De-rate & assumed Temp takeoff thrust
I apologise if this has been thrashed out previously, but I missed it; and having recently changed performance tables a question has been generated, not least by me but some students also.
There are also different rules on their use by different national authorities.
26K engines with fixed de-rates of 24K & 22K.
Not only is a fixed de-rate used, but then an assumed temp from that de-rate.
What is the benefit of this rather than using assumed temp from the top power setting of 26k? There must be an assumed temp from 26K = 24K or 22K. How low do you want to go? Is it that the 25% or CLB rule (ref 26K)will come into play before reaching the lowest possible thrust setting?
I can understand that, on some runways, the a/c performance can be satisfied by the smaller 22K engine, (the larger 26K being there for runway flexibility) and then derated. However, is there an assumed temp from 26K that equals 22k. If so, this would require only one piece of paper per falp setting instead of 3. We have 3 pages for every dry flap setting, then another 3 for wet, and then another 6 if there might be another flap option. 12 pages per runway. Surely 1 page at 26K and using only assumed temp would suffice. In UK the fixed de-rate + assumed temp is allowed, but in some continental countries it is not?? In the latter it is only assumed temp from full power.
The former seems cumbersome, the latter simple and perhaps therefore less likely to error. Certainly saves on paper and space in the books.
Equally puzzling is that under certain conditions reduced thrust is not allowed: i.e. systems failure or contamination, but a fixed de-rate with no assumed temp is allowed. In other words, you've got a 26K engine, you've got some restricting problems, but it's OK to downsize the engine to 22K if RTOW tables allow it. Capt's discretion is still in force.
Thoughts?