Vertical Modes I — Takeoff, Climb, and Altitude Capture

From setting takeoff thrust to levelling in the cruise, the vertical axis runs a relay: SRS TO hands over to (OP) CLB, constraints interpose ALT CST*/ALT CST, and the last leg ends in ALT CRZ*/ALT CRZ. This article walks that relay with the exact engagement, protection and reversion clauses. (RWY/RWY TRK, the lateral half of takeoff, is in article 09; the descent family is article 08; the reversion system as a whole is article 10.)

 ground TOGA ──► SRS TO (vertical) + RWY/NAV (lateral)
    ACCEL ALT ──► CLB (needs NAV)          [no NAV / no profile → OP CLB]
    constraint ──► ALT CST* → ALT CST (level, waiting to sequence)
    waypoint sequenced ──► CLB re-engages, climb continues
    approaching cruise FL ──► ALT CRZ* → ALT CRZ (+ A/THR SOFT mode)

1. The altitude triplets — one law, three employers

ALT, ALT CST and ALT CRZ read almost identically in the FCOM because they are the same capture/hold machinery; only the target's owner differs:

The shared internal mechanics, quoted from the ALT section. Per FCOM DSC-22_30-40-50:

‐ Has an internal V/S guidance that is a direct function of the difference between present altitude and the altitude target ‐ Has an internal protection that decreases the vertical speed when VLS or VMAX is reached. (VLS or VMAX becomes the priority target)

‐ In the case of TA, in order to avoid spurious RA, automatically adjusts the V/S to reach the target altitude

The system switches automatically to ALT mode when the altitude deviation becomes less than 40 ft.

Capture rate scales with remaining altitude difference; touching VLS or VMAX makes speed the priority target; a TCAS TA gentles the closure automatically (the "1500 ft/min in the last 2000 ft" discipline of article 15, done for you in the capture leg); and below 40 ft of deviation the star is dropped — capture becomes hold.

2. SRS TO — the speed reference system

Per FCOM DSC-22_30-40-20:

The SRS mode is a managed vertical mode, used during takeoff and during go-around.

The SRS TO controls the speed via the elevators, in order to steer the aircraft along a vertical path.

Speed via the elevators: at takeoff the thrust is a fixed setting (TOGA/FLX, FADEC's business — article 16), so pitch is the only lever left, and SRS uses it to hold a speed. Three protections are built into that trade:

‐ A flight path angle protection, that ensures a positive climb slope of 0.5 ° ‐ A pitch angle protection to reduce the aircraft nose-up (17.5 ° of maximum pitch attitude or 22.5 ° maximum in case of windshear) ‐ A speed protection limiting the target speed to V2+15 kt.

However short of speed, never descend (0.5° minimum slope); however eager, no more than 17.5° nose-up — relaxed to 22.5° in windshear, where altitude is dearer than airspeed; and never chase more than V2+15.

The speed target evolves in three stages:

When the aircraft is on ground, V2 is the speed target. When the aircraft is airborne, V2+10 kt becomes the speed target. If one engine fails, the speed target is the current aircraft speed at the engine failure detection. However, the speed target is limited by V2 and V2+15 kt.

Read the engine-failure clause twice: SRS does not command back to V2 — it memorises the speed you had when the failure was detected (your energy as-is), merely clipped into [V2, V2+15]. That is the mechanism behind "don't rush the nose down chasing V2 after an engine failure", and it matches the engine-out speed references in article 06.

Engagement — and the no-V2 trap. Per FCOM DSC-22_30-40-20:

SRS TO engages, when all of the following conditions are applicable: ‐ The aircraft is on ground ‐ The slats or the flaps are extended ‐ V2 is inserted in the PERF TAKEOFF page

(plus a thrust lever at TOGA or in the FLX-MCT detent). And:

If V2 has not been inserted, V/S mode engages 5 s after lift-off on the current V/S value.

No V2, no SRS takeoff — you lift off into V/S at whatever rate you happen to have, with the FCU speed window frozen at the engagement-moment speed (article 02). Conversely:

When V2 is inserted, or SRS TO is engaged, the speed automatically becomes managed.

Disengagement:

SRS TO mode disengages, when one of the following occurs: ‐ Automatically, at the ACCEL ALT, or if ALT* or ALT CST* mode engages (above 400 ft RA), or ‐ Another vertical mode engages.

In Engine Out conditions, the SRS mode does not automatically disengage at EO ACCEL ALT.

