Would you, or someone else who flies biz jets, tell me what performance rules you chaps use please.
I'll answer that. Light transport category airplanes use the same performance rules that large transport category aircraft use. The training standard is the same, too. Most have reduced thrust takeoff calculation data available. The same SOP's apply in business class turbojet aircraft that apply when flying a B373 or 747. The posters arguing to the contrary are doing so based on their own ideas, and certainly not what's been taught them at well recognized and industry approved training centers.
Still getting my head around the flex concept, particularly the bit where u do one set of calculations, then recalculate for the (higher?) temp.
There exist several programs for operating on reduced thrust. Assumed temperature is just one of them. It's used in business class turbojets as much as large Boeing products. As you know, an aircraft will experienced reduced takeoff performance with an increase in density altitude. With an increase in temperature, we experience an increase in density altitude. We experience an increase in takeoff distance. If we then compare that distance to our present field and find that we can meet all the safety requirements to take off (including stopping and going, appropriate climb gradients, obstacle clearance, etc), then we can operate with a thrust setting that equals that same performance.
As an example, using example numbers which are for illustration only, we have a 10,000' runway. We calculate a full thrust takeoff and find we can takeoff in 5,500'. We then run an assumed temperature thrust reduction analysis, and determine that at 40 degrees C, we would use 7,000' of runway. Our peformance calcualtions show that we can reduce power enough to use that 7,000' of runway and still climb out, or stop from V1, safely using the reduced thrust. The assumed temperature is a reference number only, and at no time are we prevented from pushing up the power to maximum thrust if required. However, the performance data assumes the reduced performance, and shows that we are able to lose an engine at that lower thrust and continue the takeoff with the reduced thrust to give all the necessary performance (climb gradients, etc).
Normally reductions based on assumed temperature are done up to 25% of maximum thrust (meaning 75% thrust avail). Other systems are available to calculate reduced thrust. We use a computer based program that takes everything into account including specific conditions, NOTAM'd runway conditions, etc. We don't use more than a 25% reduction. A whole litany of conditions exist and are clearly spelled out in our Aircraft Operations Manual detailing when we can and can't use reduced thrust. We have the option at any time at pilot discretion of using full thrust, and we're required to do so at least every seven days for no other reason than that's one of our requirements; a full thrust takeoff logged every seven days.
Assumed temp is just one method of determining a reduced thrust setting. Rather than an arbitrary number, it's a reference that compares takeoff performance under a known given set of conditions (increased density altitude), in order to start with meaningful data to compare to the runway and departure conditions in use. If the performance experienced at the assumed higher temperature could still work right here, right now in these conditions, then we can reduce thrust to replicate the same distances and performance we'd experience at that higher temp. In so doing, we reduce engine operating temperatures, save wear, tear, money, fuel, and still meet every margin of acceptable safety.