Trims, Yaw Damper and Envelope Protection - ATPL 022 — Instrumentation

Not all of automatic flight announces itself on the FMA. This lesson covers three systems that work in the background: the trims that erase standing control forces, the yaw damper that quietly kills an oscillation the aircraft is otherwise prone to, and the flight-envelope protection that stands between any control input and the edges of what the aircraft can survive.

Trim: one system per axis

The purpose of a trim system is to remove a standing control force: an aircraft that needs constant elevator to hold its attitude forces its pilot to hold a constant load, and trim re-rigs the balance so the force goes to zero, for reasons the Principles of Flight course derived in full. This course adds the systems view: there is one trim system for each control axis, depending on the complexity of the aircraft. Light aircraft may trim pitch alone; transport aircraft typically trim pitch (the stabiliser), rudder and, less used in flight, aileron.

Each trim has an indicator showing where the surface is set: a scale in units nose-up and nose-down for the pitch trim, index marks for rudder and aileron. The pitch scale carries the marking the syllabus singles out: a green band, the range of stabiliser settings within which the trim must be set for take-off. Its significance is the rotation: a stabiliser trimmed outside the band can make the aircraft reluctant to rotate, or vicious when it does, and setting trim inside the green band for the computed centre of gravity is a before-take-off item checked and warned about.

Automatic pitch trim

The autopilot lesson promised that a well-flown autopilot hands back an aircraft in trim, and automatic pitch trim is how. On a conventional aeroplane, the automatic pitch-trim system watches the standing elevator deflection the autopilot is holding: when the elevator stays displaced for more than a moment, the system runs the stabiliser in the direction that relieves it, until the elevator can return to neutral. The aircraft stays trimmed as speed and configuration change, the elevator keeps its full authority for manoeuvring, and a disconnect hands over no standing force.

On a fly-by-wire (FBW) aeroplane, where the control computers already sit between stick and surface, the automatic pitch trim is folded into the control laws and operates during manual flight as well: the aircraft trims itself continuously, and the pilot flies without touching a trim wheel at all. During certain phases, though, the automatic trim may be deliberately disabled to alter the handling characteristics: in the landing flare, for instance, the designers want the conventional feel of an aircraft that must be flown onto the runway, so the self-trimming stops and the pitch forces return.

The two systems meet the human at the trim wheel, and the autopilot lesson's rules apply. Manual operation of the trim wheel while the automatic pitch trim is engaged sets the two against each other, and normally disconnects the autopilot, with the risk of an out-of-trim aircraft arriving unannounced. The engagement side matches: the autopilot may refuse to engage until the trim controls are in the required state, an out-of-trim aircraft being exactly the standing-force situation the servos should not inherit.

Mach trim

Mach trim is automatic pitch trim against a specific high-speed misbehaviour: as the Mach number rises past the critical Mach number, the aircraft's nose tends increasingly to drop, the Mach tuck the high-speed aerodynamics course explained. The Mach-trim system senses Mach number and trims nose-up against the tendency, keeping the stick forces conventional as speed rises. The Mach-trim system can be independent of the rest of the automatic flight system: it serves manual flight as much as automatic, and it works whether or not any autopilot is engaged.

Runaway and out-of-trim

Trim moves the most powerful surface on the aircraft, so its failures are taken seriously:

runaway trim — the system drives the stabiliser without being asked and keeps driving it.
significant out-of-trim state — however arrived at, the aircraft carries a large standing force.

The implications for the pilot:

control forces that grow toward, and can exceed, what one pilot can hold.
an aircraft whose response to the elevator is skewed by where the stabiliser sits.
a drill that must be prompt, holding against the force, stopping the runaway by the aircraft's procedure (trim cut-out or the equivalent), and retrimming manually.

The earlier the runaway is caught, the smaller the force that has to be fought.

The yaw damper

The purpose of the yaw-damper system is to suppress the Dutch roll the Principles of Flight course dissected: the coupled yawing-and-rolling oscillation that swept-wing jets are prone to, wearying at best and divergent at worst. The damper is a small closed loop of its own: a yaw rate gyro senses the yawing, the signal drives the rudder servo, and the rudder is applied against the swing, each cycle of the oscillation met by a counter-input, so the motion dies away within a few cycles.

Between gyro and servo sits the part the syllabus names: the Dutch-roll filter. The rate gyro senses every yaw, including the ones that are wanted: a commanded turn, a steady sideslip correction. The filter passes only the oscillatory signal at the Dutch-roll frequency and blocks the steady and slowly changing components, so the damper fights the wobble and leaves the flying alone. That is also the clean way to state the difference between a yaw damper and a 3-axis autopilot's rudder channel: the damper only damps, opposing oscillation about a path it does not know; the autopilot's yaw channel is a genuine control channel: it commands the rudder deliberately as part of flying the aircraft, rather than merely opposing an oscillation.

Flight-envelope protection

The purpose of flight-envelope protection (FEP) is to keep the aircraft inside its safe flight envelope, the region of speed, angle of attack and attitude it is certified to fly, whatever inputs arrive from the pilot or the automatics; it is the guard the autopilot lesson's mode reversion fell back on. To know where the aircraft sits in that envelope, the FEP watches typical input parameters:

angle of attack (AoA) — from the vanes of module 2.
aircraft configuration — the envelope's edges move with flaps and gear.
airspeed information — from the air-data side.

Two protective functions are named:

Stall protection — acts as the angle of attack approaches the stalling value, backing up the warnings the stall lessons described with intervention: nose-down demand, stick pusher or law-enforced AoA limit, depending on the aircraft.
Overspeed protection — acts at the other edge, resisting excursions beyond VMO/MMO with nose-up demand or automatic recovery.

And the syllabus draws one applicability line: stall protection and overspeed protection exist on both mechanical/conventional and FBW control systems, but the other envelope functions, pitch or bank limitation among them, can only exist on FBW aircraft: a mechanical linkage can be pushed or pulled by a protection device at the envelope's edges, but only a computer that sits between stick and surface on every input can refuse to exceed a pitch or bank angle in the first place.

That closes the background workers. One machine of the module remains, the one that owns the speed: autothrust, the next and final lesson.

What the exam asks

The green band, the Dutch-roll filter and the FEP applicability line carry the marks here. The overview, following the learning objectives:

Explain the trim system's purpose and the one-system-per-axis layout

Give examples of trim indicators and explain the pitch-trim green band

Describe automatic pitch trim on a conventional aeroplane

Describe FBW automatic pitch trim: active in manual flight, disabled in certain phases to alter handling

Describe the consequences of manual trim-wheel use with automatic pitch trim engaged, and the trim conditions for autopilot engagement/disengagement

Define Mach trim and state that it can be independent

Describe the implications of a runaway trim or significant out-of-trim state

Explain the yaw damper's purpose and operation, the Dutch-roll filter, and the difference from a 3-axis autopilot's rudder channel

Explain the FEP's purpose and its input parameters: AoA, configuration, airspeed

Explain stall and overspeed protection, and which functions are FBW-only