Well.. DES is a Managed vertical mode. It can only be set when in NAV lateral mode. DES mode is the only time that you will get the Managed Speed target range (normally ± 20 Kts, limited by Vmo – 3 Kts or Mmo – 0.006 or if a descent SPD LIM or a Speed constraint is encountered, the speed is limited to the Constraint Speed + 5 Kts) displayed on the PFD Speed tape.There are three basic cases that can occur when in the DES mode:
On profile:
The aircraft is descending on the FM calculated vertical profile and so VDEV will be about zero.
By setting DES when on profile you command the AP/FD to fly the target SPEED/MACH (Managed Speed target range). The Speed is now controlled by using the elevator to change the pitch attitude by the AP/FD. As the AP/FD is controlling the SPEED/MACH the A/THR cannot – so it must control the THR(ust), which it does and sets THR DES.
Above Profile:
The aircraft is descending but is above the FM calculated vertical profile and so VDEV will be positive.
By setting DES when above profile you command the AP/FD to fly the target SPEED/MACH (Managed Speed target range and it will increase up to the upper limit of the range). The Speed is now controlled by using the elevator to change the pitch attitude by the AP/FD. As the AP/FD is controlling the SPEED/MACH the A/THR cannot – so it must control the THR(ust), which it does and sets THR IDLE in an attempt to intercept the profile from above.
Below Profile:
The aircraft is descending but is below the FM calculated vertical profile and so VDEV will be negative.
By setting DES when below profile you command the AP/FD to fly the trajectory to intercept the profile from below. The Managed Speed target range will be displayed but it will use the set Managed Speed only, not the lower limit of the range. The trajectory (or path) is now controlled by using the elevator to change the pitch attitude to set a V/S of either 1,000 ft/min (Idle segment) or 500 ft/min (Geometric segment). As the AP/FD is controlling the trajectory the A/THR must be in SPEED/MACH mode in an attempt to intercept the profile from below.
Remember GIGO (Garbage In/Garbage Out). The FMGS will only be able to achieve what you input into it. If you put crap in, you can expect a less than optimal performance.
Hope you are a little bit closer now then.. The mode interact with several System wich is helpfull to understand before you jump in to the DES mode!