• Descending above 300 ft/min
• At or below ETL
• Rotor eats its own downwash
• Recover forward cyclic and lower collective (must lose altitude), or turn out with lateral cyclic and pedals
• In extreme state, enter autorotation to break the vortex ring then recover out of that

• Fully articulated rotor system
• Bump or jolt causes asymmetric lagging
• CM of rotor displaced from center of mast, whirl mode can develop rapidly!
• Recover by loading up the rotor system (raise collective) 
• PS: If you don't have the rotor RPM to do this, you may be screwed!

• Forward speed lowers the resultant relative wind on the retreating blade, which is flapped down and has a higher AOA, until it stalls. 
• Feel low freq vibration, pitch up, and roll into retreating blade
• Recover by lowering collective to lower AOA then slow down via aft cyclic

• Pivot around (toward) skid/wheel in contact with ground
• At critical angle 5-8 deg cyclic limit exceeded, will roll
• Easy to get on slope ops, toward upslope skid
• Avoid ops that can catch a skid, use two-step lift off
• Collective most effective at reducing roll, smooth lowering over a second or two (reduces thrust vector)
• Do not dump collective!

• Rolling to the right
• Translating tendency (tailrotor thrust) assists in direction of roll

• An aerodynamic condition resulting from a control margin deficiency in the tail rotor
• Caused by an alteration in the angle or speed of airflow through the tail rotor, away from optimum
• Main rotor downflow interfering with flow entering tail rotor, worse at high power
• Main blade tip vortices entering tail rotor
• Turbulence affecting flow around tail rotor
• Slow forward speed, where translational thrust (TR) and translational lift (MR) are changing 
• Airflow relative to helicopter (see separate card)
• Avoid low and slow flight

• Low and slow flight out of ground effect
• Winds from 9:30 to 11:30 (tip vortex into TR)
• Tailwind altering onset of ETL and translational thrust require higher power and more left pedal
• Hovering in left crosswind
• Low speed downwind turns
• Large changes of power at low airspeeds
• Low speed flight near physical obstructions, buildings

• Maintain max rotor RPM
• Avoid tailwinds at airspeeds below 30 knots
• Avoid OGE ops, and high power demand situations at low altitudes, at airspeeds below 30 knots
• Be aware of wind direction when hovering in winds of 8-12 knots, sudden loss of ETL 
• Be wary of any situation where large left pedal is needed
• Be careful and make slow right pedal turns

• Forward cyclic to increase speed
• If altitude permits, reduce power
• As recovery occurs, enter normal forward flight.
• Last-chance: enter an auto, holding left pedal till rotation stops, then fly out of it if possible or land as if engine out
• If low may have to do engine out at hover (hovering auto)

Mechanical condition where main rotor blades get out of phase in the lead/lag axis.  Caused by an abrupt force applied to the airframe (run-on landing impact, skid/tire bump). Effect is to displace the center of mass of the rotor system away from mast, causing a whirl mode oscillation.

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Left crosswind (220-330°) opposes tail rotor thrust, recirculates output air back into the tail rotor. Horizontal equivalent of "settling with power". 

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http://www.aviationtoday.com/rw/training/turbine/Ask-Ray-Prouty-Tail-Rotor-Vortex-Ring-State_13637.html#.UXqePbXCZ8E
http://www.dynamicflight.com/aerodynamics/loss_tail_eff/
See also The Art of the Helicopter section 5.4

• Preferred: Load up the rotor, fly the helicopter off the ground
• Last ditch if low rotor RPM: Close throttle and lower collective to flat pitch. Cross your fingers.

• Main rotor disc interference
• Weathercock instability
• Tail rotor vortex ring state