Originally Posted by
B737900er
I never could quite understand the climb or descent 500ft if your able to maintain your FL.
If you can maintain your flight level why would you leave the track? If your diverting you would need to descend to below FL285 anyway, or am I missing something?
I understand that descending below FL285 will avoid other tracks but surely staying high as long as you can at +/-500ft would save on fuel and get the same result i.e avoid traffic?
If your ultimate strategy is to divert to an airfield `abeam` or an airfield `behind you` then you will have to leave the track centreline to ensure separation from other OTS traffic.
Because your eventual diversion route is not (yet) subject to a formal ATC clearance then changing level by +/- 500 ft (assuming you were cruising below FL410 and not thrust limited) will be necessary to give you some degree of vertical separation from other traffic.
You are NOT obliged to descend below FL285, even if diverting `across` the OTS (weather issues i.e. CB below you, or shortage of fuel?). You could divert from Track `D` to Keflavik maintaining FL355 if you wanted to (NAT doc 007 refers).
If your intention is to divert to an airfield ahead of you, then it may not be necessary to leave the NAT track in the first place. Because you are then complying with an existing oceanic clearance, if you still have sufficient thrust, no change in FL would be necessary.