Aircraft Stability and Control :
Aircraft are designed so that they are inherently stable. This means that when a properly balanced aircraft is ‘trimmed for flight,’ it will fly straight and level without needing to hold onto the controls. An aircraft, if not properly balanced, can be either too stable or too unstable. This can cause catastrophic control problems and is the reason that an aircraft has what they call an operating centre of gravity envelope (C of G).Calculating the position of the C of G, often called a trim or weight and balance calculation, is critical to every flight. To comply with the limits of a C of G envelope, an aircraft must be loaded in accordance with the information provided to the pilots on a load sheet so that they can prepare the trim required for flight.
To maintain ‘trim,’ flight attendants must not move passengers out of their allocated zones. To do so could result in catastrophic consequences. Taken to extremes, the weight of the passengers seated in the incorrect position could affect the stability of the aircraft, and subsequently, the pilot’s ability to control of the aircraft.
Pilots control the direction the aircraft travels through the manipulation of:
*Primary flight controls, and
*Secondary flight controls
The primary flight controls of the aircraft are moved by the pilots via a flybywire ‘side stick’ that sit outboard of them (on the left console and the right console), and via the rudder pedals. These controls are duplicated and wired together and are operable by both pilots. ‘Flybywire’ means that any side stick movements made by either pilot are sent to a computer that then moves the primary control surfaces on the tail and the wings to produce the desired response.
Older design planes are hardwired via cables. One of the flybywire advantages is that it saves considerable weight. Primary flight controls control movement of an aircraft around the three axes of vertical, lateral and longitudinal that control the yaw, pitch and roll movement of the aircraft.
Pitch, which acts around the lateral axis, is the name for nose up and nose down movement and is caused by the movement of the elevator attached to the horizontal stabiliser and controlled by the pilot with fore and aft movement of the side stick. Pitch has only a little to do with an aircraft going up or down.
Roll, which acts around the longitudinal axis is the name for the banking or turning left/right of the aircraft and is caused by movement of the ailerons that are attached to the outboard trailing edge of the wings and controlled by the pilot with side-to-side movement of the side stick. Spoilers, located on the top of the wings, are hinged panels that also assist roll control.
Yaw, which acts around the vertical axis, is the name for directional control and is caused by movement of the rudder attached to the vertical stabilizer and controlled by the pilot with left and right rudder pressure. Pilots use rudder pressure to control or maintain directional control, and while yaw can turn an aircraft, it is used primarily to keep an aircraft travelling straight or to balance a turn. Balancing a turn effectively keeps the C of G through the centre of the aircraft. This is much the same reason and feeling you get through your body when leaning through a corner at speed on a motor bike.
The most significant operational use of the rudder is for directional control should one engine fail. Single engine operation results in asymmetric thrust causing more ‘push’ on the side of the operating engine and resulting in yawing and rolling to the dead engine side. Opposite rudder maintains straight directional control.