Sol SF34 near Los Menucos on May 18th 2011, severe icing, stall, loss of control

Last Update: September 7, 2015 / 17:42:44 GMT/Zulu time

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Incident Facts

Date of incident
May 18, 2011

Classification
Crash

Aircraft Type
SAAB 340

ICAO Type Designator
SF34

On Sep 7th 2015 Argentina's JIAAC released their final report in Spanish concluding the probable causes of the crash were:

During a domestic commercial passenger flight, in the cruise phase, a loss of control and uncontrolled impact with the ground occurred as result of severe formation of ice due to the combination of following factors:

- entering of an area with conditions permitting the formation of ice with inadequate monitoring of warning indications by environmental factors (temperature, cloudiness, precipitation and ice accretion) and by internal factors (airspeed, angle of attack), which permitted long term operation in conditions of severe icing

- forecast of light icing that motivated the lack of perceiption of the actual weather hazard of severe icing

- inadequate assessment of risk, mitigative measures were not adopted or briefed (distribution of tasks, review of the anti ice systems, limitations, use of power, use of autopilot, strategies etc.)

- increased stress levels that contributed to a distributive attention deficit as result of unmatched expectations of operational context

- Icing conditions that exceeded the anti-ice systems' capacities

- inappropriate selection of airspeed close to the boundary of flight envelope

- inappropriate use of the autoflight systems not being switched to IAS mode while flying in icing conditions

- biased implementation of procedures laid down in the flight crew operations manual in terms of flight in severe icing conditions

- late recognitition of the aircraft entering stall due to vibrations by the airframe being confused with vibrations produced by the propellers contaminated with ice

- activation of the stick shaker according to uncontaminated wings rather than the actual loss of lift at higher speeds in icing conditions

- improper initial stall recovery technique, the priority should have been to lower the angle of attack in exchange for altitude

- Unusual reactions of the aircraft to inputs during the loss of control, probably by contamination of control surfaces with ice which prevented recovery of the aircraft

- increased stress on the crew which prevented operational decisions

Potential hazards to the operation, however not cause of the crash, were:

- outdated specifications of operations by the operator

- biased implementation of updates to the operating manuals

- crew rostering managed by a management outside operations management

- out of date aircraft manuals

- Stall Recovery Technique instructions not in line with the manufacturer's instructions

- inappropriate simulation exercise for stall recovery

- breach of two airworthiness instructions with respect to maintenance of propellers

Systemic Issue

- lack of VHF coverage along the route of the aircraft

- lack of meteorologic service availability throughout all hours of the airline's operations

The JIAAC reported that the aircraft departed Neuquen to Commodore Rivadavia, which was the last leg of their flight from Cordoba via Mendoza, Neuquen to Commodore Rivadavia. The crew intended to climb to FL190, but did not climb above 17,800 feet remaining at that altitude for 9 minutes. Due to icing conditions found at this altitude the crew subsequently initiated a descent to FL140 and levelled off at FL140 five minutes later, the icing conditions worsened progressively however. Within 2 minutes the aircraft severly accumulated ice resulting in the loss of control, the aircraft impacted ground shortly after in instrument meteorological and night conditions at position S41.0877 W67.9480 at an elevation of 2741 feet. All 3 crew and 18 passenger aboard perished in the impact.

The crew comprised the captain (45, ATPL, 6,902 hours total, 2,181 hours on type) and the first officer (37, CPL, 1,340 hours total, 285 hours on type).

The aircraft had accumulated 41,422 flight hours in 44,477 flight cycles, the aircraft underwent last inspection on Apr 22nd 2011. The aircraft was equipped with two CT7-5A2 engines, the left hand engine had accumulated 38,592 flight hours in 41,779 flight cycles, the right hand engine had accumulated 34,409 flight hours (editorial note: the JIAAC reported in 41,779 flight cycles which is obviously in error), both engines had undergone maintenance inspection and compressor section cleaning on May 5th 2011.

The aircraft had taken off with a takeoff mass of 12,873kg, 57kg below the maximum takeoff mass of 12,930kg.

The crew used following weather data for their flight preparation, the crew however was not in possession of satellite images of a cold front about to arrive in the accident area:

