AT75, vicinity Yasouj Iran, 2018

AT75, vicinity Yasouj Iran, 2018


On 18 February 2018, contact was lost with an ATR72-500 approaching Yasouj and two days later the wreckage of the aircraft was located in mountainous terrain with no sign of survivors. The flight recorders were eventually recovered and their data helped attribute the accident to descent below the designated minimum safe altitude followed by an encounter with severe mountain wave conditions which led to the crew losing control and a terrain impact which destroyed the aircraft and killed all its occupants. An apparently widespread failure to recognise the potential risk of severe mountain wave encounters was also found.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Public Transport (Passenger)
Flight Origin
Intended Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
Location - Airport
Copilot less than 500 hours on Type, Inadequate Aircraft Operator Procedures, PIC aged 60 or over
Into terrain
Inappropriate crew response - skills deficiency, Procedural non compliance
Mountain Wave/Rotor Conditions
Damage or injury
Aircraft damage
Hull loss
Non-aircraft damage
Non-occupant Casualties
Occupant Fatalities
Most or all occupants
Number of Occupant Fatalities
Off Airport Landing
Causal Factor Group(s)
Aircraft Operation
Aircraft Technical
Safety Recommendation(s)
Aircraft Operation
Aircraft Airworthiness
Air Traffic Management
Airport Management
Investigation Type


On 18 February 2018, an ATR72-500 (EP-ATS) being operated by Iran Aseman Airlines on a scheduled domestic passenger flight from Tehran Mehrabad to Yasouj as IRC3704 disappeared whilst crossing mountainous terrain in day IMC near to its destination and having established contact with the FIS there. The wreckage of the aircraft and its point of impact with terrain were located from the air two days later close to the top of the highest mountain on its likely track and when it was reached it was confirmed that the aircraft had been completely destroyed and that none of the 66 occupants had survived.

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


An Investigation was carried out by the Iranian CAO Accident Investigation Board (AAIB). An ELT signal from the aircraft was received on 121.5 MHz by a Fokker F100 en route south east of Yasouj after the accident but not by the ACC or by satellite. Two days later, a SAR helicopter detected a weak signal on 121.5 MHz (but not on the SARSAT frequency 406MHz) and the location of the wreckage was established. It was situated approximately 350 feet below the north face of a 13,412 feet summit in the chain of mountains running east-west across the likely ground track of the aircraft approximately 8½ nm from Yasouj. Evidence of an initial impact with the mountain a little over 100 feet below its summit was also identified. The SSCVR and SSFDR were located and recovered from the site of the accident two weeks after it had occurred, the delay being attributable to the deep snow at the site’s high altitude and subsequently, their data were successfully downloaded.

It was noted that the 62 year-old Captain was also the Chief Pilot of the ATR Fleet and a Training Captain who held current TRI and TRE ratings. He had flown in India from 2002 to 2007 and then returned to Iran to fly with Iran Aseman Airlines. He had a total of 17,926 flying hours including 12,519 hours on type and had made two flights to Yasouj during the previous three months. His medical was endorsed with a restriction that he must only fly with another qualified pilot if in command or alternatively as second in command. The 36 year-old First Officer, who was PF for the accident flight, had a total of 1,880 flying hours of which 197 hours were on type.

It was also noted that Yasouj “is surrounded by Zagros mountain chain and the highest peaks of the mountains named Dena are located to the north of the airport”. It was also noted that “due to the particular geography of this airport location, the majority of airlines are not willing to fly there (and) most of the airport's flights are operated by Iran Aseman Airlines”.

Relevant Navigation Procedures

Yasouj is in Class ‘G’ airspace and there are mountains up to 13,800 feet amsl only 14 nm from the airport. It had an ATZ of 7 nm diameter up to 12,500 feet, all of which was Class ‘G’ airspace. The MSA within 25 nm to the north of the airport was 15,500 feet and the AIP stated that the only instrument approach at Yasouj was the Circling NDB approach. There was no STAR, so IFR flights from the north should follow ATS route W144 and respect the MEA of FL170 until overhead the airport before descending to 15,000 feet QNH in the holding pattern and commencing an approach to land on either directly on runway 31 or by circling to runway 13.

