Modes divide the control of energy

Why does pressing a mode button not prove capture?

A selection requests behaviour; annunciation identifies what the system is doing.

Selected, armed and active are different states

A selected value is an input such as heading, altitude or speed. An armed mode waits for its capture conditions; an active mode is currently commanding the aircraft. Not every mode has an armed state, and an armed mode may never capture if the required source or geometry is unavailable. A lit selection or button position therefore cannot replace the flight-mode annunciator as evidence of actual system state.

The FMA typically separates lateral guidance, vertical guidance, thrust mode, autopilot/director status and landing capability as appropriate. Colour, size and boxing help distinguish active, armed or recently changed information. The exact arrangement is aircraft-specific. Read the mode names and understand their implication rather than relying on a colour alone or expecting every transition to produce an aural announcement.

Capture and reversion change the next action

A capture commonly includes an initial manoeuvre, a trajectory towards the selected parameter, a capture phase and a hold or tracking phase. Closure rate influences how the aircraft must reduce the error without overshoot. Changing a target during capture, losing a navigation source or reaching a flight-plan discontinuity can lead to a different mode or reversion. The crew’s original intention may remain unchanged while actual guidance has changed.

After each important selection, verify the FMA, aircraft attitude, trend and trajectory. Announce and monitor mode changes according to the operator’s procedures. In the model select an armed localiser with an intercept outside the synthetic capture condition: the previous mode stays active until the condition is satisfied. This explains why seeing a route or a selected approach is insufficient evidence that the aircraft is following that guidance.

Who is controlling speed in the selected mode?

Pitch and thrust divide the energy task differently across mode combinations.

Fixed thrust and speed through pitch

In a climb mode using a fixed thrust reference, the pitch channel may control speed while the resulting climb rate is allowed to vary. In an altitude-hold mode, pitch maintains altitude and autothrust may control speed by changing thrust. These are different allocations of the same energy problem. Reading a speed target without knowing which channel pursues it leaves an incomplete understanding of the mode combination.

A selected vertical-speed mode can prioritise a requested climb or descent rate. If the requested climb rate demands more energy than available thrust can supply, speed can decay unless a specific protection or reversion intervenes. Do not assume every aircraft provides the same speed protection in every vertical mode. FMA interpretation must be combined with speed trend, attitude, thrust and the flight path.

Autothrust is a mode-dependent controller

Autothrust can command a thrust target, speed control, idle or other phase-specific functions. Some systems move the thrust levers; others leave them stationary while commanding engine thrust within the selected lever authority. Lever position alone can therefore be an incomplete indication of current thrust or active control. The annunciation and engine parameters show how the system is actually working.

Turbulence and rapidly changing airspeed can make aggressive feedback corrections oscillate. Appropriate response follows the type-specific procedure and may require deliberate manual thrust. Do not use repeated extreme targets as a substitute for energy awareness. The laboratory compares energy available with the climb demand using explicitly synthetic excess-power data: when the requested vertical rate exceeds the supplied capability, the shortfall appears as speed loss rather than imaginary extra thrust.

Can you explain and apply the mechanism?

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