Helicopters in Turbulence: Part 2

Forensic engineers examined the main rotor driveshaft that had sheared just below the teeter stops. Teeter stops are fitted to the main rotor driveshaft to prevent the main rotor spindles from contacting the driveshaft in normal operation. The accident report noted that New Zealand’s topography and prevailing wind conditions mean that for much of the country, and particularly the South Island, turbulent conditions can be encountered most of the time. Credit: New Zealand Transport Accident Investigation Commission

The first part of this article series covered how trubulence can cause catastrophic mast bumping, main-rotor stall and loss of control.

Prior to a flight in mountainous terrain on June 8, 2002, the pilot of a Bell 47G3-B-1 obtained a weather forecast that called for winds from the southwest at 30 mph, with gusts up to 40 mph.

The flight departed and was in cruise flight at 9,000 ft. MSL (approximately 700 ft. AGL) with a 20- to 25-kt. headwind and experiencing light turbulence, when it encountered “severe turbulence.” The pilot stated that the helicopter went “up and down violently for a period of 2 to 3 sec.” The pilot performed an autorotation to sloping terrain without further incident.

The NTSB determined the probable cause was the pilot’s inadvertent inflight encounter with severe turbulence, which resulted in main-rotor-tailboom contact and the separation of the tail-rotor drive shaft.

While transiting a ridgeline at 8,100 ft. MSL near Placitas, New Mexico, on Jan. 22, 1995, a Robinson R22 encountered turbulence, resulting in a loss of airspeed and rotor rpm. The pilot was unable to maintain terrain clearance.

The helicopter settled into trees along the side of a slope. The NTSB determined the probable cause was the pilot’s decision to operate beyond the climb capability of the helicopter, resulting in inadequate altitude and terrain clearance over mountainous rising terrain. A factor was turbulence.
The localized winds that occur in the vicinity of mountainous terrain often contain rapidly changing directions and magnitudes that can compromise a rotorcraft’s performance and/or control.

On April 5, 1992, the pilot of an R22B indicated that prior to departure he had received a full weather briefing and no significant weather was forecast for his route of flight. The initial portion of the flight near Palm Springs, California, was uneventful.

However, when the pilot entered a mountain pass area at his cruise altitude of 500 ft. AGL, the winds became very strong and he experienced severe turbulence. The pilot further reported that he attempted to turn into the wind for the purpose of making an emergency landing. While maneuvering, he lost control of the helicopter and collided with the mountainous terrain at an elevation of about 3,000 ft. MSL. Fortunately, the pilot was uninjured.

The NTSB determined the probable cause was the pilot’s failure to maintain a proper cruise altitude upon entering a mountain pass area, and his failure to maintain control of the helicopter. Factors that contributed to the pilot’s loss of control were related to his encountering turbulence and downdrafts.

On June 28, 1992, a Bell 222UT EMS helicopter encountered clear-air turbulence while en route to pick up a patient in mountainous terrain. The rotorcraft’s nose pitched up rapidly to about 20 deg., which was severe enough to activate the aircraft’s ELT.

The pilot felt feedback through the controls and landed to reset the ELT. Later, the pilot noted mast torque fluctuations followed by a zero reading on the gauge. Post-flight inspection disclosed the transmission had contacted its mounts, severing several electrical leads including the torque sensor. A crack was also discovered about 2.5 ft. outboard of the main blade grip, propagating back from the blade leading edge.

The NTSB determined the probable cause of this incident was an encounter with clear-air turbulence.

Entangling External Load

Mountain-induced turbulence caused tragedy for the Honolulu Fire Department’s McDonnell Douglass 369D helicopter on July 21, 1995. It was called upon to insert two police officers into mountainous terrain to search for a missing hiker.

The two officers were carried in a rescue net 50 ft. below the helicopter. Deteriorating weather and cloud cover with turbulence, a very normal occurrence over the Hawaiian island’s mountainous terrain, was present. Hikers in a canyon near the accident site observed the net swinging like a pendulum through an arc of 45 to 50 deg. Shortly thereafter, the helicopter plummeted out of control and impacted terrain, fatally injuring all three public safety officers. Examination of the wreckage revealed that the line that secured the net to the helicopter became entangled with the main rotor and was found wrapped around the main-rotor mast, and all the main-rotor pitch change links were fractured.

The NTSB determined the probable cause was the pilot’s poor judgment by intentionally flying into adverse weather conditions in mountainous terrain with an external load, which resulted in loss of aircraft control.

On Feb. 23, 1992, near Hobart Bay, Alaska, the pilot of a Bell 214B-1 had a mechanic attach a 30-ft. “choker cable” to the external cargo hook so that when he came to a hover to pick up passengers, the cable would touch the ground and dissipate the static electricity. After picking up the last timber cutters, en route to base camp, the helicopter flew through turbulence, during which the external load cable became entangled in the tail rotor. The tail rotor and 90-deg. gearbox separated from the helicopter. The pilot autorotated into tall trees, fatally injuring six occupants and seriously injuring five others.

The NTSB determined the probable cause was separation of the tail rotor and 90-deg. gearbox due to the pilot’s failure to follow procedures/directives, which resulted in the external load cable becoming entangled in the tail rotor. Factors were the turbulent weather and the lack of suitable terrain for a forced landing.

A Need for More-Effective Training

Poor inflight weather decision-making can expose the helicopter to turbulence, which is certainly capable of causing mast bumping, main-rotor stall, compromises in control and/or performance of the rotorcraft, and external load entanglement, all of which are serious undesired aircraft states likely to end tragically. The best preventive action is simply to “avoid…avoid…avoid.”

Important points are noted in a New Zealand accident report involving an R44 mast bumping and inflight break-up. It reminds pilots of two-bladed, semi-rigid rotor helicopters to be acutely aware of the risks and effects of encountering moderate or greater turbulence in strong winds, especially in the lee of high terrain.

The accident report noted that New Zealand’s topography and prevailing wind conditions mean that for much of the country, and particularly the South Island, turbulent conditions can be encountered most of the time.

This of course could be said for many other regions around the globe, to include (but certainly not limited to) major sections of Alaska and Canada, the western U.S., and European countries such as Scotland, Norway and Switzerland.

If moderate or greater turbulence is encountered while flying a two-bladed, semi-rigid rotor helicopter, the pilot should consider landing and waiting for the conditions to improve. Pilots of this type of rotor system should be aware of the helicopters’ increased susceptibility to low-G conditions when lightly loaded, and the adverse effects that high power and a high tail rotor have on the rate of roll.

How do we train pilots to recognize and avoid these hazardous terrain-induced turbulent flows? Good aeronautical decision-making requires situational awareness, experience and skills. Yet the route to gaining “enough” experience in this high-risk mountain environment can be unforgiving. The aviation accident rate in mountainous regions clearly indicates that this is not a low-risk training environment.

Clearly there is a need for an effective training approach in which pilots can effectively learn insightful lessons about meteorological threats in the mountain environment without the risks.