內政部空中勤務總隊全球資訊網-英文網內政部空中勤務總隊全球資訊網-英文網

The causes of flight safety accidents can be divided into “environmental factors”, “mechanical factors” and “human factors”, which can happen to all personnel regardless their experiences in the industry. The causes of helicopter accidents are analyzed according to the European Helicopter Safety Team (EHEST) report that addresses 2000-2005 European Helicopter accidents. Data from the EHSAT review confirm that a continuing significant number of helicopter accidents dues to:

• Pilot disorientation in the Degraded Visual Environment
• Vortex Ring State
• Loss of Tail Rotor Effectiveness
• Static & Dynamic Rollover

The descriptions and correction measures for aforesaid factors are as follows:

1. pilot disorientation in the Degraded Visual Environment

   Research indicates that such condition can cause serious accident
   1. The aircraft suddenly loses control when attempting to leave the degraded visual environment, such as turning back, climbing up and descending.
   2. When the driving mode is changed instrument flight, the pilot fails to control the dashboard information and then loses the sense of direction or flight control.
   3. The pilot is not aware of surrounding environment and then flies towards the ocean or barrier, or collides with other objects.

   The aforesaid conditions are closely related to visual reference condition, helicopter handling characteristics and pilot ability. The research also indicates that, when an accident occurs, the instability of helicopter is the main cause of accident. Therefore, the pilot shall return the helicopter to a stable condition by relying on the strength of some reference. However, as most of the pilots only took restricted instrument aviation trainings and often forget training details within a short period of time, they are sometimes unable to respond to sudden risks.

   Circumstances that have resulted in degraded visual environment:
   1. Low brightness can result in a decrease of visible landscape. For example, sand storm or night.
   2. Cloud and mist can narrow visual range or the haze or glare sun make it impossible to see the ground or sea level.
   3. No obvious features, such as buildings, roads floors, or the streets without lamps.
   4. Tranquil sea and ground level
   5. No obvious change, such as high and low, up and down. For example: snow field.
   6. Mistaken reference line, such as street lights.
   7. Showers on the windshield.
   Risk analysis, where the following matters shall be taken into consideration when planning a visual flight:
   1. Whether visual flight can be implemented with the aircraft.
   2. Whether the pilot is only capable for instrument flight.
   3. Whether the pilot is a flying stunt and has not yet been transformed into a general flyer.
   4. Whether the maps and visual references are used as the navigation tool; whether GPS is used as the reserved navigation tool.
   5. High-altitude flight and it is impossible to identify geomorphological features.
   6. Whether the aircraft will pass through wide rural area or areas with no signature.
   7. Whether the flight takes place at night or in a ducky weather condition.
   8. Whether the flight takes place at moonless and starless night.
   9. Whether the aircraft will pass through low clouds.
   10. Whether the visibility will reach the minimum standards during the flight.
   11. Whether mist, fog and haze will be seen during the flight.
   12. Whether it will meet heavy shower.

   If 1 to 4 items have been ticked, it means that the risks involved in visual flight are acceptable.

   If 5 to 9 items have been ticked, it means that visual flight shall not be implemented.

   If 7 to 12 items have been ticked, it means that the flying mission shall not solely rely on visual flight.

   There are some potential variables and may be added to the list for evaluation:
   13. The brightness of surrounding environment decreases.
   14. The visual horizontal line disappears or glitches.
   15. Little visual reference on the horizontal plane.
   16. No change to the height and speed of visual reference objects.
   17. The horizontal plane cannot be identified even after reducing the flight altitude.
   18. Mist or haze has resulted in a degraded vision in the cabin.
   19. The pilot unwittingly reduces the flight altitude due to cloud dropping, and mistakenly thinks that the altitude has remained unchanged.

   If the conditions as described from item 13 to item 19, it is a must to end the flight mission and search for a safe landing place as soon as possible. The reason is that the pilot may lose the sense of direction when the visual environment degrades.

   Conclusion: when the visual references no longer work, the pilots must immediately pay attention to the dashboard to avoid losing the sense of direction. Weather, terrain, aircraft and pilot’s ability are the key factors of deciding whether a safe flight pattern can be quickly established. The risk analysis before the flight and the on-time decisions during the flight are extremely important for the pilots, avoiding them to be stuck in difficult situations.

2. Vortex Ring State

   Vortex ring state refers to the state when the helicopter descends into its own downwash and results in severe loss of lift, which is at least three times of the original descent speed. It happens when the air speed is below 30 nm (1 nm = 1.852 km) and, when it takes place, the aircraft would drop with the downwash speed of main rotor. However, the downwash speed can change depending on the aircraft model and weight. Normally, the descent speed shall not exceed 500 feet per minute.

