Another extra major factor influencing fuel burn happens only in the hot humid summers in the US. Our contingency fuel does not include this fairly frequent
problem.
Last summer in Houston (IAH), we waited about an hour and a half for departure, but do to the inability for the old APU to keep the cabin/c0ckpit even halfway cool
, I let an engine run at idle for the entire delay, which involved waiting at two different runways. Only the east departures were directly affected, but even northbounds were held a long time.
We were about 5-10 minutes from a min. fuel situation when we took off, and this lasted for the entire flight. Halfway to the destination, I sent a message to Dispatch stating that if ATC gave us any delays enroute, anywhere, we would divert to whichever midwestern airport was nearby, at least near the destination (mostly good weather was north of DFW/OKC). Being late to the destination resulted in almost no traffic in front of us.
You always decide to draw a line in the sand somewhere when necessary, after looking at all factors, including ideas of the other pilot(s) and Flight Engineer, if you are fortunate enough to have one behind you. Many of you are not required to choose between a liveable temperature and making it to the dest., with no accurate info on how much contingency you WILL use, just to keep semi-cool.
Only many years of experience help us anticipate just how much into that extra, new safety "corner" we might go. Any "mission-oriented thinking" is no longer on the "list". Winter offers different factors, not just the question on whether we will be inside the de-icing holdover time, or can safely land at the next nasty frozen airport with a landing weight of about 108,000 pounds and a direct 15-knot crosswind.