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After flying in cloud for 15 minutes of a 17 minute cruise at 18000 feet descent was commenced to the destination aerodrome. As the aircraft was passing 7800 feet on descent in cloud and light to moderate rain the cockpit warnings alerted the crew to a failure of the left engine. However, before any rectification action could be undertaken by the crew, all warnings ceased and power was observed to be restored. The crew reported that although approximately 15 minutes of cruise flight had been in cloud, the only visible ice accretion was a very light rime ice coating on the leading edge of the wings. There was no ice visible on the propellers or the nacelles. The investigation noted that this and all other reported incidents involving uncommanded engine shut down (flameout), occurred either when the aircraft was in moderate to severe icing and free moisture or shortly after exiting those conditions. Tests carried out at certification failed to reveal the source of a critical ice buildup in the intake duct or engine. It was revealed that a propeller was not fitted to the test unit during the ice tunnel tests and subsequent post certification tests have suggested that an unusual swirl which has been detected coming from the propeller may be a contributing factor causing a blanketing effect in the intake duct. Depite early modifications to the Electronic Engine Control Unit, the Handling Bleed Valve and the Fuel Nozzles further cases of uncommanded engine shutdown under similar weather conditions were experienced. It is considered that these modifications by themselves are not sufficient to prevent a recurrence of the flameout problem. Subsequent tests by the manufacturer using video camera equipment installed in the intake duct revealed ice buildups on the unheated flexible seal between the intake duct and the engine flange. The tests confirmed that when a critical mass of ice was reached the buildup would break off and be ingested by the engine resulting in power fluctuations. It is considered that the inflight power fluctuation incidents have been caused by flameouts originating from ice ingestion in the engine. It is considered that the methods adopted to satisfy the certification requirements for icing failed to adequately address the area of intake icing. Had they done so, the formation of ice on the unheated flexible seal between the intake duct and the engine flange would have been discovered prior to certification.

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[ Download PDF: 24KB]
General details
Date: 06 July 1988 Investigation status: Completed 
Time: 950  
Location:25nm N Cooma Investigation type: Occurrence Investigation 
State: New South Wales  
Release date: 21 February 1989  
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
Aircraft details
Aircraft manufacturer: Fokker B.V. 
Aircraft model: F27 
Aircraft registration: VH-FNJ 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Nil 
Departure point:Sydney NSW
Departure time:0907
Destination:Cooma NSW
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