The Weird and Wonderful Logic

A318/A319/A320/A321

Professional

Vol

8-03-4

10 July 2018

THOSE PESKY FCU BUTTONS

The buttons on the FCU (the Flight Control Unit, on the glare shield) can be pushed or pulled and this leads

to endless confusion for new pilots. Yet there is a great way to remember what is what:

• PUSH: you push control from you to the aircraft and go into MANAGED MODE. The aircraft will

now fly the route (with altitudes and speed etc) programmed in.

• PULL: you pull control from the aircraft to yourself and go into SELECTED MODE. You now have to

tell the aircraft what to do.

You’ll never be confused again.

LAWS AND PROTECTIONS

Our airbus project is built to be used as 99.9% of all flights are flown. Without serious failures and without

the need for the systems to leave NORMAL law mode. However it is good to know about the modes!

There are 5 laws (but each has a ground and flight mode):

1. NORMAL LAW

2. ALTERNATE LAW

3. ALTERNATE LAW (without speed stability)

4. DIRECT LAW

5. MECHANICAL BACK-UP

And you can now forget about that list because when the shit hits the fan and the systems degrade into

‘lower’ modes even extremely well trained pilots will not know in what modes they are. That’s fine as long

as you keep flying the aircraft. The laws are there to make sure you get at much help from the systems as

possible.

NORMAL LAW

In normal law the aircrafts computers will provide as much protection as possible. When the aircraft

threatens to go into an abnormal flight condition (in other words, gets too close the edge of the flight

envelope) the systems will try to prevent it. When the aircraft gets into an abnormal flight condition the

systems will try to correct it. Here is what it does:

• LOAD FACTOR LIMITATION: prevents high G-loads that are unpleasant for the passengers and

potentially dangerous for the airframe. The system will try to stay between +2.5G and -1.0G. A

serious rollercoaster will be designed with these same limits in mind btw.

• PITCH ATTITUDE PROTECTION: if there is one thing that your passengers will not like is going

steeply up or down (they might be more bothered by the latter and so should you). The system

will try to stay between 30° nose up and 15° nose down. At low speed and extended flaps the

nose up limitation will lower to 20°)

• HIGH ANGLE OF ATTACK PROTECTION: as we all know your wings will stall when your angle of

attack (the angle between the wing and the oncoming air) is too great. That’s why the systems will

try to prevent the aircraft getting into this situation.

• HIGH SPEED PROTECTION: going too slow is dangerous but going to fast as well. Your Airbus will

prevent you from racing other aircraft to the airport.