The accident site in relation to Yasouj and its ‘Circling NDB’ approach. [Reproduced from the Official Report]

Iran Aseman Airlines designated Yasouj as a Cat ‘C’ airport and the OM Part C and the applicable restrictions for all operations into Cat C airports without a STAR included that:

  • the approach and landing must be made only after not descending below the MEA until reaching the (undefined) ‘entry point’.
  • there must be no descent below the highest applicable MORA when descending from en-route in order to make an approach and landing to the designated runway.
  • descent must then only continue if the appropriate navigation facility is available and the MSA within 25NM has been published.
  • only when the runway is in sight may a visual approach be requested.

Both pilots had to be - and were - explicitly qualified to operate to Yasouj.

Relevant Aircraft Equipment included:

  • a Mark VIII EGPWS,
  • a pneumatic airframe de-icing system for the wing and horizontal leading edge surfaces supported by an aural alert to ice aerofoil ice accretion and an ‘Ice Evidence Probe (IEP)’ visible to both pilots.
  • an electrically delivered anti icing system covering the propeller blades, the windshield and forward part of the side windows, the pitot tubes, static ports, TAT probe, the AoA vanes and the aileron, elevator and rudder balance horns.
  • Stall Protection System with stick shaker and stick pusher functions which used AoA inputs to lower the activation thresholds for both functions when it is “rapidly moving toward positive values”. It also had different AoA-derived activation thresholds for icing and non-icing conditions for each flap configuration to account for aerodynamic changes.

The aircraft had a power lever quadrant with a “notch” marked TO which represented the power limit for normal operations. With the power levers “close to” this notch, the power obtained depends on the “power management system” but if they are moved beyond this, then Maximum Rated Torque (MRT-92% Torque) can be set and a further advance to 100 % is permissible to recover the desired flight path if there is no obstacle in the way.

What Happened

It was noted that the three hour flight to Yasouj, which is about 340 nm south of Teheran, was the first of the day for both aircraft and crew. During pre flight preparations, the currently-available METAR for Yasouj was obtained and gave the main cloud cover as BKN (broken) at 9000 feet. It was noted that a TAF was not routinely issued for Yasouj (or for a number of other low-use airports) but according to the AIP, one could be made available with 30 hours’ notice. Both METAR and TAF were available for the two planned alternates, Isfahan and Shiraz and their weather conditions were good.

The flight climbed to FL210 for the cruise and proceeded normally in contact with Tehran ACC. Only the final stage of the flight became challenging on account of the strong mountain wave weather conditions encountered in the lee of a mountainous ridge which would be crossed to reach the overhead at Yasouj from where the approach to land could commence. This wind was from a direction perpendicular to the mountain ridge and was subsequently assessed to have been blowing at around 60 knots, thereby maximising its mountain wave consequences. The intended final track to the Yasouj overhead has been added to the instrument approach chart (illustration shown above) and the track as actually followed is shown below against a view of the terrain orientation and the prevailing wind direction.

The accident aircraft’s flight path towards Yasouj showing the conditions for mountain waves. [Reproduced from the Official Report]

After a very detailed analysis of all the available data, the Investigation concluded that the final 10 minutes of the flight had, in summary, proceeded as follows:

Once the flight, initially still at FL210, had turned onto ATS route W144 at waypoint OBTUX, this being the direct route to the Yasouj overhead on a track of 200°M, the crew became aware that the BKN cloud at 9000 feet they had seen on the pre-departure METAR had deteriorated to OVC (overcast) cloud at 9000 feet with FEW CB 3300 feet. Since the elevation of Yasouj was 5,939 feet, the base of this overcast was at around 15,000 feet, below the charted MSA of 15,500 feet.

