Definition
A teaching model used in the Airplane Flying Handbook that compares an airplane's total mechanical energy to water held in a reservoir. The throttle controls how much energy is added (the inflow), and the elevator controls how that stored energy is distributed between altitude and airspeed (where the water is directed). The model is used to explain why throttle changes total energy while the elevator only redistributes it.
Plain English
Think of the airplane's energy like water in a tank. The throttle fills or drains the tank. The elevator decides whether that water goes toward gaining height or gaining speed. You can shift it between the two, but you can't have more than what's in the tank.
Context Anchor
Seen in energy management discussions, especially when learning how throttle and elevator work together during climbs, descents, and airspeed control.
Derivation
A reservoir is a holding place for a resource — usually water. The word comes from the French réservoir, meaning a place where something is stored for later use. The analogy borrows that image: energy is stored in the airplane and drawn on as needed.
Why Pilots Care
It helps pilots see that only the throttle adds energy and that improper elevator use merely trades one form of energy for another, often creating unwanted drag or loss of control.
Analogy
Picture two connected containers, one labeled “altitude” and one labeled “airspeed.” The elevator can shift energy from one container to the other, but the throttle is what fills or drains the overall supply.
Grounding Statement
If you raise the nose without adding power, the airplane may gain altitude for a time, but it usually pays for that by losing airspeed.
Intuition Check
The reservoir analogy does not mean the airplane has a real energy tank. It is a mental picture for understanding how speed, altitude, elevator, and throttle relate.
Example Sentence 1
The instructor used the reservoir analogy to show that pitching up without adding power simply trades airspeed for altitude.
Example Sentence 2
Applying the reservoir analogy during approach helps the pilot understand that lowering the nose without reducing power simply converts altitude into excess speed.