"Anyone able to explain what is meant by "vmc roll"?
After an entire career as a commercisl pilot this is the first time I've ever heard that expression."
Hmmm, have you ever flown a twin prop ?.
Vmc is Velocity of minimum control.
Say on landing or takeoff of a twin prop the left engine fails.
1) You have immediate asymmetric thrust from the right engine yawing the plane to the left
2) You have loss of thrust from left engine and some loss of lift as the engine airflow over the left wing reduces, reducing the lift
3) You have increase drag on left side due to windmilling prop, (need to feather it immediately)
4) So the plane yaws to the left and drops the left wing (quickly) and at low altitude you are usually dead.
So in a twin on landing stay above that speed for as long as possible, on take off get to that speed as soon as possible otherwise with an engine failure its uncontrolable.
Even above that speed a pilot needs to be alert and react quickly by putting in right rudder, reducing thrust on right engine and feathering left engine and keeping wings levels and maybe puttiing nose down to gain airspeed (if possible).
All the more complicated by temperature, humidity, air pressure, undercarriage down etc etc etc.