PBS Optimization

Commonly I have seen "replaced by lower layer pairing due to line constraints" in my Reason Reports. I understand that this occurs when the PBS computer runs an optimization due to certain situations. While I have seen it done in almost all of my awards, I don't believe there is a rhyme or reason for it. Here is what Frank Bowlin has to say about it:

Reason Report: Replaced By Lower Layer Pairing Due To

Line Constraints

Probably the least understood reason in the Reason Report is, “Replaced by lower layer

pairing due to line constraints.” The simple explanation is that PBS optimized. That is,

it looked at pairings from multiple layers within your bid as if they were in the same layer

and it picked a pairing from a lower layer that allowed it to complete your line when an

otherwise available but conflicting pairing from a higher layer did not allow a solution.

When Does PBS Optimize?

There are only three occasions when PBS will optimize:

1. You tell it to by bidding the Line Property, “Try to finish at this layer.”

2. You tell it to by bidding the Line Property, “Clear award/partial line.”

3. Your bid did not permit a full line meeting all user and system properties within

the seven layers you created.

Optimize = Try to Finish at This Layer

When you bid this property in a layer PBS still processes that layer normally. If a full,

compliant line is built in that layer, PBS stops and the bid does not optimize.

However, if PBS cannot finish your bid in a layer where you bid “Try to finish at this

layer,” PBS optimizes before moving on to the next layer. When optimizing, PBS takes

all pairings from all previous layers and treats them as if they were in the current layer,

trying to arrange them to complete your line with the current layer’s line properties.

For example, say you prefer locals. Also, say in layer 1 you bid for and got a really

sweet local that pays 5:25. Now, suppose that by the end of layer 1, PBS has only

found 42 hours of layer 1 pairings that it can place using layer 1 line properties, so it

moves on to layer 2.

Before looking at layer 2 pairings, PBS will see if the line properties in layer 2 will allow

it to award/arrange any more layer 1 pairings. PBS will then look at the layer 2 pairings

to see if it can find additional pairings to add to what it already built for you with layer 1

pairings. In your case, you bid for more locals and PBS was able to get your bid up to

just 10 minutes short of your required minimum (75 hours for RJ pilots, 80 hours for

Brasilia pilots and all flight attendants). Since your bid at the end of layer 2 is still 10

minutes short, PBS would normally move on to layer 3. But, in layer 2 you bid “Try to

finish at this layer” so before moving on to layer 3, PBS will optimize in layer 2.

In this example, say in layer 2 there was a local paying 5:40 that is available to you but

works on the same day as that sweet local from layer 1 that paid 5:25. Normally, PBS

will not replace a pairing with conflicting pairing from a lower layer. But since it is

optimizing, it can. During the optimization, PBS discovers that replacing the 5:25 credit

pairing with the 5:40 pairing allows your line to exceed the minimum required by 5

minutes and complete, so it does — it replaces the layer 1 pairing paying 5:25 with the

layer 2 pairing paying 5:40 and completes the line, stopping there.

If the optimization didn’t find a way to solve in that layer, PBS makes no changes and

goes on to the next layer normally as if it never optimized.

Clear Award/Partial Line

This property is almost the same as “Try to finish at this layer.” There are two

differences:

1. “Clear award/partial line” is the first thing PBS processes in a layer; “Try to finish

at this layer” is the last, and

2. If the optimization fails to solve the line with the previous layers, instead of

moving on as if nothing happened, PBS clears out all the pairings that were

initially awarded in the higher layers and starts over with a clean slate as if this

current layer is the first.

The “Clear award/partial line” property is useful if you’re bidding for one type of pairing

that you don’t want to mix with other types if you can’t get a full line of them. For

instance, continuous-duty-overnights (CDOs, “stand-ups”) don’t mix well with regular

pairings for most of us, so if you like CDOs you might bid them in the first few layers, but

then bid “Clear award/partial line” to get rid of all the CDOs before you bid for other

pairings.

PBS Gives You Seven Chances for Success

PBS gives you seven layers in which to express properties to build a line. To process a

given layer, PBS takes all the pairings meeting your pairing properties in that layer and

tries to arrange them (along with pairings from previous layers that have already been

awarded) according to the line properties of the current layer and those from PBS itself.

Thus, to be a legal line, PBS must meet:

1. Standard system-wide constraints such as legalities,

2. System constraints — found while processing the bids — of required minimum or

maximum credit that it must apply to you so PBS can solve the remaining bid

package with the remaining crewmembers and remaining pairings.

3. System constraints — found while processing the bids — of required coverage

dates on which you must work so PBS can solve the remaining bid package

with the remaining crewmembers and remaining pairings, and

4. Your line properties as expressed in the current layer,

Requirements 1–3 always apply, but you only get 7 layers for requirement 4. Thus,

expressing a line constraint that conflicts with a system constraint will cause that layer

to fail. If you do so in all 7 layers, all 7 layers will fail.

If PBS does not solve your line within your 7 layers PBS will optimize, layer-by-layer, as

if you bid a “Try to finish at this layer” in each layer. If one of those optimizations

succeeds in meeting all four criteria above, PBS stops and awards you that line. If none

of those produce a full line, then PBS will do two more optimizations:

1. With your pairings but with default line properties and system constraints, and

2. Using ALL the pairings that remain at your seniority with default and system line

constraints.

These last two steps usually produce very undesirable results.

Thus

You want to do whatever you can to maintain control of your bid by giving PBS the

necessary freedom to solve within your seven layers. As you can see above, if you

allow PBS to go past your seven layers, the results are often very undesirable. Thus,

by layer seven:

1. You should consider relaxing day off bids in case your desired day off falls on a

day that PBS determines is a coverage date, requiring you to work that day, (See

previous PBStips on this topic.) and

2. You should consider relaxing your “Target line credit range” (if any) in case PBS

has established a minimum required credit or a maximum permitted credit that

falls outside your range.

Optimizing under your control can produce very positive results, such as using “Try to

finish at this layer” that many people bid in all layers, or “Clear award/partial line” when

strategically appropriate. But, allowing PBS to do whatever it takes to solve your line,

including removing pairings that you could have held but conflicted in some way, can

produce very unpleasant results and is something you should avoid.