Vertical Modes II — Descent, V/S and FPA

Descent is the geometrically busiest stretch of vertical guidance: the FM draws a profile, and the aircraft behaves completely differently depending on whether it is on it, above it, or below it. This article covers DES and OP DES with their speed-margin bookkeeping, the Latch philosophy, Too Steep Path, and the basic modes V/S–FPA with their pre-selection trap. (How the profile itself is constructed — where T/D comes from — is article 24; the reversion master table is article 10.)

          ┌─ above profile: idle held, speed drifts to upper limit
          │     (+20 kt or VMO−5; only +5 in a constrained region)
 T/D ──►  ├─ on profile (Latch): path has priority — speed is yours,
          │     via speedbrakes; cap VMO−5
          └─ below profile: fixed-rate convergence (−1000 ft/min;
                −500 near the field / below the SPD LIM altitude)

1. DES — two kinds of path, one Latch

Per FCOM DSC-22_30-40-60:

DES is a managed vertical mode. DES guides the aircraft vertically along the vertical flight plan. All the altitude constraints associated to waypoints are taken into account.

When DES is engaged in idle path, the AP/FD adjusts the pitch in order to maintain a speed/Mach target. The A/THR then maintains the idle thrust. When DES is engaged in geometric path, the AP/FD controls the vertical trajectory. The A/THR is then in SPEED/MACH.

Same green DES on the FMA, two different thrust roles underneath. On an idle path the descent is energy-managed: pitch holds the speed, thrust is parked at idle. On a geometric path (the fixed-slope segments between constraints) the pitch owns the trajectory and the A/THR switches to SPEED/MACH to own the speed. So FMA column 1 hopping between THR IDLE and MACH/SPEED during one descent is not a malfunction — it is the profile changing segment type under you (article 16's pairing table, in motion).

The soul of DES is the Latch clause:

The Latch symbol computed by FMS and displayed on the PFD, informs the flight crew that the vertical profile has priority over speed management. In case of steep profile, if the FMS is not able to maintain both speed and descent rate, the FMS maintains the aircraft on the vertical profile assuming the speed is managed by the flight crew with the use of speed brakes. The aircraft is “latched on the profile”.

This behavior is applicable to both managed speed and selected speed. There is no de-latch when the flight crew switches from managed speed to selected speed.

Latched, the path belongs to the FMS and the speed belongs to you and the speedbrake lever — and switching the speed to selected does not unlock it.

2. OP DES — idle straight to the FCU altitude

Per FCOM DSC-22_30-40-60:

OP DES is a vertical selected mode. When OP DES is engaged, the AP/FD adjusts the pitch in order to maintain a speed/Mach target (selected or managed). The A/THR then maintains idle thrust. All the altitude constraints are disregarded.

OP DES engages, if the FCU selected altitude is below the aircraft altitude, and the flight crew pulls the ALT knob.

The OP prefix keeps its article 07 meaning — the FM's vertical opinion muted. Its reversion is the familiar shape:

OP DES reverts to V/S / FPA, if the flight crew sets the FCU selected altitude above the aircraft altitude.

If within 5 s after reversion to V/S / FPA, the flight crew does not confirm the altitude target change by another expected action, then: ‐ The triple click sounds ‐ The new guidance mode is boxed for 10 s ‐ The FD bars flash for 10 s.

— the same 5-second acknowledgement window as the ALT-capture reversion in article 07.

3. Arming DES — and starting down

Per FCOM DSC-22_30-40-60:

DES can only arm when: ‐ The altitude on the FCU is below the aircraft altitude ‐ NAV is armed, or engaged ‐ The FMS flight phase is cruise, descent, or approach ‐ The vertical flight plan path is valid.

One more gate than CLB had: the FMS flight phase. If the FM does not yet consider you in a descent-capable phase, managed descent is simply not on offer. And on final the door closes:

Note: DES will not engage if one of the following modes is engaged: G/S*, G/S, F-G/S *, F-G/S , FINAL APP, LAND.

The high-frequency exam point — nothing happens at T/D by itself:

The aircraft will not start its descent automatically when reaching the top of descent (T/D). In order to initiate the descent, the flight crew will set the clearance altitude by turning the ALT knob, then pushes the ALT knob.

‐ If the top of descent is not reached, the aircraft descends at a constant V/S converging on the descent path ‐ If the aircraft is at or beyond the T/D, it descends at idle thrust.

Push before T/D: a gentle constant-V/S ramp down to meet the profile. Push at or past it: idle descent. Miss it entirely and the aircraft tells you:

If the flight crew exceeds the T/D (refer to Typical Descent Vertical Profile illustration, point 2), the message T/D REACHED appears on PFD and the VDEV increases.

