Bell 212 Loss of Control – Inflight: After Windshear Encounter in the Lee of Mountain Ridge (Airspan Helicopters C-GNYI)
History of the Accident Flight
…moderate, with localized severe, mechanical turbulence and low-level wind shear were expected in the area… Though the strong winds were a concern, the flight was dispatched with the knowledge that it would be a turbulent flight. The PIC [the operator's President and Operations Manager, who had 9126 hours of experience, 1247 on type] made the decision to depart based on an improving forecast later in the day, the desire to complete the operational flight, and the observation that other aircraft were operating in and out of Sechelt Aerodrome.
The second pilot [who had 5757 hours of experience, 480 on type] then began to prepare the helicopter for the flight.
…while in the cruise… the helicopter entered wind shear and experienced a sudden, dramatic right roll with nose-down pitch.
Unrestrained items in the cabin struck the pilots, and an impact on the second pilot’s helmet made their visor cover to move temporarily blocking their vision as a critical moment.
After regaining control, the number 2 engine experienced an uncommanded in-flight shutdown and the flight controls became very hard to manipulate.
A location on nearby Bowen Island, BC, was selected for an emergency landing; however, the helicopter was still difficult to control and the pilots were unable to manoeuvre it to the desired location. During the descent, the helicopter began a rapid rotation to the right, which the pilots were unable to arrest. After several rotations, the helicopter collided with trees and came to rest on a rocky ridge approximately 270 feet above sea level on the northwest corner of Bowen Island.
Fortunately there was no post-crash fire as…
…fuel lines under the cabin floor panel were compromised during the impact and began leaking fuel onto the pilots, who were still seated in the inverted fuselage, restrained by their safety harnesses. The PIC assisted the second pilot, whose foot was pinned in the footwell, and they both egressed successfully.
The TSB Safety Investigation & Analysis
The TSB explain that:
On the morning of the occurrence, strong south easterly winds were blowing in the vicinity of the accident site. The west side of Bowen Island is dominated by a long ridge which is anchored at the south end by Mount Gardner with an elevation of 2388 feet ASL. The flight’s intended destination, Cypress Provincial Park, is approximately 6 NM east of Bowen Island. Within the park are Mount Strachan with an elevation of 4714 feet ASL and Black Mountain with an elevation of 4016 feet ASL.
The final minutes…would have been flown on the downwind, or lee side, of this terrain.
Lee effects are described in The Weather of British Columbia as follows:
When the winds blow against a steep cliff or over rugged terrain, gusty turbulent winds result. Eddies often form downwind of the hills, which create stationary zones of stronger and lighter winds.
These zones of strong winds are fairly predictable and usually persist as long as the wind direction and stability of the air stream do not change. The lighter winds, which occur in areas called wind shadows can vary in speed and direction, particularly downwind of higher hills. In the lee of the hills, the wind is usually gusty and the wind direction is often completely opposite to the wind blowing over the top of the hills. Smaller reverse eddies may also be encountered close to the hills.
- Delay your flight when the winds at mountain-top level are greater than 30 knots[…]
- Don’t try to out-climb downdrafts—your aircraft may not be able to. Turn out of the sinking air as soon as you can and locate the rising air (usually by turning upwind or downwind), which will be nearby
- Cross ridges at a 45-degree angle
- Stay away from violent turbulence from mountain waves and rotors; know what warning clouds look like
TSB concluded that:
The helicopter entered an area of severe turbulence that led to low-G conditions and a loss of control that included extreme attitudes, that resulted in excessive flapping of the main rotor blades.
Examination of the wreckage confirmed that…
…the main rotor blades made contact with the tail rotor drive system, disabling the tail rotor driveshaft ahead of the intermediate gearbox and leaving the helicopter with a loss of tail rotor thrust.
As the pilots regained control of the helicopter following the initial loss of control, neither pilot was aware that the main rotor had disabled the tail rotor driveshaft. With the helicopter moving at high speed after the upset, very little yaw control was required because the vertical fin was doing much of the work to keep the helicopter from yawing. As the speed decreased, there was a greater demand for tail rotor thrust to maintain directional control.
Additionally, just to further complicate the situation:
The helicopter’s extreme attitude during the initial loss of control likely caused the hydraulic system to malfunction… [It] is likely that air was introduced into both hydraulic systems via the unpressurized hydraulic reservoir… As a result, the performance of the hydraulic system…was likely degraded, leading to a lag in servo actuator response, increased feedback load on the controls from aerodynamic forces, and greater pilot inputs to achieve normal main rotor and tail rotor response.
It also likely caused…
…the No. 2 engine to shut down in flight, and the No. 1 engine to reduce fuel flow (resulting in less power), which subsequently reduced the main rotor speed.
TSB concluded that wearing helmets and a 4-point harness reduced the severity of injuries to the pilots.
TSB also note that:
Operational flights conducted by [Airspan] are typically single pilot. Except for training [flights], there are no company operating procedures for situations where 2 pilots are in the cockpit at the same time during a flight and where dual flight controls are installed.
When 2 qualified pilots fly a single-pilot helicopter with dual controls installed, ambiguity can arise in decision making and command if roles and responsibilities are not clearly articulated. In the event of an in-flight emergency or critical situation, there is an elevated risk of discoordination in the cockpit, resulting in a delayed response or the exacerbation of the in-flight emergency or critical situation.
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