Engine-out, SRS waits for you — the handover happens when you are ready (push the SPD knob or pull for altitude), not at a pre-set gate. The manual exit:

SRS TO reverts to OP CLB, when the FCU selected altitude is above the aircraft altitude, and the flight crew pulls the SPD/MACH knob.

(A reversion, so it arrives with the triple click, the 10 s box and the flashing bars — article 10.)

3. OP CLB — climb with the FM muted

Per FCOM DSC-22_30-40-40:

OP CLB mode is a selected vertical mode. OP CLB guides the aircraft vertically to the FCU selected altitude. All altitude constraints are disregarded.

Straight to the FCU altitude, constraints silenced — remember the prefix rule: OP = the FM's vertical opinion muted; only the FCU is heard. Speed via pitch, thrust fixed by the A/THR in THRUST mode — the same energy logic as SRS (article 16). Small changes are softened:

If the altitude change is less than 1 200 ft, the aircraft responds with a rate of climb of 1 000 ft/min.

Arming mirrors CLB's — that mirror is the exam point:

OP CLB arms, when all of the following conditions are applicable: ‐ The aircraft is on ground, or SRS is engaged (TO or GA), and ‐ NAV is neither armed nor engaged, or the vertical flight plan is not available, and ‐ The ACCEL altitude is available and below the FCU selected altitude.

In a world without NAV, the ACCEL-altitude successor is OP CLB. Engagement:

OP CLB mode engages, when one of the following are applicable: ‐ The FCU selected altitude is above the aircraft altitude, and the flight crew pulls the ALT knob, or ‐ SRS TO or SRS GA is engaged with OP CLB armed, and the aircraft reaches the ACCEL altitude.

OP CLB is always associated to an altitude mode.

That last line means the FMA's second column always shows an armed altitude under OP CLB: ALT cyan when the FCU altitude differs from the cruise level, ALT CRZ cyan when it equals it.

4. CLB — the managed climb along the profile

Per FCOM DSC-22_30-40-40:

CLB mode is a managed vertical mode. CLB guides the aircraft vertically along the FMS flight plan, to the FCU selected altitude or to the next altitude constraint, whichever comes first.

CLB can only be engaged, when NAV is engaged.

The vertical profile hangs off the lateral plan — lose the route, lose the "along what" of the climb. One boundary of managed climbing is easy to mis-remember:

The FG system does not adjust the speed/Mach target to match the altitude constraints.

Constraints are honoured by shaping the trajectory, never by pinching the speed target (if geometry can't satisfy a constraint, the FM raises its messages — article 24). Arming requires the FCU altitude above the aircraft and the ACCEL altitude below the lower of FCU altitude and next constraint; and CLB re-arms while ALT CST* is engaged — the system already queuing the climb resumption. The ACCEL altitude itself is displayed and modifiable on the PERF TAKEOFF and PERF GO AROUND pages (default written by the AMI — article 04). One instantaneous special case:

Note: If the aircraft altitude is within the capture zone of the FCU selected altitude, CLB will immediately disengage, and ALT* will engage.

And CLB's three reversion clauses (the climb row of article 10's master table):

‐ The flight crew sets the FCU selected altitude below the aircraft altitude: V/S / FPA engages ‐ The vertical flight plan becomes invalid: OP CLB engages ‐ The FMS flight phase changes to DES or APPR phase: OP CLB engages.

FCU wound below you — the climb target is nonsense, fall to V/S. Profile invalid or phase logic derailed — managed climbing has lost its basis, fall to the "muted" version. Net: CLB and OP CLB differ in exactly two things — constraints (honoured vs disregarded) and prerequisites (NAV vs none). The speed/thrust pairing is identical.

5. ALT* and ALT — capture, precisely

Per FCOM DSC-22_30-40-50:

ALT* mode guides the aircraft to acquire the selected FCU altitude. ALT mode maintains the selected FCU altitude.

Why the cyan ALT is almost always present:

ALT arms, with FD/AP engaged, when the aircraft climbs or descends toward the FCU selected altitude. However, ALT does not arm, when one of the following modes is engaged: G/S*, G/S, F-G/S* , F-G/S , FINAL APP, LAND, FLARE, ROLL OUT.

Any time you are moving toward the FCU altitude, ALT waits armed — except on final: glide-family modes and the landing script must not be interrupted by an altitude capture (article 11, article 13).

