MD Helicopters 900 N179PA

Summary

On 16 October 2003 at about 23:50 local time, a MD Helicopters 900 registered N179PA, operated by National Park Service, was involved in an accident near Fredonia, Arizona, United States. 5 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

16 October 2003 at 23:50

Classification

Accident

Location

Fredonia, Arizona, United States

Nearest airport

Bear Dip Landing Zone

Coordinates

36.8596, -112.8394

Aircraft

MD Helicopters 900

Registration

N179PA

Category

Helicopter

Engines

2

Operator

National Park Service

Operating rule

Public Use

Purpose of flight

Public Use

Phase of flight

Not recorded

Route

North Rim → Fredonia

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

LAX04TA017

Nobody was injured.

the pilot's in-flight loss of control due to the fatigue fracture and separation of the force limiting control rod (an integral part of the anti-torque system), which resulted in the helicopter's uncommanded yaw/spin and subsequent collision with terrain.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

As the NOTAR (No Tail Rotor) helicopter entered a normal descent for landing, the pilot experienced a loss of control following the failure of the anti-torque system. The helicopter entered an uncommanded and uncontrollable right spin as it descended to ground impact. During the approach, and after passing over a group of trees, while the helicopter was about 15 to 30 feet above ground level (agl) and 60 feet from the intended touchdown location, the helicopter began a rapid, uncommanded right yaw. The pilot input full left pedal pressure in an effort to stop the rotation. Despite his attempts, the pilot's control inputs had no effect on the spin rate. The pilot retarded the left engine throttle, and maneuvered the helicopter away from the trees as it continued to descend. Upon touchdown, the helicopter rolled on its left side, and the main rotor blades impacted terrain. The helicopter's NOTAR anti-torque yaw control system utilizes a transmission driven fan with variable pitch blades to supply air to circulation control slots on the tail boom and a pilot-controlled directional jet thruster nozzle. Movement of the anti-torque control pedals changes the fan blade pitch to produce more airflow to the circulation control slots and the anti-torque thruster nozzle. As the pedals are displaced from center toward either extreme of travel, the airflow increases proportionally. During post-accident examination of the NOTAR control system, a fracture and separation was found in the force limiting control rod. With the rod assembly disconnected, the NOTAR fan blades assume a neutral pitch, preventing a sufficient volume of air in the tail boom to provide anti-torque control and maintain a heading. The helicopter had previous mechanical problems with the rod assembly and it was replaced about 779 hours prior to the accident. Using a scanning electron microscope, an examination of the fracture surface revealed that the rod separated due to a fatigue crack originating from the base of the thread roots, and final tension overload. There was no evidence of damage sites, foreign material inclusions, or other potential fracture initiating conditions.

Quoted verbatim from the NTSB record.