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L410, vicinity Lukla Nepal, 2017

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Summary
On 27 May 2017, a Let 410 attempting to complete a visual approach to Lukla in rapidly deteriorating visibility descended below threshold altitude and impacted terrain close to the runway after stalling when attempting to climb in landing configuration. The aircraft was destroyed by the impact and two of the three occupants fatally injured. The Investigation concluded that the Captain had lost situational awareness at a critical time and had been slow to respond to the First Officer’s alert that the aircraft was too low. Safety Recommendations included the establishment of an independent and permanent Air Accident Investigation Agency.
Event Details
When May 2017
Actual or Potential
Event Type
Human Factors, Loss of Control, Weather
Day/Night Day
Flight Conditions IMC
Flight Details
Aircraft LET L-410/420
Operator Goma Air
Domicile Nepal
Type of Flight Public Transport (Cargo)
Origin Kathmandu/Tribhuvan International Airport
Intended Destination Lukla/Tenzing-Hillary Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed No
Flight Phase Landing
LDG
Location - Airport
Airport vicinity Lukla/Tenzing-Hillary Airport
General
Tag(s) Approach not stabilised,
Inadequate Aircraft Operator Procedures,
Ineffective Regulatory Oversight,
Inadequate Airport Procedures,
Visual Approach
CFIT
Tag(s) Into terrain,
Vertical navigation error,
VFR flight plan
HF
Tag(s) Manual Handling,
Plan Continuation Bias,
Procedural non compliance,
Violation
LOC
Tag(s) Flight Management Error,
Environmental Factors,
Aerodynamic Stall
WX
Tag(s) In Cloud on Visual Clearance,
Precipitation-limited IFV
EPR
Tag(s) RFFS Procedures
Outcome
Damage or injury Yes
Aircraft damage Hull loss
Injuries Few occupants
Fatalities Most or all occupants (2)
Causal Factor Group(s)
Group(s) Aircraft Operation,
Air Traffic Management
Safety Recommendation(s)
Group(s) Aircraft Operation,
Air Traffic Management,
Airport Management
Investigation Type
Type Independent

Description

On 27 May 2017, a Let 410 UVP E-20 (9N-AKY) being operated by Goma Air on a VFR cargo charter flight from Kathmandu to Lukla as flight 409 impacted terrain close to the 527 metre-long and up-sloping runway 06 at destination during rapidly deteriorating daylight visibility after attempting to climb in landing configuration following a premature descent below runway threshold elevation. The aircraft was destroyed by the impact and both pilots were fatally injured. The third occupant, described as “cabin crew” sustained serious injuries.

The wreckage of the aircraft. [Reproduced from the Official Report]

Investigation

An Accident Investigation Commission to examine and report on the circumstances of the accident was established in accordance with the Civil Aviation (Accident Investigation) Regulation (2016) of Nepal on the day after the accident occurred. The aircraft was fitted with a 25 hour FDR and a 2 hour CVR and both were recovered from the wreckage and their data successfully downloaded.

It was noted that the 48 year-old Captain, who was acting as PF for the accident flight, had accumulated 9,687 total flying hours which included 1,897 hours on type and had flown 97 hours in the month preceding the accident. The 27 year-old First Officer had a total of 1,311 flying hours which included 1,028 hours on type.

What Happened

It was established that the accident flight was the fifth in a pre-planned sequence of five cargo flights from Kathmandu to the STOL VFR-only airport at Lukla operated by the same aircraft and crew that morning, each sector taking around 35 minutes and operated under VFR. Shortly after the accident flight had departed from Kathmandu, the crew were advised that Lukla “was having heavy rain and the airport was closed”. However, since by this time Kathmandu was becoming congested with traffic holding both in the air and on the ground, it was decided to continue towards Lukla and a few minutes later, they were advised that the rain had stopped and the airport had reopened. An Airbus AS350 helicopter which had just left Lukla for Kathmandu had reported to Lukla TWR (which provides FIS only) that the weather conditions beyond the Lukla valley were good but those in the valley might make it difficult for a fixed wing aircraft to enter and this aircraft subsequently communicated directly with the accident aircraft. Lukla TWR subsequently communicated all available weather information when the inbound aircraft crew established contact 26 minutes after takeoff from Kathmandu, at which time VFR conditions prevailed. Then, two minutes later, the weather began to deteriorate quickly but, contrary to airport procedures, the runway was not closed and pilots including those in the accident aircraft “ventured to continue even though the weather was marginal” whilst TWR continued to provide weather updates.

