Mode Reversions and Speed Protections

Of the five ways a mode gets selected (article 06), the most easily missed is the one where the system selects for you. A reversion is not a failure — it is a design: when the basis of managed guidance disappears, or your target contradicts the current motion, or the envelope is about to be breached, the system retreats to an always-available basic mode and hands the choice back to you. Articles 07–09 met reversions one by one; this article assembles the master tables and lays out the whole speed-protection family. The standard annunciation first — the canon behind every "reversion triple" mentioned so far. Per FCOM DSC-22_30-40-40:

When the reversion occurs: ‐ The triple click sounds ‐ The new guidance mode is boxed for 10 s ‐ The FD bars flash for 10 s.

1. Three root causes

Pile up every reversion clause in the FCOM and only three root causes remain (synthesis):

Faster than memorising clauses: at any reversion, ask contradiction, lost basis, or protection? — the answer points straight at the recovery action.

2. Master table I — FCU altitude changes

Per FCOM DSC-22_30-40-110 (the FCOM's own reversion table, rearranged):

Conditions Event Consequence OP CLB engaged CLB engaged FCU selected altitude set below A/C altitude OP DES engaged DES engaged FCU selected altitude set above A/C altitude ALT* active FCU selected altitude modification (greater than 250 ft) V/S-FPA engages.

Three rows, one common thread: when the target contradicts the direction of motion, the system does not guess — it drops to V/S–FPA on the current rate and waits for you to declare yourself. The full script continues in the pieces you already have: the 5-second acknowledgement window (pull ALT / set a V/S / push ALT — article 07), and the 45-second pre-selection trap lurking inside that window (article 08).

3. Master table II — basis lost

The V/S–FPA section carries the consolidated list; learn this one clause and you have learned the book. Per FCOM DSC-22_30-40-70:

A vertical mode reverts to V/S / FPA, when one of the following occurs: ‐ OP DES, or DES is engaged and the FCU selected altitude is set above the aircraft altitude ‐ OP CLB, or CLB is engaged and the FCU selected altitude is set below the aircraft altitude ‐ DES is engaged and one of the following mode disengages: NAV, LOC*, LOC, F-LOC* , F-LOC , LOC B/C*, LOC B/C ‐ FINAL APP is engaged and NAV disengages

(The list continues: profile lost in DES/FINAL APP; the LOC family dropping out from under the G/S family; deselecting glide-type modes via the LOC/APPR pushbuttons; the ALT-capture 250 ft case; and a change of approach — which kills FINAL APP at any time, the G/S family below 700 ft RA, and the F-G/S family at any time.)

DES's own three, for the record. Per FCOM DSC-22_30-40-60:

DES reverts to V/S / FPA, when one the following occurs: ‐ The flight crew sets the FCU selected altitude above the aircraft altitude ‐ One of the following modes was engaged, and disengages: NAV, LOC*, LOC, F-LOC* , F-LOC , LOC B/C*, LOC B/C ‐ The vertical flight plan is lost.

Compare CLB's three (article 07): FCU below → V/S–FPA; profile invalid → OP CLB; phase slips to DES/APPR → OP CLB. The asymmetry is deliberate: a climb that loses its profile can safely continue climbing openly (the direction is still right), but a descent that loses its profile is not granted an unconstrained idle plunge — it gets pinned at the current rate instead. Different directional risk, different conservatism.

The lateral side (article 09): NAV at a discontinuity or with an invalid plan → HDG/TRACK, dragging the vertical chain with it (CLB → OP CLB; ALT CST*/ALT CRZ*/DES → V/S–FPA; ALT CST → ALT; constraints void). And when all APs and FDs drop, re-engagement lands on the basic pair HDG/TRK + V/S–FPA — the floor under everything.

One phase reversion carries a deliberately reduced annunciation. Per FCOM DSC-22_30-40-90:

When SRS GA reverts to OP CLB, the following occurs: ‐ The triple click sounds ‐ The new guidance mode is boxed for 10 s.

Two items, not three — no flashing bars. It is an expected handover (go-around acceleration altitude), not an alarm.

4. The speed-protection family — four fences

Fence ① — built into the capture modes. ALT*/ALT CST*/ALT CRZ* internally reduce V/S when VLS or VMAX is touched, making the limit speed the priority target (article 07); DES runs its speed-margin ledger (+20/+5/VMO−5 — article 08). Character: silent — no mode change, the target quietly yields.

Fence ② — the AP/FD speed protection (the V/S–FPA and OPEN/CLB/DES worlds). The AMM gives the full parameter sheet. Per AMM 22-11-00:

The speed protection is applied, according to the mode, in order to protect against Vmin or theta max (=22.5°) when climbing or Vmax when descending with the following load factor limits: - 0.3g max versus VMO - 0.15g max versus VFE or VLE (when landing gear is down).

The protected modes are OP CLB, OP DES, CLB, DES, ALT*, and V/S-FPA except when a level off has been performed.

- Vmax is VMO(MMO)-1kt in clean configuration, VFE/VLE+4kt else - Vmin is VLS if Vc target >or= VLS +5kt or Vmin is VLS-5kt if Vc target =VLS

Indication (article 05): the triple click plus a flashing amber box around the V/S (FPA) mode and target on the FMA — a protection variant of the reversion triple: the mode has not changed, the target has been overridden, so the box is amber, not white.

Fence ③ — the TCAS-mode V/S protection (article 15 owns the mode; the protection rows belong here). Per FCOM DSC-22_30-40-110:

Excessive V/S target in climb IAS reaching VLS (or VLS - 5 kt if target is VLS or VLS - 2 kt if target is VLS in landing configuration) Excessive V/S target in descent IAS reaching VMAX The TCAS RA V/S target is temporarily disregarded to maintain VLS in climb, or VMAX in descent.

