## 2 Background
### Conceptual Framework for Air-to-Air Refueling
The movement of the drogue can be attributed to multiple factors to include wing flex of the tanker wing, environmental conditions such as turbulence, and finally the creation of a bow wave, which may be attributed to different aspects of the receiver airframe, as it interacts with the drogue during the final moments before engagement [18]. Additionally, the pilot must account for environmental effects on the receiver aircraft and aircrew, such as turbulence, degraded visibility, or challenging lighting scenarios. Depending on environmental conditions, these approaches may become challenging for even the most experienced pilot and may require multiple attempts to safely engage the probe tip and the coupler. Throughout the approach, the pilot must assess the probability of successful engagement. Outside of the nominal outcome of a successful engagement of the probe tip into the drogue coupler, sub nominal outcomes include \"lipping the basket\" by contacting the outer ringer of the drogue or the drogue impacting a portion of the receiver aircraft, such as the nose cone or canopy. These conditions may result in damage ranging from minimal to safety of flight depending on the severity of the incident. If the pilot assesses the engagement will not be successful, the pilot must discontinue the approach by decreasing the closure rates and increasing the separation between the receiver aircraft and the drogue in a safe and controlled manner. The decision to reattempt engagements after a failed attempt will be made after an assessment of the root cause of the failed attempt. Based on any damage during the failed attempt, identification of root cause of the error, probability of error correction, and on mission planning factors to include acceptable airfield availability, allowable level of risk for success of the mission, and physiological state (fatigued, hypoxic, airsickness, etc.) of the pilot, the pilot may reattempt the engagement if deemed necessary. Once the pilot has successfully engaged the probe tip and the coupler of the drogue, the pilot must continue to decrease the range between the tanking aircraft and receiver aircraft by approximately 5 to 25 feet to enter the Fuel Transfer Zone (FTZ), as annotated on the long hose markings. The FTZ represents a corridor in space determined by the length of hose extended from the tanker aircraft. When the receiving aircraft is in the FTZ, fuel will begin to transfer from the tanker aircraft to the receiver aircraft. When the hose is fully extended prior to engagement, no fuel is being transferred. As the receiver aircraft engages the coupler with the probe tip and begins to decrease the range between the tanking aircraft and receiver, the hose will begin to retract, but fuel will not transfer until the hose reel retracts to a predetermined distance, which represents the FTZ. The pilot receives visual confirmation upon entering the FTZ through a combination of lights installed on either the fuselage of the tanker aircraft or the ARS connected to the wing, as well as the hose markings being in the correct positions. Once established in the FTZ, the pilot then transitions to formation position keeping. Directional position keeping uses the tanker aircraft wing, fuselage, or other marking to help maintain appropriate vertical and horizontal separation. Forward/aft positioning within the FTZ is done using the hose markings on the tanker hose by keeping the first long marking within the tanker hose outlet. The amount of time required to complete aerial refueling depends on a multitude of variables to include: the tanker's fuel delivery rate, the maximum fuel rate the receiver can accept, and the total amount of fuel to be transferred. The receiver can confirm fuel flow through the static green light on the drogue system signal light or on their fuel quantity management indications in the receiver cockpit. Two methods exist for determining that the receiver aircraft received the desired amount of fuel.