Why do we have Vertical Separation if ATC acts like we are on a highway?

[Lance Williams]

I just can't figure out why you need to slow someone who is 4,000 feet above and 70 knots faster than someone 15nm ahead of him when both aircraft have 800 nm left in their respective flight plans? Seems to be a pandemic on VATSIM lately. We are up in the big blue so we can go above and below each other, why slow us all for some ridiculous in-trail spacing that is not needed at all and actually ends up causing more problems?

[Mats Edvin Aaro]

ATC always have a plan or a reason for the actions and instructions they give. Even though it might not seem apparent to you as a pilot, remember that the controllers have the big picture and often plan ahead.

A tip: Depending on the controller’s workload, there’s usually no problem asking the controller about the reason for an instruction via private message. 🙂 

[Evan Reiter]

16 hours ago, Mats Edvin Aaro said:

A tip: Depending on the controller’s workload, there’s usually no problem asking the controller about the reason for an instruction via private message. 🙂 

From my perspective, I generally prefer when pilots make that request (or ask a question) on frequency. Totally appreciate this is down to individual preference but, in my view, there's nothing wrong with asking about a restriction on the frequency. It's easy for us to miss text messages via PM and even easier for people to misconstrue tone of voice on text vs. voice. 

I will get pilots occasionally not hear my instruction and instead of just saying "say again", they will PM me saying "what was that last instruction?" So I get nervous whenever I hear requests for more private messages. For me, let's get it on the frequency. Ultimately though, I agree - feel free to ask when you're unsure of an instruction.

21 hours ago, Lance Williams said:

Why you need to slow someone who is 4,000 feet above and 70 knots faster than someone 15nm ahead of him when both aircraft have 800 nm left in their respective flight plans?

It does happen that in-trail is needed, even for 800nm. The "line" of arrivals for the busiest U.S. airports can start several states away. That said, when there's only 2 of you and plenty of airspace, there is no reason to be slowing when an overtake works. Politely querying the controller, as Mats suggests, is probably the best way to figure out which situation you're in. 

[Alex Ying]

I'll echo what Evan said and emphasize that it's highly situation dependent. If it's just another day on the network then there probably isn't likely to be a need. However, if it's an FNO or some other massive event? There's a lot of traffic management and flow planning going on the background that is not readily apparent to pilots. In the real world, sequencing and flow management for LGA or JFK can start as far west as Chicago and Minneapolis. VATUSA's FNO events in particular can regularly exceed real-world demand, so it's very common to see flow management even hundreds of miles away from the event airport.

[Brad Littlejohn]

On 12/26/2020 at 7:19 PM, Lance Williams said:

I just can't figure out why you need to slow someone who is 4,000 feet above and 70 knots faster than someone 15nm ahead of him when both aircraft have 800 nm left in their respective flight plans? Seems to be a pandemic on VATSIM lately. We are up in the big blue so we can go above and below each other, why slow us all for some ridiculous in-trail spacing that is not needed at all and actually ends up causing more problems?

 

There are multiple reasons for this. Let's also use that highway analogy.

Not only do we as ATC have those highways horizontally (airways, as we all know), but we also have those same highways vertically. Take for example if you are trying to get onto an airways that is already crowded at FL340 and FL360, but has little traffic on that same airway at FL380, and no traffic at all on that same airway at FL400 and higher. Without rerouting you or taking you off the airway, we could have one aircraft reduce their speed relative to the traffic ahead of them on that airway, to give you a window to climb through their altitude to get you to your assigned flight level on that airway. Once you're through, the traffic that slowed can resume normal speed.

The same would have to happen in reverse. If ABC123, you, and XYZ456 are all headed to the same destination, and you're at FL400 while they are at FL320, and all of you would be descending on the same STAR to the same airport, something will have to give for you all to make the same crossing restrictions for the STAR. As ATC would be losing the vertical separation, only lateral separation would be available, unless you are taken off your flight path or the airway, which would defeat the purpose of meeting the transition to the STAR. So a hole in that airway would need to be made for you by either having the first aircraft speed up, or having the second aircraft slow down, so you can descend into their flight level on that airway. That sequencing would have to take place well before the transition into the STAR so you can meet the first set of crossing restrictions on the STAR.

This way, we can control the separation both horizontally, as well as vertically, allowing access to the same airway across multiple flight levels.

BL.

 

[Dustin Rider]

Center controllers, especially on VATSIM where we're typically working the entire facility combined, have lots of options to effect MIT separation (if it's required) besides slowing faster aircraft down--most of the time. Say, for example, I have two aircraft at separate altitudes but on the same route, and the second aircraft is overtaking the first by 70 knots. Say that I need to give the next facility 15 MIT. Even if the trailing aircraft is 15 miles behind the leader, with that overtake, it will take less than a half hour for the trailing aircraft to be 15 miles in front of the lead. Say the faster aircraft is going 400 knots across the ground, it only takes 200 NM or so to make this happen.

Now 200 NM is about half the distance between the western and eastern boundaries of ZAU, for example. Some facilities have much more airspace, and therefore time, to effect the required MIT; others less. Sometimes you can shortcut the trailing aircraft to accelerate the overtake, sometimes you can use speeds to make it happen. Sometimes it just works out that you'll have to slow everyone down to the leader. That happened to me at my rw facility where I had a CRJ2 that was simply too many miles in front of the other aircraft to justify turning it. I didn't have the room to use speeds and offloading to another arrival stream wasn't an option. I had the pilot maintain his maximum mach number and slowed everyone else down to match. Sometimes the luck runs out.

[Charan Kumar]

I too get asked this by pilots. I will let you be, if you are heading to one of my airspace airports or if there is no controller next sector. The moment a controller logs on, I ask the controller first and then it’s vectors or speed based on what the controller requests. 99% of the time the pilot will just comply because they also would have noticed the new controller. Sometimes you are not in range yet, but we can see on the scope and just take some advanced steps. 
 

But if I notice a gnd speed difference, then you be on your merry way without a concern.