Accident of AS 350B Helicopter: 9N-AIA of Fishtail Air
at Annapurna Base Camp (South), on 29 June, 2008
1. SYNOPSIS
The helicopter AS350B with registration number 9N-AIA of Fishtail Air Pvt. Ltd. was on a domestic charter service. As per the schedule, the helicopter departed Kathmandu at 01:09 UTC but landed at Chomrong village en route to Annapurna Base Camp due to bad weather. After waiting there for about two hours it took off again for Base Camp at 04:15 UTC, assessing some improvement in weather. According to PIC, it was a short flight of 08 minutes to the Base Camp where he boarded three passengers with their personal belongings, without switching off the engine. The surface wind was not significant with light drizzle. Facing east along the valley, there were two small houses in the take off path within the vicinity of the helipad. The PIC applied maximum collective pitch to hover high, before initiating take off in order to clear these obstacles. Now with almost no power reserve in hand, the PIC pushed the cyclic to get forward speed and thereby to acquire translational lift. Therefore the helicopter started to sink progressively as it moved forward. The PIC instead of pulling the maximum power available pushed the cyclic further to gain more forward speed in order to achieve translational lift. Thus, PIC was unable to check the sink rate and as a result the tail rotor struck the boulder within fifty feet of the take off path. Due to considerable loss in the anti torque value, the helicopter swung viciously to the left. After the second impact the tail rotor assembly got detached. The helicopter, almost out of control, plunged into a small pond where the cold dense water cushioned the effect of further vicious swing to the left. The main rotor blades rotating at high speed of this helicopter struck the water surface of higher viscosity than the free air. A sudden hindrance to the free rotating blades resulted in damage of the Main Rotor Head Assembly. The misaligned Blades drooped down loosing its support and further cut off the tail boom cowling, tail drive shaft, push pull rods, the vertical fin and finally broke the MGB mounts. The pilot and three passengers survived with minor injuries. There was no fatality but the helicopter was substantially damaged.
Initially, after landing at Chomrong, the PIC was able to report his position to Pokhara Tower on VHF but he was unable to report his departure both in VHF and HF.
Immediately after the mishap, one of the eyewitnesses ran down to Chomrong village, which took him about three hours, from where he notified Fish Tail Air officials at Kathmandu through land line phone. Subsequently the Civil Aviation Authority of Nepal, Ministry of Culture and tourism and Civil Aviation Nepal were also informed at approximately 08:15 UTC. The Civil Aviation Authority of Nepal did not forward notifications of the accident to the State of Design and the State of Manufacture as required by Annex 13 to the Convention on International Civil Aviation. All the occupants of the ill fated helicopter including the remaining passengers at the Base Camp were evacuated to Pokhara in the late evening of the same day by an Army Helicopter. Next day in the afternoon the whole group was flown back to Kathmandu by the same helicopter.
The GPS co-ordinate of the first impact point of the ill fated helicopter is N280 31’49.72” E 0830 52’39.62”. The crash site is situated at 13,000’AMSL on a radial of 3530 and at an aerial distance of 20 nm from Pokhara.
This report has been prepared by the three members of the commission constituted by the Government of Nepal, Ministry of Tourism and Civil Aviation abiding by the Civil Aviation (Accident Investigation) Rules, 2024B.S.
The investigation commission, after an intensive study over the case of 9N-AIA has concluded that the probable cause of this accident is a case of human factor as a result of excessive loading of the helicopter with respect to the nature of helipad at high altitude which does not allow the normal take off, poor selection of the helipad and poor flight preparation. The PIC was not fully conversant with the procedure of limited power flight procedure and was reluctant to pull remaining available power during the take off which turned out to be very crucial. The Helicopter Manual permits to apply Collective Lever to an extent where RRPM drops to a minimum permissible RRPM value of 360 which is annunciated by a provision of low RRPM continuous audio horn .Eventually the helicopter tail struck the ground resulting in an accident. Therefore the contributory factors for the accident are:
- Insufficient high altitude flight experience of the PIC on type after a long gap.
- Overlook in the laid down criteria of the renewal of license by CAA
3. Optimum weight calculation in IGE configuration with no power margin available for take off from Annapurna Base Camp.
4. Reluctant to use maximum power available where the helipad demands OGE power calculation to check sink rate during take off.
5. Time constraint due to marginal weather condition to complete the mission.
6. PIC not having adequate rest to undertake high altitude flight which demands a high state of physical fitness.
7. Standard OAT correction was not considered for take off weight calculation.
2. Damage to aircraft
Immediately after the mishap, the helicopter was lying ditched into the pond but at present the pond is found to be dry as the local people have drained the water to have an access to the helicopter.
