Date:	Sunday 18 February 2018
Time:	09:32
Type:	ATR 72-212
Owner/operator:	Iran Aseman Airlines
Registration:	EP-ATS
MSN:	391
Year of manufacture:	1993
Total airframe hrs:	28857 hours
Cycles:	28497 flights
Engine model:	Pratt & Whitney Canada PW126
Fatalities:	Fatalities: 66 / Occupants: 66
Aircraft damage:	Destroyed, written off
Category:	Accident
Location:	15 km NE of Yasuj Airport (YES) -   Iran
Phase:	En route
Nature:	Passenger - Scheduled
Departure airport:	Tehran-Mehrabad Airport (THR/OIII)
Destination airport:	Yasuj Airport (YES/OISY)
Investigating agency:	AAIB Iran
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
An ATR 72-200 operating Iran Aseman Airlines flight 3704 from Tehran to Yasuj, Iran, impacted a mountainside in the Mount Dena range to the northeast of Yasuj Airport. All on board (60 passengers, two flight crew members, two cabin crew members and two security officers) were killed.
The flight departed Mehrabad Airport at 08:05 hours local time and climbed to the cruising altitude of FL210. At 09:22 the aircraft was over the Isfahan (IFN) VOR. The flight requested permission to descend from FL210 to FL170. This was approved by Tehran ACC. The flight followed Airway W114 from the OBTUX navaid to the Yasuj DVOR/DME. Minimum altitude on that airway is FL170. Minimum safe altitude (MSA) over the sector close to Yasuj Airport is 15500 feet.
Weather reported to the crew indicated a cloud ceiling of 15000 feet. Despite their clearance for FL170 and the MSA in the area, the crew subsequently selected FL150 as their target altitude.
The flight levelled off at FL150 at 09:28 hours and the de-ice & anti-ice systems were switched on. Subsequently the flight crew selected the autopilot's altitude mode to FL150. The aircraft then entered a mountain wave with vertical (updraft) winds reaching up to 3000 ft/min with associated icing conditions. Since the autopilot attempted to maintain FL150, engine power reduced to flight idle and the aircraft pitched down -4°. In these conditions the speed gradually reduced from a Calibrated Airspeed (CAS) of 205 knots.
At 09:29:44 the flight crew switched off the de-ice & anti-ice systems. The aircraft then entered a mountain wave downdraft. Engine powered increased and the nose pitched up because the autopilot was attempting to get back to FL150. The CAS meanwhile dropped to 130 knots.
The pilot then selected FL140 on the autopilot as the airspeed dropped to 118 knots. The stall warning sounded and the autopilot disconnected automatically. Airspeed increased to 134 knots as the aircraft descended at a rate of -3200 ft/m. At 09:31:22 the flight crew re-engaged the autopilot and set FL150 as their target altitude.
Four seconds later the aircraft became close to the surrounding mountainous terrain and the EGPWS alarm sounded. Radio altitude at that point was 3643 feet (pressure altitude 13869 ft).
The autopilot was disengaged again as the aircraft continued to lose altitude. Unable to regain altitude the flight crew noticed the mountain side ahead rolled the plane -86° to the left. The nose pitched up and the aircraft impacted the side of a mountain at 09:31:41.
The wreckage was located the following day in the snow, 350 feet below the top of a 13,412 ft high mountain.

Probable causes (from final report):
The main cause of accident:
The accident was happened due to many chains of considered causes but the 'Human Factor' had main role for the conclusion of the scenario. The Cockpit Crew action which has caused dangerous conditions for the flight is considered as main cause. Based on provided evidences, the errors of cockpit crew were as follows:
- Continuing to the Yasouj airport for landing against Operation manual of the Company, due to low altitude ceiling of the cloud and related cloud mass. They should divert to alternate airport.
- Descending to unauthorized altitude below minimum of the route and MSA
- Lack of enough CRM during flight
- Failure to complete the stall recovery (flap setting, max RPM).
- Inappropriate use of Autopilot after Stall condition
- Inadequate anticipation for bad weather based on OM (Clouds, Turbulence, and Icing ...)
- Quick action to switch off anti-ice system and AOA
- Failure to follow the Check lists and standard call out by both pilots

Contributing Factors:
The contributive factors to this accident include but are not limited to the following:
- the airline was not capable to detect systematic defectives about :
- effectiveness of crew training about Meteorology, OM, SOP, ...
- enough operational supervision on pilot behaviors
- The lack of SIGMET about Mountain Wave or Severe Mountain wave
- Unclear procedure for stall recovery in FCOM
- Lack of warning in aircraft manuals by manufacturer for flight crew awareness about mountain wave.
- Lack of APM System to alert crew about performance degradation

05:30 UTC / 09:00 local time:
OISY 180530Z 09004KT 9999 FEW035CB SCT040 OVC090 14/M01 Q1022

06:00 UTC / 09:30 local time:
OISY 180600Z 13004KT 9999 FEW035CB SCT040 OVC090 13/M00 Q1021

06:30 UTC / 10:00 local time:
OISY 180630Z 14006KT 9999 FEW035CB SCT040 OVC090 15/M02 Q1021

PressTV
Mehr News

photo (c) aerospacetalk.ir; near Yasuj Airport (YES/OISY); 20 February 2018

photo (c) aerospacetalk.ir; near Yasuj Airport (YES/OISY); 20 February 2018

photo (c) Google Earth / ASN annotations; near Yasuj Airport (YES/OISY); 18 February 2018

photo (c) CAO Iran / ASN annotations; near Yasuj Airport (YES/OISY); 18 February 2018

photo (c) CAO Iran; near Yasuj Airport (YES/OISY); 18 February 2018

photo (c) CAO Iran; near Yasuj Airport (YES/OISY); 18 February 2018

photo (c) Aeroprints.com; Dubai Airport (DXB/OMDB); 01 October 2006; (CC:by-sa)

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