Flex < MCT; 10 minutes; and 25% ±
Like FE Hoppy, I don't think Flex Thrust has to equal or exceed MCT, but can't quote chapter and verse yet. On the A310-200 with CF6-80A3 engines in 1984, "TLIM" (TMAX-FLEX) was ISA+57. After using it for a normal take-off with A/THR, and subsequently selecting CLB thrust via the TRP, the throttles usually advanced very slightly, reflected by the N1. This always seemed to discredit the benefit of using so high an assumed temperature, and I don't remember it happening on other jet types.
So in that case, the engine was in practice often using slightly less than CLB thrust for take-off. CLB was, of course, lower than MCT at low altitudes, but we still complied with the 10-minute rule. I suppose that it would be impracticable to have one rule for a Flex temp of ISA+57, and another for ISA+20 (say), when the thrust would exceed MCT.
Like mutt, I have always assumed that comparing the TMAX-FLEX of various turbofans would give a fairly good indication of the percentage of maximum thrust-reduction, as most of them seem to be flat-rated up to a similar TREF of about ISA+15 to 20. I now notice that the CFM-56 on our A320s had a TMAX-FLEX of ISA+46, 31° above its TREF of ISA+15; whereas the CF6-80A3 had values of ISA+57 and ISA+18 respectively, a greater difference at 39°. This might explain why the CF6-80 at maximum thrust reduction seemed to use slightly less than CLB thrust for take-off, whereas the CFM-56 did not; but my course notes suggest that both engines are supposed to be limiting thrust reduction to 25%. So I must admit to being a bit perplexed on this one.
Last edited by Chris Scott; 2nd November 2008 at 01:31.