Quote from
sispanys ria:
ISA + 15 flat rating doesn't make sense to me.
It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
[Unquote]
I would wager that there is a high proportion of pilots who have little idea what the thrust ratings of their engines are, without looking them up. We just don't need to know it. The people who calculate the Performance graphs/charts/tables/FMC-algorithms, of course, DO.
411A is right, charts or tables have been a quick and accurate way of calculating the Flex Temps and V-speeds. On my last type (I retired 6 years ago), T
MAX-FLEX was ISA+57, and our tables went right up to that level. There was shading to show that anything above TMAX was for Assumed (Flex) Temperature calculations only, not for actual OAT, and that the lowest permitted Flex Temp was TREF (the top end of the flat-rating).
In an earlier incarnation, on VC10s, we copilots used to calculate the Flex Temp (we called it something else) and all the other T/O performance data from a handbook of Perf A
graphs (like you might still do in an ATPL exam?). Sounds difficult, particularly at some third-world airfield in the middle of the night, but we used to get very good (and quick) at it, using a proforma provided for the purpose. If the aeroplane was very light, even T
MAX (we didn't have TMAX-FLEX in those days) permitted a range of possible V1s, and some of the more pernickity captains woud ask us to calculate them. If the departure airfield had no engineering facilities, the lowest V1 would be chosen, so we had to make sure it was above VMCG.
"You guys have it easy nowadays..."
