LIMITED POWER OPERATIONS

AIM:

To be able to operate the helicopter in the safest way possible when the power is limited due to either temperature, altitude etc. To also understand the power requirements for operating in confined areas.

THEORY:

Limited power can be classified as either a limited power approach or a limited power takeoff.

Limited Power Takeoff

  • Running/rolling takeoff

A running takeoff is sometimes used when conditions of load and/or density altitude prevent a sustained hover at normal hovering altitude. However, you should not attempt this manoeuvre if you do not have sufficient power to hover, at least momentarily. If the helicopter can not be hovered, its performance is unpredictable. You MUST use your performance graphs to determine your in and out of ground effect performance. 

If the helicopter cannot be raised off the surface at all, sufficient power might not be available to safely accomplish the manoeuvre. If you cannot momentarily hover the helicopter, you must wait for conditions to improve or off-load some of the weight.

To accomplish a safe running or rolling takeoff, the surface area must be of sufficient length and smoothness, and there cannot be any barriers in the flight path to interfere with a shallow climb.

  • Normal takeoff

It is also mostly when in the hover the helicopter has at least 1 inch spare of manifold pressure to be able to execute a standard departure.

  • Maximum performance/steep takeoff

A maximum performance takeoff is used to climb at a steep angle to clear barriers in the flight path. It can be used when taking off from small areas surrounded by high obstacles.

Before attempting the exercise you must know thoroughly the capabilities and limitations of your equipment. You must also consider the wind velocity, temperature, altitude, gross weight, centre-of-gravity location, and other factors affecting your technique and the performance of the helicopter.

To safely accomplish this type of takeoff, there must be enough power to hover, in order to prevent the helicopter from sinking back to the surface after becoming airborne. This hover power check can be used to determine if there is sufficient power available to accomplish this manoeuvre. There should be at least 2 inches of manifold pressure spare in the hover.

The angle of climb depends on existing conditions. The more critical the conditions, such as high-density altitudes, calm winds, and high gross weights, the shallower the angle of climb. In light or no wind conditions, it might be necessary to operate in the crosshatched or shaded areas of the height/velocity diagram during the beginning of this manoeuvre. Therefore, be aware of the calculated risk when operating in these areas. An engine failure at a low altitude and airspeed could place the helicopter in a dangerous position, requiring a high degree of skill in making a safe autorotative landing.

  • Vertical takeoff

Vertical takeoff requires the helicopter to climb vertically out of a confined area before executing a normal type of departure.

All the same check as a maximum performance takeoff need to be considered, however, in addition, it is recommended that there is at least 3 inches manifold pressure spare.

Have a definite plan and abort the procedure.

Limited Power Approaches and Landings

  • Shallow approach and running/roll-on landing

Use a shallow approach and running landing when a high-density altitude or a high gross weight condition, or some combination thereof, is such that a normal or steep approach cannot be made because of insufficient power to hover. To compensate for this lack of power, a shallow approach and running landing make use of translational lift until surface contact is made. The glide angle for a shallow approach is approximately 5°. Since the helicopter will be sliding or rolling to a stop during this manoeuvre, the landing area must be smooth and long enough to accomplish this task. Accurate pedal control is essential.

  • Normal approach and landing

Standard landing by executing it very smoothly the helicopter should only require 1 inch manifold pressure extra than hovering.

  • Steep approach to a hover

A steep approach is used primarily when there are obstacles in the approach path that are too high to allow a normal approach. A steep approach permits entry into most confined areas and is sometimes used to avoid areas of turbulence around a pinnacle. An approach angle of approximately 15° is considered a steep approach.

  • The normal approach to hover OGE with vertical landing

To enter an OGE hover and slowly lowering the helicopter to the ground. Being aware of Vortex and the power available.

AIR EXERCISE:

Limited Power Takeoff

  • Running/rolling takeoff

Refer to the above figure.

To begin the manoeuvre, first align the helicopter to the takeoff path. Next, increase the throttle to obtain takeoff r.p.m., and increase the collective smoothly until the helicopter becomes light on the skids (position 1). Then, move the cyclic slightly forward of the neutral hovering position, and apply additional collective to start the forward movement (position 2). To simulate a reduced power condition during practice, use one to two inches less manifold pressure than required to hover.

Beware pedal control essential danger of dynamic rollover and the danger of dynamic rollover

  • Maximum performance/steep takeoff

Refer to the above figure.

Before attempting, bring the helicopter to a hover, and determine the excess power available by noting the difference between the power available and that required to hover, have at least 2 inches manifold pressure spare. Then position the helicopter into the wind and return the helicopter to the surface. Normally, this manoeuvre is initiated from the surface meaning, do not stop momentarily in the hover – keep the upwards momentum going. After checking the area for obstacles and other aircraft, select reference points along the takeoff path to maintain ground track. You should also consider alternate routes in case you are not able to complete the manoeuvre.

Begin the takeoff by getting the helicopter light on the skids (position 1). Pause and neutralize all aircraft movement. Slowly increase the collective and position the cyclic forward. Continue to slowly increase the collective until the maximum power available is reached. This large collective movement requires a substantial increase in pedal pressure to maintain heading (position 2). Use the cyclic, as necessary, to control movement toward the desired flight path and, therefore, climb angle during the manoeuvre (position 3). Maintain rotor r.p.m. at its maximum, and do not allow it to decrease since you would probably have to lower the collective to regain it.

Maintain these inputs until the helicopter clears the obstacle, or until reaching 50 feet for demonstration purposes (position 4). Then, establish a normal climb attitude and reduce power (position 5). As in any maximum performance manoeuvre, the techniques you use affect the actual results. Smooth, coordinated inputs coupled with precise control allow the helicopter to attain its maximum performance.

