In defining trigger events, the division divided the operating parameters into general categories such as departure, vertical speed, attitude control, altitude and landing. For example, parameters on departure included excessive pitch angle on takeoff, excessive climb and premature turn before reaching 300 ft. AGL.
Vertical speed trends examined excessive descents and climbs. Further, an excessive descent at altitudes greater than 500 ft. AGL was 1,250 fpm and higher, while 750 fpm and above were excessive when the aircraft was within 500 ft. of the ground.
Attitude control triggers included excessive bank angle, steep turns, and yaw and roll rates in excess of 30 deg./sec.
Altitude trends included low cruise over land, low cruise offshore, premature departure turns and low turns to final.
Landing parameters included an excessive pitch angle on landing — that is, greater than 18 deg. nose up when less than 150 ft. AGL — and turns to final at less than 300 ft. AGL.
Since negative g loading that is less than 0.2 g is a concern for semi-rigid rotor systems because of the threat of mast bumping, that was also on the watch list.
So, what did the monitoring uncover? In January 2009, the fleet experienced a total of 784 trigger events. The most common was an excessive descent below 500 ft. AGL (256 events; 33%). Excessive descent rates are a symptom of settling with power, a situation in which the helicopter descends within its own airflow, or with the rotor in the vortex ring state. Depending on the type of rotors, settling with power can begin with as little as a 300-fpm rate of descent.
Other contributing conditions to the phenomenon include an airspeed less than effective translational lift and power between 20% and 100%. Common situations that often lead to settling with power include attempting to hover above hovering ceiling, steep approaches with tailwind and hovering OGE without precise attitude control.
The remaining trigger event, presented by total number at percentage they represent, were: excessive bank angle (133; 17%); low turn to final (105; 13%); excessive descent above 500 ft. AGL (95; 12%); excessive pitch angle landing (56; 7%), excessive pitch angle on takeoff (53; 7%); premature departure turns (41; 5%); and the remaining events (6%) scattered among the other categories.