Question: Helicopters can autorotate back to earth in case of emergency. The FAA recommends a flight profile similar to the following procedure:

[![enter image description here](https://i.stack.imgur.com/jATGq.png)](https://i.stack.imgur.com/jATGq.png)

Notice that in section (2) there is forward speed: vertical autorotation is not recommended. Why is that?

Answer: 
During autorotation, forward speed is used to control rotor head speed. In auto, the engine is disconnected (in the case of tail rotor failure) or just plain dead, so it can't be used to control rotor speed. 

In a vertical descent, the rotor head will eventually overspeed from the air rushing past the blades, and the rotor blades will depart from the aircraft. Not good.

The air hitting the blades from forward flight is used to induce drag on the rotor blades, slowing them down. The faster the helicopter moves forward, the greater the drag. 

So the pilot, in auto, uses forward airspeed to control rotor speed. If the rotor head speed builds up too much, nose down a bit to gain airspeed and slow the rotor down. If the rotor head starts slowing down, then back off the forward velocity to let the rotor head pick up speed.

Forward flight also aids in maneuvering the helicopter when choosing a touchdown point, but mainly, it is used to keep the rotor speed in the green. 

Every helicopter has an ideal autorotation forward flight velocity, that should keep rotor head speed in the green. In a Bell JetRanger, it's around 60 kts I believe, though this is a general guideline, not an absolute. 

Theoretically, one could increase blade pitch to an overspending head to slow it down, but forward flight is a more precise way of doing this, and avoids the risk of inadvertently slowing the head down too much.
There are many wrong answers in this thread but I want to address one that any rated helicopter pilot should be able to answer (which judging by the answers in this thread may be not as true as I’m hoping) and that is:

YOU CANNOT GET VORTEX RING STATE IN AN AUTOROTATION

The three things you need for VRS or “Settling with Power” (FAA definition) are:

* Rate of descent greater than 300fpm
* Speed less than ETL
* POWER APPLIED

Obviously if the engine has quit you cannot have power applied. 

That being said, you do still have energy stored in the helicopter in the form of potential energy (in altitude above ground level) and kinetic energy (in the helicopters airspeed). Kinetic energy increases exponentially with airspeed i.e. if you double your airspeed (ex. 30 to 60 kts) you triple the amount of kinetic energy you can use to maintain your rotor RPM in the flare (that you will need a lot of to arrest the rate of descent and forward airspeed prior to touching down) 

If you have no airspeed you have a higher rate of descent in the auto and less energy to arrest that rate of descent with prior to landing. 

Answers to these questions and more can be found in the Helicopter Flying Handbook which is free to download from the FAA website! 

-FAA RW Commercial Pilot and CFI-I