Recalculating an ETP after a track change

[pythagoras]

Hi - I was just wondering what is the best way of quickly and simply recalculating an ETP when given a track change - taking into account wind. I have heard several different methods of doing this and thought I would see what the general consensus was here.

Thanks

[TopBunk]

Good question!

I will ask my next co-pilot how to do it:E

OK my starting suggestions:

I assume that you are talking about a situation like on the Atlantic when you get a different NAT track to that initially planned?

Let's say you get NAT B rather than NAT A (1 deg further south) and you are looking at EINN then BIKF or Lajes then CYQX.

I would argue that practical aviation would suggest that you draw a tangent cutting through your new track line at right angles that passes through the original ETP on the original track. I really can't believe that it would be out by more than 1 minute or so.

An alternative solution would be to look at the ratio of the distance on the original track from say EINN to ETP : ETP to BIKF, and apply that ratio to the new track. I guess that would apply a better wind correction factor. Again I don't think the difference would be too significant.

[BOAC]

the ratio of the distance - that's the way I've done it on several re-routes, and of course your carefully calculated fuel calcs are 'oot the windy':)

[rab-k]

From the ATC point of view, it seldom exceeds 5 minutes either way from the original estimate and on occassion is no different whatsoever.

While we're on the subject - any clue as to why some crews will accept a 6000' drop just to stay on the same track, while others are prepared to change track to keep their requested FL as opposed to say a 3000' drop on the FPL track? Is it simply down to company SOP's? Just curious...