Another N. Atlantic Track question

[av8boy]

In that my ATC experience has all been tower and approach, and I’ve got only the slightest clue when it comes to operations in oceanic sectors, a question came to mind yesterday…

I happened to notice that there was more than a little NAT traffic with a final altitude of FL280. If I recall, there were a couple of DC10/MD11s and other similar airframes. I’m guessing that they were stopped at that level because they didn’t meet MNPS criteria for higher, but that’s just a guess. If I’ve guessed correctly, can someone please explain to me the rationale for continuing to fly heavy aircraft across the pond at these lower altitudes? There must be a trade-off somewhere between the cost of refitting the airframe to meet the spec and fly higher, and additional fuel costs brought about by the lower altitude, but I don’t have the expertise to quantify it. Have I missed something here? Is the additional fuel burn less than significant? Is the cost of refitting that high? Are these aircraft that are on their way out anyway, meaning that the cost of the retrofit isn’t worth the benefit of the higher altitude over the rest of the short life of the airframe? What am I missing?

If this question has been addressed before, I apologize.

Thanks,

Dave

[Human Factor]

I’m guessing that they were stopped at that level because they didn’t meet MNPS criteria for higher, but that’s just a guess.

Perhaps but not necessarily. Operators will file a flight plan for the most efficient track and level for their sector. However, at the planning stage there is no co-ordination between operators so potentially you can end up with two (or more) aircraft with the same ETA and level at the Oceanic entry point. It's then up to the Oceanic controllers to co-ordinate how they deal with this traffic.

Often, if you have filed at FL330 (say) you may be offered higher or lower in order to maintain your route. Depending how heavy you are, your maximum altitude may be quite restrictive forcing you to go lower than you'd like. Alternatively, you may be offered a re-route in order to get a higher level. If you're going across at a peak time, it can easily be possible to get four or five aircraft at a particular entry point within a couple of minutes of each other and clearly they have to be at different levels.

Hope this helps. An Oceanic controller may be able to do a better job....

[av8boy]

Thanks.

I saw the NAT advisory for yesterday and the altitudes were the traditional, higher-than-FL280 levels I've come to expect. Of course, there's always the remarks section which says...

MNPS AIRSPACE EXTENDS FROM FL285 TO FL420. OPERATORS ARE REMINDED THAT SPECIFIC MNPS APPROVAL IS REQUIRED TO FLY IN THIS AIRSPACE. IN ADDITION, RVSM APPROVAL IS REQUIRED TO FLY BETWEEN FL290 AND FL410 INCLUSIVE.


...which I read as a clue rather than the word of God when it comes to wondering about traffic at FL280.

Alternatively, you may be offered a re-route in order to get a higher level. If you're going across at a peak time, it can easily be possible to get four or five aircraft at a particular entry point within a couple of minutes of each other and clearly they have to be at different levels.
Which makes sense. What seemed odd to me, however (and probably something I would have mentioned before if I'd remembered) was that two of these aircraft were MD11s from the same company, and one was a couple of hours ahead of the other--both at 280. What's more, the one that caught my eye was probably 300 miles away from the nearest traffic, and that traffic was at FL340 (IIRC). In other words, I could see no traffic-related reason for staying at 280 (however, I'll be the first to admit that I didn't have the whole picture on this... obviously there was a LOT going on that I wasn't aware of!).

I appreciate the info, HF. Don't get me wrong... I'm not taking issue with what you've described. It's just that it still seems like it was something else going on.

Thanks again,

Dave

[PPRuNe Radar]

The aircraft may have also been going against the 'flow' on the tracks. In which case, their clearance will be below or above the track levels which are reserved for the aircraft going in the opposite direction.

If they were freighters, it's pretty plausible.

[CR2]

Or in a hurry? We've sometimes gone for M.87 "low" since we needed the aircraft back pronto & to heck with the burn off.

[tug3]

If you don't want to follow a track structure and route on a 'random' track, you'll generally have to fly above or below the tracks, or at the very least in the lower or upper 30s. For example, this could account for a flight requesting F340 being cleared at F280 if the requested route infringed 3 or more tracks. This of course depends upon time of day, traffic volume, conflicting traffic, etc.

One other possibility is that the aircraft in question may not be RVSM approved, hence the F280. This would come back to your question of cost of avionics upgrade vs fuel savings at higher FLs.

Forecast turbulence can also account for unusual altitudes e.g. AA B777s have requested and got F260 as their Ops deemed it too bumpy at higher FLs.

Rgds
T3