There was no evidence that the required approach brief had been given in accordance with prescribed SOP, which explicitly included “any deviation from standard procedure, applicable minimum altitudes during descent, arrival and initial approach and any operational impact of the local situation, weather and aircraft deficiencies not yet covered”. The only remarks recorded on the CVR were about the go around procedure.

The ACC controller cleared the flight to descend to FL170, the lowest permitted level on the route and the limit of Class ‘D’ airspace, and terminated radar service due to imminent loss of coverage. It was then handed over to the Yasouj FIS. Whilst passing FL193, the crew then decided on further descent below FL170 to FL150 on the basis that they expected to become clear of cloud at around 15,000 feet. There was no evidence from CVR data that the descent checks were completed. During this descent, the airframe de-icing system was selected for 2½ minutes at FL156 and switched off at altitude15,000 feet, the Local QNH of 1021 having been set by this time. The variation in vertical acceleration recorded on the FDR “was consistent with turbulent conditions” prior to reaching 15,000 feet, which occurred at 15nm from Yasouj.

Soon after reaching 15,000 feet, a strong updraft was encountered and to maintain the speed at around 200 knots and stay level, the power levers were retarded to flight idle and pitch attitude changed to around -5°. Then, after little more than a minute of this, the updraft became a strong and more persistent downdraft. Despite the power levers being progressively moved towards the Maximum Continuous Torque position, the airspeed continued to decrease and as it reached a minimum of 117 knots, the stall warning was triggered and stick pusher activation immediately followed. The crew stall recovery actions were only partially completed and although airspeed did increase, the angle of attack remained high and control of the aircraft was quickly lost as a descent at over 3000 fpm developed and persisted. Within 10 seconds of the stall warning being activated, a series of EGPWS Warnings of ‘TERRAIN AHEAD’ and ‘PULL UP’ Warning began and continued for almost 20 seconds until impact occurred in a left-wing-down attitude with the left wing first at approximately 13,300 feet about 8.5 miles north northeast of Yasouj. The reconstructed vertical profile described is shown below.

The accident aircraft vertical profile over the terrain below up to the point of impact. [Reproduced from the Official Report]

The role which encountering moderate airframe icing conditions had played in control difficulties could not be established but no evidence could be found to suggest that it might have contributed significantly to the terminal loss of control for which the underlying context had very clearly been shown to be an encounter with mountain wave conditions.

It was surmised that in the attempt to become visual overhead the destination, the effect of risk of mountain wave air movement capable of exceeding the means available to control the aircraft flight path had either been underestimated or unappreciated.

The performance of the flight crew was reviewed and in general it was found that there had been significant non-compliance with existing SOPs. The most potentially relevant such non-compliances included a failure to action the stall recovery procedure in its entirety, both the requirement to set the flaps to 15° and to use maximum power and maximum propeller rpm having been ignored. It was also concluded that in general, CRM had not been consistently effective with missed standard call outs and a prioritisation on indicators and related warnings being “major factors which unfortunately were missed” thereby reducing pilot situational awareness.

The underlying context for the accident was clearly the attempt to become visual overhead the destination. It could be surmised that an experienced and senior pilot had apparently not understood that severe mountain waves are quite capable of making it difficult or even impossible to control the flight path of a modestly-powered turboprop aircraft and/or had not appreciated their potential effect on the accident flight. A considerable number of findings therefore related to a lack of appreciation of mountain waves as a hazard to safe flight not only by pilots (whether or not there was compliance with prevailing navigation procedures) but much more widely.

