Description

On 24 February 2016, an almost new Viking DHC6-400 (9N-AHH) being operated by Tara Air on a daytime scheduled domestic passenger flight from Pokhara to Jomsom on a VFR flight plan did not arrive at the intended destination and was subsequently found to have crashed into high terrain en-route resulting in its destruction with this and a post crash fire resulting in fatal injuries to all 23 occupants. 

Investigation 

An Investigation was carried out by an Aircraft Accident Investigation Commission (AAIC) established for the purpose by the Government of Nepal the day after the accident had occurred in accordance with provisions of the Aircraft Accident Investigation Regulation 2014. The wreckage of the aircraft was located at 10,982 feet amsl on a sloping hillside near the entrance to the valley in which Jomsom Airport is situated. 

The 53 year-old Captain, joined the Operator when it began operations in 2009 as an Instructor Pilot on the DHC6. He had accumulated a total of 20,108 flying hours, of which 18,500 hours were on type. All his airline flying experience had been gained with Nepalese operators. He was the Tara Air DHC6-300 and -400 Deputy Chief Pilot and had piloted the delivery (ferry) flight of the accident aircraft from Canada in September 2015. Including that flight, he had 217 hours experience on the -400 variant. 

The 29 year-old First Officer joined the Operator in 2014 on the DHC6-300 in his first job as a professional pilot.  He had a total of 760 flying hours of which 560 hours were on the DHC6 but only 14 of those on the Viking -400 variant. 

Both pilots had received differences training on the Viking -400 variant of the DHC6 and had completed a “familiarisation flight” on it. Both were also paired for a routine 5 day/4 night posting to operate from Pokhara after travelling from their base in Kathmandu.

What Happened

The flight departed from runway 22 at Pokhara (2,700 feet amsl) on a VFR clearance to destination climbing to 10,500 feet QNH with the First Officer-- acting as PF. Pokhara had only just opened after the prevailing visibility rose above 5km and the designated alternate, Bhairahawa, was closed due to poor weather.

On reaching 500 feet agl, the aircraft turned right to take up a heading of 305° towards the GHORI (Ghorepani) waypoint - the ground track of the flight is on the illustration below. Whilst climbing through 10,100 feet around five miles before reaching the ‘GHORI’, the Captain was recorded as observing that “cloud cells were still present” and instructing the First Officer to continue the climb to 12,000 feet. He then stated that they would continue until reaching waypoint ‘TPANI’ before deciding whether to continue or divert. 

After reaching GHORI ten minutes after takeoff, the aircraft turned right and initially maintained a heading of 330°. At 11,500 feet, the area in the vicinity was in cloud and just after passing the waypoint, a brief EGPWS ‘TERRAIN’ PULL UP Warning was annunciated. The climb was continued and the flight emerged from cloud soon afterwards. Half a minute later, with the flight now “just below cloud”, the Captain instructed the First Officer to maintain a heading of 330° and “a shallow descent was initiated”. In response to a question from the Captain as to whether his side was visual, the First Officer said he was “somewhat visual” and the Captain then instructed him to continue descent to 10,000 feet QNH. One minute after the EGPWS Warning had ceased and passing 11,000 feet on heading 321°, a two second-long ‘OVERSPEED’ warning sounded as the airspeed reached 152 KCAS.

The reconstructed ground track (green), the VFR flight plan track (mauve) and crash site (red). [Reproduced from the Official Report] 

Two minutes later, with 200 feet to go before reaching 10,000 feet, an EGPWS TERRAIN caution was annunciated. The rate of descent was reduced but after eight seconds had passed, with the aircraft now at 10,100 feet, an EGPWS ‘PULL UP’ Warning commenced and continued. The Captain was recorded immediately responding by remarking “nothing to worry about” but on reaching 10,000 feet, he “took over control, initially continued descent and asked the First Officer whether his side was visual”. A gentle climb was then started and at 10,150 feet whilst the PULL UP warning was continuing to sound, the Captain informed the First Officer that he intended to turn towards the waypoint ‘LETTE’. Pitch up attitude was only 7° as the right turn commenced, but was slowly increased during the turn and once the wings were levelled, it was increased to 12° degrees nose up.

