NOTES ON STEEP INSTRUMENT APPROACHES IN A HELICOPTER

Precision instrument approaches have been made in a single-rotor helicopter at approach angles as high as possible for a speed range from 65 knots to 25 knots. Sixty-five knots was the cruise speed and the minimum speed of 25 knots was determined by limiting aerodynamic behavior of the helicopter and the fact that the difficulty of instrument flying in general increased as speed was reduced. Possible approach angles increased progressively as the speed was reduced. In general, it was found that the difficulty in following the glide slope increased as the angle steepened and the rate of descent increased. With steeper glide slopes, the variation in rate of descent to correct for horizontal wind gusts increases, imposing a very definite limit on the maximum approach angle practicable under gusty conditions. Also, at high angles the effects of wind gradients in both direction and velocity with height are appreciable in affecting control of the glide path. As the rates of descent become high the difficulty of initially establishing the desired glide path increases and the time for bracketing and correcting becomes shorter to the point of being limiting. Also at high rates of descent, the pilot could not safely observe reasonable weather minima. In view of these factors, the operational limits would seem to be defined by rates of descent between 500 fpm and 700 fpm for the range of speeds considered. These rates of descent correspond to maximum angles between 12° and 16° for the lowest speed found practicable in this case, 25 knots. It was found that lag due to friction in descents for some standard 50,000-foot altimeters could result in large errors not compatible with low approaches. Surprisingly enough the vibration in the helicopter was not great enough to reduce the friction errors to those found in standard calibration procedures. There are altimeters now available, fortunately, with very small friction errors. Assuming no altimeter errors, 100 feet and 1/4 mile would seem to be practical weather minima for a speed of 65 knots if a landing area approximately 200 feet by 400 feet were available. These weather minima could probably be reduced to threshold visibilities of 1/16 mile (330 ft.) if a speed not greater than approximately 25 knots were used and the landing area were easily identified and properly marked. A low-speed spiral-type descent might be a practical means of making essentially vertical approaches, given a suitable method of navigation for the pilot.