Slope Limits
Goodpost Ian C, and an excellent article by James McCollough from the Bell Test Pilots Office.
Just to add an observation as to why I think those limits are worth posting in operators manuals: Hopefully, any pilot's checkout in a new vehicle includes slope landings, and those landings ought to be to the limits, so that the pilot becomes familiar with the aircraft behavior and control positions. Not all machines have similar limits and have varying amounts of control margin at those limits. In some cases, the limits are set by rules that may surprise. True story follows.
US Army requirement for UTTAS ( UH-60A ) was that the machine had to able to land on 12 degree slopes at any azimuth to the slope and laterally ( sideways onto slope ) at 15 degrees. Design gross Weight ( 16,500 lbs for the SA UTTAS version ). US Army guidance ( written in the Request for Proposal Handling Qualities section ) was that full control could be used, i.e., stick on the stop. For the 15 degree left wheel up slope landing, the cyclic control was hard on the left stick stop with the downhill wheel still a foot or a bit more in the air ( copilot graciously moved his leg ) so the collective was lowered to get that gear on the slope and that was determined to be in compliance with the requirement. Aside: at 15 degrees, there is real and appropriate concern for foliage and any obstacle on the uphill side as the rotor tip path is getting fairly low.
Now in the field, there is no way to eyeball a slope and determine if it is within your envelope or not, but if you have been trained to the envelope, you'll know, as you go down onto the slope, what to expect of your aircraft, and when to quit*. And you won't be placed in a position of not having been there before.
*There are a number of experienced and savvy aviators who peek into these pages, so inevitably, someone will ( correctly so ) remonstrate that the limits were written and tested to in zero wind, and what about that? I believe that observation is akin to the problem of not knowing what the real slope angle is, in the first place, and is another reason for thorough training.
( Minor point of interest to some: the lateral control range at the UH-60 rotorhead was set by this lateral slope landing requirement! )