Another great fuel saving feature on the Airbus is SOFT ALT.
The FMGS engages the “soft altitude” (SOFT ALT) mode 2 minutes after the cruise flight level is captured and Mach speed stabilized.
(SOFT ALT) mode allows the aircraft to deviate ± 50 ft from the target altitude by limiting A/THR authority, by setting a maximum rate of thrust target change to minimize the thrust variation and reduce the fuel consumption.
This mode is automatically set 2 minutes after these main conditions are met:
• A/THR active in MACH control law
• ALT Track mode engaged in Cruise phase
• Delta-Speed (difference between CAS and speed target) < 4 kts.
If engaged, it disengages as soon as:
• Mach target is changed
• Or ATHR or AP logic changes
• Or CAS reaches VMO -5 kts
• Or during 1 minute Delta-Speed > 4 kts & the estimation of altitude excursion to the Delta-Speed is > 50 ft (in term of aircraft total energy)
A newer logic changes the conditions based on the gap between target and actual speed
It disengages above 10kts in 4s and above 4kts in 10s for faster response of the ATHR system to prevent a very high speed excursion.
Author – Arjun Singh
