Sidestick Priority — Dual Input, the Takeover Button and the Lights

The two sidesticks are not mechanically linked and give no force feedback — neither pilot can feel the other's input. So the EFCS adds the inputs together, caps the sum, and provides a clear takeover/priority logic with a "DUAL INPUT" callout and dedicated red/green lights. This article covers the design rationale, the priority mechanism, the CRM "I have control" procedure, and why armrest adjustment affects handling. It is as much a CRM article as a systems one.

The sidesticks are springloaded to neutral. They are not mechanically linked, and do not receive feedback from the flight control surfaces... if both flight crewmembers use their sidesticks simultaneously, their orders are algebraically added. — FCOM DSC-27-20-30

1. Why a side-mounted, uncoupled stick

Per FCTM AOP-10-30, the side-mounted stick's operational benefits are: a non-obstructed view of the main instrument panel; adaptation for emergency situations (incapacitation, stick jamming, control failures); comfort with a correctly adjusted armrest; and the possibility of a sliding table. With the autopilot engaged, the sticks are locked in neutral (immediate tactile feedback), there is no simultaneous crew + AP input, and the AP can be disconnected instinctively at any time by firm pressure on the stick.

[!note]- The uncoupled stick is a deliberate trade — and the system replaces the missing feedback (integrative synthesis) A linked yoke gives tactile awareness of the other pilot but obstructs the panel and couples to the AP; the side-mounted, uncoupled stick clears the panel and decouples the AP but removes the tactile cue of the other pilot (FCTM AOP-10-30). The EFCS deliberately compensates: AP-engaged neutral lock with tactile feedback, the DUAL INPUT callout, and the priority lights all substitute for the feel a linked yoke would have given. Trusting "I'd feel it if they were on the stick" is exactly the trap this design must guard against.

2. The sidesticks — independent and silent

Per FCOM DSC-27-20-30, each sidestick is spring-loaded to neutral, not mechanically linked, and gives no surface feedback. With the autopilot engaged the sticks are locked at neutral; a force above threshold unlocks the stick, disconnects the AP and triggers AUTO FLT AP OFF — the AP disengages at > 5 daN in pitch or > 3.5 daN in roll (though this is not the recommended way to disconnect). Each stick carries the PTT and the sidestick pb (AP-disconnect / priority).

3. Dual input — orders are summed and capped

Per FCOM DSC-27-20-30, if both pilots move their sticks at once, the orders are algebraically added, and the flight-control laws limit the combined order to the full deflection of one sidestick. In this case the two green SIDE STICK PRIORITY lights on the glareshield illuminate and the "DUAL INPUT" voice message sounds.

[!warning]- Two opposite inputs can partially cancel — DUAL INPUT is the warning Because inputs are summed, two pilots pulling opposite ways partly cancel, and two pulling the same way are capped at one-stick authority (FCOM DSC-27-20-30). Neither pilot gets the response they expect. The FCTM is explicit: "dual inputs must be avoided" — the "DUAL INPUT" aural + both green lights are the alert to resolve it: one pilot takes priority and the other lets go.

4. The takeover/priority pushbutton, and the CRM procedure

Per FCOM DSC-27-20-30, a pilot takes full control by pressing and holding the sidestick pb, which deactivates the other stick. The rules:

• holding the pb takes priority for as long as it is held;
• press for 40 s to permanently deactivate the other stick — it stays off until any pilot presses their pb;
• if both press, the last pilot to press gets priority;
• a stick deactivated on the ground triggers CONFIG L(R) SIDESTICK FAULT at takeoff power / during the TO CONFIG test.

Per FCTM, the tasksharing rule: "Only one flight crew flies at a time. If the PM wants to act on the sidestick, the PM must clearly announce 'I have control' and press and maintain the sidestick pushbutton... Either flight crew can make an input on their sidestick at any time. Either flight crew can deactivate the other flight crew's sidestick by pressing on their sidestick pushbutton."

[!warning]- The takeover is a CRM action: announce "I have control" AND press-and-hold the pb The systems logic (press-and-hold to deactivate the other stick) is incomplete without the CRM half: clearly announce "I have control" as you take it (FCTM). Because the priority is last-press-wins (FCOM DSC-27-20-30), the announcement is what prevents a silent tug-of-war — either pilot can take it back by pressing, so the verbal protocol, not the hardware, is what assigns control. In an incapacitation, pressing your own pb always lets you take (or take back) control.