— the profile has started down without you.

4. Above the profile — the speed-for-height ledger

Per FCOM DSC-22_30-40-60:

If the aircraft is above the descent profile, the speed will increase toward the upper limit of the managed speed range. If the speed reaches the upper limit, the aircraft will maintain the speed but will deviate from the profile (autothrust at idle).

Speed is spent to buy descent rate — up to a limit, and at the limit the speed wins and the profile loses (the deviation is allowed to grow). The limits, precisely:

If aircraft not in constrained region the Speed Upper Limit is minimum of: ‐ Speed target + 20 kt, ‐ VMO – 5 kt. If the aircraft is in a constrained region, the Speed Upper Limit is Speed target + 5 kt. The Speed Lower Limit is Speed target – 5 kt, whatever the aircraft is in a constraint region or not.

Inside a constrained region (think 250 kt below FL100) the ledger only stretches +5 kt — which is why high-and-fast situations low down run out of options so quickly. The ND's profile-intercept prediction is built on three assumptions:

The FMS assumes that the aircraft will return to the profile using: ‐ Idle thrust ‐ Half speedbrakes extension ‐ ECON speed plus a margin (until intercepting the profile).

No half speedbrake from you = an optimistic intercept symbol. When more drag is needed, the system says so:

If necessary, the message “EXTEND SPD BRK” comes up on the FMA and the MCDU, scratchpad and remains there as long as more drag (speedbrakes) is still required.

And the note that answers a very common cockpit puzzle:

Note: With DES mode engaged, the speedbrakes extension will not necessarily increase the descent rate. It increases only if the aircraft is above path.

"I extended the boards and the V/S didn't change" — correct behaviour. Latched on the profile, the speedbrakes are a speed tool, not a height tool: the path has priority, so extra drag buys a lower speed, not a steeper path. Want steeper? Change mode (OP DES or V/S) and take the geometry yourself.

5. Below the profile — fixed-rate pursuit

Per FCOM DSC-22_30-40-60:

‐ If the aircraft flies at an altitude that is higher than both the descent speed limit altitude and the destination elevation +5000 ft: The FMGES maintains the V/S at -1 000 ft/min and the target speed, until the aircraft reaches the altitude constraint, or intercepts the descent profile. ‐ If the aircraft flies at an altitude that is lower than either the descent speed limit altitude, or the destination elevation +5000 ft: The FMGES maintains the V/S at -500 ft/min and the target speed, until the aircraft reaches the altitude constraint, or intercepts the descent profile.

Two gears: −1000 ft/min in the open descent, −500 ft/min once you are into the terminal region (below the speed-limit altitude or destination +5000 ft) — gentler the closer to the field. This is also the world you enter by pushing ALT before T/D (section 3). Holding patterns use the same machinery:

In the holding pattern, the DES mode commands a V/S of -1 000 ft while A/THR maintains the holding speed. The aircraft levels off at the next altitude constraint if it is reached during the hold.

6. Capture zone, wide capture, and the N+1 rule

Per FCOM DSC-22_30-40-60:

The capture zone is considered as wide when the width, above or below the aircraft is more than +/-540 ft (PFD Altitude scale half range).

If the aircraft is above the flight path, and reaches the capture zone (refer to Typical Descent Vertical Profile illustration, point 4), the speed margin no longer appears and the Latch symbol appears. The aircraft is Latched on the profile and the FMS gives priority to the vertical profile over speed management (autothrust MACH or SPEED). The maximum speed that the aircraft can reach is VMO – 5 kt.

Entering the capture zone from above: margins vanish, the Latch appears, and the cap becomes VMO−5. And the rule that explains "there's a VDEV but the aircraft isn't chasing it":

the FMS guides the aircraft along the extension of the subsequent segment (N+1), when all of the following conditions are met: ‐ The aircraft is above or below the descent profile ‐ The DES mode is active, or the FINAL APP mode is armed ‐ The aircraft intercepts the extension of the subsequent segment (N+1) of the vertical profile, before the aircraft reaches the initially expected segment (N), as illustrated below.

If your present path meets the next segment's extension first, the FMS simply flies that extension — no doubling back to chase segment N; the VDEV converges by itself. With one tightening for final:

Note: For the particular case of FINAL APP mode, the FMS guides the aircraft along the extension of the subsequent segment only if the VDEV is less than 150 ft.