A few seconds later, a shallow left bank began and by 10,550 feet, the left bank angle had reached 20°. Six seconds later, on a heading of 335° with the left bank angle increased to 25° and the nose-up pitch angle reduced to 7°, with the EGPWS ‘PULL UP’ warning active, the lower rear fuselage of the aircraft contacted the up-sloping terrain at 10,700 feet and after a further 77 metres it was then completely destroyed by a further and more direct impact at 10,982 feet. The accident site is shown below and once located, it became clear that impact had been followed by a fuel-fed post crash fire which had consumed the whole main fuselage structure which incorporated the fuel tank in its belly.

The wreckage as found the following day. [Reproduced from the Official Report]

Why It Happened

There was no evidence to suggest that any airworthiness deficiency had played any part in the accident. The prevailing weather conditions clearly were relevant but only insofar as they created the circumstances which made the accident possible. The weather encountered did not include any potentially hazardous phenomena such as windshear, severe turbulence, up/down draughts, thunderstorm and/or icing conditions. All the factors involved were found to be within the general area of “human factors” and effectively encouraged “plan continuation bias”:

• The departure into what were evidently marginal weather conditions for a VFR flight to the intended destination was evidently not unusual. There was “a tendency amongst flight crew to disregard the alternate weather conditions and the weather at the departing aerodrome”. The previous day’s flight had been commenced in similar circumstances.
• The acceptance of flight in conditions where VFR could not be maintained was evidently commonplace despite the strict flight plan and clearance condition.
• The complete failure to respond as required to EGPWS activation when in IMC was a serial habit rather than a one-off which was supported by the fact that the operator did not require such activations to be reported by crew or - in the specific case of the DHC8-400 only - have an OFDM process which would capture them.
• Pilot Training on the use of the EGPWS installed on the -400 variant was inadequate as was Operations Manual content on the requirement to respond to both cautions and warnings. Due to the frequent occurrence of EGPWS activations, the operator’s flight crew were habitually ignoring them.
• The -400 version of the DHC6 was, unlike the -300 version, equipped with modern flight deck instrumentation including modern avionics which, amongst other things, facilitated GNSS based navigation.  
• There was no formally documented ‘escape route’ in the event that a Jomsom-bound flight was unable to land there although such routes were documented for other Nepalese STOL airports such as Pokhara.

In respect of what represented “operations normal” for the accident aircraft flight crew, in particular for the Captain, the Investigation examined the CVR and FDR data for the Pokhara-Jomsom rotation which the same aircraft and crew had completed the day prior to the accident. This was found to show that:

• Although Pokhara was opened for departures only and Bhairawaha airport was closed due to poor weather, the flight departed for Jomsom anyway.
• On the outbound flight, the aircraft was in IMC until reaching Ghorepani after which VMC prevailed.
• EGPWS cautions and warnings were activated several times during the outbound flight.
• During the return flight, VMC was maintained until Ghorepani after which the flight descended in IMC until reaching 7,800 feet.
• After completion of this flight duty, both pilots had a rest period in excess of 18 hours during which they were accommodated overnight in company-provided crew accommodation.

The Investigation determined that the Probable Cause of this CFIT accident was “the fact that despite of unfavourable weather conditions, the crew repeatedly decided to enter into cloud during a VFR flight and deviated from the normal track due to their loss of situational awareness aggravated by spatial disorientation”.

It was additionally determined that there had been five Contributing Factors:

1. Loss of situational awareness
2. Deteriorating weather conditions
3. A skill-based error by the crew during critical phases of flight
4. The crew’s failure to utilise all available resources (CRM) and especially their insensitivity to EGPWS cautions/warnings 
5. he reluctance on the part of crew to follow the VFR rule  

One Interim Safety Recommendation was made during the Investigation on 3 April 2016 as follows:

• that the Civil Aviation Authority of Nepal (CAAN) should immediately enforce the cessation of the practice of an aircraft commander acting as Pilot Monitoring and the First Officer acting as Pilot Flying during VFR flight in domestic sectors in the event of serious and critical en-route circumstances such as IMC Conditions, EGPWS Terrain Warnings, TCAS RA, Stall Warnings, Severe Turbulence/Thunderstorm, low visibility and cloudy conditions etc.