   Impacts of vortex ring state:
   1. Wingtip vibration
   2. The turbulence can blunt the control of pitch roll.
   3. The power demand can change abruptly due to the big change of obstruction.
   4. The expansion of vortex will make the descent speed abnormally large, event to 3000 feet per minute.
   Recovering from the vortex ring state:

   It can be done using rotating rod or collective pole. However, recovery made by the rotating rod is insufficient to obtain the speed. It is a must to lower the collective pole to the lowest level in order to leave the vortex ring state. However, the loss of altitude caused by the pitch of collective pole is bigger than that of rotating rod. Therefore, it is suggested to:

   1. Push the rotating rod forward to accelerate
   2. In case of failing to accelerate, put the collective pole in when entering the automatic spiral state and then push the rotating rod forward to speed up.
   Preventing the vortex ring state:
   1. Do not approach the site in a confined area.
   2. Do not approach the site before the wind.
   3. Do not approach the site precipitously.
   4. Do not hover out of ground effect.
   5. Do not spiral automatically at low speed.
   6. Do not brake abruptly before the wind.

3. Loss of Tail Rotor Effectiveness

   In general, the main rotor rotates clockwise. For aircrafts whose main rotor rotates clockwise, their right rudder is then designed to resist the torsion and the loss of tail rotor effectiveness will occur on the right rudder in the full travel position. Tail rotor can easily lose effectiveness in 30 nm (1 nm = 1.852 km) because:

   1. The tail fin has low aerodynamic effectiveness.
   2. The airflow and downwash can disturb the inlet airflow of tail rotor.
   3. The aircraft is having high horse power and the control is close to the full travel position.
   4. The tail control’s demand for anti-torque shall be increase before the wind.
   5. Disturbance requires rapid and huge horse power.

   In overall, low altitude, low speed, high horse power and unstable wind speed can result in the loss of tail rotor effectiveness. In other words, it often occurs when the aircraft is implementing the following missions:

   1. Checking the electricity and pipeline networks.
   2. An object is hoisted in the ventral position of the aircraft.
   3. Hosting personnel.
   4. Fire rescue.
   5. Site survey.
   6. Low speed, and take off / land at a high altitude.
   7. Take off from or land on the deck.

   To prevent the tail rotor from losing effectiveness, it is a must to take the altitude and wind direction into consideration when planning the flight route, as it can affect the aircraft’s performance. During the flight, it is important to pay attention to the wind direction at any time and prevent the following actions:

   1. Drive at low speed before the wind.
   2. Running out without control.
   3. Quickly lift the collective pole with a big force and change the lateral direction at low speed.
   4. Drive the aircraft at low speed when the wind direction changes frequently.
   Recovering the effectiveness of tail rotor

   Where the aircraft has any of abovementioned conditions, it means that its tail rotor can potentially lose its effectiveness and the pilot shall immediately make corrections. If the altitude is allowed, the pilot shall increase the forward speed without increasing the power (or even reducing the power, if possible). This will help to correct the problem. However, as this action can result in a big loss of altitude, it is a must to plan for an evacuation route beforehand.

4. Static & Dynamic Rollover

   If the gravity of the helicopter exceeds the pivot point, even if we immediately remove the external force that tilts the aircraft, the aircraft will still be overturned. The overturn generally happens when the aircraft takes off, lands or hovers with one single foot at the angle of 30° with one point touched to the ground. The said contact point can be one side of landing skid or wheel as it can be stuck to the ground or be entangled by, for example, pitch or mud. The critical flip angle is between 13° and 17°. If the angle exceeds this range, it is impossible to stop the aircraft from overturning even if we reverse the rotating rod. The critical flip angle is the aircraft’s tilting angle when the rotating surface of its main rotor is maintained in horizontal direction.

   The processes of dynamic overturn

   Dynamic overturn does not take place only when the critical flip angle is reached. If there is any over-turning movement, over-lifting the rod will give sufficient momentum to overturn the aircraft. The processes are as follows:

   1. Increase of the collective pole to increase a lift force.
   2. The right landing skid is entangled and becomes the turning point.
   3. Move the rotating rod to the left side to keep the rotating surface of main rotor in horizontal.
   4. A small roll rate will be created at this moment.
   5. Continue to increase the force of collective rod to create more lift power.
   6. Reaching the critical flip angle.
   7. Move the rotating rod to the left further to keep rotating surface of main rotor in horizontal.
   8. The horizontal component of the thrust of main rotor will increase and result in an increase of roll rate.
   Recovering from dynamic overturn:

   It is a must to immediately release the collective pole to remove the horizontal component of the thrust of main rotor. Otherwise the inertia of rotation will continue and cannot be stopped.

   Preventing dynamic overturn:
   1. Be aware of the horizontal gravity as any change of it will affect the operations of rotating rod.
   2. The hovering shall be made after the wind.
   3. Paying attention to barriers when the aircraft is hovering or sliding.
   4. Slowly taking off or landing the aircraft while preventing lateral actions.
   5. Ensure that the aircraft has entirely left the ground when it hovers above a slope.
   6. When taking off from or landing on a kickboard, be extremely careful about it.

   To summarize it, the poor visual environment, low altitude, low speed and high horse power can largely increase the flying risk of helicopters. However, most of the conditions of NASC missions, including offshore island EMS, sea rescue, personnel hoisting and fire rescue missions, are extremely dangerous. Therefore, it is a must to make risk assessment before implementing the mission to prevent potential risks, and carry out relevant trainings in ordinary days in order to enhance all personnel’s capability to respond to emergency and dangerous conditions.