The Probable Cause of the accident was formally documented as “the actions of the flight crew which created dangerous conditions for the flight”, their errors being identified as follows:

  • Continuing to Yasouj for landing which was contrary to the requirements of the Company Operations Manual due to the low height of the cloud ceiling and the related cloud mass and which should have led to a diversion to an alternate airport.
  • Descent to an unauthorised altitude below the minimum for the route and below MSA
  • Lack of an adequate level of CRM during the flight
  • Failure to fly the complete the stall recovery (flap setting, max RPM).
  • Inappropriate attempt to use the Autopilot after the stall
  • Inadequate anticipation of adverse weather based on Operations Manual guidance (clouds, turbulence, and icing)
  • The unjustifiably quick action to switch off the anti-ice system and thus the loss of the lower (icing) threshold for stall protection
  • Both pilots’ failure to follow Checklists and use Standard Callouts

Five Contributory Factors were also identified as follows:

  • The airline was not capable of detecting systematic deficiencies in the effectiveness of crew training on Meteorology, the content of the OM and awareness and compliance with SOP and overall there was not enough operational supervision of pilot behaviour.
  • The lack of a SIGMET about Mountain Wave or Severe Mountain Wave in the area of the accident.
  • A lack of clarity in the procedure for stall recovery in the FCOM.
  • The absence of a warning in the manufacturer’s aircraft manuals to increase flight crew awareness about mountain wave risk.
  • The fact that the aircraft was not equipped with an Aircraft Performance Analysis (APM) System to alert the flight crew about performance degradation.

A Preliminary Report was issued in the early stages of the Investigation when some of the operational context of the accident had already become apparent included five Interim Safety Recommendations as follows:

  • that the Iran Civil Aviation Organisation takes a decision about operation of the Aseman Airlines’ ATR72 fleet based on non-compliance with AD 2009-0170 and ensures that introduced Alternative Methods Of Compliance (AMOC) for safe flight and related training and operational procedures are complied with.

NOTE: EASA AD 2009-0170 became effective on 24 August 2009 and made the installation of an Aircraft Performance Monitoring System (APM) system mandatory on ATR 42 and ATR 72 series aircraft equipped with a Multi Purpose Computer (MPC) and was intended to minimise hazards associated with the inadvertent encounter of severe icing conditions beyond the prevailing certification requirements for Part 25 aeroplanes “by providing the flight crew with measurable and objective evidence and timely alert when such severe ice conditions are encountered”. This requirement did not alter existing AFM emergency procedures for the encounter of severe icing conditions which “remained valid and must be applied”. The problem with them was that their application is based on the subjective detection of severe icing conditions by pilots.

  • that the Iran Civil Aviation Organisation takes the necessary action to review operation manuals of airlines and restrict the flight mode from the IFR to the Visual Mode (V App) to enhance the safety of flights.
  • that the Iran Civil Aviation Organisation improves procedures for verifying the implementation of technical requirements relating to aircraft airworthiness and ensures that necessary enhancements to aircraft operator safety requirements are adopted by the airlines.
  • that All Crew and Airlines are requested to research/monitor about the limitations of the crew (medical) certificates before planning of crew for the flight.
  • that All Airlines should review required process of briefing between dispatcher and pilots on weather information before dispatch release of the flights to ensure safe operation of flights with consideration of available weather conditions.

A total of 28 new Safety Recommendations - which were not uniquely identified in any way - were issued as a result of the findings of the Investigation upon its completion and presented in the following order:

  • that ICAO considers implementation of Chicago Convention Annex 8 Standards to ensure that the State of Design and Manufacture supports other Contracting States with necessary information and the components required for the safety of their aircraft and remove civil aircraft from related embargoes.
  • that ICAO defines the extent of the responsibility which aircraft designers and manufacturers have in addressing the hazard of mountain waves in flight crew aircraft documentation.
  • that the European Aviation Safety Agency (EASA) ensures aviation authorities of EU countries which are States of Design and Manufacture support operating airlines’ flight safety requirements and separate civil aviation activities from sanctions.
  • that the European Aviation Safety Agency (EASA) clearly distinguishes the definitions of ‘qualified co-pilot’ and ‘fully qualified pilot’ as covered in the section on Operational Multi-pilot Limitations (OML) under the Aircrew Regulation.
  • that the European Aviation Safety Agency (EASA) ensures all aircraft manuals have a full description about mountain wave hazards and preventative requirements and guidance.
  • that the European Aviation Safety Agency (EASA) revises the stall recovery procedure in the ATR72-212 [ATR72-500] FCOM based on findings of this report and inform aircraft operators accordingly.
  • that the Iranian Interior Ministry defines responsibilities of the organisations involved in crisis management for participation in Search and Rescue following aircraft accidents and the related training and exercises which should be observed.
  • that the Iran Civil Aviation Organisation reviews aviation personnel certification regulation in accordance with the findings of the report and having clarified the definitions of ‘qualified co-pilot’ and ‘fully qualified pilot’ issue the necessary instructions to airlines.
  • that the Iran Civil Aviation Organisation improves procedures covering the lack of compliance of airlines with the requirements and mandatory technical publications aimed at ensuring the safety of aircraft operation.
  • that the Iran Civil Aviation Organisation defines source of technical publications (the State of Design) for each type of aircraft that need to be followed by airlines operating those aircraft.
  • that the Iran Civil Aviation Organisation sets a required plan for auditing airports that does not include the Aerodrome Certification requirement.
  • that the Iran Civil Aviation Organisation develops the national aviation Search and Rescue programme in coordination with related government organisations, especially in respect of its implementation away from airports.
  • that the Iran Civil Aviation Organisation empowers regulatory supervision on Annex 3 requirements.
  • that the Iran Civil Aviation Organisation provides suitable oversight of the Iranian Aeronautical Information Publication (AIP).
  • that the Iran Meteorological Organisation researches requirement for issuing airport weather forecasts (TAF) based on ICAO Annex 3.
  • that the Iran Meteorological Organisation researches the possibility of issuing mountain wave hazard warnings in SIGMET form.
  • that the Iran Meteorological Organisation coordinates with the Iran Civil Aviation Organisation concerning the implementation of ICAO Annex 3 Standards.
  • that the Iranian Airports and Air Navigation Company facilitates weather forecasts at airports in accordance with the provisions of ICAO Annex 3 in coordination with the Iran Meteorological Organisation and update information provided in the AIP.
  • that the Iranian Airports and Air Navigation Company ensures air traffic controllers use standard phraseology.
  • that the Iranian Airports and Air Navigation Company ensures the communication of pilot reports (PIREP) about adverse weather phenomenon to other flights.
  • that the Iranian Airports and Air Navigation Company clearly defines responsibility for the controlling of flights between ACC handoff and a Class ‘G’ airspace ATZ..
  • that Aseman Airlines revise/update the Operations Manual to ensures it contains restrictions for all airports which may be used on planned flights.
  • that Aseman Airlines plans timely additional training courses for all operational personnel on the subject of the Operations Manual.
  • that Aseman Airlines plans advanced meteorological training courses for pilots and dispatchers.
  • that Aseman Airlines revises their Operations Manual in respect of the responsibility of the dispatch unit to review the file of all flight information after briefing by the pilots so that if any of the requirements contained in the OM are not available, the decision about flight despatch should be referred to the Operations Control Centre (OCC) of the airline.
  • that All Airlines implements a meteorological training program which includes the subject of mountain waves for operational personnel and include corresponding detail in their operations manuals.
  • that All Airlines have an up-to-date copy of the country's Aeronautical Information Publication (AIP) at their airline and establish necessary related training for operational employees.
  • that All Airlines review their flight routes in accordance with aircraft performance focusing on the prevalence of Mountain Waves in relation to the principles of the safety management system and related risk assessment.

The completed Final Report was published on 17 February 2019 nominally as a Draft with any comments requested within two months. As of September 2019, it had not been formally reclassified as a ‘Final Report’.

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