Connetts
A most worthy question. There is specific language that describes the process, but the first step is to divide the flight into the planning phase and the operational phase. During planning a number of factors govern the decision about which en-route divert fields to use. For a twin like the 777 one would consider the mechanical status of the aircraft and the weather and accessibility of the proposed divert field (Is the runway open? Can the aircraft be safely handled?). Other factors along with these determine whether the flight will be 60, 120, or 180 minutes from a suitable alternate. There is a special 204 minute etops used for North Pacific/Alaska but it is very much a case by case decision). I do not know of any operator of the 777 who has more than a 204 minute etops allowance. I know Lindbergh flew for hours on one motor over the water, I prefer not to...
Great. You launch off and now in operation before you enter the etops area you and the dispatcher make sure that the proposed alternates are still usable. If not, you make another plan.
If you are already in the etops area and one of the alternates becomes unusable, you again make a new plan. This of course raises the question of what to do if another suitable alternate is not available. Different companies have different language for this but the overarching intent of the rules is that the captain and dispatcher are required to determine if the flight can safely be continued on its current routing--a different track might be required.
Long answer I realize but it is something that crews and dispatchers pay particularly keen attention to during the planning and operational phase of the flight. Not only is it legally mandated to do so, but it has to already be in mind when dealing with an emergency.
Had Ascension not been available for the Delta diversion, the crew would have had another field in mind--Liberia etc. St. Helena does not yet have an airport.
Apparently there are statistics out that show that more diverts are caused by problems other than engine difficulties. Perhaps, but having shut an engine down in flight on a 777 I would prefer to review the mathematics from a nice comfortable chair on the ground.
Last edited by Uncle Fred; 11th Jan 2013 at 23:48.