Hang onto your hats chaps, I’m about to get mildly technical!
When an aeroplane is in flight it is considered to be possible to manoeuvre it in 3 axes.
Firstly imagine a line running approximately from wingtip to wingtip. This line is called the lateral axis and rotation around this line is called pitch. In essence I have described a nose up/down motion and this is controlled by the elevator on the horizontal tailplane at the back.
Secondly imagine a line from the tip of the nose to the tail. This is the longitudinal axis and motion around this is called roll or bank. This wing rock motion is controlled by ailerons on the rear outer wing panels.
Lastly imagine a line running vertically downwards through the center of the aeroplane. This is called the directional axis and this motion is called yaw. It is in effect the ability to slew the nose left or right by use of the rudder which is mounted on the rear vertical surface called the tail fin.
To fly the aeroplane we use the primary controls (elevator, aileron and rudder) to steer the aircraft around the three axes. Little aeroplanes with basic autopilots may have what is called a single channel (or single axis) autopilot which just stabilises the aircraft in roll while the pilot maintains altitude by ‘trimming’ the aircraft to fly level hands off by use of a settable tab on the elevator. This is a fairly basic set up but used quite commonly in air-taxi aircraft.
Larger aircraft (typically turboprop size) may use a 2 channel (or 2 axes) autopilot which controls the aircraft in pitch and roll. Some may have a yaw damper fitted which just acts as stabiliser for the remaining axis.
Modern jets have 3 channel (axes) autopilot which take full control of pitch, roll and yaw. They would usually be accompanied by a further piece of automation called an autothrottle which is a separate set of computers which automatically set the required amount of thrust for you.
Now for something that’s quite useful to understand.
In a non-FlyByWire aircraft (eg A300/310, B737, B747, B757/B767) the aircraft can be flown either manually or with the autopilot engaged. When flown manually the aircraft is controlled through the control column via hydraulic lines to the control surfaces.
In a FBW aircraft (eg A318/319/320/321, A330/340, A380) you can still fly either manually or on autopilot however the definition of ‘manually’ is open to interpretation. When the autopilot is engaged the aircraft follows a routine of positioning itself in the air at a certain preprogrammed speed, position and altitude. When the autopilot is disengaged the guidance stops following the preprogrammed route and follows requests from the joystick/control column. In effect the autopilot has not really disengaged, it just takes its inputs from elsewhere. The joystick/control column provides input to the autopilot computers which send electrical signals to the motors that power the control surfaces.
Because on the FBW aircraft there is a computer in between the joystick/control column and the control surfaces (rather than just hydraulic fluid) the designers can program in certain features. An example of this would be an inability of the aircraft to reach the stalling angle as sensors would put in overiding inputs to prevent this. Another useful feature is something called ‘gust alleviation’ which automatically damps out turbulence (to a degree). What this means is that if a gust causes one wing to rise (for example) the aircraft will automatically sense this and provide an equal and opposite control input without the pilot having to do anything.
Now I think what Kabir is describing is possibly some sort of sensor that provides input (primarily for gust alleviation) to provide a damping input to the pitch axis for the autopilot/flight control system. I have never heard this being referred to as a hook before but then I am no expert on the more modern FBW jets.
Hope you are all still awake and what I’ve written makes sense!
regards
wys
Preston – Alicante, Banjul, Faro, Monastir, Tenerife, Palma, Malaga, Faro and Las Palmas for me in the remainder of this month. I’ve got a few standby’s so there’s a slim chance I may end up in Arrecife!
Sign In
