When a plane experiences a decrease in weight must there also be decrease in lift? I'm thinking the answer is yes in order to maintain equilibrium.

Yes. Lift must equal weight at equilibrium. Lift can be lowered by increasing altitude, lowering speed, decreasing the angle of attack, or changing aileron/flap settings

Great! thanks a lot.

To answer your question, let's first discuss the concept of lift and weight in the context of an airplane.

Lift is the force that acts in an upward direction and opposes the force of gravity. It is generated by the wings of an aircraft as air flows over and underneath them. Weight, on the other hand, is the force of gravity pulling the airplane down towards the Earth.

In order for an airplane to maintain equilibrium and level flight, lift must equal weight. When the airplane experiences a decrease in weight, such as through the consumption of fuel, there are two possibilities:

1. If the decrease in weight is accompanied by a proportional decrease in lift, the equilibrium is maintained, and the airplane's flight path remains unchanged.

2. If the decrease in weight is not accompanied by a decrease in lift, the equilibrium is disrupted, and the aircraft will experience a change in its flight path. For example, if weight decreases without a corresponding decrease in lift, the airplane will experience an upward acceleration or climb.

Therefore, in order to maintain equilibrium, it is necessary for a decrease in weight to be accompanied by a decrease in lift. This ensures that the aircraft remains in level flight and does not experience any unwanted changes in its flight path.