Slope limits

September 26, 2012 by Tim McAdams

Since helicopters land in areas that have not been previously approved, the pilot must make some last minute decisions regarding the landing site. One of these is the slope of the land where the helicopter will be touching down. Depending on the model helicopter the flight manual might have published limits.

The Bell 206 Jetranger is one helicopter that does not have slope limits listed in the limitations section of the flight manual. Bell’s approach is that slope landings are a function of available cyclic margin. In other words, if the pilot determines that the limit of cyclic control (close to or at the physical stop) will be reached before the helicopter is completely seated on the slope, then the slope is too steep and the landing should be aborted. (The proper technique to execute a slope landing is another discussion coming up.)

However, in the case of Eurocopter’s AS350 AStar the helicopter’s flight manual contains limitations on the amount of slope (in degrees) depending on the direction the pilot wishes to land. This is due to stress placed on the mast when landing on a lateral slope greater than 8 degrees.









The maximum slope when the ground is sloping down is 6 degrees. The shallower slope limitation in this direction is due to a 2 degree forward tilt that is built into the rotor mast. 




Also, the 2 degree tilt allows the maximum slope when the ground is sloping upwards to be 10 degrees 



Trying to determine the exact angle of a slope while hovering is difficult at best, however, with enough experience in a making off airport landings in a specific helicopter a pilot can become fairly good at judging the safety of landing on sloped terrain.

  • Avi Weiss

    While available cyclic margin is the primary factor dictating acceptable slope angle, it is also a good idea to keep in mind surface conditions (reduce coefficient of friction) which may not be an issue on level terrain, but could be an issue while at an angle. Such conditions include wet grass, wet hard surfaces close to freezing, and slightly soft terrain.

  • Alexis Geacintov

    Another thing to consider is the critical angle at which dynamic rollover will occur, which changes based on the changing center of gravity due to the amount of fuel on the aircraft. Normally, having more fuel on board will cause the helicopter to be heavier, and bring the center of gravity down along the vertical axis. The lower center of gravity will result in a greater critical angle. The difference can be significant.

  • Alan D. Resnicke

    While flying military aircraft, especially the UH-1N “Twin Huey,” we routinely practiced slope landings. It was a great contest between pilots to determine how much slope we could set down on and how smoothly we could do so. Occasionally it wasn’t cyclic limited but we’d be right on the edge of mast bumping (semi-rigid rotor system). For that, the Sikorsky HH-53 was much more capable. Still, the comments about surface stability that Avi Weiss references can make a huge difference even without mechanical limitations.

  • don mclean

    Also; translating tendancy and wind will affect how much cyclic you need to use. Which is another reason Bell doesn’t give a number to the degree of allowable slope.

  • dom

    As an H-60 driver for the army, we have tiltled tail rotors which means left pedal application on the ground = right roll, although having a crew chief or two poking their heads out of the window clearing the aircraft down for you and letting you know exactly how high each of your wheels are lightens the work load. Air Assault!

  • Ron Stahla

    While sitting on a lateral slope the fuel will run to the low tank and will affect how much cyclic you need to use.

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