Event: CPT 5

Last updated: 7 April, 2003 22:22
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  1. Dynamic rollover - An accelerated roll about a ground pivot point (landing gear/skids). When AOB or side drift during takeoff or landings occur, lateral cyclic control becomes sluggish and less effective. If AOB passes 15°, the helo will enter a roll which can not be stopped with cyclic. Without lateral trim, aircraft may exceed angle in less than 2 sec.

    During slope or crosswind landing and takeoff maneuvers exceeding the critical rollover angle (15o) or exceeding 10o per second will cause the helo to roll over onto its side regardless of cyclic corrections introduced by the pilot.

    For dynamic rollover to occur 2 essential elements must exist:
    1. Ground pivot point
    2. A side force

    • Side force is always present due to right tail rotor thrust, which is offset with left cyclic. Left cyclic tilts the lift vector to the side creating a sideward force to balance the thrust from the tail rotor. Critical rollover angle is reduced for a right skid down condition, crosswind, lateral CG offset, and left rudder pedal inputs.
    • When landing or taking off, keep aircraft trimmed and do not allow aircraft roll rates to build. If roll rates begin to build recover by smoothly lowering the collective.
    • Lowering the collective will eliminate the lift vector and hence the sideward force
    • The static roll-over angle for the TH-57 is approximately 31°.

    Critical rollover angle is reduced (made worse) for a right skid down condition, cross winds, lateral center-of-gravity offset, and left rudder pedal inputs.

    To avoid entering roll, pilot should
    1) Maintain trim
    2) Not allow aircraft rates from becoming large
    3) Not allow aircraft bank angle from becoming too large
    4) Fly smoothly
    To recover, pilot should use a smooth, moderate collective reduction less than 40% (full up to down in 2 sec). Lowering collective too quickly may cause mast bumping or dynamic roll in the opposite direction.

    During slope landings
    1) Descend slowly, placing upslope skid down first
    2) Lateral cyclic into slope to maintain level TPP
    3) Set cyclic to neutral position once assured helo will remain stable
    4) Do not land on a slope greater than 7.5 degrees
    5) If helo rolls upslope side (5 to 8 degrees of level), collective down

    In crosswind maneuvers, hold cyclic into wind and land/takeoff downwind skid first until firmly on ground.
    Static rollover (if aircraft were pushed with no thrust from blades) is 31°.

    With one skid on the ground and thrust approx. equal to weight, if the lateral control becomes sluggish or ineffectual, contacts the lateral stop, or if bank angle or roll rates become excessive (15 degrees or 10 degrees/sec) the aircraft may roll over on its side. Reduce the collective to stop the roll and correct the bank angle to level.
    When landing or taking off, with thrust approx. equal to the weight and one skid on the ground, keep the aircraft trimmed and do not allow aircraft roll rates to build up. Fly the aircraft smoothly off (or onto) the ground, carefully maintaining trim.

    Note - Do not attempt to level the skids prior to takeoff/landing as this will aggravate the side drift and possibly lead to dynamic rollover [NATOPS 11.8, AERO 5-8]

  2. Rotor blade stall - All helicopters have a tendency to have their retreating blade stall in forward flight, which limits their forward speed. The airspeed of the retreating blade slows down as forward airspeed increases. To equal the lift of the advancing blade, the retreating blade must have an increased AOA, which will cause stall earlier than the advancing blade. Stall angle is considered to be 14 degrees for this rotor system. Upon entry into blade stall, a two-to-one vibration will begin, loss of longitudinal control will occur, and severe cyclic feedback will result. Also, the nose will pitch up as loss of lift at 270 position is felt at tail.

    As airspeed increases the retreating blade linear flow is reduced. In order for the retreating blade to generate the same amount of lift the pitch angle is increased and it flaps down (increasing induced flow). This causes the AOA on the retreating blade to increase. Eventually as airspeed increases the blade will exceed the critical AOA and will stall. The effect on the helicopter is:

    1. 2:1 Vibration level increases
    2. Pitch-up of the nose (Left blade 270° stalls, due to phase lag the loss of lift is felt over the tail causing the tail to drop and the nose to pitch up)
    3. Rolling tendency toward the stalled side (left)

    Three factors, during blade stall:
    1. Up collective (increased power for forward airspeed-increase blade pitch)
    2. Forward Cyclic (increased forward airspeed-causes retreating blade pitch angle to increase)
    3. Increased blade flapping due to high airspeed

    Factors which increase the potential for blade stall:
    High blade loading (i.e. high gross weights / G loading)
    Low rotor RPM
    Turbulent air
    High Density Altitude
    High airspeed

    Factors which affect stall include:
    1. Airspeed
    2. Gross Weight
    3. Density Altitude
    4. G loads
    5. Nr.

