The crew of a Boeing 777-300 rejected the take-off run from Melbourne airport as a result of a failure within the left (number-one) engine.
The failure was reported by the flight crew as being characterised by a single loud `bang' and the immediate left yaw of the aircraft. A flash of flame from the engine intake and other short-lived visual indications of fire were observed by passengers and air traffic controllers, however no fire warnings were received on the flight deck.
Following the rejection of the take-off, the left engine was immediately shut down. After assessment from the attending fire-fighting services to confirm the absence of fire, the aircraft was able to be returned to the terminal using thrust from its remaining serviceable engine.
The passengers and crewmembers were not injured.
Damage to the Engines and Aircraft
Failure of the Rolls-Royce RB211 892-17 (Trent 800 series) engine (S/No. 51197) fitted to the aircraft was attributed directly to the release of a single low-pressure compressor (fan) blade (S/No. RGG16936) from the rotor disk. Liberation of the blade caused extensive damage to the remaining blades and the engine intake linings. A large compressor surge associated with the event and the quantity of ingested debris produced severe internal damage to the engine core. The escape of a small quantity of low energy debris caused minor damage to the aircraft fuselage and the fan of the right (number-two) engine.
The subject engine had been installed since the aircraft was manufactured and at the time of failure had accumulated 5,765 hours and 907 cycles. It is understood that the blades had been fitted since new. The blade configuration was to Service Bulletin RB211-72-C629 standard, incorporating an undercut radius along the length of the root block. The intention of the radius was to alleviate "edge of bedding" stresses within the dovetail root - a mechanism that had been implicated in two previous failures of RB211-524 fan blades. The blade failure was reported to be the first of its type in the Trent 800 series engines.
Data recovered from the quick access recorder (QAR) showed that the aircraft reached a maximum ground speed of 41 knots before the take-off was rejected. Blade release occurred at engine turbine N1/N2 speeds of 91.4 / 97.1 % respectively, whereupon the speeds rapidly increased to 110 %, accompanied by a slow decrease in engine power ratio. The engine was still accelerating and had not reached the commanded power output at the time of failure. Immediately prior to the blade release, broadband vibration levels within the left engine increased markedly above those of the right; that being the first indication of a problem with the engine. The flight crew observed elevated exhaust gas temperatures of more than 770 degrees Celsius during the application of reverse thrust, moments after the failure. The temperatures were not associated with any fire indications and no fire warnings were received. The absence of fire was verified by the emergency services personnel in attendance.
The cabin crew reported that immediately following the loud bang from the engine, a number of passengers released their seatbelts and vacated their seats. The cabin crew also reported that the Passenger Entertainment Landscape Camera displays within the cabin remained on during the event, showing the approach of the emergency services vehicles. That distressed some passengers. The displays were subsequently switched off by the Cabin Services Director. The landscape camera system was designed to provide an outside view for passengers during take-off, landing and ground manoeuvring. The system was part of the in-flight entertainment system and was controlled by the cabin crew. There was no flight crew interface with the in-flight entertainment system, and the system could not be controlled from the flight deck.
Tests & Research
The ATSB examined the released blade in conjunction with authorised representatives from Rolls-Royce.
Release of the fan blade from the rotor disk occurred as a direct result of cracking within the dovetail root block. Large transverse fatigue cracks had developed within the undercut radius on the concave (suction) side of the blade, extending into the section from both sides of the shear key slot. The cracks had progressed to a point where the remaining sound material was insufficient to support the loads, with final overload failure and blade release occurring.
Crack growth initially presented a smooth, woody appearance to a depth of around 10-12 mm, before transitioning to a broader, uniform surface leading to the regions of final fracture. Many finely spaced crack arrest marks were observed within the initial areas of cracking, however they became further apart towards the transition point, and only one or two widely spaced marks were observed beyond the boundary. Although the initiation sites were obliterated by post-failure damage, there was no evidence to suggest the contribution of pre-existing defects in those areas. Blade material and manufacturing aspects were also acceptable.
The blade dovetail root faces showed extensive galling and micro-welding type damage in all items examined. Breakdown of the lubricant coating was also apparent and was characterised by a blotchy, irregular colouration on the contact faces. At the time of the occurrence, no requirements existed for the periodic re-lubrication of the blade seating surfaces.
Technical Analysis report No. 8/01 is available on the ATSB website or from the Bureau on request.
Fatigue cracking initiates and propagates in response to the application of repeated tensile stresses. In general terms, the greater the magnitude of the stresses, the fewer cycles are required to produce cracking. It is possible to mathematically model the distribution of service stresses within a component to gather some appreciation of any areas of stress concentration that may predispose the item to fatigue cracking.
