If FLEX is still a reduced set of thrust, why can you select Max. Thrust with FLEX but you can't with D-TO?
You are still missing the difference between flex and derate - if you are at the derate setting, you
ARE ALREADY at Max Thrust.
Think in the following way -
(a) for an aircraft, the certification set of rules is addressed considering the maximum power (thrust) which the engine is able to produce.
If, by some means, you operate at power output levels
higher than those used for the certification then you invalidate the certification basis for the aircraft ie you are not allowed to do this.
(b)
full power T/O - if you conduct a T/O using the maximum power permitted from the engine, you are conducting a full power T/O
(c)
flex - you
may operate the engine at a lower power setting by using less than full throttle. This is what you are doing in a flex T/O - conducting the T/O
intentionally at a power setting less than the maximum that the engine is capable of producing on the day. That doesn't cause any certification problems (and, providing you have done the sums correctly, won't produce any operational problems). It follows that you will not produce any certification problems if, during the T/O you elect to return to the maximum power output envisaged by the certification basis.
(d)
derated T/O - now pull the original engine out and install a modified engine of smaller power output capability. This is quite common within a family of aircraft (motorcars, etc). If you now conduct a T/O using the new full T/O power capability, you are operating to the maximum power available for this particular model engine but, compared to the higher powered version, it is a lower power output, for which the term we use is
derated (ie the new max thrust rating is less than [hence derated from] the higher/highest rating. However, for the derated engine, if you are operating at max power (thrust) you are operating at full thrust
FOR THAT DERATE.
(e)
derated and flex T/O - exactly as at (c), you
may operate the derated engine at a lower than maximum (now the derated level) power setting by using less than the throttle setting necessary to achieve the derated power output level. It follows that you will not produce any certification problems if, during the T/O you elect to return to the maximum power output envisaged by the certification basis. However, this maximum power level is now the derated level, not the original engine's higher level.
If you exceed the derated level, you will invalidate the certification basis for the aircraft. Keep in mind that derate requires the OEM to schedule separate AFM data for the derate ie you have, in effect, multiple AFMs and you must use the one appropriate to your chosen level of derate on the day.
(f) with an aircraft engine, we want to be able to have our cake and eat it too - ie sometimes we want to operate to the maximum rating and, on other times, to a derated rating. However, we don't physically swap engines back and forth - that just wouldn't work in practice.
What we can do to achieve the same thing, in principle, is constrain the engine
not to operate at power outputs higher than the declared reduced rating level. This can be done either by computer control or manual setting of lower power output using the engine gauges.
In doing this we are conducting a derated T/O -
intentionally operating a bigger (higher thrust) engine at a lower certificated power setting. This must be done in a manner to emulate installing a lower power engine ie you must not permit the engine to operate at a power output higher than the new, lower certification rating
otherwise you invalidate the certification basis for the derate setting.
Does this seem to be a round about way to achieve a goal ?
Absolutely so. Wouldn't it just be simpler to keep reducing the flex setting and not worry about the intermediate step of setting a derate setting ? Probably, but there are a couple of important things which prevent us using that simpler approach in the first instance and would deny us an advantage in the second.
(a) maximum flex setting. The rules prevent our using more than a specified maximum flex thrust reduction. See, for example,
FAA AC25-13 which amplifies the FAR 25 requirements that the actual thrust be no less than 75% of rated thrust. Given that, with some of the larger engines, it is feasible to use significantly greater flex settings than this, we need to use the "trick" of derated flex to get past the initial certification restriction.
(b) as the certification is based on the maximum power output, if we use a derate then we can get some side benefits, such as a reduction in Vmcg/Vmca. Such a benefit shows up in a reduced minimum T/O speed schedule .. which may require a lesser TOD for low weight takeoffs, allowing better payload out of shorter runways.
[Caveat - I have used power and thrust interchangeably but intend power for turboprops and thrust for turbofans]
Is it because Vmc speeds with FLEX are not calculated to the respective thrust reduction, but to TO/GA thrust?
Precisely - and the important thing is that you can have several sets of Vmcg/Vmca data, provided that the OEM choses to schedule these in the AFM.
you can continue on TO/GA up until the time limit is almost up, whether it is 5 or 10 mins
really a matter for the operator to choose for routine use.
In the event that you REALLY need the extra thrust on the day because of some out of left field consideration, I suggest that the 5/10 minute limit is not going to be very high in your list of priorities on the day. A case of whether we are talking routine or emergency conditions.