Bearskin SW4 at Red Lake on Nov 10th 2013, impacted ground on final approach

Last Update: August 4, 2015 / 20:59:47 GMT/Zulu time

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Incident Facts

Date of incident
Nov 10, 2013

Classification
Accident

Aircraft Registration
C-FFZN

ICAO Type Designator
SW4

Airport ICAO Code
CYRL

The Canadian TSB released their final report concluding the probable causes of the crash were:

Findings as to causes and contributing factors

- A first-stage turbine wheel blade in the left engine failed due to a combination of metallurgical issues and stator vane burn-through.

- As a result of the blade failure, the left engine continued to operate but experienced a near-total loss of power at approximately 500 feet above ground level, on final approach to Runway 26 at the Red Lake Airport.

- The crew were unable to identify the nature of the engine malfunction, which prevented them from taking timely and appropriate action to control the aircraft.

- The nature of the engine malfunction resulted in the left propeller being at a very low blade angle, which, together with the landing configuration of the aircraft, resulted in the aircraft being in an increasingly high drag and asymmetric state. When the aircraft’s speed reduced below minimum control speed (VMC), the crew lost control at an altitude from which a recovery was not possible.

Findings as to risk

- If pilots believe that the negative torque sensing (NTS) system in the SA227 aircraft will activate in the event of any power loss or that NTS activation alone can provide adequate anti-drag protection in the event of an engine power loss, there is a risk that flight crews operating these aircraft types may not initiate the Engine Failures In Flight checklist in a timely manner.

- If there is no requirement for a boroscope inspection of the TPE331-11U-612G’s internal engine components in conjunction with the 450-hour fuel nozzle inspection, there is an increased risk that premature internal engine damage will not be detected.

- If there are discrepancies between the fuel nozzle testing procedures described in the TPE331-11U-612G maintenance manual and the corresponding fuel nozzle overhaul manual, there is a risk that unserviceable fuel nozzles may be evaluated as serviceable and re-installed on aircraft.

Other findings

- The SA227’s negative torque sensing (NTS) system may not always activate in response to an engine failure. The nature of the engine failure and aircraft profile may affect whether or not NTS activation parameters are reached.

The TSB reported the aircraft was on approach to runway 26, the captain (ATPL, 5,155 hours total, 3,550 hours on type) was pilot flying, the first officer (CPL, 2,200 hours total, 1,060 hours on type) was pilot monitoring. When the aircraft descended through 500 feet AGL about 1.4nm before the runway threshold the crew detected an aircraft malfunction but could not identify what had occurred. Both engines were selected to maximum power, the gear was retracted, an emergency was declared when the aircraft did not climb, the gear was selected down again before it could completely retract. A short time later the aircraft rolled left, descended, struck trees, continued through the trees taking out six hydro lines parallel to Ontario Highway 125 before coming to rest across the highway.

Both crew and 3 passengers died in the crash, one passenger received serious, the other minor injuries.

The aircraft was destroyed by impact forces and post impact fire. The aircraft's ELT was destroyed by impact forces as well and did not activate.

The aircraft's engines were equipped with a Negative Torque Sense (NTS) system which provide partial anti-drag protection, however would not auto-feather the propeller in case of a loss of torque. The TSB however found training manuals that described the function in a way, that could lead to interpretation of a full anti-drag protection provided by the system.

The flight data recorder revealed that the left hand engine (TPE331) lost power 56 seconds prior to impact. The torque reduced to -1%, while N1 remained nearly stable at 98.5% - the right hand engine at that time indicated a torque of 22% at 99% N1. When maximum power was set the left hand engine reached 97% N1 and delivered 0% torque, the right hand engine produced 102% torque at 101% N1. The left hand engine continued to run until impact but at around 0% torque only.

Examination of the propellers showed that both were in a about 6 degrees blade angle position, both propellers showed deformations typically associated with mid level rotational energy absorption.

