23L8 applies as soon as you are scheduled to release more the 4 hours late. 23L9 applies on that same timeline only if the duty period interrupts an X day.

So, assume 1600 release. CA gets rerouted into an extra day. As soon as he is scheduled to release later than 2000, he gets 23L8 pay for the duty period and all following duty periods. So block + m/u for the original last day and new last day.

Same setup for a reserve…the original last day is LC so the trigger is not met unless the release (adjusted for base time) interrupts an X day. Only the new last day interrupts your X day so you get block + m/u for the last day.

If your reroute on the original last day goes past 0000 base time for release before the break in duty, then 23L9 is exactly the same as 23L8.

 

This might be what the discrepancy is. Our release, after timing out in the outstation, didn't interrupt an X-day (original release was 2145, RR'd release was 2200. I'm rounding but that's close enough), although the trip was originally a 3-day that turned into a 4-day with us operating back to ATL on my X-day.

 

Edge case example I'm curious about:

RES, trip originally ends on a LC day followed by X-days. Reroute (tag-on, legal) on the end of the trip (on an LC-day) that adds a day (additional duty period on X-day) to the trip. The rerouted duty period on that last LC day is "scheduled" to release late that day, but delayed ops carry that duty period beyond midnight and into the next (X) day (but still less than 4 hours past original release.) Then, an additional duty period the next day (on the X-day) to complete the reroute/trip.

Obviously the duty period on the X-day would pay L9. But what does it take for the duty period on the last LC day (the day the reroute began) to pay L9? What if it was scheduled to end duty before midnight, but actually ended duty after midnight (X-day.) Does the LC day reroute duty period ALSO have to be scheduled to extend beyond 4 hours from original release AND infringe an X-day to pay L9? L8 pay doesn't have such a restriction and gets paid on the original last day of the trip even if that duty period didn't exceed >4 hours (scheduled or otherwise.) The reroute itself pushes scheduled release beyond 4 hours by adding a new duty period, and L8 gets triggered.

At the risk of confusing my example above, here are a few more. All for RES, all on last LC day:

1) Tag-on reroute scheduled to release <4 hours and before X-day, actually releases after midnight (into X-day) and < 4 hours after original (I don't think L9 because scheduled <4.)
2) Tag-on reroute scheduled to release <4 hours and before X-day, actually releases after midnight (into X-day) and >4 hours after original (I don't think L9 because scheduled <4.)
3) Tag-on reroute scheduled to release >4 hours and before X-day and after midnight (into X-day.) Actual release.... probably not important (I think actual release is irrelevant for this, it's due L9.)
4) Tag-on reroute scheduled to add additional duty period on X-day well beyond >4 hours, but last LC day is from #1-#3 above.

 

The key is “scheduled to release more than 4 hours late” and “interrupt an X day.” If the scheduled flight delays it does not trigger 23L9.

But if the delayed flight is cancelled and replaced say with a ferry that is scheduled to release more than 4 hours late and interrupt an X day, then that reroute can trigger 23L9 for a LC day.

Difference is reroute vs delayed ops.

 

Here’s an example of L9 pay on a LC day:

Scheduled for a 1 leg day, FL shuttle -> ATL, blocking in around 2200 on last LC day. In flight rerouted to operate ATL 2230 -> LAS 2345 (0245 base time), then DH LAS -> ATL on the X day.

The duty period on the LC day interrupts the X day. The next duty period interrupts the same X day. 23L9 is paid for both duty periods.

 

My question was more about the "trigger" for L9.

If a RES is rerouted into an X-day (and >4 hours), L9 is clearly triggered.

If a REG is rerouted into an off day, L8 is clearly triggered and L8 pay includes the last day of the original trip in which it was triggered regardless if the original last duty period of the trip is rerouted >4 hours or not.

What does it take for a last (original) day reroute for a RES pilot on a LC day that is rerouted into an additional duty period on an X-day to trigger L9 pay treatment for the duty period on the last LC day when the duty period on that last LC day ALSO infringes an X-day?

 

To put the iron to fire, I submit:

SRH pg 162. Reroute example #6.

REG pilot is rerouted into a 5th day of a 4 day trip. They get L8 pay for All of day 4 and all of day 5. It did not matter whether the duty period on day 4 ended within 4 hours of original release or not. The fact that day 5 extended release >4 hours beyond original release triggered L8 pay.

Now imagine a RES pilot in a similar scenario, except that the duty period release on day 4 passed midnight and infringed on an X-day. Day 5 is clearly L9 pay, but why wouldn't day 4 (duty period infringed on an X-day) pay L9 also?

What is the trigger for L9 pay?

 

The trigger is a rerouted duty period was scheduled to release more than 4 hours late and infringed on an X day.

A rerouted duty period that delayed into an X day does not trigger.

 

Regarding SRH Example 6:

Look at the release for the duty period. Is it after midnight base time (and infringes on an X day)? If yes, then that duty period gets L9 pay. If no, then no L9 pay.

 

While I think you are probably correct, this is where the SRH example #6 gets weird. In that example, the duty day for the REG pilot on day 4 didn't even get extended, in fact it was shortened. Yet L8 is paid for that day 4 duty period.

So it appears that for a REG pilot, only the footprint of the rotation needs to extend beyond >4 hours for L8 to kick in, and that L8 pay includes the duty period where the reroute began, regardless of the end of that duty period. But for RES, L9 only pays as you say, for duty periods that both end >4 late, AND infringe an X-day.

That's a massive difference that doesn't seem to be represented in the language of the PWA, the SRH, or any scheduling alerts.

Both L8 and L9 include the phrase: "single pay no credit for any duty period(s) that extends beyond such time limitation.". SRH example #6 gives L8 pay to a REG pilot for a duty period that didn't extend >4 hours (day 4 of the rotation.) This suggests that the duty period itself going >4 hours ISN'T the trigger, rather the footprint of the rotation being rerouted >4 hours is. And if that's true for REG, I don't understand why it wouldn't be true for RES if the footprint of their rotation is rerouted >4 hours and into an X-day.