Primary controls and fly-by-wire architecture

Follow the command and the source of control force

Distinguish the control-command path from the energy supplying surface force in reversible, powered and fly-by-wire systems.

Mechanism and relationships

In a manual reversible control, cables or rods transmit pilot force to a surface and aerodynamic hinge moments can return to the cockpit. A powered irreversible actuator supplies surface force from hydraulic or electrical energy and largely prevents those hinge forces returning through the linkage. Artificial feel then provides a designed cockpit force cue. A servo valve meters hydraulic flow according to command error; position feedback closes the loop so the surface stops at the demanded position rather than continuing to travel.

Reason through the evidence

Fly-by-wire describes electrical signalling, not a guarantee of a particular control law. Computers interpret pilot and sensor inputs and command actuators; mechanical linkage, computation, power and actuation are separate architectural questions. Redundant computers do not protect against a common incorrect sensor input unless the architecture detects or contains it. A degraded control law can change protections or handling cues without implying that every actuator has failed.

Apply the explanation to a different case

A cockpit lever can send an electrical position demand to a computer, which then commands a hydraulic valve. In that one chain, the information is electrical but the surface force is hydraulic. A loss of signalling and a loss of pressure therefore predict different evidence. The computer could command correctly while the surface remains still, or the actuator could remain powered while receiving an invalid demand. Drawing the command path and energy path separately makes those possibilities visible. It also prevents the common mistake of calling every problem in an electrically signalled system an electrical actuator failure.

Keep this distinction clear

A jam and a runaway are different control failures: a jam prevents intended movement, while a runaway creates unintended movement. Position monitoring, actuator isolation and command-channel comparison address different parts of those risks. Redundancy must preserve controllability rather than merely duplicate component names.

Use feedback without assuming unlimited authority

Use demanded position, actual position and feedback error to explain actuator lag, rate limits and common-source failures.

Mechanism and relationships

A basic actuator loop compares demanded and measured surface position. A larger error opens the metering valve farther until limits intervene. As the surface approaches the demand, feedback reduces flow. Actuator rate and travel limits still apply, and high external loads can reduce available movement. A surface-position indication reports a measured position, whereas a command indication reports an intention. Confusing those two removes the very comparison needed to detect a jam or runaway.

Reason through the evidence

Redundancy can involve separate hydraulic circuits, independent electrical feeds and physically separated command paths. Two actuators supplied by one failed pressure source are not fully independent. Reversion and isolation arrangements differ between types, so a generic architecture diagram cannot be used as an aircraft failure checklist. In the lab, reduce actuator rate and observe how lag develops even though the command itself remains valid.

Apply the explanation to a different case

Consider a 24° command and an actuator limited to 6° per second. After one second, even an ideal actuator has moved only 6° and the error is still 18°. Position feedback is doing its job by reporting the difference; it cannot create extra flow or bypass a mechanical stop. If an actuator is jammed, the same comparison may show a persistent error with no movement. A monitoring design decides how long and how large an error is acceptable. The teaching loop has no authority to set that real fault-detection threshold.

Keep this distinction clear

Artificial feel can use speed-related or configuration-related inputs to create an appropriate force gradient. It is not simply the actuator transmitting its hydraulic pressure to the pilot. Losing a feel input can change the cue even if surface position control remains physically available.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.