[FF-5] Slopes and Dynamic Rollovers

Slope Operations. When the helicopter rests on a slope, the mast is nearly perpendicular to the inclined surface, while the plane of the main rotor must parallel the true horizon or tilt slightly up-slope (refer to the flight manual for specific slope operation limitations). Thus, the rotor tilts with respect to the mast. Normally, the cyclic control available for this rotor tilt is limited by cyclic control stops or by contacting the droop stops. These control limits are reached much sooner in down-slope wind conditions. Also, since the H-60 characteristically hovers slightly left side low, there will be less control travel when landing with the left side up-slope. 

Landing down-slope is usually the most difficult operation. The helicopter will have a tendency to roll down the slope. Setting the parking brake prior to attempting the slope operation will help prevent this rolling tendency. Due to the normal tail-low hover attitude, a great deal of aft cyclic must be applied to accomplish a down-slope landing. The tailwheel will touch very early, while the main gear is still at a considerable height above the ground. Droop-stop contacts can easily occur when the nose is down-slope, and the collective is minimized with aft cyclic applied. Landing up-slope provides certain advantages. Due to the tail-low attitude in a normal hover, an up-slope landing enables the tailwheel and main gear to touch almost simultaneously with minimal cyclic input. The disadvantages of landing up-slope are the reduced main rotor clearance off the nose (due to mast tilt) and a tendency to roll downhill (backwards). When landing cross-slope, the uphill main gear is placed on the ground first, and the collective is slowly reduced while cyclic inputs are made toward the hill to keep the helicopter on the landing point. Positive heading (pedal) control on a forward reference point is critical during this maneuver.

Abort the landing attempt if cyclic control travel limits are reached (or droop-stop contact occurs) before the downhill main gear is firmly on the ground. Once the aircraft has weight on all three wheels, the collective should be carefully lowered as far as corrective cyclic inputs permit. Once safely on the ground, some pilots maintain the cyclic and rotor tip path plane with the last required tilt for the landing, while other pilots attempt to level the disk with the slope (perpendicular to the mast). Maintaining the tilted tip path (that was required for landing) ensures stable ground contact, but provides marginal clearance from the tip path to the up-slope. This could be very hazardous to crewmembers/teams entering/exiting the rotor
path from the up-slope side. The technique of leveling the tip path (with respect to the slope) avoids this hazard, but may be less stable, with a greater tendency to roll down-slope.

The takeoff from the slope is basically the reverse of the landing procedure. Instead of "guessing" where the cyclic and rotor tip path plane should be, always adjust the main rotor disk to a near level (true horizon or VSI) position, or perhaps very slightly biased to the left rear. This is approximately the cyclic/tip-path position in a normal hover. Slowly increase collective to raise the down-slope main gear, then carefully bring the helicopter up vertically off the slope.

Dynamic Rollover. Dynamic rollover may occur on level ground, but is more critical during slope or crosswind landing and takeoff maneuvers. Every helicopter has a critical rollover angle beyond which recovery is impossible. If this angle is exceeded, the helicopter will roll over on its side regardless of cyclic corrections introduced by the pilot. The rate of rolling motion is also critical. As the roll rate increases, it reduces the critical rollover angle at which recovery is still possible. The critical rollover angle is dependent on which wheel is touching the ground, crosswind component, lateral offsets in center of gravity, and left pedal inputs for torque correction. Dynamic rollover starts when only one main wheel is on the ground and that wheel becomes a pivot point for lateral roll. The wheel may become a pivot point for a variety of reasons. It may be caught on something projecting from the landing surface, it could be stuck in ice or mud, or it could be forced into the slope by improper landing techniques.

Prevention of down-slope rollover during landing. If the combination of slope, wind, and center of gravity (CG) conditions exceed lateral cyclic control limits, the mast forces the rotor to tilt down-slope, resulting in lift with a down-slope component, even with full up-slope cyclic applied. To prevent downslope rollover during landing, slowly descend vertically to a light ground contact with the up-slope main gear. Slowly and cautiously lower the down-slope wheel. As the cyclic nears the lateral stop, compare the distance to go with the lateral control travel remaining. If it appears that the cyclic will contact the upslope control stop before the down-slope gear is firmly on the ground, return the helicopter to a level flight
attitude and abort the slope landing. The wind direction/velocity, CG, or amount of slope must be changed. 

Prevention of down-slope rollover during lift-off. This is not usually a concern, because if adequate control was available for the slope landing, it should be adequate for the lift-off. If winds have become adverse, or if CG is now adverse (due to passenger onload) or the gear is stuck or sunk in soft ground, this type of rollover could be a possibility. In these situations, you should reduce power at the first sign of lateral roll around the down-slope wheel, and abort the lift-off attempt. Before making another lift-off attempt, await different wind conditions, change CG loading, or dig out under the up-slope gear.

Prevention of up-slope rollover during lift-off. This primarily results from excessive use of cyclic to hold the up-slope gear against the slope. Improper use of collective pitch could then result in a rapid pivoting around the up-slope main gear to the point of rollover. To prevent up-slope rollover, the pilot should cautiously lift the down-slope side of the helicopter to the level point and simultaneously work the cyclic control to neutral. Once the cyclic is neutral and/or the up-slope main gear has no side pressure applied, a vertical ascent can be continued.