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ASN Wikibase Occurrence # 190761
Last updated: 11 August 2019
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Date:04-MAY-2014
Time:
Type:Silhouette image of generic AT76 model; specific model in this crash may look slightly different
ATR 72-600 (72-212A)
Owner/operator:Caribbean Airlines
Registration: 9Y-TTC
C/n / msn: 989
Fatalities:Fatalities: 0 / Occupants: 71
Other fatalities:0
Aircraft damage: Minor
Location:near Piarco Airport -   Trinidad and Tobago
Phase: Landing
Nature:Domestic Scheduled Passenger
Departure airport:Tobago-A.N.R Robinson International Airport (TAB/TTCP)
Destination airport:Piarco Airport
Narrative:
In descent, the crew reduced engine power to the minimum possible in flight by setting the levers to Flight Idle (FI). The aircraft speed was 246 kt, close to the maximum operating speed (VMO) of 250 kt. The crew then felt strong vibrations followed by an alarm associated with the electronic controller (PEC) which controls the right propeller.
After the flight, the drive shaft of the engine's AC generator right was found broken and was replaced. A maintenance team carried out maintenance tests on both engine/propeller assemblies. No vibration or operation abnormal was detected.
The next day's flight was normal. During the taxi on landing, the crew reported a loud vibration noise as they move the power levers from the idle position to ground idle.
Following this flight, various maintenance operations were undertaken. Three tests on the ground of the engine/propeller assembly ware carried out and did not highlight any abnormal operation. An element of the propeller control system (the propeller valve module) was replaced. A fourth ground test was performed, during which the power levers were positioned in reverse thrust.
Vibrations appeared and the engines were immediately shut down. After the stop of the engines, blades n1, 2, 5 and 6 of the right propeller were feathered while blades n3 and 4 seemed to have remained in the reverse thrust position. Removal of the right propeller blades revealed, among other things, the failure of the blade pin No. 4 and damage to the propeller blade angle change cylinder plate.

Other incidents
The circumstances and damage observed were similar to those observed in an investigation of a serious incident on 18 September 2013 in Indonesia involving an ATR 72-500 registered PK-WFV. The Indonesian investigating authority, NTSC, had initiated an investigation and issued an immediate safety recommendation to the aircraft operator regarding the verification of the condition of the propeller blade pins and the search for indications of cracks in part of the fleet.

On November 30, 2014, a new similar incident occurred in Sweden involving an ATR 72-500 registered SE-MDB for which the Swedish investigating authority, SHK, had opened an investigation. Shortly after this incident, the BEA issued four safety recommendations to EASA.

Causes of the incident
Seven vibration phenomena on ATR 72-212A have been reported in recent years. In almost all cases, the failure of the pin of one of the blades and damage to the front plate of the propeller blade angle change cylinder were found. The investigation revealed the existence of alternating overloads causing damage to the plates and a final overload in a direction resulting in the failure of the crank pin.
The origin of these overloads and the precise timing of the damage and vibration could not be determined. Nevertheless, several elements may have contributed to this:
- a retention force caused by friction in the area of the blade foot bearings (ball bunching);
- significant forces caused by the impact of the crank pin on the platform ear during cyclic force phenomena on the front platform that occur when the aircraft speed is close to the VMO and the power levers in the idle position flight;
- unexpected operation of the servo loop of the propeller control system disrupted by cyclic forces on the front plate and friction phenomena.
The maintenance work carried out on the 9Y-TTC following the vibration phenomena did not identify this damage.

Sources:

https://www.bea.aero/index.php?id=40&L=1&news=13629&cHash=def2f4779463ad964fb8fd416d732e3b
http://aerossurance.com/safety-management/bea-propeller-vibration-atr72/
19 Sep 2013 incident: https://aviation-safety.net/wikibase/190762
30 Nov 2014 incident: https://aviation-safety.net/wikibase/172073


Revision history:

Date/timeContributorUpdates
20-Oct-2016 07:02 harro Added
27-Jul-2019 12:39 harro Updated [Date, Total occupants, Location, Nature, Departure airport, Destination airport, Source, Narrative, Accident report, ]
11-Aug-2019 12:14 Aerossurance Updated [Source]

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