When the wind blows in the same direction as the aircraft moves, it is called tailwind.
Flying in tailwind conditions increases the groundspeed and reduces the airspeed of an aircraft.
Impact on Operations
Tailwinds impact all phases of the flight. This is sometimes beneficial to operations while in other cases it can reduce the available options or create a hazardous situation.
During take off and landing, tailwinds ireduce the airflow. Consequently, the necessary lift is achieved later and at higher speeds (the wind speed is added to the aircraft speed). Therefore, longer runways are required to perform a safe take off or landing. Another factor to be considered is that in case of rejected take off, the speed of the aircraft will generally be higher, so it will need more distance to decellerate. Take offs and landings with tailwind component exceeding certain value (usually 10 kts) are avoided (the precise value may differ depending on the aircraft type and other factors, e.g. runway surface condition).
A climb or descent in tailwind conditions results in a reduced gradient (i.e. the level change over the distance travelled) even though the rate of climb or descent (i.e. the level change over time) remains the same. Hence, aircraft reach their cleared levels later compared to the calm wind scenario. This may lead to e.g. an aircraft not being able to reach its expected level at the transfer of control point which in turn may contribute to a loss of separation.
In the cruise phase, tailwinds make flights more efficient by reducing fuel burn (due to the reduced drag). Another effect is that flight time is reduced. While this is generally considered advantageous, it may have the side effect of overloading certain ATC sectors. Also, reaching the destination aerodrome faster sometimes means more time will be spent in the holding stack. Nevertheless, the overall impact of tailwind on the cruise phase is considered beneficial.
These effects during take off, approach and landing are described in detail in the dedicated article Tailwind Operations.
The primary risk of tailwind operations is runway excursion. It may happen at take-off (especially in reduced take-off thrust scenarios), landing (especially in the late touchdown case) or rejected take-off. In the landing case, tailwinds may also result in higher number of go arounds due to the increased probability for an unstable approach. This increases both pilots' and controllers' workload. Additionally, landing aircraft would be likely to remain longer on the runway and vacate via later-than-normal taxiways. Therefore, a go around may occur due to insufficient spacing between arriving aircraft.