Automatic flight

Navigation · Chapter 32 · DGCA CPL and ATPL

The flight director, how an autopilot closes the loop through servos to the controls, engaging and overriding it, autoland redundancy, the yaw damper and envelope protection.

Remember

FD shows commands; the pilot or autopilot flies them.
Autopilot: closed loop: sensor → computer → servo → surface. Roll ailerons, pitch elevator, yaw rudder.
Engages through interlocks; synchronises (no snatch); CWS steers through it.
LAND 2 fail-passive; LAND 3 fail-operational.
Yaw damper: rudder against yaw rate (Dutch roll). FBW protects AoA, speed, pitch, bank, g.

What does the flight director do?

Advice, not action

The deck, and the knobs • On the glareshield is a row of knobs: speed, heading, height. • You dial where you want to go. Then two helpers can do the flying for you.
Command bars • The flight director works out how to get there and shows it as two command bars on the PFD (magenta on a Boeing, green on an Airbus). • It moves nothing. The pilot moves the stick to keep the bars on the little aeroplane.
The stick follows the bars • Watch the sidestick. The pilot steers towards the bars, and they settle.
Same brain, one more step • Press AP1 and the same commands drive the controls instead. • Same directions. Now nobody has to hold the stick.

How does an autopilot hold the wings level?

A closed loop

A gust, with nobody helping • A gust lifts one wing. Without help the aircraft stays tipped. • Watch the line on the chart stay off the middle.
Sense, compute, move • A sensor feels the roll. The computer tells a servo to move the aileron. The roll comes out. • The gold dots are the signal. • That circle never stops, so it is a closed loop.
Pitch: the elevator • A bump lifts the nose. The elevator on the tail pushes it back. • Look at the side view.
Heading: the rudder • A gust swings the nose. The rudder swings it back. • From above you can see it.