Shared Flashcard Set


Hazards in Helicopter Flying
Questions relating to rotary wing hazards

Additional Aviation Flashcards




What is vortex ring state and how do you  recover
  • 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
What is ground resonance and how do you recover?
  • 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!
What is retreating blade stall and how do you recover?
  • 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
What is dynamic rollover and how do you avoid it?
  • 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!
Which direction (left or right) is the critical one for dynamic rollover (CCW rotor) and why?
  • Rolling to the right
  • Translating tendency (tailrotor thrust) assists in direction of roll
What is loss of tail rotor effectiveness (LTE) and what are the causes?
  • 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
What maneuvers are susceptible to LTE?
  • 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
What are some ways to avoid LTE?
  • 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
What is the general recovery from LTE?
  • 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)
What is ground resonance and what can cause it?

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.


What can cause tail-rotor vortex ring state (a type of LTE)?

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

See also The Art of the Helicopter section 5.4

What can cause weathercock instability (a type of LTE)?
Tailwind (120-240°) causes helicopter to weathervane and if no anti-torque input can allow yaw rate to develop and can accelerate rapidly.
How do you recover from ground resonance?
  • 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.
What can cause main rotor disk interference (a type of LTE)?
Quartering left crosswind (285-315°) 10 - 30 knots causes main rotor tip vortex to be blown into the tail rotor, disrupting tail rotor airflow.
What are the three types of LTE?
  • Main rotor disc interference
  • Weathercock instability
  • Tail rotor vortex ring state
Why are airplane pilots considered "high risk" when flying helicopters?
Airplane pilots' ingrained emergency reactions may lead to deadly reactions in a helicopter.
What is a Low G condition? When will it occur? How do you recover?
The main rotor disk unloaded in flight, commonly caused by cyclic "push over", turbulence. Recover with slow and smooth aft cyclic to re-load the rotor. Do not attempt to correct for rolling motion under low G (mast bumping or droop stop pounding)
What is low rotor blade stall? What can cause it? How can it be identified? How do you recover?
Main rotor blades fail to produce lift due to insufficient rotor RPM (rotational relative wind). Low frequency vibrations, left yaw, divergent descent. Trick question, there is no recovery.
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