Actuation Control and Monitoring — Active, Damping, Centering and the Fail-Safe Default
Between the computer order and the moving surface sits the electrohydraulic servo-control with its operating modes. This article covers how a computer drives a jack, the three modes (active / damping / centering), and the crucial fail-safe property: on any electrical or hydraulic loss, a jack reverts to damping — following the surface to prevent flutter, never fighting it.
Each servojack has three control modes: Active — jack position is electrically controlled; Damping — jack follows surface movement; Centering — jack is hydraulically maintained in neutral position. — FCOM DSC-27-10-20
1. The three modes
Per FCOM DSC-27-10-20, a servo-control jack operates in one of:
| Mode | Behaviour |
|---|---|
| Active | Jack position is electrically controlled (follows the computer order) |
| Damping | Jack follows the surface freely — provides flutter damping, no commanded position |
| Centering | Jack is hydraulically held in neutral (elevators only) |
Per AMM 27-14-00, the mechanism: a solenoid valve drives a mode selector valve. Energised → selector to active: the actuator chambers connect to the servovalve user ports and the feedback transducer closes the servoloop. De-energised → spring moves the selector to damping: the two chambers interconnect through a damping restriction + anticavitation valves.
[!note]- The feedback transducer also drives the SD position display (integrative synthesis) The same feedback transducer that closes the servoloop in active mode reports the piston-rod position to display the surface on the SD (AMM 27-14-00). So the F/CTL-page surface positions you read are the actuators' own feedback sensors — the display and the control loop share the sensor.
2. Normal configuration — one active, one damping
Per FCOM DSC-27-10-20 / AMM 27-14-00, each surface has two jacks: in normal operation one is active, the other damping, and some manoeuvres bring the second jack active to add power. The reconfiguration on failure:
- active jack fails → the damped jack becomes active, and the failed jack switches to damping;
- neither jack controlled electrically nor hydraulically → both go to damping;
- (elevators) none of the four jacks controlled electrically → all four go to centering.
[!warning]- Damping is the fail-safe default — the jack follows the surface rather than fighting it The headline property: on a hydraulic failure the blocking valves isolate the jack and the selector spring drives it to damping; on an electrical failure the de-energised solenoid does the same (AMM 27-14-00). Damping prevents flutter in multiple failures (notably dual-electrical or dual-hydraulic). The system never leaves a dead jack rigidly locked — it lets the surface move and damps it. This is the actuator-level expression of the computer reconfiguration chains: as control sources drop out, jacks fall back to damping, then (elevators) to centering.
3. Which computer drives which jack
Per AMM 27-14-00, the jack-to-computer mapping is deliberately split: an inboard aileron jack connects to two computers (one FCPC + one FCSC), while an outboard aileron jack connects to one computer (one FCPC or one FCSC). Each computer runs a command channel and a monitoring channel; the mode-selector-valve position transducer reports the jack's current mode back so the computer knows whether its jack is active or damping.
[!note]- Command + monitor is the EFCS integrity principle (integrative synthesis) Every computer flying a surface watches itself: a command path issues the order and a monitoring path checks the result, with the mode-selector transducer confirming the jack's state (AMM 27-14-00). A disagreement trips the channel and the reconfiguration hands the surface to the next computer. This self-checking is why "one computer of any type can fly the aircraft" is trustworthy — each is continuously proving itself.
4. Hydraulic isolation and the accumulator
Per AMM 27-14-00, after a hydraulic failure the inlet and return blocking valves close, isolating the jack from the aircraft system; an accumulator stays connected to the servovalve return, and a return relief valve keeps the jack's LP line above the aircraft return so the accumulator stays charged — preserving damping even if the aircraft return line ruptures. The jack thus damps reliably on its own stored fluid after a hydraulic loss.
5. Counterintuitive points
[!warning]- Both electrical and hydraulic failure send the jack to damping Two different failures, one safe outcome — the jack follows the surface and damps flutter (AMM 27-14-00), never locking rigidly.
[!warning]- Elevators have a fourth-mode endpoint: centering If none of the four elevator jacks is electrically controlled, all four go to centering (hydraulically held neutral) — distinct from damping (FCOM DSC-27-10-20).
Self-test
[!note]- Q1. The three jack modes and their meaning? Active (electrically controlled position), Damping (follows surface, flutter damping), Centering (hydraulically held neutral, elevators).
[!note]- Q2. What does a jack do on electrical or hydraulic failure? Reverts to damping (solenoid de-energised / blocking valves isolate + selector spring) — preventing flutter.
[!note]- Q3. Normal jack configuration per surface, and failure handover? One active, one damping; active fails → damped becomes active, failed → damping.
[!note]- Q4. How does a computer know its jack's mode? The mode-selector-valve position transducer reports the mode; the feedback transducer closes the servoloop and drives the SD display.
Key takeaways
| Point | Detail |
|---|---|
| Modes | Active (electrical), Damping (follows surface), Centering (neutral, elevators) |
| Mechanism | solenoid → mode selector valve; feedback transducer closes loop + drives SD |
| Normal | one jack active, one damping; 2nd active on demand |
| Fail-safe | electrical or hydraulic failure → damping (flutter prevention) |
| Elevator endpoint | no electrical control of 4 jacks → all centering |
| Integrity | command + monitoring channels; mode-selector transducer confirms state |
| Hydraulic isolation | blocking valves + accumulator + return relief → damps on stored fluid |
References
- FCOM DSC-27-10-20 (Flight Controls — Architecture) — servojack modes: Active (electrically controlled), Damping (follows surface), Centering (hydraulically held neutral); normal one jack active/one damping, active fail → damped becomes active; neither electrical nor hydraulic control → damping; four elevator jacks with no electrical control → centering.
- AMM 27-14-00 (Aileron and Hydraulic Actuation — Description and Operation) — two electrohydraulic servo controls per surface; active mode (solenoid energised → mode selector valve → chambers to servovalve, feedback transducer closes servoloop); damping mode (solenoid de-energised → selector spring → chambers interconnected via damping restriction + anticavitation valves, prevents flutter on dual electrical/hydraulic failure); hydraulic-failure isolation (inlet/return blocking valves close, accumulator + return relief valve preserve damping); inboard aileron jack to one FCPC + one FCSC, outboard to one FCPC or FCSC; mode-selector-valve transducer identifies mode, feedback transducer drives SD position.
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.