    If blade stall is encountered the pilot should:
    1. Reduce airspeed (reduces pwr req, reducing pitch/AOA)
    2. Decrease collective pitch (reduces AOA)
    3. Descend to a lower altitude (decreases pwr req.)
    4. Decrease the severity of the maneuver (reduces G loading)
    5. Increase rotor rpm (increases rotational velocity)

    CAUTION • Entry into severe blade stall can result in structural damage to the helicopter. [NATOPS 11.9, AERO 5-3]

  3. Vortex Ring State - The uncontrolled rate of descent caused by the helicopter rotor encountering disturbed air as it settles into its own downwash, also known as power settling. This condition may occur in powered descending flight at low airspeeds while out of ground effect, when rate of descent approaches or equals the induced flow rate. At 300 - 600 ft/min descent, vortex ring state may begin and will not clear until exceeding 1500-3000 ft/min. Glide slope of 70 degrees (nearly vertical) seem increase the possibility of settling. When these conditions are met, the rotor pumps air into a large bubble underneath it, which then bursts, disturbing air flow and blade thrust. [AERO 5-6] Because approach angles less than 50 degrees and airspeeds of 15-30 kts allow enough new air to enter the rotor system, the TH-57 is limited to descent rates less than 800 ft/min, with airspeeds greater than 40 KIAS, and approach angles less than 45 degrees. [AERO 4-2]

    1. Forward cyclic to gain airspeed
    2. Decrease collective
    If impact is imminent:
    3. Level aircraft to conform to terrain

    Warning • Increase collective has no effect toward recovery and will aggravate vortex ring state. During approaches at less than 40 KIAS, do not exceed 800 feet/min descent rate.

  4. Power required exceeds power available - When power required for a maneuver exceeds power available under the ambient conditions, an uncommanded rate of descent will result.

    Pilots can avoid situation by:

    1. Preflight planning to calculate expected a/c performance
    2. Avoiding excessive maneuvering, esp. during high/hot and or high gross weight/marginal pwr avail. situations
    3. Avoiding high descent rates at low altitudes which will require large power inputs to arrest helicopter's descent
    4. Avoid downwind landings and takeoffs
    5. Maintaining awareness of windspeed and direction, especially during low altitude/low airspeed maneuvers
    6. Maintaining awareness of the factors leading to pwr req. exceeding pwr available and the effects on a/c and perf

    Factors that can cause or aggravate this situation are:
    1. Hi G loading (i.e. level turns)
    2. High gross weight
    3. High density altitude
    4. Rapid maneuvering (i.e. quick stops)
    5. Spool up time from lower power settings to high power settings (i.e. power pull at the completion of a power recovery auto rotation)
    6. Loss of wind effect (i.e. descending below a tree line during a confined area landing)
    7. Change of wind direction (i.e. during lower altitude / low airspeed flight terrain following)
    8. Loss of ground effect (i.e. transitioning to forward flight from the deck of a ship with a heavy internal load)

    Power required exceeding power available becomes dangerous to the crew and helicopter when operating in close proximity to obstructions where the pilot may not have enough altitude / maneuvering space to recover prior to impacting an obstacle. This condition will be aggravated by rotor droop and loss of tail rotor effectiveness associated with excessive power demands.

    Indications: Uncommanded descent with associated maximum torque, Rotor rpm droop, Possible loss of tail rotor effectiveness

    1. Nr - Maintain
    2. RPM switch - FULL INCREASE
    3. Airspeed - 50 KIAS (min pwr req.)
    4. Angle of bank - Level Wings
    5. Jettison stores - As Required
    If impact is imminent:
    6. Level aircraft to conform to terrain
    7. Cushion the landing

  5. Sprag clutch malfunctions - The sprag clutch assembly is the main component of the freewheeling unit and provides et means to disconnect the power train from a failed or secured engine. The sprags are held in a cage assembly and rotation of the outer race {Nf} by the engine jams the sprag between the inner {Nr} and outer races {Nf}. If the outer races stops because of engine failure/shutdown, the inner race is free to turn because of sprags. [SYSTEMS 4-8]

    When the twist grip is full open the pilot may experience:
    Indications: Nf indication higher than Nr, Low torque indication, Ng and TOT indications lower than normal and not responsive to collective

    1. Autorotate
    2. Twist grip FLIGHT IDLE
       If time and altitude permit:
    3. Twist grip Smoothly Rotate to Full Open
       If Nf/Nr are married:
    4. Collective Increase
       If sprag clutch continues to slip:
    5. Autorotate
    6. Twist grip Closed
       If the sprag clutch reengages:
    7. Land as soon as possible

    CAUTION • After completing the autorotative landing, ensure the twist grip is secured. Failure to do so may result in sudden reengagement of the sprag clutch, causing severe damage to the drive system.

    Note • Multiple attempts to reengage the sprag clutch are permitted dependent on time and altitude.

    Indications: Nf/Nr married during shutdown, Nf/Nr married above 100% during autorotational flight

    1. Ensure twist grip is full open
    2. Land as soon as possible

    Warning • If suspected during an autorotation, execute a waveoff before Nr decays below 85%.

    Note • In a normal autorotation, Nr and Nf may be matched together between 92-96% steady state.


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  1. All FAM stage checklists and voice reports - PRESTART CHECKLISTS
  2. Normal starting/shutdown procedures - PRESTART CHECKLISTS
  3. Abnormal starts - same as from CPT1
  4. Overspeed, high Nr - CPT4
  5. Underspeed, low Nf - CPT4
  6. Compressor stall - CPT4
  7. Engine failure - CPT4
  8. Engine restart - CPT4
  9. Engine fire - CPT3
  10. Electrical fire - CPT4
  11. Smoke and fume elimination - CPT2
  12. Suspected fuel leakage - CPT2
  13. Main drive shaft failure - CPT4
  14. Fuel system malfunctions - CPT3
  15. Post shutdown fire/internal - CPT1

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