Copyright D.P. Browne 2011
The altitude hold mode causes the aircraft to fly at a target altitude. The altitude is maintained throughout the speed range. The target altitude is either the FCU selected altitude, or an Flight Management constraint altitude.
Arming
Altitude Hold ( ALT, ALT CST, ATL CRZ )
The altitude hold mode is automatically armed whenever the aircraft climbs or descends towards the target altitude. The FMA displays the armed mode as ALT, in either blue or magenta. The Target altitude is also shown on the altitude tape on the PFD, again, either in blue or magenta.
When the target altitude is the managed altitude constraint from the Flight Management
When the target altitude is the selected altitude from the FCU.
Engagement
The altitude hold mode has two sub modes, and acquire sub mode and a hold sub mode. The acquire sub mode allows the aircraft to modify the pitch angle to slow the rate of climb/descent in order to be able to hold an altitude. The altitude hold sub mode engages when the altitude is difference between the aircraft and target is less than 20 feet.
Altitude acquire occurs when the altitude rate and the Delta Altitude meet the capture criteria.
Disengagement
The altitude hold mode disengages when any other vertical mode engages. Note that the FCU altitude must be selected to a different value from that being held, in order to engage a Climb/Descent mode.
Guidance
Should the A/P be engaged when the F/D mode is in Alt Hold, then the A/P commands the aircraft to the target altitude if it is within 250 ft of that target ( A/P engagement 1). Otherwise the A/P levels the aircraft out , ( A/P engagement 2)
When altitude acquire occurs, the FMA shows ALT * and the V/S is gently bleed to about 100 ft/min
At 20ft difference from the Target Alt, the Alt hold mode engages.
The Alt hold mode holds the Barro uncorrected alt plus the remaining difference between Baro corrected alt and the target alt at the Alt Hold engagement event. Therefore it is immune to changes of baro correction at the Nav Control Panel
Altitude Hold Cruise
After 120 seconds of holding the Cruise altitude as defined on the Progress page
of the MCDU, the Flight Guidance mode changes to soft altitude hold mode. The FMA
changes from ALT to ALT CRZ. In ALT CRZ, the aircraft is allowed to deviate by +/-
