Microburst2002;
Some very good explanations here. I'll chuck my 2c in.
THR DES mode is a SPEED/MACH mode. It is like the DES mode in the A320.
The mode has a +/-20kt speed latitude from the calculated target descent speed in order to energy-manage the calculated descent path, essentially to conserve fuel, (same idea as any manual descent we've ever done in any other airplane, by either speeding up a bit or slowing down a bit or if that wasn't enough, by either adding a bit of power if slightly low or using speedbrakes if slightly above what we wanted).
You can imagine the FMGEC-calculated descent path as a geometric, "hard line" in the sky so to speak, and the autoflight/autothrust tries to keep the airplane on that hard vertical path.
The mode is intended to manage aircraft energy and conserve fuel. The FMGEG's calculate the required descent path taking into account all altitude and speed constraints in the programmed arrival route as well as the pilot-entered winds, and sets a descent point.
The speed range with which THR DES manages aircraft energy is plus or minus 20kts from the calculated (target) descent speed. You already know what the airplane does if above or below the calculated path...it speeds up (and the descent rate increases) if above the path or it slows down (and the descent rate reduces) if below the path.
If too high and the speed is at the upper limit, the A/THR mode reverts to THR IDLE, and if still too high the "EXTEND SPD BRK" message is displayed on the top of the PFD. If too low, the pitch is increased slightly to use aircraft energy to reduce the descent rate to stay on the path. If the lower limit is reached, thrust is added because, as stated, the THR DES mode is a SPEED/MACH mode.
PJ2
Last edited by PJ2; 9th Jun 2012 at 15:58.