Jump to Content

While climbing through 1,500 ft after take-off from Hobart, the crew of the Boeing 737 advised air traffic services that the number-one engine had failed and was being shut down. The crew returned the aircraft to Hobart. There was no evidence of fire.

 

While climbing through 1,500 ft after take-off from Hobart, the crew of the Boeing 737 advised air traffic services that the number-one engine had failed and was being shut down. The crew returned the aircraft to Hobart. There was no evidence of fire.

Disassembly and inspection of the CFM56-3C1 engine, serial number 856135, traced the failure to the loss of a 15 x 20 mm segment of trailing edge from a single high-pressure turbine blade. The passage of the segment through the turbine resulted in extensive damage to all four stages of the low-pressure turbine assembly, rendering the engine inoperative. The subject blade was subsequently removed and examined by the Australian Transport Safety Bureau.

The failed blade, serial number 849M8, had accumulated 26,576 hours and 17,928 cycles since new. Maintenance records indicated that the blade received a "full" repair in the manufacturer's facilities in Singapore in June 1997 and was subsequently installed into the subject engine where it accumulated 10,226 hours and 5,332 cycles. The repair involved the use of Rene 80 alloy. The blade also received a "mini tip" repair in July 1995. On that occasion Inconel 625 alloy was used.

The reason for the "full" and "mini tip" repairs was to rebuild the blade tip area eroded due to the high operating temperatures and pressures. Any thermal cracks in the area were checked and removed at that stage. The "full" and the "mini tip" repairs were carried out in approved repair facilities in accordance with the appropriate manufacturer's repair documents. There was no difference between the repairs as far as the extent of the blade weld repair, materials and techniques used. The only difference was in the area of the blade coating.

The subject blade and the high-pressure turbine were periodically inspected in accordance with the manufacturer's and operator's maintenance programs. Those programs required the high-pressure turbine to be inspected at intervals not exceeding 1,140 hours. The inspection required detailed examination of the convex and concave mid-chord tip areas for radial cracks and of the blade trailing edge for axial cracks. The procedure specified the permissible crack length and imposed engine service limitations if cracks exceeded the limits.

The subject blade had accumulated 660 hours since it was last inspected on 11 August 2000. Following that inspection the high-pressure turbine was certified as serviceable.

The operator advised of a similar failure to a high-pressure turbine blade from another CFM56-3 engine. That item showed the loss of a similar section from the blade trailing tip corner but without the catastrophic consequences for the rest of the turbine and the engine.

The engine manufacturer indicated that that type of blade failure was not new. Although not every blade failure was reported by the engine operators, the manufacturer received reports of a number of similar cases each year. The manufacturer indicated that no formal records of the failed blades, their time in service, and repairs, were maintained.

 

Metallurgical examination of the failed blade (refer to ATSB Technical Analysis Report No.3/01) indicated that the loss of the blade section was due to the progression of fatigue cracking into the blade section from an area of cracking and notching on the blade tip. The cracking was found to have progressed into the blade parent Rene 125 material through an extensive Inconel 625 weld repair beneath the tip notching.

The Inconel 625 alloy is inferior to the Rene alloys in terms of its fatigue strength. Cracks within the Inconel 625 alloy will develop and propagate much more rapidly than equivalent cracks developed within the Rene alloys.

The Rene alloys are successfully weldable only under tight procedural control and require high levels of preheat to reduce hot cracking. Under conditions of restraint, such as deep crack repairs, the use of a ductile Inconel 625 alloy as a filler improves the success rate in producing sound repairs. The latest revisions of the appropriate repair documents that came into affect on 19 October 2000, cautioned the repairers that certain cracks are not repairable with Inconel 625 weld filler and that Rene 80 or 142 alloys are necessary for trailing edge welds below the blade tip shelf. The documents did not prohibit the use of Inconel 625 alloy.

 

Use of the inferior Inconel 625 alloy consumable for mini tip repair in 1995 allowed the tip crack to continue rapidly through the region of the repair into the parent material below.

 

Local safety action

In April 2001, in response to this occurrence, the operator reduced the high-pressure turbine inspection interval, requiring the turbine to be inspected at intervals not exceeding 625 hours.

ATSB safety action

As a result of this occurrence, the Australian Transport Safety Bureau makes the following recommendations:

R20010121

The Australian Transport Safety Bureau recommends that General Electric Aircraft Engines:

a) Review the suitability of using Inconel 625 alloy consumables during the weld repair of high-pressure turbine blades.

b) Review the life estimate and inspection intervals for high-pressure turbine blades repaired under the procedures using Inconel 625 alloy consumables.

c) Notify all operators using CFM56-2C, CFM56-2A/2B, CFM56-3 or similar engines of the possibility of catastrophic engine failure due to failure of high-pressure turbine blades repaired using Inconel 625 alloy consumables.

R20010122

The Australian Transport Safety Bureau recommends that the US Federal Aviation Administration:

a) Review the life estimate and inspection intervals for high-pressure turbine blades repaired under the procedures using Inconel 625 alloy consumables.

b) Notify all operators using CFM56-2C, CFM56-2A/2B, CFM56-3 or similar engines of the possibility of catastrophic engine failure due to failure of high-pressure turbine blades repaired using Inconel 625 alloy consumables.

R20010123

The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority notify all operators using CFM56-2C, CFM56-2A/2B, CFM56-3 or similar engines of the possibility of catastrophic engine failure due to failure of high-pressure turbine blades repaired using Inconel 625 alloy consumables.

 
General details
Date: 14 October 2000 Investigation status: Completed 
Time: 0615 hours ESuT  
Location   (show map):Hobart Investigation type: Occurrence Investigation 
State: Tasmania  
Release date: 12 November 2001  
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
 
Aircraft details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 737 
Aircraft registration: VH-TJN 
Serial number: 24439 
Type of operation: Air Transport High Capacity 
Damage to aircraft: Minor 
Departure point:Hobart, TAS
Departure time:0615 hours ESuT
Destination:Melbourne, VIC
 
 
Share this page Provide feedback on this investigation
Last update 13 May 2014