18 - 22:00 BARILOCHE 320/10 KT 7KM FBL RA CONS 4St1000FT 4Sc2000FT
8Ns5900FT 08/05 Q1010.1
18 - 22:00 COMODORO RIVADAVIA 320/06KT 30 KM 2Ac9900FT 8Sc19800FT
10/M04 Q1008.0
18 - 22:00 ESQUEL 290/15KT 30KM 2Ac9900FT 7Cs19800 07/M06 Q1007.1
18 - 22:00 NEUQUEN 050/01KT 20 KM 1Sc6900FT 4Ac9900FT 15/04 Q1010.7
18 - 22:00 RIO GALLEGOS 320/06KT 30KM 3Sc3500FT 5As6900FT 04/M01
Q997.3
18 - 22:00 TRELEW 270/02KT 30KM 2Cu4500FT 3Cs19800FT 11/02 Q1009.2
b) TAF
18 – 16:00 – TAF SAZN 181600Z 1818/1918 29003KT CAVOK TX15/1918Z
TN08/1910Z BECMG 1822/1823 34015KT BECMG 1904/1906 36005KT BECMG
1916/1918 25015KT
18 – 16:00 – TAF SAZC 181600Z 1818/1918 29020KT 4000RADZ BKN018 BKN
050 TX10/1818Z TN 08/1910Z BECMG 1905/1908 30005KT 2000 RASN
6Sc1000FT 5Sc2000FT
18 – 16:00 – TAF SAVC 181600Z 1818/1918 25025KT CAVOK TX15/1819Z
TN07/1911Z BECMG 1823/1901 30015KT

The Sigmets indicated a cold front entering the southwest of the control zone with stratiform clouds and precipitation, moderate turbulence between FL50 and FL200 respective between FL200 and FL250 in the area around Ezeiza, and icing between FL080 and FL150.

Editorial Note: The JIAAC used the satellite image, GOES-E May 18th 23:45Z by NASA, produced and published by The Aviation Herald on May 19th 2011.

According to communication transcripts the crew was unable to establish contact with Commodore Rivadavia and therefore called Ezeiza South Center informing the center they had descended to FL140 due to icing. Ezeiza acknowledged the aircraft had already descended to FL140, then forwarded the information to Commodore Rivadavia.

Following the loss of control the first officer radioed a Mayday call, which was heard by another aircraft at FL350 enroute from Buenos Aires Aeroparque to Commodore Rivadavia.

The JIAAC reported that both cockpit voice and flight data recorder were severly damaged including the interior and recording mechanicsm, the tapes were cut. In manual restoration of the tape pieces the investigation was able to recover 26 minutes of the 30 minutes audio recording, however in poor quality, and was able to recover about 2 hours 29 minutes of the 25 hours recording of the flight data recorder with numerous drop outs (gaps). The data recovered represent 43 minutes of the accident flight, the last 18 seconds of the flight are missing.

The JIAAC analysed that there is no evidence of any malfunction of the aircraft systems prior to impact, both engines were delivering power at the time of loss of control. Although the violent impact destroyed the propellers the investigation was able to determine that two airworthiness directives concerning the propellers had not been completed, a causal link to the accident sequence could not be established however.

Evidence suggests that all aircraft anti-ice systems were operating as expected throughout the entire flight.

The JIAAC analysed that the TAF predicted light icing between FL030 and FL150. It can be assumed that the crew conducted a risk assessment on that foundation and subsequently got surprised by the magnitude of the meteorological phenomen of icing found enroute. The flight crew was unable to obtain more recent weather information from the weather office at Neuquen which was closed at the time of the arrival of the aircraft. In addition the dispatch office of the operator had not provided the crew with the satellite image showing the arrival of the cold front, thus providing the crew with incomplete information according to manuals and requirements. In addition a later Sigmet, which would have alerted the crew of the severe icing conditions, was not transmitted to the crew.

The JIAAC analysed that the crew had used the autoflight systems in climb mode with the target of FL190, the aircraft however did not climb above 17,140 feet MSL, outside air temperature was -11 degrees C at that altitude. The aircraft was maintaining 144-146 KIAS at that time. The crew selected CLIMB LOW mode, which permitted the aircraft to climb to 17800 feet, the operating manual however required to select the autoflight systems into IAS mode in icing conditions. At 17,800 feet the autopilot was selected into vertical speed mode, then the autopilot was disconnected while the crew descended the aircraft to FL140 and was re-engaged at FL140. At FL140 the airspeed was 145 KIAS, then dropped to 129 KIAS below onset of stall (137 KIAS), the stick shaker however did not activate. The aircraft was in cloud accumulating ice and about to leave the clouds when the airspeed rapidly dropped from 145 KIAS to 137 KIAS in 13 seconds, the crew increased power but insufficient to maintain speed and observed vibrations which the crew interpreted to be caused by the propellers contaminated with ice rather than the onset of stall. The autopilot disconnected automatically and the aircraft encountered several roll events and a nose down movement. The roll movements could not be stopped and wings level was not achieved. When the crew attempted to counteract the nose down movement the effect of the control surfaces was insufficient probably due to ice accretion. Attempts by the crew to overcome the nose down resulted in secondary stalls in addition and the activation of the stick pusher for five times. The JIACC concluded that the crew did recognize the directions of rolls and pitch changes however was unable to control the movements due to abnormal reactions of the aircraft to the control inputs.
Incident Facts

Date of incident
May 18, 2011

Classification
Crash

Aircraft Type
SAAB 340

ICAO Type Designator
SF34

This article is published under license from Avherald.com. © of text by Avherald.com.
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