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Applicable to:
AIRCRAFT BELOW THE DESCENT PROFILE
If the aircraft is below the descent profile, its speed will be maintained at target speed until it
reaches the descent profile. The lower margin becomes effective when the aircraft is on the
descent profile but has to loose speed in order to stay on it.
The intercept point on the navigation display is based on the following assumptions:
�� If the aircraft is flying an idle segment:
The FMGS maintains V/S = -1 000 ft/min and target speed, until it reaches the constraint
altitude or intercepts the profile.
�� If the aircraft is flying a geometric segment:
The FMGS maintains a constant flight path angle (half of the theoretical FPA of the profile)
until it intercepts either the altitude constraint or the profile.
A318/A319/A320/A321 FLIGHT CREW
OPERATING MANUAL
AIRCRAFT SYSTEMS
AUTO FLIGHT - FLIGHT GUIDANCE
AP/FD VERTICAL MODES - DESCENT MODE
A318/A319/A320/A321 FLEET DSC-22_30-70-50 P 19/22
FCOM 19 AUG 10
Applicable to:
,
,
MSN,,,
AIRCRAFT BELOW THE DESCENT PROFILE
If the aircraft is below the descent profile, its speed will be maintained at target speed until it
reaches the descent profile. The lower margin becomes effective when the aircraft is on the
descent profile but has to loose speed in order to stay on it.
The intercept point on the navigation display is based on the following assumptions:
�� If the aircraft is flying at an altitude that is higher than both the descent speed limit
altitude and the destination elevation +5 000 ft:
The FMGS maintains the V/S at -1 000 ft/min and the target speed, until the aircraft reaches
the altitude constraint, or intercepts the descent profile.
�� If the aircraft is flying at an altitude that is lower than either the descent speed limit
altitude, or the destination elevation +5 000 ft:
The FMGS maintains the V/S at - 500 ft/min and the target speed, until the aircraft reaches the
altitude constraint or intercepts the descent profile.
Applicable to:
,
,
MSN,,
4538,,
VERTICAL DEVIATION (VDEV) MANAGEMENT CHARACTERISTIC
In some cases, when the aircraft is above or below the vertical profile, the ND may not display any
intercept point (pseudo waypoint), even if the VDEV appears on both the MCDUs and PFDs. In
addition, the FMS guidance may not immediately attempt to decrease this VDEV (contrary to flight
crew expectations).
This is due to the fact that the FMS guides the aircraft along the extension of the subsequent
segment (N+1), when all of the following conditions are met:
The aircraft is above ‐ or below the descent profile
‐ The DES mode is active, or the FINAL APP mode is armed
‐ The aircraft intercepts the extension of the subsequent segment (N+1) of the vertical profile,
before the aircraft reaches the initially expected segment (N), as illustrated below.
The VDEV will gradually decrease until the aircraft reaches the beginning of the subsequent
segment.
A318/A319/A320/A321 FLIGHT CREW
OPERATING MANUAL
AIRCRAFT SYSTEMS
AUTO FLIGHT - FLIGHT GUIDANCE
AP/FD VERTICAL MODES - DESCENT MODE
A318/A319/A320/A321 FLEET DSC-22_30-70-50 P 20/22
FCOM 19 AUG 10
Applicable to: ALL
LEVELING OFF AT A CONSTRAINT
If the aircraft levels off at an ALT CSTR, the DES mode arms and remains armed until the
aircraft passes the constraint, then reengages (if the FCU altitude is set below the altitude of the
constraint).
If the FCU selected altitude is that of a constraint, the flight crew may continue the descent below
that altitude by turning the ALT knob and pushing it in. This arms the DES mode, which reengages
when the aircraft passes the constraint waypoint.
Applicable to: ALL
GUIDANCE IN A HOLD
Just before the aircraft enters a holding pattern, the speed target becomes the holding speed.
In the holding pattern, the DES mode commands V/S = -1 000 ft/min while A/THR maintains the
holding speed. The aircraft will level off at the next altitude constraint if it is reached during the
hold.
The current vertical deviation VDEV is based on the altitude at which the aircraft is supposed to
cross the exit fix in order to be properly positioned on the descent profile.
Until the flight crew exits the hold, the FMGS in DES mode will maintain V/S = -1 000 ft/min
considering downpath vertical constraint. This means that the aircraft will not descent below the
next altitude constraint, neither the FCU selected altitude. If the aircraft reaches the next altitude
constraint it will level off and ALT CST mode will engage.
A318/A319/A320/A321 FLIGHT CREW
OPERATING MANUAL
AIRCRAFT SYSTEMS
AUTO FLIGHT - FLIGHT GUIDANCE
　
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