There's lots of interesting stuff being mentioned here with regard to how much gas we're planning to arrive at destination with, but the fact of the matter is, that we don't have access to the 'crystal ball' or 'twenty-twenty hindsight' whilst at the planning stage prior to departure. It's my belief that it's the 'experience' and good judgement by the commander that will win the day.
You have only to experience a few departures from the US Eastern Seaboard and inbound for an early arrival slot into LHR to become acquainted with the issues in hand. You will certainly expect a minimum of twenty minutes holding. A departure from JFK will most certainly mean that you will be taxying for up to one hour prior to departure. The opposite can also happen... some years ago pushing back from Philly we actually taxiied straight to the departure runway, so we were airborne within fifteen minutes of pushing back (and not the expected one hour!). Having got airborne and looked at the FMS arrival time we found ourselves to be one hour early; plus the fact that slack tailwinds were in fact not true, but a fifty knot tail component... great news; but not really, because we were now well within the approach ban time prior to 6.00am. 'Company' wouldn't allow us to arrive early and, although we had the extra fuel on board, we couldn't hold for the one hour, so we were directed to land at Shannon for a refuel and go; therefore arriving into LHR approximately forty-five minutes later than our scheduled arrival time. Deterioration of the Shannon weather from being CAVOK to a howling crosswind right on the B747 limits didn't make life any easier... during the landing (still in the dark and lashing with rain) the No.2 engine reverse surged and pushed out a huge plume of flame; witnessed by a newspaper reporter sat in the aft of the aeroplane, who got straight onto his mobile to report the incident directly to his newsdesk! The news was immediately reported on UK TV 'breaking news' of a B747 landing in flames after a transatlantic crossing... the company chairman witnessed the news whilst he was having breakfast at home! As you can perhaps well imagine, all hell broke loose within company headquaters... the moral of the story is; even with the extra gas on board, it won't guarantee you having a nice day! Missinformation, either by a computer screen or a 'nurd' sat in seat-row 49A trying to get a news scoop, won't be discounted until the actual story is released by those in the know... even on PPrune by all accounts! The Irish authorities kept us in Shannon for the day/night whilst the incident was investigated... we 'blacked' the runway; not withstanding the fact that the wind increased in force to warrant the closure of the airport for a few hours! As for the crew... just a routine day at the office. The Guinness was fabulous!
TCF