All,

I wanted to clarify a couple of things from my last email.  The attached explanations came from TMU and still contain some issues that we might need to work through.  Main one being whether or not we route a/c direct to BTG when we are done with them.  The 7110 requires all routes to be updated and TMU wants us to call them every time we route key a locked in aircraft so something has to give.  Making that many calls to TMU is not operationally feasible.  I haven't heard any issues coming from the floor about this in awhile so it seems like it is something we are working through with a modicum of success.  Just know that when you route key aircraft to BTG there are things that could happen to make the time jump.  Be prepared for that possibility.  Again, these are TMU's responses, I just want to get them out to you all so you know their thoughts on all of this stuff.  Feel free to join in on slack to discuss.

Metering Explanations:

Metering Delay/Sequencing Delay

  • TBFM calculates delay to meet an arrival rate (which is programmed via a spacing matrix by the TMCs).  It also calculates delay in order to sequence aircraft at each of the metering fixes and/or metering arcs.
  • During metering, an aircraft may have a small delay indicated in the D area, but none in the B area.  This delay is due to TBFM sequencing the aircraft at the outer Meter arc.  This will only be an issue if the D area controller decides to meter more time than the indicated delay.  That would result in the aircraft being behind the assigned meter time and that will push back (waterfall) the delay backwards into the other arrivals.

Updating routes and Frozen times jumping

  • Updating a Frozen aircraft’s route: If TBFM projects the aircraft will arrive at the Meter fix (arc) later than the currently scheduled time after a route amendment, the aircraft might jump to the back of the list and the slot that the aircraft had in the TBFM timeline could potentially disappear. If the TMC is notified prior to a controller routing an aircraft, the current slot can be noted/preserved so that the aircraft can be placed back in it quickly if it were to jump out.
  • TBFM will adjust times even after they are frozen. The adjustments are small, but they are large enough to potentially lose a slot when an aircraft jumps out of the assigned slot.  If the TMC is not aware that the aircraft was routed direct, and that aircraft jumps to the back of the list, he/she may not know where that aircraft was originally.
  • TMC Techniques:
    • Swapping slots with aircraft that have negative times to keep the list up to date.
    • Shuffling slots with internal departures (swapping the aircraft in the list).  This is done because the towers can release aircraft two minutes earlier and one minute later than their CFR time, so the departures do not always land in the correct slot once departed.
    • List stability.  TBFM will jump aircraft around when frozen aircraft are mixed with unfrozen aircraft.  Since ZSE’s arrival streams have varying freeze horizons, it is common for TMCs to freeze aircraft further out to create stability.
  • Oceanic, Alaskan, Hawaiian, and sometimes Canadian flights do not appear in TBFM until they are within ZSE radar. These flights will suddenly “pop” into the TBFM list since they appear a lot later in radar than flights on the other arrivals. The TMC may shuffle the flight down the metering list to put the aircraft in a more equitable arrival slot.  This would affect the frozen times on all streams.

Delay Countdown Timer (DCT)

  • When an aircraft is in level flight and on its FP route, the DCT is accurate. Once an aircraft’s speed is changed, the DCT changes to reflect the new ground speed (by turning, descending or climbing an aircraft, it’s speed will change).  TBFM constantly updates the DCT based on the aircraft direct the next fix in the FP with the current speed. 
    • For example:  An aircraft has a 5 minute delay according to the DCT.  This aircraft is turned into a strong head wind, which drops the aircraft’s speed dramatically. TBFM recalculates, which drops the DCT to 0 after two minutes of actual time has passed.  Once that controller sees a 0 for the DCT, that aircraft is turned back to the fix on it’s FP and the DCT now jumps back up to 3 minutes. This is because TBFM has recalculated again based on the speed, which increased once that aircraft was out of the wind.

When aircraft are slowed too much or slowed when there is no delay: 

  • Over delaying an aircraft results in negative delays on the TBFM timeline. Once an aircraft has fallen behind its slot, that slot is very difficult to fill. Over delaying an aircraft creates compacted demand where there originally wasn’t because it pushes that aircraft back into the aircraft behind it. This may result in times being shuffled, the metering list being rescheduled or even ZSE being shut off by TRACON due to an excess of aircraft.
  • Having aircraft cross a metering fix/arc with 0 to +1 is better than -1.

 

Again, this is directly from TMU.  Please don't hesitate to jump on slack and discuss.

 

 

PS - You can also venmo money for Al's retirement gift to garretwilk@gmail.com