Well spotted, mutt...
Quote from me:
Actually, using OAT or VREF as the assumed temperature is pointless - you might as well select Rated Thrust in the conventional manner.
Quote from mutt:
Chris, what VREF are you talking about?
Good to know someone is paying attention. It was a typo - I meant TREF. In case I've confused anyone, I had better explain. If my memory serves, TREF is the highest temperature for the maximum thrust rating (the right end of the flat part of the line). On the day, OAT can be higher, or lower.
[Hope no one minds, but I'm going to EDIT my post to remove that error.]
Quote from mutt:
Yep they are, in fact the pencil is so sharp that Boeing will offer you 4-8000 kgs additional takeoff weights in some hot/high airports, same aircraft, same AFM, just using a sharper computer powered pencil. [Unquote]
Sorry for any confusion, but I was not suggesting that the old manual method enabled us to get a HIGHER RTOW than a computer might calculate. Quite the reverse.
That's why I wrote: "We always used to use the "thickness of the pencil" on the safe side. I don't think computers are capable of doing that!"
That was in response to FE Hoppy's anecdote about the Nimrod crew, who got 4 different answers to the manual calculation, and sensibly decided to use the most conservative (least optimistic) figures.
When a human is running a "pencil" horizontally along the graticule of the graph, it's very easy to slip the line - up or down. It's usually safer to err on the 'down' side...
I say again, a computer would never do that. So a computer may remove that EXTRA margin of safety that you or I might instinctively apply, but which we would not admit to our company commercial department! The regulations, of course, already build in fat in most cases (e.g., only 50% of a headwind to be taken into account, etcetera..).
But, when you load the data into the FMC performance-computer, make sure you don't over-estimate the strength of that headwind!