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The Sikorsky S76C helicopter was in cruise flight with the automatic flight control system engaged, when the flight crew noted a loud noise and the helicopter yawed to the left, rolled left, and the nose pitched down. The flight crew disengaged the automatic flight control system and resumed flying the helicopter manually, stabilising it in level flight. The right engine-out and fire-warning annunciators were illuminated, with the engine-out aural warning sounding. The right engine instruments displayed zero rotational speed of the gas generator (GG) and extremely high turbine outlet temperature (measured at point T4 within the engine). The crew activated the right engine fire bottles and simultaneously closed the fuel firewall shut-off valve. The fire indication extinguished. They then configured the helicopter for single engine flight with the remaining engine operating approximately ten seconds into the two and one-half minute One Engine Inoperative (OEI) limitation. The flight crew adjusted power requirements for the OEI condition and then completed an uneventful single engine landing at their Longford base.

Examination of the helicopter revealed minor shrapnel damage to the right engine exhaust extension, and fracture separation of the engine oil pressure switches and rear bearing external oil vent and return pipes.

The Turbomeca Arriel model 1S1 engine comprised five modules. Module three (or the high-pressure section) contained the gas generator first and second stage wheels. The left side of the right engine, forward of the external rear bearing oil return line near the outer surface of module three, displayed evidence of fire and oil residue.

The right engine was removed and shipped to the engine manufacturer's Australian facility for disassembly and examination with Australian Transport Safety Bureau (ATSB), operator, and engine manufacturer representatives in attendance.

Engine examination

Disassembly and preliminary examination of Arriel 1S1 engine, serial number 15038, revealed a separation of one GG second stage turbine blade. Blade number sixteen was separated above the blade 'fir tree' attachment point, below the blade platform, and had punctured the second stage nozzle guide vane turbine ring. The rear bearing of the GG had collapsed and was significantly damaged. Separated pieces of the centrifugal diffuser of module three were found inside the module. There were indications that several fracture surfaces of the separated sections were pre-existing before the incident. In addition, the engine exhibited signs of severe overheating and significant damage in the air path downstream of the turbine blade separation.

The fracture surface of the separated blade was typical of ductile tensile overload, with the exception of the small corner area of fatigue cracking. The dendritic patterns within the fracture were indicative of the normal underlying microstructure of the blade casting. Failure of the blade in that mostly ductile overload manner indicated exposure to a transient or sustained stress level above the ultimate strength of the blade material at its operating temperature. Refer to ATSB Technical Analysis Report 200103038 (BE/200100017) for further details.

Engine history

The engine was installed on 4 March 2000 and had accumulated 7,935.0 hours and 6,784.1 cycles since new. It had been overhauled on 12 February 1999, and had accumulated 1,992.0 hours time since overhaul (TSO) and 1,878.1 cycles since overhaul. The GG assembly second stage turbine disc, serial number DC3666YC, had been installed during the overhaul with zero hours and cycles accumulated. The turbine disc and blades were well within the life limit of 10,000 cycles established by the manufacturer. Arriel engine modification TU204 (GG turbine blade plasma coating) had been incorporated.

Previous Australian occurrences

Occurrence report 200100584

On 7 February 2001, a Sikorsky S76C helicopter belonging to the same operator, with two crew and ten passengers on-board, was in a hover with the flight crew completing before take-off checklist items. The pilot reported that while trimming the engines, a "pop" was heard. He then noted that the left engine turbine gas temperature (measured at point T4 within the engine) was in excess of 1000 degrees C. The helicopter was then landed uneventfully. The flight crew reported that the only cockpit indication of imminent failure was the almost simultaneous illumination of the left engine chip (magnetic particle) detector advisory.

Examination of the helicopter revealed minor shrapnel damage to the left engine exhaust extension and engine cowling. There was no reported engine fire. The left engine was removed and sent to the engine manufacturer for disassembly and examination. The manufacturer's final report noted a separation of turbine blade number six of the GG second stage disc. The blade was separated above the 'fir tree' attachment point but below the blade platform, and had punctured the second stage nozzle guide vane turbine ring. One adjacent blade (number seven) in the direction of turbine wheel rotation was also noted as cracked.

