Investigation Summary
The flight was operating in an area known for complex mountainous terrain and significant meteorological hazards. As the aircraft approached Yasuj, the crew was cleared to FL170, but they unilaterally decided to descend to FL150 to get clear of the cloud ceiling, which was reported at 15,000 feet. This altitude was below the sector's Minimum Safe Altitude (MSA) of 15,500 feet. While at FL150, the aircraft encountered a mountain wave, a phenomenon where horizontal airflow is disturbed by mountains, creating powerful vertical currents. Initially, an updraft caused the autopilot to reduce engine power to flight idle to maintain the target altitude. When the aircraft then entered a severe downdraft (reaching 3,000 ft/min), the autopilot pitched the nose up to maintain FL150, causing the airspeed to bleed off rapidly. The crew, likely suffering from reduced situational awareness and lack of training regarding mountain waves, failed to apply full power or set the correct flap configuration during the subsequent stall. The Enhanced Ground Proximity Warning System (EGPWS) sounded, but the aircraft was already too low for a successful recovery in the terrain.
Final Conclusions
The Iranian Civil Aviation Organization (CAO) concluded that the primary cause was human error. Specifically, the crew continued the flight to Yasuj despite weather conditions being below minimums, descended below the authorized altitude and MSA, and failed to follow standard stall recovery procedures. Contributing factors included the airline's inadequate supervision and training regarding mountain waves, the lack of SIGMET warnings for the phenomenon, and the absence of an Aircraft Performance Monitoring (APM) system to alert the crew to performance degradation.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
Identified as Hojatollah Foolad, age 62.
First Officer
Identified as Kaveh Khalili, age 36.
Systems & Failure Modes
Autopilot
The autopilot attempted to maintain the unauthorized altitude of FL150 during vertical wind shifts, leading to a dangerous reduction in airspeed when countering a downdraft.
EGPWS
The Enhanced Ground Proximity Warning System provided a terrain alert four seconds after the crew re-engaged the autopilot, but the aircraft's low energy state and altitude made impact unavoidable.
Anti-ice System
The crew switched off the de-ice and anti-ice systems shortly before entering the downdraft, despite being in an area favorable for severe icing.
Interesting Facts
- 01The aircraft had been in storage for seven years and was restored to service only three months before the accident.
- 02The captain had over 17,000 total flying hours, with 12,000 hours on the ATR 72.
- 03The minimum ceiling for landing at Yasuj was 11,000 ft, but the actual ceiling was 9,000 ft at the time of dispatch.
- 04The wreckage was located 350 feet below the top of a 13,412 ft high mountain peak.
- 05The investigation found the crew had never been trained on how to handle mountain wave phenomena.
Safety Actions & Advisories
Mountain Wave Training
The Iran CAO recommended that all Iranian airlines include training programs specifically addressing the hazards of mountain waves.
Stall Recovery Revision
A recommendation was made to EASA to revise the stall recovery procedures for the ATR 72-212 to ensure clarity and effectiveness.
Search and Rescue Coordination
The Iran CAO was tasked with developing a better Aviation Search and Rescue program to improve coordination during disaster responses.
Fleet Suspension
Source ↗On February 23, 2018, the Iran Civil Aviation Organization temporarily suspended all Iran Aseman Airlines ATR 72 operations pending further investigation.
Known Controversies & Unanswered Questions
Impact of Economic Sanctions
Reference ↗Journalists and Iranian officials suggested that international sanctions prevented the airline from obtaining modern aircraft and spare parts, forcing the continued use of older airframes like EP-ATS.
Location
ACCIDENT SITE · Coordinates taken from the official investigation report.