Examination of the left hand engine found the #9 fuel nozzle had developed cracks permitting cross flow between primary and secondary flows affecting atomization of the fuel. Debris was caked on the fuel nozzle that could not be removed, hence a test for fuel flow was not possible.

Further examination revealed that the first turbine stage had been stripped of all its blades as result of a blade failure, that had separated as result of high cycle fatigue in an area of high porosity. The first stage stator was found burnt through.

The engine manufacturer indicated that there had been two previous cases of similiar engine failures during which a first stage turbine blade had cracked as result of high cycle fatigue in an area of high porosity resulting also in the #1 stator to burn through. The TSB summarized the manufacturer's assessment: "The turbine blade failures have been attributed in some instances to porosity issues in the casting process and/or fatigue issues caused by excessive turbine distress due to stator vane burn-through. A burn-through in the stator vane will produce a vibrational response (one per revolution) in the blades that can lead to the blades separating in a high-cycle fatigue mode. Stator vane burn-through has been attributed to streaking fuel nozzles or the blockage of the first-stage stator cooling tubes which could cause hot spots on the stator vane."

The TSB analysed: "The teardown of the left engine revealed a burnt-through stator vane. The burnt-through stator vane created a one-per-revolution vibrational excitation and allowed excessive heat stress on the first-stage turbine wheel blades. This, combined with higher porosity in one of the blade castings, inadequate fatigue capability and robustness of the blade material, and a minor increase in the mean stress in the blade fir tree region due to blade platform contact, resulted in a high-cycle blade failure. The separated portion of the failed blade damaged the remaining first-stage blades causing them to separate from the first-stage turbine wheel and pass through the remaining turbine wheels, damaging them in the process. Although the engine was severely damaged, there was enough remaining air and fuel flow so that the engine continued to run, but it produced little or no power to drive the propeller."

The TSB continued to analyse:

The following factors likely contributed to the crew’s difficulty in identifying the nature of the malfunction:

- The right engine was at a low power setting when the left engine power loss occurred, which would have made it difficult for the pilot flying to sense the yaw resulting from the malfunctioning engine;

- The left engine continued to run, which resulted in engine readings of 98% engine rpm, with likely normal oil pressure, exhaust gas temperature, and fuel flow. The low torque indication in the cockpit would have provided some indication of the engine problem, but it was not noticed; and

- There was little time available to identify the nature of the malfunction.

The loss of power and drop in N1 speed to 98% would have commanded the left engine propeller governor to attempt to maintain a constant engine speed of 100% by reducing the propeller blade angle. As a result, the left engine and propeller went from a low thrust condition to a high drag condition, with the fining out of the propeller blades. The left engine negative torque sensing (NTS) system was likely not operating because the engine had not completely lost power and was developing torque greater than the −4% value required to activate it. With the landing gear extended and flaps at ½, the aircraft was in a high drag asymmetric state.

The SA227’s NTS system may not always activate in response to an engine failure. The nature of the engine failure and aircraft profile may affect whether or not NTS activation parameters are reached. If pilots believe that the NTS system in the SA227 aircraft will activate in the event of any power loss or that NTS activation alone can provide adequate anti-drag protection in the event of an engine power loss, there is a risk that flight crews operating these aircraft types may not initiate the Engine Failures In Flight checklist in a timely manner.

Because the exact nature of the engine malfunction was not identified, the crew did not follow the standard operating procedures (SOPs) prescribed action of calling out the associated emergency procedure, which required them to stop and feather the propeller of the affected engine. This may have resulted from a belief that the NTS system would always activate in the event of a power loss and that NTS activation alone would provide adequate anti-drag protection from a windmilling propeller. Feathering the failed engine’s propeller would have decreased the drag associated with it and likely would have allowed the crew to maintain control of the aircraft.
Incident Facts

Date of incident
Nov 10, 2013

Classification
Accident

Aircraft Registration
C-FFZN

ICAO Type Designator
SW4

Airport ICAO Code
CYRL

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