Metallurgical examination by the manufacturer attributed the blade failure to a low-cycle fatigue cracking mechanism. The manufacturer concluded that abnormal loading was the major contributing factor in the failure, given the reported absence of anomalous material features or evidence of high-temperature operation. Dimensional inspections failed to reveal any sign of non-conformity that could have led to the development of the abnormal loads. However, the manufacturer stated that turbine blade platform/GG disc interferences were also a potential factor that could have aggravated the fatigue failure of the blade.

At the time of the occurrence, Arriel 1S1 engine, serial number 15522, had accumulated 4,737.4 hours and 4,471 cycles since new. It had accumulated 1,740.0 hours TSO and 1,615 cycles since overhaul. Following overhaul, the engine was installed on March 11, 1999. Module three did not have turbine blade plasma coating modification TU204 incorporated.

Occurrence report 199602839

On 9 September 1996, a Sikorsky S76C helicopter belonging to the same operator, experienced an in-flight engine failure of the right engine while taking off from an oil platform. A loud noise was heard before the engine failure. The right engine was shut down and the crew completed an uneventful single engine return to the Longford base. There was no reportedassociated engine fire. The right engine was removed and sent to the manufacturer fordisassembly examination.

At the time of the occurrence, Arriel 1S1 engine serial number 15513, had accumulated 2,282.0 hours and 1,949 cycles since new. The manufacturer provided the operator with a final report noting the rupture (separation) of one GG turbine blade with subsequent rear bearing damage and GG seizure. Their report stated that the separation was suspected to be the result of blade rubbing with the second stage nozzle guide vanes with no signs of fatigue or abnormal over temperature operation. Module three had turbine blade plasma coating modification TU204 incorporated.

Other overseas occurrences

The French airworthiness authority, Direction Generale de l'Aviation Civile (DGAC), reported knowledge of three other overseas occurrences involving GG second stage turbine blade separation failures. Of those three incident engines, all had the TU204 modification. Cycles since overhaul on those incident engine turbine discs and blades varied from 1,978 to 5,933 cycles.

Engine service bulletin history

Turbomeca Service Bulletin (SB) 292 72 0151 was originally issued on 5 June 1992 specifying the incorporation of modification TU204, the protection of the GG second stage turbine blades from corrosion or erosion with a Heurchrome low pressure plasma coating. That modification also permitted a performance improvement by allowing the more accurate machining of the turbine tip diameter to control the tip clearance. The service bulletin addressed all Arriel variants, with Arriel 1S1 engines having incorporated TU204 from the first production engine. For all other variants, TU204 implementation was optional and installed at the customers' request.

In July 1998, the engine manufacturer implemented internal documentation and procedures to remove all GG turbine blades with TU204 installed during overhaul of module three. Consequently, SB 292 72 0151 was amended on 18 August 2000, to recommend removal of all TU204 modified blades, citing possible weight mass increases and suspected increased stress on the turbine blade root. The manufacturer stated that if the plasma coating was not applied as per drawing requirements, the resulting stresses could be more than anticipated, resulting in abnormal loading of the blade root. Both incorporation and removal of modification TU204 required removal of the engine and/or module and shipment to the manufacturer.

External oil pipe description

Three external oil related pipes provided lubrication of the GG rear bearing. Those pipes passed through hollow support struts and were then physically secured to module three. The supply oil pipe provided oil from the engine driven gear type oil pump to the bearing after passing through a restrictor and a tube screwed into the bearing housing. Oil was then sprayed onto the bearing. After lubricating the bearing, the oil fell by gravity to the bottom of the housing, through a tube and was returned to the tank through an oil pipe to the scavenge pump. The air/oil mist that resulted from the lubrication of the bearing was vented overboard through a vent pipe attached to the top of the housing.

Engine oil flashpoint/autoignition

The engine oil temperature of a normally operating Arriel 1S1 engine in a S76C helicopter was approximately 100 degrees Celsius (C). The flash point of the turbine engine oil was approximately 223 degrees C. The flash point of a liquid was defined as the lowest temperature at which a material would produce a flammable vapour, and was a measure of the volatility of the material.