5. The lights and callouts

Per FCOM DSC-27-20-30:

[!warning]- Red arrow = you have lost authority; it faces the pilot who is locked out The red arrow lights in front of the pilot who has lost control (the deactivated side), pointing at them; the green light marks the pilot who has priority or flags the dual input (FCOM DSC-27-20-30). "PRIORITY LEFT/RIGHT" announces a takeover; "DUAL INPUT" announces both are flying. If you see a red arrow in front of you, you are not flying the aircraft — press your pb to take it back.

6. Armrest adjustment and over-controlling

Per FCTM, correct armrest adjustment is a handling prerequisite: the forearm should rest comfortably with no gap and the wrist not bent, so the pilot moves the wrist, not the forearm. "Symptoms of incorrect armrest adjustment include over-controlling, and not being able to make small, precise inputs."

[!note]- Over-controlling often starts at the armrest (integrative synthesis) The Normal-law pitch rewards small, precise inputs — and the FCTM identifies a physical root cause of over-controlling: a poorly adjusted armrest forces the pilot to lift the whole forearm instead of articulating the wrist, costing precision (FCTM). So the "make small inputs" technique starts with the seat and armrest set up so the wrist alone flies the stick.

7. Counterintuitive points

[!warning]- Inputs add, they don't average Dual inputs are algebraically summed and capped at one-stick deflection (FCOM DSC-27-20-30) — opposite inputs partly cancel.

[!warning]- The last pilot to press the pb has priority — so announce "I have control" Priority is recoverable; the CRM call, not the hardware, assigns control (FCOM DSC-27-20-30 / FCTM).

[!warning]- A poorly adjusted armrest causes over-controlling Move the wrist, not the forearm (FCTM).

8. Self-test

[!note]- Q1. What happens if both pilots move their sticks, and how is it annunciated? Orders are algebraically added (capped at one-stick deflection); two green lights + "DUAL INPUT" voice; dual inputs must be avoided.

[!note]- Q2. The takeover procedure — systems and CRM? Announce "I have control" AND press-and-hold the sidestick pb (deactivates the other); 40 s for permanent deactivation.

[!note]- Q3. If both press the priority pb, who gets it, and why does the call matter? The last pilot to press — so the verbal "I have control" protocol (not the hardware) assigns control and prevents a silent tug-of-war.

[!note]- Q4. What do the red and green priority lights mean? Red arrow = pilot losing authority; green CAPT/F/O = pilot who took priority (or both on dual input); audio "PRIORITY LEFT/RIGHT" / "DUAL INPUT".

[!note]- Q5. Why does armrest adjustment matter to handling? A poor adjustment forces forearm movement instead of wrist articulation → over-controlling and loss of precision (FCTM).

9. Key takeaways

References

• FCOM DSC-27-20-30 (Flight Control System — Controls and Indicators — Sidesticks / Priority Logic) — sidesticks spring-loaded to neutral, not mechanically linked, no surface feedback; AP locks sticks at neutral, force above threshold unlocks + disconnects AP (>5 daN pitch / >3.5 daN roll); dual input algebraically added, limited to one-stick full deflection, two green SIDE STICK PRIORITY lights + "DUAL INPUT" voice; takeover by pressing and holding sidestick pb (deactivates other), 40 s for permanent deactivation until any pb press, last press gets priority; on-ground deactivation → CONFIG L(R) SIDESTICK FAULT; red arrow light in front of pilot losing authority, green CAPT/F/O light for pilot taking priority or both on dual input, "PRIORITY LEFT/RIGHT" audio.
• FCTM AOP-10-30 (Fly-by-Wire Design Principles) — side-mounted stick benefits (non-obstructed panel view, emergency adaptation — incapacitation/jamming/control failures, armrest comfort, sliding table); AP engaged sticks locked neutral with tactile feedback, no simultaneous crew+AP input, instinctive AP disconnect.
• FCTM (Tasksharing / Sidestick) — only one crew flies at a time; PM announces "I have control" and presses-and-maintains the pb to take control; dual inputs algebraically added, must be avoided (aural/visual alerts); either crew can input or deactivate the other's stick anytime; armrest adjustment (wrist not forearm) — poor adjustment causes over-controlling and loss of small-input precision.

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.