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Climb/Cruise Performance


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I attempted to climb to FL400 at a 37,000lbs gross weight, but I was well under 100fpm climb at about FL390. This was using a climb profile of 250/M0.72 at -10 ISA. I descended down to FL380 and accelerated to M0.80 with no problem. I decided to climb back to FL400, and I'm managing to maintain FL400, but I'm only able to accelerate past M0.72. This should be possible at this GWT, ISA, and climb profile according to the Operations Reference. I crosschecked this with a CL604 QRH and I'm seeing similar book numbers. I double checked the GWT in the XP default W&B menu and it only differs by a couple hundred pounds. I just wanted to report this to make sure this isn't a bug. Thanks again for the wonderful aircraft!


EDIT: I saw the comment Toto made to a similar issue about maximum N2 in regards to TAT. However, it seems that if I disconnect ATS and go to max power, I'm not meeting that close to the maximum N2 shown in the chart. Perhaps this is why I'm struggling to accelerate? I was seeing about 87% N2 maximum when I should be seeing around 90% N2 maximum for the given TAT.





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As I've learned recently, this is indeed intended behavior with very low TATs. To put it extremely simply, the maximum N2 the engines can achieve is limited with temperature, becoming less as TAT decreases. You can find a graph of the engine fuel control in the "Fuel Control Map" study menu. I believe there will be a graph put into the operations reference showing the maximum *corrected* N2 values for a given TAT. Make sure to keep an eye on the TAT during the climbout and keep the speed up to have an easier time accelerating once you reach cruise.


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If you look carefully she isn't making the rated thrust.

There is quirk with this aircraft in that if you fly too slowly in cold temps, the FADEC believes the air density is higher than it is, so reduces thrust. The problem is the air is not that dense, so it is no longer producing the *required* thrust. Consequently, you decelerate, which causes it to further reduce thrust.

By flying faster, you increase TAT, which makes the FADEC think it is warmer, and so it increases thrust on the engine enabling it to maintain higher speeds at higher altitudes.

I found it better to climb at 280/0.76.

If SAT is very low (ISA-10 or lower) I find it beneficial to cruise at Mach 0.8 rather than anything faster as it doesn't have the thrust. I had a TAT of -48 the other night (ISA-18).

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