Wow, what do you guys do when your busy? seven miles? thats a massive spacing!
When busy, in theory we "type match". So the required distance is 4nm. But since there isn't enough room to have a/c pass each other on the threshold taxiways, (thanks for building the taxiways too close together) it usually falls into the "too hard" basket. By the time it's been organized, the situation that made it seem a good idea is often irrelevant.
When really busy, everything just slows right down. The morning and afternoon rush period lasts longer. It's a PITA.
So why is this used? isn't it a bit like boxing with one arm tied behind your back?
Yep. I believe it is referred to as
best practice.
But seriously, it works in the worst case scenario, and provides a bit of a buffer. Frequently the space provided by the procedure seems excessive, but TWR staff are not approach rated and have no authority to vary it. (If we were, I believe we could possibly make departure rate increases of the order of 10 maybe 20% on many "below circling" days.)
Surely SSR radar has the required accuracy to provide proper separation inside the terminal area and on approach to make this process look a bit antiquated?
The minimum radar spacing is 3nm. Shaving it by .5nm won't make a huge difference.
Using Wellington as the example....how often do you get anything bigger than a 767-300 operating on a regular basis? Even at Max landing weight your approach Vref is still only about 140ish knots..which is well below the 175 you quote....
Whats wrong with 180kts till 8 miles...reducing to 160kts till 4 miles etc...? It works bloody well in Europe, and appears to be gaining traction in the USA as well.
Generally the largest sched a/c is a 757 (maybe 3-7X a week.) The bread and butter is 737/A320, lots of DH8's/ATR72/B190, a fair few C208 and light twins.(PA31 etc.) 767+ usually only run through WN if diverting. The arrival of a 777 or 747 etc is rare, maybe a once or twice a year event. (Perhaps needless to say, the poor C208 etc is at the bottom of the food chain when it's like this, and I've seen one of them wait well over an hour for a departure gap at times.)
Partly the reason this seems antiquated is that the wx around here is unusually changeable, can get quite windy, windshear and turbulence is likely, and terrain to the north prevents radar vectoring below 3000'. The procedure is tailored to the worst case scenario, and in that scenario (overshoot likely due wx, preceding climbing poorly due turb, preferred approach airspeed higher rather than lower due w/s) you absolutely would not want to shave it. That it is excessive in a situation of light wind but cloud below the circling minima, where everyone is performing normally, is an unfortunate consequence. I guess alternative and more expeditious procedures are just too hard to dream up and implement. I don't know,but I imagine that such a procedure would have to be nightmarishly complicated to satisfy the regulators' requirement. So in fair weather we have to guard against an event that happens maybe once a fortnight, using procedures that are designed against the worst case scenario. Sad, innit?