AZFlyer

I just got back from Tokyo and it got me wondering a bit about navigation and separation for ocean crossing flights.

I understand that there are specific routes or tracks that are used and that their specific locations can vary depending on weather and time of day (east vs west flows).

Mostly, I'm just curious how separation between aircraft is maintained and controlled. About halfway through the flight back to the U.S. I noticed a contrail very close to our right and it stayed there for a good 2 hours or so, which led me to believe that we were in trail behind a plane not far ahead, and thus the question about maintaining separation when you're thousands of miles from a radar site.

We non-revved on Delta and my girl and I were put in business class. Very comfortable flight! Many thanks to the excellent Delta cabin crew.

edit: in hindsight this should probably be in the 'Technical' forum.

HSLD

Oakland and Anchorage CTA/FIR have used a 30nm lateral separation test (change from 50nm) since 2007, although 50 nm is still sometimes used depending on RNP ability of the aircraft. Vertical spacing is 1000 ft and on track in-trail spacing is 10 minutes when using an assigned mach number.

The contrails that you saw were most likely 1000 feet above or below. If they were at the same alt, the preceding aircraft had a least 10 minutes on you.

This guy passed us last week a 1000 feet above:

Twin Wasp

And the NoPac track tracks don't move like the North Atlantic ones do.

AZFlyer

Hmm. I had no solid visual reference point for distance, but it appeared to be only slightly higher than our altitude and very large (i.e. close) in the window.

Curious also, by what means do you navigate when over the ocean? (type of waypoints, GPS? INS? etc)

Thanks for the input.

HSLD

That's right for airways like R220 or G580, although there are daily tracks built similar to the NAT tracks. I tend to see the random tracks eastbound, and the airways westbound.

Wingtip vortices sink at around 300-500 fpm and stabilize (at altitude) around 800-1200 feet below where they are generated. I'd guess contrails might follow that profile too. As for the navigation question, it depends on the type of aircraft and what airspace it operates (RNP). The plane I fly has several ring laser gyros, two GPS receivers, VOR, and ILS all tied into a box that looks at all the inputs and blends those inputs to arrive at the most probable location. That's the position it feeds to the FMC.

bcrosier

HSLD has pretty well nailed it - the contrails become entrained in the wing vortices, which sink below the generating aircraft's flight path. They were most likely slightly off to the side due to the wind (remember, the air mass is moving, but both you and the aircraft in front of you are navigating from one fixed point to another, so the vortices and entrained contrails drift where the wind takes them (in your case to the right of your flightpath).

Also, regarding the longitudinal separation, we use what is know as "Mach Number Technique." Essentially, everyone is assigned a specific Mach number to fly, which insures that we will maintain the 10 minute separation from preceding/trailing aircraft.

Over the years there have been a variety of navigation systems utilized (Doppler, Omega, LORAN), but as HSLD said, most newer aircraft use multiple sensors. Some of those utilizing blended position, others select what the computer determines to be the most accurate position. Older aircraft often still use triple INS/IRS, though as time goes by more and more of them are having GPS added to improve navigational capability. Ironically, it's more of an issue over land where closer tolerances are required (RNP-2, RNP-1, or less for approaches) than over water (where RNP-5 or RNP-10 is the standard). What is RNP you ask - if you have too much free time on your hands:

Required navigation performance - Wikipedia, the free encyclopedia

As far as how things are controlled - it's essentially the same way it was done before RADAR. We report our position, time at that position, altitude, and time estimate for our next waypoint to air traffic control. Actually, usually to a High Frequency radio operator who relays it to the air traffic controllers. The kids with new toys like SATCOM use CPDLC (Controller-Pilot Data Link Communications). The air traffic controllers monitor this to insure every one is where they are supposed to be, and can issue instructions to change altitude, routing, or cross a fix before or after a specified time to maintain separation.

In reality, ATC works very hard to get all of their little ducks (us) in a row, on speed and altitude while we're still in radar contact so they don't have to make non-radar adjustments out over water. In over ten years of flying in oceanic airspace and hundreds of crossing, I can probably count on one hand the number of times I've had to make any changes enroute due to ATC.