Landing on a Slope

December 4th, 2009 Leave a comment Go to comments

Landing on a Slope

This maneuver is used to transition the helicopter from hover to a landing on a slope.

Maneuver Description

First the helicopter must be positioned over the slope. Care must be taken not to place the tail rotor in a position where it will strike the ground. Most slope landings are performed parallel to the slope. Landings can be done nose in to the slope if tail rotor clearance is assured. We will describe a parallel approach here.

With the helicopter positioned above the intended landing area, and aligned parallel to the slope, the pilot should descend by lowering collective. When the uphill skid contacts the slope, pause, and add just a little cyclic into the hill. This will help to lock in the landing gear. Both the front and rear (toe and heel) of the gear should be in contact with the ground. If not, the helicopter will tend to yaw around the single pivot point in contact with the slope. If the slope and the skid are not at the same angle, the helicopter can be pivoted slightly using pedal to find an angle where the skid will be alighned with the slope. Care must be taken not to swing the tail rotor into the hillside. The landing can be done with only a single contact point on the uphill skid, but it will be much harder. The pilot should first see if there isn’t another section of slope that will allow the skid to be properly planted.

Once the skid is planted, the pilot lowers the downhill skid toward the ground by lowering collective. Rather than a continuous rolling motion, an iterative process of lowering the skid a few inches then pausing before lowering it some more prevents too much of a rolling momentum from building up. This is important if the slope turns out to be too steep to land on.

As the downhill skid is lowered, the cyclic needs to be deflected into the hillside in order to keep the rotor disk horizontal. Since the swashplate is connected to the fuselage, failure to displace the cyclic into the hillside will cause the rotor to tilt toward the downhill side, and the usual result will be skidding the helicopter sideways down the hill (not such a great idea!). Cyclic inputs should be coordinated with fuselage roll, not with collective pitch motions.

After the downhill skid makes contact, the pilot should continue to use caution lowering the collective in case there is any tendency for the helicopter to tilt toward the downhill side. Some people recommend centering the cyclic after the collective reaches flat pitch, other people recommend keeping it displaced into the hillside for the entire duration of the slope landing (including shutdown if the helicopter is to be parked on the slope).

Pilots who fly helicopters with twin, cross feeding fuel tanks mounted high on the fuselage (like the Bell 47) need to consider the fact that while parked on the hill the fuel will cross feed from the upper tank to the lower tank, and the CG shift may be severe enough to roll the helicopter over while it is parked, or to prevent a safe liftoff later on.

Center of Gravity Considerations

If passengers or cargo are to be loaded or unloaded while parked on the hillside, the pilot needs to take into account the effect the shift in CG will have on his ability to take off again. Unloading weight from the uphill side and then trying to take off could cause the pilot to run out of uphill cyclic authority and could cause a dynamic rollover downhill to occur. Generally, if weight is going to be offloaded, it’s a good idea to land with that side of the helicopter downhill. Similarly, if weight is going to be added to the helicopter, it’s a good idea to add it to the uphill side.

Wind and Tail Rotor Considerations

The factor which usually defines the slope limit for a helicopter is the amount of cyclic authority available to the pilot. If the wind is blowing downhill, less cyclic authority will be available to the pilot, because some of it will have to be used to counter the downslope wind.

Similarly, if tail rotor translating tendency requires left cyclic to counter, less left cyclic authority will be available. Thus landing on a slope right skid upslope may allow a steeper slope landing. Manufacturers tend to tilt the masts to counter translating tendency, so this guideline may be more or less true given different makes and models of helicopters.

Common Mistakes

First of all, the same mistakes pointed out in Slope Takeoffs are common during landings, namely not using enough uphill cyclic, using too much uphill cyclic, rolling downhill too fast, or overcontrolling the cyclic. Also, the following unique errors are often made:

Failure to lock in the uphill skid before lowering the downhill skid

People in a hurry will often just try to plant the uphill skid quickly and then start lowering collective. If the uphill skid is not properly planted both front and rear, the result is usually a wobbly landing. It makes sense to take the time to properly plant the uphill skid.

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