Six minutes after checking in with TWR, the crew reported to TWR that they were entering the valley (at an altitude subsequently found to have been 9,200 feet rather than the normal 10,500 feet). They advised that they would decide whether to continue to a landing when nearer. One minute after that, and with just over a minute to go to impact, the aircraft was 200 feet above runway elevation when CVR data showed that both pilots had simultaneously sighted the runway from the west. Descent was continued to approximately 100 feet above the threshold elevation and this altitude was maintained for about 20 seconds during which time the TWR FISO advised that the surface wind was westerly at 4 knots and runway was clear. The Captain responded apparently “still in doubt” and asked if it was raining, and on receipt of the reply that it was not, the aircraft restarted its descent and deviated to the right. At this point, it was concluded that the Captain had been following the GPS map and no longer had the runway in sight and that he had “lost situational awareness whereas the First Officer had not lost sight of it. When the aircraft was descending through 8,650 feet (already 250 feet below the runway threshold elevation), the CVR record showed that the First Officer had warned the Captain that they were too low but this call was not followed by any immediate corrective action and the descent continued for a further 150 feet after which the First Officer again warned the Captain “in panic” and this time elicited a similar “in panic” response from the Captain asking where the runway was. The First Officer gave directions towards the runway and a climb in landing configuration was begun although without the immediate application of maximum thrust. Two short stall warning annunciations occurred and then a continuous stall warning began and lasted for 13 seconds until impact. The prescribed response to this was not forthcoming and by this point may anyway not necessarily have facilitated a successful recovery. It was estimated that the aircraft had fully stalled when it was just 40 metres short of the runway threshold. The last words on the CVR were “do not pull too much”. The data from the final moments indicated that excessive drag led to the aircraft stalling. Its left wing then hit a small tree branch some 55 metres short of the runway threshold and the whole aircraft then hit sloping terrain just 30 metres short of the runway and 30 feet below it. The impact and the subsequent slide 70 feet down the slope collectively destroyed the aircraft but there was no post crash fire.

No evidence was found of airworthiness or loading issues which might have been relevant to the loss of situational awareness or the ability to recover from the resulting premature and off-track descent. It was evident that the flight crew had failed to comply with their VFR flight plan or the VFR only restriction on Lukla operations and that the aircraft had “repeatedly entered into cloud. It was also noted that with a fuel endurance of 2 hours on departure from Kathmandu, the flight could have returned there from Lukla or reached either of the alternates specified in the Flight Plan - Ramechhap and Phaplu.

The Airport

It was noted that a CCTV camera was installed at Lukla TWR for the specific purpose of providing a visual record of the cloud cover and visibility along the runway 06 final approach to those not at Lukla and that its images were not visible to those in the TWR, only to remote users with authorised access. However, the sequence of recorded images from this camera was available to the Investigation and they confirmed local reports of a sudden deterioration in visibility as low cloud began to move across the runway 06 final approach from the south east. It was considered that this deterioration may well have led to increased workload with necessarily increased attention being given to flight deck instruments triggering loss of situational awareness at a critical time.

In respect of the STOL runway at Lukla and its restricted VFR only use, it was noted that all landings must be made uphill in the 06 direction and all takeoffs in the downhill 24 direction because of steeply rising terrain to the southwest of the 24 threshold. This also means that a go around cannot be flown once committed to a final approach to runway 06. The Lukla TWR has a console equipped with QNH and air temperature readouts but as neither is calibrated, they are not used. The console does have an indication derived from the surface wind sock at the runway midpoint but not from the one at the 06 threshold. It was noted that the Nepal CAA had developed a “generic SOP for fixed wing aircraft operations which included guidance to FISOs on determination of both the required visibility and minimum cloudbase and that this also “authorises a duty ATS Officer to declare the airport closed” if any of the following occur:

  • the visibility is less than 5,000 metres or
  • the cloud ceiling is less than 5,000 feet or
  • the tail wind component exceeds 10 knots or
  • there is light rain.

However this had not happened at the time of the accident. It was noted that runway 06 also had a Visual Augmented Guidance System (VAGS) and an A-PAPI installed as landing aids but neither was in use because of “calibration issues”.

Operational Safety at the Aircraft Operator

It was concluded that:

  • there was no adequate system of monitoring the pilot adherence to prescribed Standard Operating Procedures (SOPs) or the performance of individual pilots and noted that there was no OFDM system in place
  • the reporting culture was poor
  • the absence of hazard identification and risk management on a regular basis had resulted in a systemic lack of effective safety management by the Operator
  • management pilots were doubling as line pilots and that limited numbers of pilots validated for STOL operations had hindered the expansion of the Operator
  • recurrent pilot training for LET 410 crews was found to have been obtained using a procedure trainer rather than a full flight simulator although since the accident, arrangement had been made to correct this and the simulator to be used will be configured with visuals for Nepalese airports used by typе
  • the 30 minute block time assigned to the Kathmandu-Lukla sector was inadequate.

Rescue and Fire Fighting Response

It was found that response to the crash was hampered by a lack of cutting instruments. The small Fire Tender available at the airport did not carry basic access equipment and available trained manpower was inadequate. In respect of the aircraft ELT, it was noted that it had activated but as the antenna had been damaged, it was not transmitting.