7. Level at a constraint — descent on hold

Per FCOM DSC-22_30-40-60:

If the aircraft levels off at an altitude constraint, the DES mode arms. DES remains armed until the FMS sequences the waypoint that holds the altitude constraint. Then DES mode engages again (if the FCU altitude is set below the altitude of the constraint).

The rhythm of a constrained descent is therefore DES ↔ ALT CST alternation, with the FCU altitude acting as the floor gate of every leg (article 07 told the same story from the ALT CST side). If the FCU happens to sit exactly at the constraint altitude: wind it lower and push the ALT knob — DES arms, and past the fix the descent resumes.

8. Too Steep Path — the slope the FMS cannot fly

Per FCOM DSC-22_30-40-60:

A descent segment is called “too steep path” when the FMS predicts that the descent segment between two constraint waypoints is impossible to fly at the planned descent speed with half speedbrakes extended.

the MCDU does not display any prediction between the upper and the lower points of the too steep path. The message “TOO STEEP PATH AHEAD” appears on the MCDU scratchpad, and the marker "TOO STEEP PATH" appears on the flight plan.

The FMS raises a white flag in advance: no predictions inside the segment, a scratchpad message, a flight-plan marker. What happens if you enter it depends on which side of the speed-limit altitude you are:

‐ VDEV rapidly increases when the aircraft enters the Too Steep Path ‐ Latch symbol no longer appears ‐ THR IDLE is displayed on FMA ‐ Speed margins appear and the aircraft accelerate to the Upper Limit ‐ EXTEND SPD BRK message appears on PFD.

Above the SPD LIM: de-latched, back to the ordinary above-profile playbook (idle, margins, accelerate to the limit, boards out). Below it:

‐ VDEV appears outside the altitude scale range when the aircraft enters the Too Steep Path ‐ Latch symbol remains displayed ‐ SPEED is displayed on FMA ‐ No speed margins displayed and the aircraft accelerates (VMO – 5 kt is the Upper Limit) ‐ EXTEND SPD BRK message is displayed on the PFD.

Below the SPD LIM: still latched — the +5 kt constrained-region margin could never catch this slope, so the FMS keeps the latch, the VDEV pegs off-scale, and you may see the odd combination of "Latch lit, VDEV exploded". Not a system fault. Both versions share one action item: EXTEND SPD BRK — the boards must come out (better: anticipate with early drag and early speed reduction before the segment).

9. V/S–FPA — the basic modes, and the 45-second trap

Per FCOM DSC-22_30-40-70:

V/S and FPA are selected vertical modes. They are the basic modes of vertical guidance.

Every vertical reversion ultimately lands here (article 10). On arrival, the target synchronises:

When V/S / FPA engages following a reversion, the V/S / FPA target synchronizes on the current V/S / FPA of the aircraft, or the preselected V/S / FPA that is displayed in the V/S / FPA window of the FCU.

Note the second clause — it is a loaded gun:

Note: The aircraft targets the preselected V/S / FPA that is displayed on the V/S / FPA window of the FCU instead of the current value, if: ‐ The reversion occurs within 45 s after the preselection of the V/S / FPA value, and ‐ The V/S / FPA mode was not engaged by the flight crew. As a consequence, this new V/S / FPA value may lead the aircraft to a path opposite to the current one, or to a steeper one.

Article 02 said a turned-but-not-pulled value evaporates in 45 seconds. This note is why those 45 seconds are not harmless: if a reversion fires inside the window, the aircraft flies your pre-selected number — possibly the opposite of your current path. Teaching rule: don't spin the V/S window as a calculator. If you dialled a value you are not going to use, keep hands off until the dashes return. Engagement has its own no-go zones:

V/S / FPA will not engage if LAND is engaged, or SRS GA is engaged below 100 ft RA.

Exits mirror article 07's: manual — pull ALT (OP CLB/OP DES), push ALT (CLB/DES if conditions met), push the ALT pushbutton (ALT), TOGA (SRS GA); automatic — ALT*/ALT CRZ* capture, glide-family/FINAL APP capture, and AP/FD TCAS mode engagement (article 15 — which also returns to V/S when it ends). An excessive V/S target triggers the AP speed protection (triple click, flashing amber box — article 05; FD-only guidance adjusts pitch silently — article 10).

A configuration footnote: the Honeywell-configuration AMM text contains an "Expedite Mode" paragraph, but the A330 FCU has no EXPED pushbutton and neither FCOM mode system lists such a mode — generic documentation text, no such function on the aircraft; it is not taught in this series.