Note: When flying with FD bars only (AP OFF), the FMGES adjusts the pitch so that VLS in climb, or VMAX in descent is maintained. However, no triple click is generated and the V/S target displayed on the FMA remains unchanged.

Read that note twice: it is the chapter's only deliberately silent protection. Hand-flying the bars during an RA, the commanded pitch has already changed (speed is being protected) while the FMA still shows the old V/S target — no click, no box. The philosophy matches the TCAS mode's whole design (no flashing, no aural, don't distract the avoidance manoeuvre). The only way to notice: when the bars and the FMA target disagree, believe the bars — that is the protection pulling you.

Fence ④ — the A/THR layer. The A/THR reverts THRUST → SPEED/MACH when the speed escapes while thrust is fixed: below VLS−2 in DES/OP DES, above VMAX+4 in CLB/OP CLB — triple click, and if an FD was engaged it drops with it (article 17). The FGE's pre-emptive version — kill the FD so the A/THR can flip to SPEED — is article 05's clause. And the last fence of all, ALPHA FLOOR, fires TOGA regardless of A/THR state (article 17).

5. The family at a glance

6. The standard response

1. Call it — the white or amber box and flashing bars are the callout trigger: "V/S plus five hundred — reversion" (the "flying now" answer just changed — article 06).
2. Name the cause — contradiction, lost basis, or protection (section 1).
3. Declare yourself — contradiction cases give you the 5-second window (pull/set/push); lost-basis cases need the basis restored (clear the discontinuity, DIR TO, check the profile) or a deliberate stay in selected; protection cases: do not fight the protection — recover the energy first.
4. Rebuild managed if still wanted — push ALT/HDG, confirm the FMA goes green.

Frequent matches: told to "maintain present level" in a descent, you wind the FCU above yourself → DES reverts to V/S (section 2). A late approach change on final → FINAL APP drops to V/S (section 3's change-of-approach clause; article 11). Dual FM failure → with FG healthy, the AP holds HDG-V/S or TRK-FPA (article 19).

[!warning]- Four misconceptions this article corrects (1) A reversion is not a malfunction — it is the system declining to guess, retreating to modes that always work, and returning the choice to you. (2) CLB and DES do not revert symmetrically — a profile-less climb continues as OP CLB, a profile-less descent is pinned at V/S; the conservatism follows the directional risk. (3) Not every reversion gives the full triple — SRS GA → OP CLB omits the flashing bars, and the FD-only TCAS speed protection is entirely silent with a stale FMA target: believe the bars. (4) The amber flashing box is not the white news box — amber means the mode stayed but its target was overridden by a protection.

Self-test

[!note]- Q1. The three reversion annunciations — and which two reversions come with fewer?

Triple click + new mode boxed 10 s + FD bars flashing 10 s. SRS GA → OP CLB omits the flashing bars (click + box only); the TCAS-mode speed protection with FD only is fully silent, FMA target unchanged.

[!note]- Q2. The three rows of the FCU-altitude reversion table — and the ALT-capture red line?

OP CLB/CLB engaged + FCU set below the aircraft → V/S–FPA. OP DES/DES engaged + FCU set above → V/S–FPA. ALT capture active + FCU changed by more than 250 ft → V/S–FPA.

[!note]- Q3. Profile lost: what does DES fall to, what does CLB fall to — and why the asymmetry?

DES → V/S–FPA; CLB → OP CLB. A climb without a profile can safely keep climbing openly; a descent without one is not granted an unconstrained idle descent — it is held at the current rate. Conservatism scales with directional risk.

[!note]- Q4. The AP/FD speed protection's exact Vmax/Vmin — and the protected-mode list?

Vmax = VMO(MMO)−1 kt clean, VFE/VLE+4 kt otherwise. Vmin = VLS when the target is at least VLS+5, or VLS−5 when the target is VLS. Protected: OP CLB, OP DES, CLB, DES, ALT capture, and V/S–FPA (except after a completed level-off). Correction authority: 0.3 g against VMO, 0.15 g against VFE/VLE.

[!note]- Q5. Hand-flying FD bars during an RA, the speed protection activates — what do the FMA and aurals show, and how do you detect it?

Nothing: no triple click, and the displayed V/S target remains unchanged while the FMGES quietly adjusts pitch to hold VLS (climb) or VMAX (descent). Detection: the bars command something different from the FMA target — believe the bars.

[!note]- Q6. The two speed gates for the A/THR's THRUST → SPEED reversion?

Below VLS−2 kt in DES/OP DES; above VMAX+4 kt in CLB/OP CLB. Triple click; an engaged FD drops with the reversion.

[!note]- Q7. A change of approach kills which vertical modes — and with what height condition for the G/S family?

FINAL APP at any time; the G/S family below 700 ft RA; the F-G/S family at any time (function not installed on the baseline airframe). The fall is to V/S–FPA.

Key takeaways

References

Reversion annunciations per FCOM DSC-22_30-40-40; the FCU-altitude reversion table and the TCAS V/S-protection rows (including the FD-only silence note) per FCOM DSC-22_30-40-110; the consolidated V/S–FPA reversion list per FCOM DSC-22_30-40-70; DES reversions per FCOM DSC-22_30-40-60; SRS GA reversion per FCOM DSC-22_30-40-90; the lateral chain map per FCOM DSC-22_30-40-30 (article 09). AP/FD speed-protection parameters per AMM 22-11-00; A/THR reversion gates per FCOM DSC-22_30-50-50 (article 17). The three-cause taxonomy and the fence panorama are integrative syntheses of the quoted clauses. F-LOC/F-G/S and LOC B/C mode names are quoted for completeness; the functions are not installed on the baseline airframe of this series.

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.