as turn radius, turn rate and the specific energy of the aircraft. Position of aircraft must quickly be assessed, including direction, angle off tail (the angle between flight paths), and closing speed. Also, the pilot must be aware of his wingman's position and maintain good communication. A pilot in combat attempts to conserve his aircraft's energy through carefully timed and executed manoeuvres. By using such manoeuvres, a pilot will often make trade offs between the fighter's potential energy (altitude) and kinetic energy (airspeed), to maintain the energy-to-weight ratio of the aircraft, or the "specific energy". A manoeuvre such as the "low yo-yo" trades altitude for airspeed to close on an enemy and to decrease turn radius. The opposite manoeuvre, a "high yo-yo", trades speed for height, literally storing energy in "the altitude bank", which allows a fast moving attacker to slow his closing speed. An attacker is confronted with three possible ways to pursue an enemy, all of which are vital during chase. "Lag pursuit" happens in a turn when the nose of the attacker's aircraft points behind an enemy's tail. Lag pursuit allows an attacker to increase or maintain range without overshooting. "Lead pursuit" in a turn occurs when the nose of the attacking aircraft points ahead of the enemy. Lead pursuit is used to decrease the distance between aircraft, and during gun attacks when the cannons must be aimed, not at where the defender is, but where he will be when the bullets get there. "Pure pursuit" happens when the nose of the attacker points directly at the defender. Pure pursuit is when most missiles will be fired, and is the hardest position to maintain. These are known as pursuit curves. The turning battle of a dogfight can be executed in an infinite number of geometric planes. Pilots are encouraged to keep their manoeuvres out of the strictly vertical and horizontal planes, but to instead use the limitless number of oblique planes, which is much harder for an adversary to track. This infinite number of planes around a fixed point about which the aircraft turns is termed the "post and bubble". A fighter that can maintain position between an aircraft and its imaginary post cannot be attacked by that aircraft. The imaginary bubble, however, is misshapen by gravity, causing turns to be much tighter and slower at the top, and wider and faster at the bottom, and is