Now the four numbers that make ALT* the most tightly-timed mode in the AFS:

ALT* engagement is inhibited during 3 s after the selected alititude is changed on the FCU.

The aircraft is at the altitude target when the difference between the aircraft altitude and the altitude target is less than 40 ft for more than two seconds.

ALT* reverts to V/S / FPA when the flight crew changes the FCU selected altitude by more than 250 ft.

If within 5 s after reversion to V/S / FPA, to confirm the altitude target change, the flight crew does not: ‐ Pull the ALT knob, or ‐ Set a new V/S / FPA target, or ‐ Push the ALT pb on the FCU. Then: ‐ The triple click sounds ‐ The new guidance mode is boxed for 10 s ‐ The FD bars flash for 10 s.

String them into one story: change the altitude (3 s cooling-off before capture can start — your window to re-wind a mis-set value passing through present altitude); mid-capture, change it by more than 250 ft and the target is yanked away — reversion to V/S on the current rate; you then have 5 seconds to acknowledge (pull ALT, set a new V/S, or push ALT — any one counts as "I know"); silence buys you the triple click, the box and the flashing bars. Capture completes at less than 40 ft held for 2 seconds.

One more note for transition altitudes:

Note: If the baro setting is changed during ALT* mode, this may lead to an FCU target overshoot due to the change of the current value of the altitude. However ALT* mode will enable the FCU altitude to be regained.

Re-setting QNH/STD mid-capture steps the measured altitude, and the aircraft can overshoot before ALT* reels it back — avoid baro changes inside the capture leg when you can.

6. ALT CST — the constraint edition

Per FCOM DSC-22_30-40-50:

ALT CST* guides the aircraft to acquire an altitude constraint. ALT CST maintains the altitude constraint delivered by the FMS.

Same law, different employer — hence the magenta arming. Its exclusive clauses:

‐ The aircraft sequences the waypoint, for which the altitude constraint is associated

— the first auto-disengagement: sequence the constraining waypoint and (with CLB re-armed and waiting) the climb resumes. The "level at a constraint, then climb past the fix" behaviour is exactly this clause.

‐ The flight crew removes the altitude constraint: CLB or DES engages

‐ NAV disengages: ALT engages

Delete the constraint and the profile re-connects instantly; lose NAV and the constraint loses meaning — the mode falls back to plain ALT semantics. For QFE-fitted aircraft (the baseline airframe of this series has the QFE option — article 01), the FCOM adds: a switching from STD to QFE (or QFE to STD) in ALT CST* mode, will change the target value and a reversion to V/S may occur if the target value is modified by 250 ft or more. — a reference switch is a target change, and the 250 ft tripwire applies.

7. ALT CRZ — same altitude, different identity

Per FCOM DSC-22_30-40-50:

ALT CRZ* mode guides the aircraft to acquire the cruise altitude, as selected on the FMS PROG page and on the FCU.

Once ALT CRZ engages, the A/THR SOFT mode activates (Refer to DSC-22_30-50-50 A/THR SOFT Mode).

ALT CRZ engages, when all of the following occur: ‐ ALT is engaged, and ‐ The aircraft altitude is equal to the FCU selected altitude, and ‐ The flight crew sets the cruise altitude to the aircraft altitude.

"Cruise" is defined jointly: PROG-page CRZ FL and FCU agree. The third quote describes an in-place promotion — level in ALT, set the PROG cruise level to your present altitude, and the FMA flips ALT → ALT CRZ without re-capturing (bringing the SOFT mode's ±3 kt thrust-soothing with it — article 17). The difference between ALT and ALT CRZ is therefore identity, not altitude: at the same level, the PROG page's opinion decides whether you get cruise treatment. Preparing descent and finding ALT where you expected ALT CRZ? First look: has the PROG CRZ FL been wound down (with the knock-on of premature descent-phase logic — article 08, article 24).

8. Flying the relay

Before takeoff, the FMA check belongs to the FCU setup: column 2 should show CLB armed in a NAV departure — and if it shows ALT cyan/magenta instead, recall article 06's note about clearances at or below the acceleration altitude before "fixing" anything.

The callout chain for a standard relay (synthesis — every call is traceable to a clause above): "SRS, RWY" at thrust set → positive climb, AP above 100 ft → at ACCEL ALT "CLB" with speed managed and clean-up on the speed tape → "ALT CST magenta" armed before a constraint → "ALT CST star" capturing → sequenced, "CLB" again → "ALT CRZ star … ALT CRZ" at the level. Armed with colour, active without (article 06).