Aim to have the bottom of the skids to well clear the highest part of the obstacle. Remember try to get the maximum speed for the height required.

  • Vertical takeoff

Self-explanatory, ensure approximately 3 inches manifold pressure spare for execution. Beware of drifting and have a contingency plan for emergency use.

Limited Power Approaches and Landings

  • Shallow approach and running/roll-on landing

Refer to the above figure

A shallow approach is initiated in the same manner as the normal approach except that a shallower angle of descent is maintained. The power reduction to initiate the desired angle of descent is less than that for a normal approach since the angle of descent is less (position 1). As you lower the collective, maintain heading with proper pedal pressure. Maintain approach airspeed until the apparent rate of closure appears to be increasing. Then, begin to slow the helicopter with aft cyclic (position 2).

As in normal and steep approaches, the primary control for the angle and rate of descent is the collective, while the cyclic primarily controls the ground speed.

However, there must be a coordination of all the controls for the manoeuvre to be accomplished successfully. The helicopter should arrive at the point of touchdown at or slightly above the effective translational lift. Since translational lift diminishes rapidly at slow airspeeds, the deceleration must be smoothly coordinated, at the same time keeping enough lift to prevent the helicopter from settling abruptly.

Just prior to touchdown, place the helicopter in a level attitude with the cyclic, and maintain heading with the pedals. Use the cyclic to keep the heading and ground track identical (position 3). Allow the helicopter to descend gently to the surface in a straight-and-level attitude, cushioning the landing with the collective. After surface contact, move the cyclic slightly forward to ensure clearance between the tail boom and the rotor disc. You should also use the cyclic to maintain the surface track, (position 4). You normally hold the collective stationary until the helicopter stops; however, if you want more braking action, you can lower the collective slightly.

Keep in mind that due to the increased ground friction when you lower the collective, the helicopter’s nose might pitch forward. Exercise caution not to correct this pitching movement with aft cyclic since this movement could result in the rotor making contact with the tail boom. During the landing, maintain directional control with the pedals until the helicopter comes to a complete standstill.

  • Steep approach to a hover

Refer to the above figure

On final approach, head your helicopter into the wind and align it with the intended touchdown point (position 1). When you intercept an approach angle of 15°, begin the approach by lowering the collective sufficiently to start the helicopter descending down the approach path and decelerating (position 2).

Since this angle is steeper than a normal approach angle, you need to reduce the collective more than that required for a normal approach. Continue to decent by smoothly lowering the collective to maintain the approach angle. As in a normal approach, reference the touchdown point on the windshield to determine changes in approach angle. This point is in a lower position than a normal approach.

Aft cyclic is required to decelerate sooner than a normal approach, and the rate of closure becomes apparent at a higher altitude. Maintain the approach angle and rate of descent with the collective, rate of closure with the cyclic.

Loss of effective translational lift occurs higher in a steep approach (position 3), requiring an increase in the collective to prevent settling, and more aft cyclic to achieve the proper rate of closure. Terminate the approach at hovering altitude above the intended landing point with zero groundspeed (position 4). If the power has been properly applied during the final portion of the approach, very little additional power is required in the hover.

  • The normal approach to hover OGE with vertical landing

Self-explanatory, the difficulty comes trying to maintain a steady hover without any sideways or rearwards flight

COMMON MISTAKES:

Limited Power Takeoff

Running/rolling take-off

  • Failing to align heading and ground track to keep surface friction to a minimum.
  • Attempting to become airborne before obtaining an effective translational lift.
  • Using too much forward cyclic during the surface run.
  • Lowering the nose too much after becoming airborne, resulting in the helicopter settling back to the surface.
  • Failing to remain below the recommended altitude until airspeed approaches normal climb speed.

Maximum performance/steep take-off

  • Failure to consider performance data, including height/velocity diagram.
  • Nose too low initially, causing horizontal flight rather than more vertical flight.
  • Failure to maintain maximum permissible RPM.
  • Abrupt control movements.
  • Failure to resume normal climb power and airspeed after clearing the obstacle.

Vertical take-off

  • Not having sufficient power
  • Low RRPM during execution
  • Too abrupt on the controls
  • Stopping in the hover, not keeping the upwards momentum going

Limited Power Approaches and Landings

Shallow approach and running/roll-on landing

  • Assuming excessive nose-high attitude to slow the helicopter too much down near the ground
  • Insufficient collective and throttle to cushion the landing.
  • Failing to add proper pedal as collective is added to cushion landing, resulting in a touchdown while the helicopter is moving sideward.
  • Failing to maintain a speed that takes advantage of effective translational lift.
  • Failing to touch down in a level attitude.
  • Poor directional control during touchdown.

Steep approach to hover

  • Improper use of collective in maintaining the selected angle of descent.
  • Slowing airspeed excessively in order to remain on the proper angle of descent.
  • Inability to determine when the effective translational lift is lost.
  • Failing to arrive at hovering altitude and attitude, and zero groundspeed almost simultaneously.
  • Low RPM in the transition to the hover at the end of the approach.
  • Using too much aft cyclic close to the surface, which may result in the tail rotor striking the surface.

Normal approach to hover OGE with vertical landing

  • Not having sufficient power to sustain a hover
  • Descending too rapidly, vortex
  • Spatial orientation of obstacles around the helicopter

AIRMANSHIP / SAFETY:

The most important aspect of this exercise is to understand the different power requirements depending on the mission to be accomplished. Always remember when landing in a confined area there is normally a reason which will directly affect the power requirement i.e. you could be landing to pick up a passenger or fuel or vice versa you could be dropping a passenger off. This needs to be considered to be able to eliminate certain dangers.

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