10. Flying the descent

Starting down, the standard sequence: descent clearance → set it on the FCU → push the ALT knob (managed DES) or pull (OP DES when constraints don't matter). The monitoring trio: FMA vertical column + VDEV (PFD scale and PROG page) + the ND intercept symbol.

Mode choice, as rules of thumb (synthesis): constraints to honour → DES. "Descend now to X, no restrictions" and you want it expeditious → OP DES (remember: idle and constraint-blind). Precise rate control (rebuilding after an RA, losing height in a hold) → V/S. The QRH emergency descent is the selected world's extreme application (article 31).

Abnormal hooks: NAV or profile lost, or FCU wound above you in DES → reversion to V/S (article 10); dual FM loss → the whole managed family gone, selected only (article 05); prediction trustworthiness (article 24, article 29).

[!warning]- Four misconceptions this article corrects (1) FMA column 1 alternating between THR IDLE and SPEED during one managed descent is the profile changing segment type (idle vs geometric), not a fault. (2) Speedbrakes in DES do not necessarily steepen the descent — on the profile (latched) they only lower the speed; the path has priority. (3) In the below-SPD-LIM Too Steep Path, "Latch lit with VDEV off the scale" is designed behaviour, not a frozen system — and in both versions the boards must come out. (4) A V/S value dialled but not pulled is not harmless for 45 seconds — a reversion inside the window flies your pre-selection, possibly opposite to your current path.

Self-test

[!note]- Q1. In DES, what is the A/THR doing on an idle path versus a geometric path — and what shows in FMA column 1?

Idle path: pitch holds speed, A/THR holds idle — THR IDLE. Geometric path: pitch holds the trajectory, A/THR in SPEED/MACH holds the speed. Column 1 changes with the segment type.

[!note]- Q2. Does the aircraft descend by itself at T/D? How does pushing the ALT knob before T/D differ from after?

No — the crew initiates: set the clearance, push the ALT knob. Before T/D: constant-V/S convergence onto the profile. At/after T/D: idle-thrust descent. Missing T/D brings T/D REACHED on the PFD and a growing VDEV.

[!note]- Q3. Above the profile: the speed upper limit outside and inside a constrained region — and at the limit, what is kept and what is given up?

Outside: min(target +20 kt, VMO −5). Inside: target +5 kt (lower limit always target −5). At the limit the speed is held and the profile deviation is allowed to grow — speed outranks path.

[!note]- Q4. Latch is displayed and you extend the speedbrakes — does the descent rate change? Why?

Not necessarily — only if above the path. Latched on the profile, the path has priority; extra drag lowers the speed instead. Speedbrakes are a speed tool while latched.

[!note]- Q5. The two below-profile catch-up rates and the boundary between them?

−1000 ft/min when above both the descent speed-limit altitude and destination +5000 ft; −500 ft/min once below either — gentler in the terminal area.

[!note]- Q6. Define Too Steep Path. Why does the below-SPD-LIM version stay latched with no speed margins — and what action do both versions demand?

A segment the FMS predicts cannot be flown at the planned descent speed even with half speedbrakes. Below the SPD LIM the constrained-region margin (+5 kt) could never catch the slope, so the latch is kept, VDEV goes off-scale and the aircraft accelerates toward VMO−5 without displayed margins. Both versions: EXTEND SPD BRK.

[!note]- Q7. The two trigger conditions of the V/S pre-selection trap — and the possible consequence?

A reversion within 45 s of pre-selecting a V/S–FPA value, and the mode not having been engaged by the crew. The aircraft then targets the pre-selected value instead of the current one — potentially a path opposite to, or steeper than, the present one.

[!note]- Q8. Can V/S engage while LAND is engaged? And below what height during SRS GA?

No while LAND is engaged, and not below 100 ft RA with SRS GA engaged — landing and the first segment of go-around must not be interrupted by a basic mode.

Key takeaways

References

OP DES and DES clauses — arming gates, T/D behaviour, speed margins and limits, return-to-profile assumptions, EXTEND SPD BRK, speedbrake note, catch-up rates, holding descent, capture zone and Latch, N+1 rule, constraint alternation and Too Steep Path (both variants) — per FCOM DSC-22_30-40-60; V/S–FPA definitions, reversion synchronisation, the 45-second pre-selection note and engagement restrictions per FCOM DSC-22_30-40-70. AP speed-protection indications per AMM 22-11-00 (article 05). The Honeywell-configuration "Expedite Mode" paragraph is noted as generic documentation with no corresponding A330 FCU control or FCOM mode. The three-worlds framing and mode-choice rules of thumb are integrative syntheses. FLS-family mode names in inhibition lists are quoted for completeness; the function is 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.