Plain English Meaning
It's the speed a helicopter should fly to climb the most feet per minute when starting the climb from a low hover near the ground.
Full Definition
VYI is the helicopter airspeed that produces the greatest gain in altitude per unit of time when the aircraft is climbing from an in-ground-effect (IGE) hover. It is the rotorcraft equivalent of a fixed-wing VY but referenced to a departure made from a low hover where ground effect is still available at the start of the climb.
Context Anchor
Seen in helicopter instrument procedures, including copter GPS approaches and missed approach climb-performance discussions.
Derivation
V stands for velocity (speed), Y is the standard aviation code for best rate of climb, and I indicates the climb begins from an in-ground-effect hover. So VYI reads as 'velocity, best rate of climb, in-ground-effect.'
Why Pilots Care
Provides the fastest altitude gain when climbing out of an approach in low visibility or after a missed approach.
Intuition Check
Do not assume VYI is always the same as ordinary VY. The I means the speed is tied to instrument-flight climb use, so use the value published for that helicopter.
Example Sentence 1
After lifting into a low hover, the pilot accelerated to VYI to climb out at the best rate of climb.
Example Sentence 2
The procedure called for maintaining VYI until reaching the published climb altitude.