The helicopter was found in a single fuselage section with all three main rotor blades completely damaged,
- Tail boom remains attached to the fuselage but the tail portion looks badly damaged,
- Tail rotor blades completely damaged due to tail rotor strike,
- Wrinkles were noticed on tail boom due to tail rotor strike,
- Tail rear portion of tail boom behind horizontal stabilizer is chopped off possibly due to main rotor blade hit.
- Tail gearbox along with vertical stabilizer found detached from the tail boom assembly.
- Tail rotor drive shaft and tail rotor control tube are completely broken.
- Main gear box mount detached from its mounting points. All four of the MGB supporting struts and bi-directional crossbeam mounts were completely broken.
- All main rotor head assembly parts, star flex, upper and lower sleeves, main rotor shaft along with MGB were completely damaged.
- Some metallic pieces of helicopter were scattered around.
- Engine front mount is wrinkled and slightly twisted.
- Engine is slightly twisted to the right hand side. Possibility of severe damage to the module-5 and module-1 is suspected due tail rotor strike.
- Instruments at the cockpit seem to be intact.
3. Cause
The investigation commission, after an intensive study over the case of 9N-AIA has concluded that the probable cause of this accident is a case of human factor as a result of excessive loading of the helicopter with respect to the nature of helipad at high altitude which does not allow the normal take off, poor selection of the helipad and poor flight preparation. The PIC was not fully conversant with the procedure of limited power flight procedure and was reluctant to pull remaining available power during the take off which turned out to be very crucial. The helicopter manual permits to apply collective lever to an extent where RRPM drops to a minimum permissible RRPM value of 360 which is annunciated by a provision of low RRPM continuous audio horn. Eventually the Helicopter tail struck the ground resulting in an accident. Therefore the contributory factors for the accident are:
- Insufficient high altitude flight experience of the PIC on type after a long gap.
- Overlook in the laid down criteria of the renewal of license by CAAN.
- Optimum weight calculation in IGE configuration with no power margin available for take off from Annapurna Base Camp.
- Reluctant to use maximum power available where the helipad demands OGE power calculation to check sink rate during take off.
- Time constraint due to marginal weather condition to complete the mission.
- PIC not having adequate rest to undertake high altitude flight which demands a high state of physical fitness.
- Standard OAT correction was not considered for take off weight calculatio
4. Safety Recommendations
- There should be safety awareness within the Operator to verify the clearance provided by a regulatory body and superimpose some extra training or dual flying as and where applicable for safe flying.
- CAAN is advised to check on the procedure to clear an experienced PIC from upgraded aircraft of same series to an underpowered version, especially in a condition where one had had a long discontinuation from flying duties on type.
- CAAN should review the mandatory requirement of HF equipment in the light aircraft operating in remote areas, the commission recommends that a good working order satellite phone or any appropriate means is aboard the helicopter for each flight to ensure communication from the ground when most needed.
- CAAN should amend the NCAR to ensure the provision of only aviator oxygen mask with built in microphone for aircraft to be used in high altitude flights.
- After landing in remote areas, outside the confines of controlled airspace, pilot must give position report every hour to the nearest ATC & given his/her intention.
- If there is no communication on VHF or HF, PIC must use satellite phone or any appropriate means to give his/her position report.
- If there is no position report as mentioned above, the concerned ATS unit should initiate necessary action in this regard.
- CAAN should review existing ATC communication procedures for helicopter operating in remote areas, which are seen not adequate.
- Pilot should take additional precaution for preparation on calculating take off and landing weight in terms of power reserve and prevailing weather condition.
- High altitude proficiency check for high altitude (10,000' or above) cleared pilot should be included in pilot proficiency check at least once in a year.
- Operators should finalize the required documents such as SOP. Training Program Manual, Accident Prevention and Flight Safety Program Manual etc. and submitted for approval immediately.
- CAAN should ensure submission and approval of required documents such as SOP. Training Program Manual, Accident Prevention and Flight Safety Program Manual etc. by the operators.
- Fishtail Air is recommended to conduct 3 hours of high altitude (10,000ft or above) training flight for PIC and 3 hours of ground class in high altitude flight procedures and performance limitations before being released for normal dut
The flight check must include the following:
- Approaches and landings at altitude 10,000 ft. or above.
- A review of emergency procedures.
- Limited power take-off.
- Autorotation
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