The auto-ignition temperature of engine oil was approximately 388 degrees C. Auto-ignition temperature was defined as the temperature at which auto-igniting materials spontaneously combust. According to the engine manufacturer, during normal operation, the external surface temperatures of number three modules ranged between 280 to 450 degrees C, dependent upon location on the module, with a maximum surface temperature of 450 degrees nearest the rear bearing. The surface temperature maximum values of module three were well within the auto-ignition temperature of the engine oil.

 

Turbine blade separation

The failure mode of the gas generator (GG) rear bearing collapse was attributed to an imbalance condition of the GG second stage disc following the separation of a GG second stage turbine blade. That imbalance condition resulted in high vibration loads and damage to the rear bearing, resulting in module three failure. The damage to the centrifugal diffuser was determined to be a secondary failure and not considered a safety of flight concern.  

Modification TU204

During the period 6 September 1996 to 11 July 2001, there were five reported incidents worldwide of turbine blade separation failure possibly related to modification TU204. Of those five documented failures, all had modification TU204 incorporated. 

The engine manufacturer had identified a possible mass increase and resulting stress increase on the blade roots of turbine blades with the modification TU204 plasma coating applied. They addressed those concerns by discontinuing its incorporation and removing TU204 compliant blades from service during overhaul of number three modules. The recommendation by the manufacturer to remove all TU204 modified modules was not made a mandatory requirement by any airworthiness authority. Arriel 1S1 engine number three modules (and other Arriel variant engines with modification TU204 incorporated) that have not passed through an approved overhaul facility since July 1998 may currently have modification TU204 installed. Those engines and modules may be subject to abnormal blade loading stresses. The imminent separation of a turbine blade is not detectable by any on-board instrumentation or flight crew observations.

Engine fire

After activation by the crew, the engine compartment fire bottles successfully extinguished the fire that occurred following rear bearing collapse and subsequent fracture of the return oil pipe. The external oil pipes were exposed to the high vibration loads imposed by the out-of-balance GG turbine disc and, as a result, two fractured. Following the fracture of the return oil pipe, a heated flammable liquid (oil), was sprayed onto the heated external outer surface of module three. During normal operation, the outer surface of module three experienced surface temperatures within the auto-ignition range of the engine oil. The oil ignited causing an in-flight fire. The fuel source of the fire was the heated engine oil escaping from the fractured return oil pipe. The ignition source of the fire was the hot outer surface of module three. If the flight crew had not secured the engine, or the engine had not stopped rotating, the supply of flammables for combustion would have been limited only by engine oil system capacity.

 
  1. The gas generator (GG) second-stage turbine blade incurred a fatigue fracture and separated.
  2. The engine manufacturer retained in service modified GG turbine blades, which from past experience could encounter unknown stress levels at the blade root. Those stresses could then possibly cause the blade to separate.


 

Local safety action

Operator

Following this occurrence, the operator conducted a one-time borescope inspection of its fleet centrifugal diffusers for cracks. No cracks were discovered.

Engine manufacturer

The engine manufacturer has discontinued the installation of all modification TU204 GG turbine blades at its factory and at repair centres. On 26 November 2001, Turbomeca issued service bulletin (SB) 292 72 0258 with applicability to all Arriel 1B engines (single engine variants), which recommends removal of modules with modification TU204 embodied.

In addition, the manufacturer has implemented dimensional checking on all new Arriel model engine module three assemblies, for turbine blade platform/GG disc interferences and on this operator's engines with more than 1,000 hours time accumulated on a module three.

RECOMMENDATIONS

As a result of the investigation, the Australian Transport Safety Bureau has identified a safety deficiency related to Turbomeca Arriel engine fire propagation following turbine blade failure and rear bearing collapse. The Australian Transport Safety Bureau therefore issued the following recommendations.

R20010192 issued on 18 September 2001

The Australian Transport Safety Bureau recommends that the Australian Civil Aviation Safety Authority assess the adequacy of the Turbomeca Arriel engine module three bearing lubrication installation to determine if it meets the applicable design standard requirements to ensure the continued airworthiness of relevant Australian registered aircraft.

Australian Civil Aviation Safety Authority response received on 25 February 2002:

"CASA advised the Direction Generale de L'Aviation Civile (DGAC) of the ATSB determination that an engine fire occurred as a result of engine failure. CASA notes that the DGAC does not support the ATSB determination. CASA has no evidence of the Turbomeca Arriel Module 3 bearing lubrication system not satisfying turbine engine certification standards. CASA notes that the DGAC, in advice to the ATSB dated 31 December 2001, has determined the Arriel 1 engine complies with the latest requirements of JAR-E-530 "Fire"."

