Aviation In Canada

Wednesday, June 29, 2005

OMNI Separation

This topic is an interesting one, and is widely regarded as an elegant method of getting an outbound aircraft through an inbound aircraft in a non-radar environment without actually putting them through one another. There are uses as well that go beyond opposite direction aircraft. I'll try to explain a little bit.

The basics are the radials. I'll assume a certain level of knowledge of VORs when I write this, so please feel free if you want me to explain this part a little more.

The radials have to be 15° apart, and not necessarily even numbers like the 010R and the 025R. It's often more convenient, since an aircraft is normally tracking an airway or proceeding directly to a VOR anyway, to start with a radial that is already in use. In the Moncton FIR, the most common place we use OMNI separation is at CYGR, Les Iles de la Madeleine. The airway most commonly used there is V382, which is the 332R. So with an aircraft inbound from Gaspe (CYGP), we would generally make sure he is on the 332R and get a departure out either west of him (this would be 15° west, meaning no higher than the 317R), or east of him (clockwise, higher than the 347R).

It's obvious that there has to be at least one other condition when you think about this. Since the radials actually converge at the facility, having two aircraft established on different radials at 2 DME isn't going to give you much separation. That condition is actually 15 NM. If the two aircraft are established on their respective radials and will pass outside of 15NM from the VOR facility, then you have established OMNI separation, a form of lateral separation. This is, of course, only useable for opposite direction aircraft in this sense. Thus, a vertical restriction is specified to ensure that the aircraft will pass outside of 15NM before discontinuing vertical separation. For those who have flown this and remember hearing a different number, ATC uses different values depending on altitude if using DME to prove the 15NM. Most commonly, OMNI is used at lower levels, and therefore 17DME is the value most commonly referred to.

For example, the inbound aircraft is on V382 (the YGR 332R) from CYGP, while someone wants to depart and head to CYGP also on V382. The inbound aircraft is at 11,000, but wants descent. The outbound aircraft wants to climb to 14,000. The clearance for the outbound aircraft would read something like, "... maintain 14,000. Make climb from 4,000 to 12,000 established on the YGR 315R, not above 4,000 til 17 DME." The aircraft is cleared up to his flight planned altitude, simply because we can. His restriction for the radial climb means that he can't climb above 4,000 until he's established outbound on the 315R (which, incidentally, was chosen as a nice round number that gives at least 15° separation) and that he may not leave the 315R until he reaches 12,000, which is 1,000 feet above the inbound's current altitude. As the inbound aircraft descends, we can ask him for altitude reports; as he leaves 8,000, for example, we can let the departure go off the 315R and on course out of 9,000. This holds our established lateral separation until vertical is proven. As for the DME restriction, the outbound aircraft may not climb above 4,000 until he's on the radial AND outside 17 DME. This restriction is on there in case the outbound aircraft is slower than expected in getting away, allowing us to keep our vertical separation since the lateral provided by OMNI doesn't exist inside the 17 DME mark. Once our departure is airborne, we look at the position of the inbound (well, we ask for a position report) and determine if they will pass outside of 17 DME. If yes, then we can break the 17 DME restriction right off the top and allow the departure to climb, since OMNI lateral will be provided at the point where they pass. Once altitude reports indicate they have passed vertically and are separation by 1,000 feet, we can now cancel the radial restrictions on both, clearing the departure on course and, perhaps, clearing the inbound for an approach.

There are other cases where OMNI can be useful, such as when two departing aircraft are heading in almost the same direction. We can use 30° between them under certain circumstances, but this rarely seems to be a useful application, since other forms of separation are easier to set up and prove. OMNI can also be useful for two inbound aircraft when, for instance, the further is faster and you want to let him overtake the closer one. This is a bit tougher with today's aircraft and their normal speeds, though, as not only does the 17 DME apply again, but there is an additional condition: The second aircraft has to be at least 10 minutes from the facility by the time vertical is established. A restriction would be issued including a time to make good an altitude, giving the opportunity to let the pilot figure out if he can make such a restriction.

Anyway, there are the basics. OMNI is your friend, provided you get to know it.