What is the vertical speed limit when altitude mode is engaged?

Question

This question applies to Airbus' autopilot but may not be restricted to just this manufacturer (there must be equivalent modes on ATR, Embraer, Dassault Falcons, etc.)

For the vertical plan, the autopilot offers different modes. Among those, there are V/S where it tries to keep the selected vertical speed, and Alt, where it tries to keep the selected altitude.

When in Alt mode, if the aircraft is not at the required altitude, then the autopilot commands a vertical manoeuvre to reach the desired altitude. My question is: during this manoeuvre, is the vertical speed limited and if is this limit configurable in flight by the pilots? (If not, I imagine it is limited to what can be achieved in open climb or open descent modes.)

EDIT: The question comes from the hypothetical situation where aircraft is flying towards a target altitude and the pilots want to limit the vertical speed for any reason (e.g. a better vertical separation with another traffic)

Answer 1

What you are describing is the managed climb mode of the Airbus, where the Flight Management Computer (FMC) determines the optimum climb speed and the autoflight system will then pitch to maintain this speed.

From the A320 FCOM (Autoflight - AP/FD Vertical Modes):

Climb Mode (CLB)

CLB mode guides the aircraft in a managed climb, at either a managed or a selected target speed, to an FCU selected altitude, taking into account altitude constraints at waypoints. The system also considers speed constraints if the target speed is managed. [...]

The pilot can arm the CLB mode during the takeoff, go around, climb and cruise phases and engage it during the climb and cruise phases.

The vertical speed is therefore determined by the difference of the current and the target airspeed. All protections like stall protection based on angle of attack still apply, which may limit the actual climb rate.

The same logic also applies when descending instead of climbing, where the aircraft will maintain a given descent speed by pitching at idle thrust instead of climb thrust.

Once you get close to the selected altitude, the ALT* mode will take over, which follows a vertical speed instead of pitching for speed:

Altitude Acquire Mode (ALT*)

ALT* (green) mode guides the aircraft to acquire the FCU selected altitude.

ALT* (magenta) guides the aircraft to acquire an altitude constraint provided by the Flight Management. Once the aircraft has reached the altitude, the altitude mode (ALT green or magenta) engages.

Engagement Conditions

The mode engages when the aircraft reaches the altitude capture zone defined by the aircraft vertical speed (among other parameters).

[...]

Guidance

The ALT* mode has internal V/S guidance that is a direct function of the difference between present altitude and altitude target. This V/S internal guidance has priority over the target speed. Therefore if a thrust reduction occurs in ALT* (Engine out), the vertical speed order will not be modified and a speed variation will occur. The ALT* mode does not have a mode reversion that results from speed variation.

[...]

ALT* and ALT CSTR* modes have internal protections that decreases the vertical speed when VLS or VMAX is reached (VLS or VMAX becomes the priority target).

[...]

The system switches automatically to ALT (altitude hold) when the altitude deviation becomes less than 20 feet.

The vertical speed is therefore only limited by the internal guidance logic, but will be overwritten when VLS or VMAX are reached providing the usual flight envelope protections.

Answer 2

The situation behind your question, "the aircraft is flying towards a target altitude and the pilots want to limit the vertical speed for any reason," would arise when an aircraft is climbing or descending at a high V/S:

Though TCAS algorithms have become far more sophisticated, there are cases where the vertical closing rate could cause an issue with legally cleared traffic not far above (climb case) or below (descent case) the target altitude. In such case, pilots were cautioned to limit their V/S approaching the target altitude.

Here's an extract from a Eurocontrol document – ACAS II Bulletin N°21 (February 2017):

Key learning points this issue:

• All RAs must be followed. The traffic in visual contact may not be the threat that triggers the RA.
• TCAS II will generate RAs in 1000-foot level-off encounters if aircraft approach their cleared levels with high vertical rates or due to combined vertical rates.
• To reduce the chance of unwanted RAs, reduce the vertical rate to 1500 ft/min. or less in the last 1000 feet before level-off. Particularly, if another aircraft is at an adjacent flight level.
• TCAS II will issue RAs against all threat aircraft with altitude reporting transponders.

(consult the latest documents and procedures for actual inflight use)

V/S mode could be used, but it maybe more expedient to just override the auto throttle clutch for a few moments during the capture phase of the level out at end of a climb. During the descent case most systems would already have declutched the auto throttle allowing the pilot 'manual' control, i.e the ability to push the throttles without changing the engaged modes and the auto throttles would automatically re-clutch upon altitude capture. Changing the vertical mode to VS shortly before level off does not fit well with the FMC based automation philosophy and these automated systems are getting more and more sophisticated to accommodate such eventualities.

As an example one some Boeings (e.g. 747-400 and 777) the rate of climb during go-around is limited to 2000fpm by limiting the thrust to the thrust required to meet the required gradient, even though the airplane may be capable of 3 or 4 times greater than this (note: this limit is easily overridden by another press of the TO/GA switches).