Abnormal hooks, each developed in its own article: no V2 inserted → no-SRS takeoff (article 30); dual FM loss → CLB/DES unavailable, selected modes only (article 05); engine-out → SRS holds until you hand over, bank schedule tightens (article 06); baro-change overshoot in ALT* (section 5); TCAS TA softening in the capture leg (article 15).

[!warning]- Four misconceptions this article corrects (1) After an engine failure SRS does not fly you back to V2 — it holds the speed at failure detection, clipped between V2 and V2+15; don't push for V2 out of reflex. (2) OP CLB versus CLB is not "faster versus slower" — the only differences are constraints (disregarded vs honoured) and prerequisites (none vs NAV engaged); the speed/thrust pairing is identical. (3) ALT and ALT CRZ can be the same altitude — the difference is whether the PROG page recognises it as the cruise level (and grants SOFT mode), not the height itself. (4) A baro-setting change during ALT* can overshoot the level even though nothing is "wrong" — the measured altitude stepped, not the aircraft; keep reference changes out of the capture leg.

Self-test

[!note]- Q1. SRS TO's three built-in protections — numbers included. What does the pitch protection become in windshear?

Positive climb slope of at least 0.5°; maximum pitch 17.5°, relaxed to 22.5° in windshear; speed target capped at V2+15 kt.

[!note]- Q2. At the instant an engine fails during SRS TO, what does the speed target become — and its limits?

The current aircraft speed at failure detection, limited between V2 and V2+15 kt. Not a commanded deceleration to V2.

[!note]- Q3. Takeoff with no V2 inserted: vertical mode after lift-off, its target, and the FCU speed window?

No SRS. V/S engages 5 s after lift-off on the current vertical-speed value; the FCU speed target is the speed at V/S engagement — the windows hand you a flyable number.

[!note]- Q4. What must be engaged before CLB can engage, and what does "the FG does not adjust the speed target for constraints" mean?

NAV. Altitude constraints are met by shaping the vertical trajectory only — the FG never pinches the speed/Mach target to make a constraint; if geometry can't do it, the FM annunciates instead.

[!note]- Q5. CLB's three reversions — which one falls to V/S, and which two to OP CLB?

FCU altitude set below the aircraft → V/S–FPA. Vertical flight plan invalid → OP CLB. FMS phase slips to DES/APPR → OP CLB.

[!note]- Q6. Tell the 3 s / 250 ft / 5 s / 40 ft+2 s story of the ALT capture in one breath.

Change the FCU altitude: capture engagement is inhibited 3 s (re-wind window). Mid-capture change of more than 250 ft: reversion to V/S–FPA. You then have 5 s to acknowledge (pull ALT, set a V/S target, or push ALT) or the triple click sounds with a 10 s box and flashing bars. Capture ends — the star drops, ALT holds — when within 40 ft for more than 2 s.

[!note]- Q7. ALT CST magenta: where does the target come from? What happens on constraint deletion, NAV loss, and waypoint sequencing?

From the FMS (a constraint — magenta says so). Delete it: CLB (or DES) engages at once. NAV disengages: ALT CST becomes ALT. Sequence the constraining waypoint: ALT CST disengages and the armed CLB resumes the climb.

[!note]- Q8. Level in cruise, the FMA flips from ALT to ALT CRZ. Did the aircraft move? What changed?

No movement — the crew set the PROG cruise level to the present altitude, so the same level acquired cruise identity: ALT CRZ engages in place and A/THR SOFT mode activates.

Key takeaways

References

Takeoff section and SRS TO clauses per FCOM DSC-22_30-40-20; OP CLB and CLB per FCOM DSC-22_30-40-40; ALT/ALT*, ALT CST and ALT CRZ (including the 3 s/250 ft/5 s/40 ft parameters, TCAS TA adjustment, QFE-switching note and baro-overshoot note) per FCOM DSC-22_30-40-50. Engine-out speed references and bank schedule per FCOM DSC-22_20-10-50 (article 06); cruise-mode arm/capture/hold phases per AMM 22-10-00. The relay diagram and callout chain are integrative syntheses of the quoted clauses. FLS-family mode names appearing in arming lists (F-G/S) are quoted for completeness; the function is not installed on the baseline airframe of this series (article 01).

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.