The Probable Cause of the accident was determined as “aircraft stall as a result of excessive drag created by sudden increase in angle of attack of the aircraft supplemented by low speed (below Vref) in an attempt to initiate immediate climb on a landing configuration (full flap and landing gear down) warranted by the critical situation of the final phase of flight”.

Four Contributory Factors were also identified:

  1. Critical terrain and rapidly deteriorating weather conditions.
  2. The Pilot's loss of situational awareness.
  3. The improper response of the Pilot to the stall warning which included failure to advance the power levers to maximum without delay.
  4. Violation of SOP s by both Air Traffic Services and the Pilot.

A total of 20 Safety Recommendations were made as a result of the Investigation as follows:

  • that the Civil Aviation Authority of Nepal and Airline Operators should launch a Voluntary Reporting System as a part of the regular Aviation Safety Campaign emphasising the existence of a non-punitive environment so that unsafe behaviour by all aviation personnel including pilots, ATS personnel and maintenance staff is promptly identified.
  • that the Civil Aviation Authority of Nepal and Airline Operators should, in the light of increased stress and fatigue as a result of present air traffic congestion in Kathmandu Tribhuvan International Airport and its direct impact on pilot proficiency, take this into consideration when assigning flights to flight crews and initiate remedial action as necessary.
  • that Goma Air should review and strengthen the Company Flight Crew Training Policy and Programme so as to emphasise airport-specific training which incorporates unforeseen situations and includes effective crew response to a stall warning during simulator training.
  • that Goma Air should review their aircraft Operations Manual and SOPs and incorporate a stabilised point and a committed point into the approach procedure of the Category C airport so that clear-cut guidance is available to the flight crews as to whether they should continue an approach or divert.
  • that Goma Air should develop a mechanism to monitor and ensure compliance of flight crews with the requirements for VFR flight, SOPs and other relevant safety directives.
  • that Goma Air should review the block time for each flight sector to ensure that enough ground time is provided between each flight.
  • that Goma Air should enhance flight dispatcher capability so that they can fulfil their responsibilities as per the provisions of Company SOPs.
  • that Goma Air should review and further strengthen the effective implementation of the SMS process in particular by embracing the crew reporting system.
  • that Goma Air should, in view of available number of aircraft in its fleet, ensure that the number of available crew meets the flight requirement and that all crew are trained to identify hazards, changes in system and the operational environment and the consequential risk to safety.
  • that the Civil Aviation Authority of Nepal (CAAN) should ensure that, irrespective of the air traffic volume, assigned numbers of ATS personnel are available for each shift of an ATS Unit (AFIS Tower).
  • that the Civil Aviation Authority of Nepal (CAAN) should, depending upon the volume of air traffic, declare the minimum experience requirement for ATS personnel manning each shift at a particular airport.
  • that the Civil Aviation Authority of Nepal (CAAN) should, in conjunction with the management of the local hospital or health post close to a busy airport like Lukla, initiate the upgrade of their essential emergency medical facilities such as readiness of oxygen and ventilators.
  • that the Civil Aviation Authority of Nepal (CAAN) should strengthen the Rescue and Fire Fighting Service at Lukla Airport with adequate manpower and equipment.
  • that the Civil Aviation Authority of Nepal (CAAN) should encourage on-the-spot inspection focusing on critical safety areas including violation of SOP, proper CRM, CFIT, and ensure effective enforcement mechanism.
  • that the Civil Aviation Authority of Nepal (CAAN) should initiate immediate action to set up and maintain essential meteorological equipment at Lukla Tower which includes the display of the real-time surface wind information at the runway threshold.
  • that the Civil Aviation Authority of Nepal (CAAN) should, owing to the critical nature of the approach at Lukla, immediately initiate action for the possible extension of the runway threshold further to the south.
  • that the Civil Aviation Authority of Nepal (CAAN) should, recognising the critical situation that might arise as a result of sudden and rapid lifting of fog or cloud during final approach at Lukla, install an appropriate runway visual aid or lighting system.
  • that the Ministry of Culture, Tourism and Civil Aviation should continuously coordinate and monitor the effective implementation of safety recommendations issued by investigation commissions with CAAN.
  • that the Ministry of Culture, Tourism and Civil Aviation should ensure that sufficient resources of finance, personnel and technical experts are available for the investigation of an aircraft accident to be performed in an effective and timely manner.
  • that the Ministry of Culture, Tourism and Civil Aviation should establish an independent and permanent body for the purposes of aircraft accident and incident investigation. The proposed Body should also be given the responsibility of monitoring the implementation of Safety Recommendations issued by Accident Investigation Commissions in the past and also act as the research body for the enhancement of safety in the aviation sector.

The Final Report was finalised on 11 April 2018 and subsequently published but did not include the Appendix referred to at various points in the text of the Report.

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