Australian Transport Safety Bureau response classification- CLOSED-NOT ACCEPTED

Physical evidence and pilot reports substantiate the occurrence of fire. The failure mode of oil tube separation has still not been proven to meets the applicable design standard requirements to ensure the continued airworthiness of relevant Australian registered aircraft.

R20010193 issued on 18 September 2001

The Australian Transport Safety Bureau recommends that the Direction Generale de l'Aviation Civile assess the adequacy of the Turbomeca Arriel engine module three bearing lubrication installation to determine if it meets the applicable design standard requirements.

Direction Generale de l'Aviation Civile response received on 15 January 2002:

"In light of the incidents related in your referenced document, DGAC determined that the Arriel 1 engine complies with the latest airworthiness requirement, i.e. JAR-E-530 "Fire" under "Notice for Proposed Amendment "NPA-E-24 and interpretative material NPA-E-37 (note: these requirements result from the harmonisation with FAR 33, but are not significantly different from current JAR-E requirements)."

Australian Transport Safety Bureau response classification- MONITOR

The Direction Generale de l'Aviation Civile response proposes a change to the applicable regulation. The Australian Transport Safety Bureau will monitor that proposed change.

The Australian Transport Safety Bureau has also identified a safety deficiency related to Turbomeca Arriel engine gas generator turbine blade failures. The Australian Transport Safety Bureau therefore issued the following recommendations.

R20010196 issued on 18 September 2001

The Australian Transport Safety Bureau recommends that the Australian Civil Aviation Safety Authority take appropriate action to ensure the continued airworthiness of Australian registered aircraft fitted with Turbomeca Arriel engines incorporating modification TU204.

Australian Civil Aviation Safety Authority response received on 25 February 2002:

"CASA has been advised that the engine manufacturer, Turbomeca, cancelled the incorporation of Modification TU 204 in 1998. The DGAC has advised of action to be taken to address engines in service incorporating TU204. CASA notes that the DGAC, in advice to the ATSB dated 31 December 2001, advises the DGAC will be issuing an Airworthiness Directive to require the mandatory removal of turbine blades incorporating modification TU 204. The Directive is to be limited to single engine helicopters. CASA will review the DGAC Airworthiness Directive on its receipt and advise the ATSB of the results of that review. CASA looks forward to receiving a final briefing on the conclusions of the ATSB investigation of Occurrence No. 200103038."

Australian Transport Safety Bureau response classification- MONITOR

The Civil Aviation Safety Authority response proposes a review of the DGAC airworthiness directive when issued. The Australian Transport Safety Bureau will continue to monitor that proposed action.

R20010197 issued on 18 September 2001

The Australian Transport Safety Bureau recommends that the Direction Generale de l'Aviation Civile take appropriate action to ensure the continued airworthiness of aircraft fitted withTurbomeca Arriel engines incorporating modification TU204.

Direction Generale de l'Aviation Civile response received on 15 January 2002:

"Taking into account the possible occurrence rate (probability calculation) of a double engine failure on twin engine helicopters and the fact it is no longer possible to install or repair blades modified by TU 204, there is no need to take a specific action for twin engine helicopters. However, as a conservative approach, DGAC will mandate by airworthiness directive the replacement of all these blades on single engine helicopters."

Australian Transport Safety Bureau response classification- MONITOR

The Direction Generale de l'Aviation Civile response proposes an airworthiness directive to address single engine variant helicopter engines only. The Australian Transport Safety Bureau will continue to monitor this proposed action.

 

Technical Analysis Report: Examination of Components from a Failed Turbomeca Arriel 1S1 Turboshaft Engine
Sikorsky S76 Helicopter, VH-EXX

1. FACTUAL INFORMATION
1.1 Introduction

A Sikorsky S76C helicopter (VH-EXX) sustained a failure of the number-two engine during cruise flight. The failed engine was a Turbomeca Arriel 1S1 turboshaft engine, serial number 15038 and had accumulated 7,935 hours and 6,784 cycles since new.

