Flight Controls

Why are large aircraft primary flight controls often split?

To lower control authority and provide redundancy

What is done to flight controls to prevent damage (specifically wing warping)?

The amount they travel can be reduced with speed or with ailerons, out ailerons are locked at high speed

What is it called when the flight control is the whole of the surface?

All-flying tailplanes, all moving wingtips and all moving fins

What are the types of secondary flight controls?

- Trim tabs

- Balance tabs

- Servo tabs

- Anti-servo tabs

- Spring tabs

- Variable incidence tailplanes

What is the purpose of a trim tab?

Trims the controls via trim wheel in the cockpit, holding controls in the desired position

What is the purpose of a balance tab?

Reduces control loads (aids control)

What is the purpose of a servo tab?

Assists movement (manually connected to control stick)

What is the purpose of anti-servo tabs?

Increases control feel making it more difficult to move the controls

What is the purpose of spring tabs?

Like a servo tab but the spring varies the load

What are the auxiliary controls?

- Spoilers (can be part of primary flight controls)

- Speed brakes

What are high lift devices?

- Flaps

- Slats

- Slots

- Leading edge flaps (Kruger, droop)

How are primary flight controls mainly mechanically actuated?

- Push-pull control rod system

- Cable and pulley system

- Combination of the above

What helps the pilot to move the flight controls?

- Aerodynamic balancing and/or tabs

- Power actuators (hydraulic jacks) either power assisted or power operated

What are power assisted controls?

- Still have direct mechanical link

- As pilot's control moves, control surface moves and pilot/spool valve directs hydraulic fluid to an actuator

- Actuator assists moving control

- Valve resets to a neutral/locked position

How will shockwaves affect the control system for power assisted?

Causes buffeting which is fed back through the control system

How do power operated controls work?

Pilot control directly linked to actuator control valve. Actuator provides all effort needed. Pilot has no actual feel of control forces, it requires artificial feel

What is a power control unit (PCU)?

A pilot/spool valve and actuator incorporated together

What happens if there is a complete hydraulic failure?

Hopefully emergency hydraulic systems are available, but if not, differential engine control

What is a fly-by-wire system?

Pilot controls/movements transmitted (with synchro's or digital data bus) electronically to actuators. Like a power operated system but incorporates control valves to direct fluid to actuators (uses a feedback system)

What is artificial feel?

"q" feel system, based off existing dynamic pressure. Computer calculates forces based on control surface position and airspeed, uses hydraulic pressure to oppose pilot movements

What is actuator gust snubbing (gust locks)?

Large aircraft can't fit external control locks so they have internal control column locks or a gust suppression system fitted in the control actuator. The flight control is free to move but won't hit the primary stop with force (causing damage)

What is the take-off configuration warning system?

An audible warning if the aircraft isn't configured for take-off and the thrust levers are advanced to take-off. It uses position sensors and the thrust management computers

When will a stall warning typically go off?

Warning of approach stall, the margin is typically 5% above the stall airspeed

How does the stall warning work?

Stick shaker, stick pusher and audible warnings only cancelled when AoA if reduced

What is the pitch limit indicator (PLI)?

Estimates the aircraft pitch at which the stick shaker will operate

What are the inputs for the stall warning?

- N1/EPR (Engine power)

- AoA

- WOW switch

- ADC

What is the overspeed warning system?

- Warns if airspeed will exceed Vmo/Mmo

- Airspeed/mach system

- Master warning lights, OVSPD light, EICAS overspeed message, aural warnings

- Remains active until the aircraft airspeed is reduced

What is envelope control?

In a fly-by-wire system, the control is by computers and they can be designed to keep the aircraft inside the flight envelope, limiting pilot input and prevent overstress, overspeed, excessive AoA and excessive altitude. AFCS prevents structural and aerodynamic limitations being exceeded

What are the inputs for envelope control?

- Control wheel

- Control column

- Rudder pedals

- Speed-brake lever

- Pitch trim switches

- Position sensor (feedback)

- ADIRU/ADAHRS

What is the automatic flight control system (AFCS)?

Combines flight director and autopilot. Gets navigation information from the FMS/IRS/GPS, transmits to servos which get feedback from flight control position and displays aircraft attitude via flight director

What are the modes and controls of AFCS?

Auto land, yaw damper, auto trim system, auto throttle, all controlled within design envelope (Green is active, white is armed)

What are the inputs for the AFCS?

- ADC (speed and mach number)

- Encoding altitude (altitude)

- Rate gyro (yaw damping)

- IRS (heading)

- FMS (track)

When will the autopilot disengage?

- When pilot input is detected

- When switch is activated

- If turbulence is sever

- System malfunction

What is the autoland system?

Fully automated landing with crew supervising and only intervene if a system fails, in an emergency or after landing

What is the alert height (AH)?

Height above the runway, based on aircraft type and its fail operational automatic landing systems. Above it, an approach must be discontinued if a specific failure occurs

What are the AFDA status annunciations (ASA)?

- Land 2

- Land 3

- No auto land

What is land 2 for autoland?

AFDS redundancy reduced (one autopilot unavailable)

What is land 3 for autoland?

All autopilots engaged and all system operating normally

What are the two failure modes for autoland?

- Fail operation

- Failure passive

What is fail operation?

- Must have at least two autopilots engaged for the approach

- Failure of one you can still autoland

- No decision height approach

- Land 3, AFDS has redundancy below AH

- At AH check ASA and discontinue if failure

What is fail passive?

- Single autopilot approach

- Failure won't cause immediate deviation from desired flight path

- Immediate control must be assumed and carry out missed approach unless visual

- With land 2 annunciated, AFDS has redundancy below AH

What is the lowest allowable DA for a fail passive system?

50ft

Why is a yaw damper required?

To control dutch roll and limit pilot input as airspeed increases

What are the inputs for the yaw damper?

- Primary control system

- IRS rate gyro

- ADC

- Gust pressure transducers

- Rudder ratio changer

What is the automatic trim system?

Automatically adjusts pitch trim with autopilot engaged, confined to pitch trim. Trim servomotor operates in parallel with autopilot pitch control servo. Limited to half the rate of manual trimming, it typically adjusts tailplane incidence

What makes the automatic trim system required?

Fuel burn, changing speed, changing configuration, any CoG changes

What are the components to the automatic trim system?

- Pitch trim servomotor

- Stabiliser trim panel

What is the pitch trim servomotor?

Separate hydraulic screw type actuator or electric motor actuator

What is the stabiliser trim panel?

Provides pilot trim switches and ability to isolate 2 hydraulic systems