the advantages of the other, so pilots learn to refine their BFM skills to make use of the opponent's weaknesses. Using BFM as the building blocks for multiple aircraft maneuvers, such as the finger four, loose deuce, and Thach weave, pilots learn how to maneuver in situations involving one against one, one against two, two against two, two against many, or even one against many. This type of training, introduced during the last stages of flight school, is more like actual combat, and is the most beneficial for aircrew once basic BFM skills are mastered. Principles Specific energy Energy is a primary factor in controlling and maneuvering an aircraft. If an attacker has too much energy, it may be easy to get in range but difficult to prevent an overshoot. Too little energy and the attacker may not be able to get in range at all. If the defender has more energy than the attacker, an escape may be possible, but too little energy and the defender will lose maneuverability. In aviation, the term "energy" does not refer to the fuel nor the thrust it produces. Instead, thrust is referred to as "power". Energy is the state of the fighter's mass at any given time, and is the result of the power. Energy comes in two forms, which are kinetic and potential. Kinetic energy is a function of the fighter's mass and speed, while potential energy is a function of its mass, gravity and altitude. The combined potential and kinetic energy is called the total energy. Because the energy package is the combination of mass, speed and altitude, a fighter flying at low altitude but a high speed may have the same total energy as a fighter of equal mass, but flying at a low speed and high altitude. One of the inputs to the formula for total energy is the mass of the object, in this case the aircraft. This means that two aircraft flying under identical conditions of speed and altitude will have different energy; the heavier aircraft will have higher energy. However, this does not imply that the heavier aircraft will be more maneuverable, as that mass will require more energy to accelerate. For this reason, a more useful measure is the specific energy, the energy per unit weight. Lighter aircraft generally have higher specific energy for any given operational conditions. Energy state can be changed through