Attitude, Airspeed, and Torque during an Instrument Approach to a Hover
Credit to Paul Ratte, a CG H60 pilot in the 90’s and early 2000’s, who first introduced me to what I consider the most important concept in flying stabilized instrument approaches to a hover – the relationship between attitude, airspeed, and torque.
The relationship between attitude, airspeed, and torque, as a helicopter descends and decelerates, is represented in the graph below. The numbers were not derived from performance data and are generic for demonstrable purposes.
The graph starts with altitude, airspeed, and torque constant.
To initiate an instrument approach to a hover, pilots decelerate with an increase in pitch and then apply a corresponding decrease in power to start the descent and avoid a climb. An abrupt pitch increase requires a rapid and appreciable decrease in torque, whereas a gradual pitch increase demands a less appreciable and smoother decrease in torque. Because instrument approaches to a hover start around bucket airspeed, shortly after the increase in pitch, the helicopter reaches the lowest power requirement of the approach, due to the “flare” from the pitch increase and the helicopter being at or near the airspeed requiring the least amount of power. Consequently, when the increase in pitch is rapid, requiring a subsequent rapid decrease in torque, a small “collective check,” or power increase, may be required to affect a steady descent and deceleration. After the initial check, the pilot smoothly and steadily adds just enough torque to hold a constant glide path.
The torque line on the graph shows a near instantaneous reduction of power at approach initiation, followed by a systematic increase of power. On the backside of the power curve, when torque requirements exponentially increase as the helicopter decelerates, to maintain a constant glidepath, the rate of power application must continually increase as the helicopter slows.
Phrased differently, the torque line is curved because the rate of power application per knot of deceleration increases the closer the aircraft gets to a hover. Therefore, while pilots attempt to produce a linear descent with respect to glide path, the rate of collective pull will not be linear over time because the pilot will be adding power more quickly toward the end of the approach. It is important to remember that in an ideal instrument approach to a hover, once the descent is established after approach initiation, power is not reduced again until the descent is stopped, and a hover is achieved. The torque setting peaks slightly above hover power (to eliminate the descent rate), then decreases slightly to a steady state hover power setting, once established in the hover.
To fly a precise, stabilized instrument approach to a hover, after the initial pitch and power adjustments, ideally, the pilot does not chase vertical speed by decreasing and then increasing the collective. Rather, if during the deceleration the helicopter descent rate is too slow, the pilot freezes the collective and continues to decelerate. As the helicopter decelerates on the back side of the power curve with a constant power setting, the rate of descent will increase. When the helicopter returns to the appropriate rate of descent, the pilot can then resume adding power. Although during the ideal instrument approach to a hover power application would never stop, during a stabilized hand flown approach, in reality, power is applied, held steady, then applied again in small, short increments as the helicopter slows at the beginning of the approach. When the helicopter slows toward a hover, the rate of application increases exponentially to a smooth power application, peaking just above the power required to hover, to stop the descent. In contrast, a destabilized approach is marked by alternatively increasing and decreasing power to chase the appropriate rate of descent, which is demanding as large rates of descents can result from simultaneously slowing and decreasing power.
Regarding attitude, the larger the pitch changes, the more dynamic the maneuver, however ideally, pitch changes are minimal (no more than 5 degrees above hover attitude) throughout the instrument approach to a hover. “Back in the day,” pilots experimenting with different techniques compared approaches with small, smooth, controlled pitch changes and larger changes in torque to approaches with larger pitch changes and more consistent torque. They discovered that the first portion of the approach can be managed with pitch if the pilot systematically trims the nose higher to descend at the appropriate rate, while maintaining a fairly constant low power setting. However, because power required to hover remains relatively high, trimming the nose higher will result in a destabilized second portion of the approach because as the helicopter bleeds off its inertia, a large, rapid power application at the end of the approach is necessary to stop the descent in a hover. Not only is the timing of this power application difficult, but it can also induce vertigo, resulting in a high rate of descent low to the water. This can feel more like a power recovery to a practice autorotation than a smooth, controlled, establishment of an instrument hover.
