail and agent-based models don't. What's the problem --- overhead bins are not assigned. It only takes about 15 of 100 passengers boarding to cause "turbulent flow" (back and forth searching and bin repacking) among the last (most forward) boarding group. In a back to front zoned boarding a large fraction of the passengers stow their bags in the forward bins. This creates extreme "turbulent flow" throughout most of the boarding process. WilMA is not as bad because some of each third of the passengers (a boarding group) are going to sit in the forward seats and stow their bags above them. Open seating does have some "turbulence" but its minor (short distances and few passengers) and generally less than WilMA.

    So now, having done all this simulating of stowing and seating and found the answer, why don't airlines with assigned seating use WilMA. Well, the invents of WilMA did. United used it in the 90's and on Ted. But that was then, why not now. Here's the answer -- upstream processes involving both aircraft and people movement.

    The hub and spoke airlines may be saying, what's the hurry if most of the flights are scheduled to arrive more than 40 minutes before departure. For us, with our usual load factor, on-time performance, and open-seating boarding, only 5% of our flights could have been helped by our WilMA scheme. And that's disregarding three other major upstream processes -- boarding standby's, boarding pass reading, and boarding elite and special Customers.

    Two of these processes cause the Customers to "trickle" on the aircraft. Delta has implemented automated, standby processing that reduces this bottle neck. And we found that 25% of the time the boarding pass reading process itself was slower than our WilMA scheme. So there needs to be some software improvement along these lines among all the assigned seating airlines.

    Finally, special boarding. This is not going away; people want to be treated special; they want to be among the elite. Dividing each of these boarding groups in three (W-M-A) won't work (unless everybody is special and among the elite -- uhm) because of the randomness with which people will join the boarding groups. What you would get is open boarding with assigned seating which is the slowest "practical" way to board (if you work hard at it, you can make it worse).

    So there you go. Until these upstream processes are "smoothed out" and the line in the jetbridge comes right back into the waiting area, there probably won’t be much change in assigned-seating boarding. Like I said earlier, why not board about as fast as you can and be able to choose the surrounds of you seat and --- fly Southwest.


    I love this guy!
    -Paula Berg
    Southwest Airlines