South Bay, FL, USA
N318DB
SUD AVIATION SA 318C
During a deer population survey flight over swampy terrain, the pilot descended the airplane from 200 feet to a hover about 25 feet above ground level and then side-stepped the helicopter right to maneuver over an island. As he did so, the helicopter began to yaw right and descend. The pilot corrected the yaw and increased collective pitch; however, the helicopter continued to descend. He then moved the cyclic forward in an attempt to fly through effective translational lift, but the descent continued, and the helicopter impacted the swamp and came to rest on its right side. Examination of the wreckage revealed that the helicopter had adequate fuel and was about 400 pounds below its maximum gross weight. No evidence of any preimpact mechanical malfunctions were found that would have precluded normal operation. However, the examination did reveal that the main rotor pitch angle cockpit indication was at 20 degrees (16 degrees when power was applied), which was beyond the redline of 15 degrees. The helicopter would have required more power to hover in sideways flight, below effective translational lift, than it would have required for a stationary hover. One of the common errors in hovering sideways flight is the failure to maintain proper rotor rpm. The excessive main rotor blade pitch angle and the pilot's report that the helicopter continued to descend after he increased the collective pitch and that he had to correct a right yaw indicate that it is likely that he failed to maintain proper rotor rpm during the hovering sideways flight.
On May 8, 2013, at 0913 eastern daylight time, a SUD Aviation SA 318C (Aerospatiale Alouette II) helicopter, N318DB, operated by Mile Hi Inc, was substantially damaged during impact with a swamp, following an uncontrolled descent from a hover near South Bay, Florida. The commercial pilot and three passengers were not injured. The local aerial observation flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the flight that departed a boat dock near Weston, Florida, at 0713. The passengers were conducting a deer population survey. The pilot reported that prior to departure, the helicopter was fueled to approximately 105 gallons. Most of the flight was conducted about 200 feet above ground level (agl) at 60 knots. Toward the end of the flight, a passenger asked to get a closer look at a particular area of vegetation. The pilot descended from 200 feet to a hover about 25 feet agl, approximately 30 yards from the intended spot, on a southerly heading, over saw grass. He then side-stepped the helicopter to the right, over to his intended spot, which was above willow trees. As the helicopter neared the willow trees, it began to yaw right and descend. The pilot corrected the yaw and increased collective pitch; however, the helicopter continued to descend. He then moved the cyclic forward in an attempt to fly through effective translational lift, but the descent continued and the helicopter impacted the swamp, coming to rest on its right side. During the impact, the main rotor blades and fuselage sustained substantial damage. The three passengers reported that they did not hear any warning noises or notice anything abnormal as the helicopter descended to the ground. A handheld GPS receiver was recovered from one of the passengers and forwarded to the NTSB Vehicle Recorder Laboratory, Washington, D.C. Data were successfully downloaded and plotted; however, the data points did not include time or altitude. Review of weight and balance information recovered from the cockpit revealed that at the time of the accident, the helicopter weighed about 3,250 pounds (lbs), which was 400 lbs below its maximum gross weight of 3,650 lbs. The wreckage was examined by representatives from the airframe and engine manufacturer, under the supervision of a Federal Aviation Administration (FAA) inspector. The examination did not reveal any preimpact mechanical malfunctions with the airframe or engine. Adequate fuel remained in the fuel tank and both the fuel control unit and emergency fuel shut-off valve were in the off position, consistent with the pilot securing the helicopter after the accident. The examination also noted that the cockpit indication of the main rotor pitch angle was 20 degrees (16 degrees after power was applied), with redline at 15 degrees. Review of FAA-H-8083-21A, Helicopter Flying Handbook (HFH), revealed that one of the common errors of hovering sideward flight was failure to maintain proper rotor rpm. The HFH further stated, "Under certain conditions of high weight, high temperature, or high density altitude, a pilot may get into a low rotor rpm situation. Although the pilot is using maximum throttle, the rotor rpm is low and the lifting power of the main rotor blades is greatly diminished. In this situation, the main rotor blades have an AOA that has created so much drag that engine power is not sufficient to maintain or attain normal operating rpm…As soon as a low rotor rpm condition is detected, apply additional throttle if it is available. If there is no throttle available, lower the collective. The amount the collective can be lowered depends on altitude…since the tail rotor is geared to the main rotor, low main rotor rpm may prevent the tail rotor from producing enough thrust to maintain direction control…" The recorded wind at an airport located about 25 miles east, at 0853, was from 290 degrees at 3 knots.
The pilot’s failure to maintain proper rotor rpm while hovering in sideways flight.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