Reports from the flight crew indicated that the engine failure was associated with a loss of gas-generator turbine speed and an escalation of turbine outlet temperatures. Fire warnings for the engine were also received, prompting the pilot commanded shutdown of the engine and discharging of the fire suppression system.

1.2 Engine examination

Disassembly of the engine (figure 1) was carried out at the Bankstown (NSW) facility of Turbomeca Pty Ltd, in the presence of representatives from the engine manufacturer, the helicopter operator and the Australian Transport Safety Bureau. The examination revealed the following significant damage to the operating components of the engine:

  • Outer wall of the centrifugal diffuser cracked and separated into seven pieces over half the circumference (figure 2).
  • First-stage gas-generator turbine blades oxidised and burnt over the outermost third of their length (figure 3).
  • Second-stage nozzle guide vanes extensively overheated and partially melted on the convex (trailing) face and on the trailing edges (figure 4).
  • Second-stage gas-generator turbine blade number 16 fractured beneath the platform. Remaining blades burnt and mechanically damaged on tip edges (figure 5).
  • Second-stage NGV housing indented and punctured, circumferential cracking extending from this area (figure 6).
  • Power turbine NGV missing two vanes; the remainder showing mechanical damage (figure 7).
  • Number-three (rear) bearing collapsed, showing extensive overheating and out-of-balance damage to races and adjacent seals (figure 8).
  • Rear bearing air vent and oil return lines fractured from outside of housing (figure 9).
  • Two of the three T5 thermocouples burnt away completely (figure 10).

Figure 1. Arrial 1S1 engine, serial number 15038, as removed from the aircraft.

Arrial 1S1 engine, serial number 15038, as removed from the aircraft.

Figure 2. Diffuser assembly, showing break-up of the outer housing.

Diffuser assembly, showing break-up of the outer housing. 

 Figure 3. First-stage gas-generator turbine blades oxidised and burnt over their outer length.

First-stage gas-generator turbine blades oxidised and burnt over their outer length.

Figure 4. Second-stage nozzle guide vanes extensively melted and disrupted in a localised area.

Second-stage nozzle guide vanes extensively melted and disrupted in a localised area.

Figure 5. Second-stage gas-generator turbine blades damaged and oxidised, with one blade missing. Item in upper left corner is a guide vane from the power turbine NGV.

Second-stage gas-generator turbine blades damaged and oxidised, with one blade missing. Item in upper left corner is a guide vane from the power turbine NGV.

Figure 6. Second-stage NGV housing with a large puncture and cracking from the released turbine blade.

Second-stage NGV housing with a large puncture and cracking from the released turbine blade. 

Figure 7. Power turbine NGV assembly, missing a vane (see Figure 5).

Power turbine NGV assembly, missing a vane

Figure 8. Rear bearing race and rotating air seals, showing extensive out-of-balance damage.

Rear bearing race and rotating air seals, showing extensive out-of-balance damage.

Figure 9. Rear bearing air vent line, fractured at point of connection with the bearing housing. The oil return line had failed in a similar way.

Rear bearing air vent line, fractured at point of connection with the bearing housing. The oil return line had failed in a similar way.

Figure 10. Thermocouple assembly - thermocouples at arrows burned/damaged.

Thermocouple assembly - thermocouples at arrows burned/damaged.

From these observations, the axial compressor diffuser assembly and the second stage turbine rotor were selected for further examination.

 

 
General details
Date: 11 July 2001 Investigation status: Completed 
Time: 0845 hours EST Investigation type: Occurrence Investigation 
Location   (show map):83 km E Longford, (HLS)  
State: Victoria  
Release date: 14 June 2002 Occurrence category: Serious Incident 
Report status: Final Highest injury level: None 
 
Aircraft details
Aircraft manufacturer: Sikorsky Aircraft 
Aircraft model: S-76 
Aircraft registration: VH-EXX 
Serial number: 760435 
Type of operation: Business 
Sector: Helicopter 
Damage to aircraft: Minor 
Departure point:Fortescue Platform, VIC
Departure time:0835 hours EST
Destination:Longford, VIC
Crew details
RoleClass of licenceHours on typeHours total
Pilot-in-CommandATPL886013921
Other PilotCommercial5661490

Safety recommendations

 
 
 
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Last update 13 May 2014