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Today is IFR conditions around LA. I was listening to KVNY ATC while I drove in to work.

The tower kept holding aircraft short of 16R for 5,10,20+ minutes for IFR releases. Some of the pilots actually got a bit snotty about it over the radio.

What is going on during this time? Is it a traffic congestion thing when it's IFR?

Just thought I'd ask those who know.

This all relates to the TMU (Traffic management unit) at the TRACON. In each TRACON, there is a desk with TMU personnel who are responsible for ensuring that the National Airspace System does not get over-crowded, making the sky unsafe. When an IFR aircraft is ready for takeoff, the tower typically has to call the TRACON and the TRACON (with the help of a computer) determines a window of time (typically 3-5 minutes wide) in which the aircraft is released. When the weather is IFR, greater separation minimums apply for approaches into airports thus their maximum arrival rate per hour decreases which snowballs to having less aircraft takeoff per hour in order to keep the sky flowing smoothly.

IFR releases are different at each airport, though. VNY must share the departure corridor with BUR because of their proximity. So, when both airports are busy, you'll see a VNY departure, then a BUR, then a VNY, then a BUR and so on. However, airports like SMO are 2nd in line to LAX due to their proximity. I have personally seen IFR aircraft (from C172s to C750s) have to wait up to 45 min on the ground at SMO because LAX is just slammed. In order for SMO to release an IFR departure, LAX must stop all of their operations on the north side for about 3-5 minutes which could be killer during rush hour.

I have personally seen IFR aircraft (from C172s to C750s) have to wait up to 45 min on the ground at SMO because LAX is just slammed

45 minutes, that's cute. My personal record is 3 hours in the CRJ trying to get to the home of flow control, ATL.

Seriously though, KC has it correct. To expand it out a bit more, each TMU is controlled by the ATCSCC (Air Traffic Control System Command Center). They do a lot, so much you can take a virtual tour http://www.fly.faa.gov/Products/Informa ... ation.html (it even works during the shutdown!)

Kevin/Peter,
Your replies were hugely informative! Thanks! So do the pilots have no other option than just sit there holding short burning fuel? If it's going to be 30 mins can they at least go back to parking and not waste insane amounts of fuel?

vfr departure if weather permits and you're part 91. that's an option. I use it to get around the problem from my home airport.

they activate your ifr once you are clear of all the traffic, until then, it's on you.

djrisc wrote: If it's going to be 30 mins can they at least go back to parking and not waste insane amounts of fuel?

It's not uncommon that you'll hear of airliners shutting down at least one engine if it will be a while. Or, if things don't look good from the start, they might only start one engine at the gate and keep the other one off until they get notification from tower at the runway that it's almost their turn. ATC tries their hardest to keep the pilots informed of the time frame for just this reason. It's also important for ATC to keep the pilots up to date so when their release does come through, the pilots are ready to go otherwise they'll come back and say "Uhhh, we need a minute to start up the engine" in which case they could miss their window and have to start from scratch!

djrisc wrote:Kevin/Peter,
Your replies were hugely informative! Thanks! So do the pilots have no other option than just sit there holding short burning fuel? If it's going to be 30 mins can they at least go back to parking and not waste insane amounts of fuel?

I once shut down at the hold short line at SMO and had the tower wake me up with a light gun when my release was ready. Those long hold times get expensive when you're renting hobbs time!