Line Constraint: The Final Word

Line constraints has always been my number one enemy and my last bid was really hurt by "line constraints." I called Frank Bowlin up on the PBS help line last month for a follow-up on my December award. Frank has been very kind to me. He explained it to me over the phone and he was kind enough to send me a copy of an article written on the SAPA forums about line constraints. I think this should be the final post on line constraints as I was wrong on what they are and why they occur. Thanks Frank, hopefully I'll be able to learn from my mistakes!

"Replaced by lower layer pairing due to line constraints"

On the face, it appears as if your seniority could hold a pairing and PBS took it away from you, which is true. However, the reason it was taken away is that your line could not be completed as required under your direction.

For instance:

Let's assume that you are bidding #1 in your bid package. You get everything you want, right? No, not necessarily. Your seniority will not trump legality, PBS limits nor run-time constraints placed on your line.

With that in mind, imagine that in layer 1 you bid specific pairings on date. That's fine; as #1 all the pairings you want should be available to you. Let's also imagine that you construct your perfect line and it adds up to 75:01. (You're an RJ pilot). Well, if everything goes as you planned, PBS will award you exactly what you bid in layer 1 and you'll have your 75:01 of credit for the month. SuuWEEEET!

EXCEPT, you failed to take into account that 6 day work block you're working at the end of the previous month that you put there to get a big block of days off earlier. And your current bid starts with some consecutive days as well. In fact, between your last month and this bid, you've ended up asking for 8 days in a row. OOPS!!

But, being #1 in base, you assumed that anything you wanted you could get, so you only bid one layer.

Here's what PBS did:


It gave you everything you asked for in layer 1 EXCEPT it didn't give you one pairing at the beginning of the month because of its requirement for a calendar day off in any consecutive 7 days.

But, by not giving you that one pairing you don't have enough hours. (PBS minimum for RJ pilots is 75 hours.) So, PBS goes to layer 2. Oh, there isn't a layer 2, so it goes to layer 3. Nope. No layer 3 either, and so on through layer 7. It falls through layer 7 looking for something, anything, that you'll let it add to your schedule to make a legal line. It finds nothing.

Since it finds nothing in your bid, you lose all control of your bid. Once you lose control of your bid by letting it fall off the 7th layer, PBS gets to do what works best for IT without regard to what you bid within your 7 layers. Seven strikes; you're out!

Suppose the calendar geometry of what you bid doesn't let PBS add anything to your schedule. So, the only way it can add more credit to your line is to replace a pairing you got with one of its choosing, and that's what it does. The reason reported for that is "Replaced by lower layer pairings because of line constraints."

This is a simple example that clearly shows that PBS has to do something, even though this particular scenario is a bit unlikely.

This process of replacing a pairing with one from a lower layer (or one of its own choosing if it exhausts all 7 of your layers) results from an optimization. An optimization occurs:

After failing to complete a line within your 7 layers

When you specifically ask for one by bidding "Try to finish at this layer" and

as the first step of a "Clear award/partial line"

Unfortunately, you can't always easily tell when you've been optimized by looking at your award. Usually you'll just see an LN solution and that's a sure sign. However, if the optimization allowed it to solve by replacing a pairing with one from one of your lower layers rather than going off the end, it will show a line solution from that lower layer, L5 for instance.

There are lots of reasons that a bid that otherwise looks reasonable might not solve within your 7 layers. The most common are:
You bid a target line credit range that was outside of the credit limit that PBS needed to impose. For example, you bid 90-120 hours but PBS limited you to 85 hours max.

When there are a limited number of bidders left to work on a specific day on which that same number of pairings fall, you WILL BE REQUIRED to work that particular day and your bid did not permit PBS to work you. (Remember, the company's requirements trump your seniority. PBS's first responsibility is to cover the flying.) In this case, PBS requires you to work that day but if your bid prevents doing so in all 7 layers, it'll fail off the end.

Your bid is too restrictive and there simply aren't enough pairings left with the right dates to make a full line within your bid. Kinda like when a junior line holder only bids for high-paying pairings with weekends off.