A further 23 Safety Recommendations were made upon completion of the Investigation as follows:

• that Tara Air should maintain an effective operational control of its aircraft even when away from the main operations base.
• that Tara Air should provide adequate skill-based training for pilots when new technology is introduced on the aircraft. 
• that Tara Air should ensure compliance with the provisions of VFR flight as stipulated in their OM. 
• that Tara Air should review the content of its OM to make it clear that in marginal weather conditions (rain, moving cloud or haze) with low visibility and a low cloud ceiling, there must be no attempt to continue a VFR flight.
• that Tara Air should devise some mechanism for determining en-route weather conditions to ensure safe operation along the routes serving remote airports. 
• that Tara Air should comply effectively with ELT registration and other technical requirements such as compass swinging when importing new aircraft.
• that the Civil Aviation Authority of Nepal (CAAN) should reinforce and strengthen its safety oversight capability. 
• that the Civil Aviation Authority of Nepal (CAAN) should facilitate and develop procedures relating to navigation and communication along the routes serving remote airports. 
• that the Civil Aviation Authority of Nepal (CAAN) should devise some mechanism for en-route weather service to ensure safe operation along the routes serving remote airports. 
• that the Civil Aviation Authority of Nepal (CAAN) should implement Safety Management Systems at all airports. 
• that the Civil Aviation Authority of Nepal (CAAN) should effectively implement registration and other technical requirements such as compass swinging for imported aircraft.
• that the Civil Aviation Authority of Nepal (CAAN) should ensure that the visibility observed by the ATC tower and reported from Met office using same visibility check point chart is consistent.
• that the Civil Aviation Authority of Nepal (CAAN) should improve its supervision of pilot training and documentation (differential training and validation of type certificate etc.).
• that the Civil Aviation Authority of Nepal (CAAN) should examine the feasibility of installing an in-built antenna type ELT or any other alternate means in addition to the standard antenna to facilitate a timely search and rescue operation for Nepalese-registered aircraft. 
• that the Civil Aviation Authority of Nepal (CAAN) should carry out a study to gradually introduce some requirements for the installation of suitable cockpit image recorders in aircraft already equipped with an FDR and a CVR. 
• that the Civil Aviation Authority of Nepal (CAAN) should, in coordination with operators, initiate a review of the existing Flight Data Monitoring programmes to ensure that they are capable of monitoring operating standards applicable to the en-route phase of STOL flight operations.   
• that the Ministry of Culture, Tourism and Civil Aviation should establish a permanent and separate Aircraft Accident Investigation Commission within the Ministry of Culture, Tourism and Civil Aviation.
• that the Ministry of Culture, Tourism and Civil Aviation should ensure that sufficient resources (financial, personnel, technical) are made available to the commission for its effective and timely investigation.
• that the Ministry of Culture, Tourism and Civil Aviation should develop and adopt an Accident Investigation Procedure Manual. 
• that the Ministry of Culture, Tourism and Civil Aviation should  continuously monitor the activities of the Civil Aviation Authority of Nepal (CAAN) and airlines in order to augment the standard of aviation safety in Nepal. 
• that the Ministry of Culture, Tourism and Civil Aviation should coordinate with the responsible meteorological authorities to facilitate en-route weather information for STOL operations.  
• that the Department of Hydrology and Meteorology should develop and facilitate the provision of en-route weather information on various routes to STOL aerodromes.  
• that the Nepal Oil Corporation should develop a fuel supply and sampling system for all aircraft operated in Nepal.

The Final Report was submitted by the AAIC to the Ministry of Culture, Tourism and Civil Aviation on 31 July 2016 and was subsequently published in English in July 2017. 

Related Articles

• CFIT
• Post Crash Fires
• OFDM
• EGPWS
• Visual Flight Rules
• Continuation Bias
• Authority Gradients