Typically, a downdraft refers to a downward flow of air within a column of air, typically found in weather phenomena like

• thunderstorms or
• microbursts.

It can cause a sudden decrease in altitude(less lift) and airspeed for helicopter flying through it, potentially leading to unsafe condition.

Supposing that the downdraft we are talking about is part of a microburst, then the sequence of events typical for a helicopter during an approach within a microburst is as follows:

• First, the helicopter will meet a sudden increase of headwind, which will cause an increase in airspeed and a decrease in rate of descent, which takes the helicopter above the desired approach path.
• At the same time the helicopter enters a reducing headwind and increasing downdraft. A loss of energy leads to reduced airspeed, less lift and an even greater rate of descent. In addition, if the helicopter flies into a downdraft, the air is no longer striking the blades horizontally but at a small angle to the horizontal which depends on the relative magnitudes of the airspeed and the vertical component of the wind (Angle of attack decreases). Hence, the angle af attack has effectively been changed without any change in the attitude, cyclic or collective position.
• As the aircraft continues, the outflow from the microburst is now coming from behind and the increasing tailwind reduces the airspeed even more, accompanied by an increase in the descent rate.