The lift vector is said to act perpendicular (at a right angle) to the wingspan. When an aircraft is banked, the lift vector is tilted sideways and we now have two components of lift – the vertical component and the horizontal component.
The horizontal component of lift pulls the aircraft in the direction of the bank and the aircraft begins to turn.
As some of the lift is now used to turn the aircraft, the vertical component of lift is no longer strong enough to oppose weight and the aircraft will begin to descend.
For a pilot to maintain altitude during a turn, the lift needs to be increased until the vertical component is again equal to the weight. This increase in lift is necessary in all turns and the pilot achieves this by applying back pressure on the control column to pitch the nose up, increasing the angle of attack.
As angle of bank is increased in a turn, the lift required to maintain altitude increases rapidly.