In this case, the fatigue crack origins correlated closely with the regions of highest stress concentration as predicted by the mathematical analysis. This directly implies that the cracking was a response to blade stresses induced in service, and not a product of an isolated defect or anomaly within the component. Indeed, no such deficiencies were found within the failed component.
Service stresses within the blade section vary in response to the thrust rating and operation of the engine, with higher ratings and power settings equating to higher component stresses. Operations of high thrust rated variants of the Trent 800 series engine, fitted to high gross weight aircraft, would thus be expected to produce the highest level of service stress related failures. The Boeing 777-300 subject aircraft was a typical example of this situation.
The uniform transmission of service loads through a multiple component assembly requires that the contact surfaces of each item are closely matched and that these surfaces behave in a similar way when loaded. Irregularities within these surfaces can interfere with the even transmission of load, leading to the development of areas of elevated stress. The surface galling and welding damage found across the blade dovetail faces is such an irregularity and would increasingly have interfered with the desired uniform bedding across the tapered blade seat. Titanium and related alloys have a high potential for surface galling and require efficient and effective surface lubrication to prevent damage. Dry film lubricants are often used in these applications and, while effective, they must be maintained within the interface in order to remain so. The surface movement that produces the galling damage also tends to progressively force out any lubricant from the interface, necessitating periodic re-lubrication of the assembly if the joint is to be maintained in a low friction condition.
A modification of the dry film lubricant system was introduced on a non-mandatory level by service bulletin RB.211-72-C905 (June 2000). This recognised the deterioration of the lubricant film and incorporated a base layer of plasma spray coated anti-galling compound above which a reduced thickness of the dry lubricant is applied. The base coating acts to improve the bonding and retention of the dry lubricant. The blades of the subject engine did not have this modification.
The premature release of seat belts by some passengers increased the risk of injury, even though no injuries were reported.
The landscape camera system may be of value as a tool to gather information in an emergency situation. However, camera images have the potential to disturb or distract passengers at a time when flight or cabin crew members may need to issue emergency instructions or pass on important information.
- The breakdown and loss of the dry film lubricant applied to the blade seating surfaces contributed to the development of galling damage.
- Surface galling and micro welding between the blade and disk was a major factor contributing to the disruption of the seating surfaces and the non-uniform distribution of transmitted loads.
- Fatigue cracking developed within the blade root in response to non-uniform service loads stemming from irregular blade seating and extended periods of high power application.
- Release of the blade was due to the development of fatigue cracking within the dovetail root section, which resulted in a loss of support within the rotor slot and the liberation of the blade under centrifugal loads.
- The left engine of the aircraft failed as a result of the release of a single low-pressure compressor (fan) blade during acceleration for take-off.
Local safety action
As a result of the investigation, the manufacturer, together with the UK CAA, implemented the following requirements for all Trent 800 series engine installations:
- Mandatory `once round the fleet' inspection for blade root cracking (Service Bulletin RB.211-72-D344).
- Mandatory periodic non-destructive inspection of the blade root. Mandatory service bulletin RB211.72-344 specifies the interval for repeat non-destructive inspection of the blade root on the basis of engine rating, aircraft category and blade root standard. In the case at hand (B777-300, Trent 892 with undercut, dry film lubricant and standard blade root), the threshold for inspection and re-lubrication was set at 600 cycles, with a repeat every 80 cycles.
- Mandatory periodic re-lubrication of the blade roots. Mandatory service bulletin RB211.72-D347 specifies re-lubrication intervals with frequency dictated by engine rating, aircraft category and blade root standard.
In addition, Service Bulletin RB.211-72-D672 specifies a revised blade design incorporating a re-work to the undercut blade and re-defining the surface anti-galling system to incorporate the `Metco 58' interface layer. The service bulletin was undergoing review by the UK Civil Aviation Authority at the time of release of this report.
Australian Transport Safety Bureau safety action
As a result of the investigation, the Bureau issues the following Safety Advisory Notice:
Safety Advisory Notice SAN20010222
The Australian Transport Safety Bureau advises airlines that Passenger Entertainment Landscape Camera systems have the potential to cause passenger distraction during non-normal and emergency situations.
Emergency procedures should ensure that the system is operated in a way that will not divert passengers' attention from instructions given by the flight or cabin crew in such situations.
|Date:||30 January 2001||Investigation status:||Completed|
|Time:||2055 hours ESuT|
|Release date:||12 February 2002|
|Report status:||Final||Occurrence category:||Serious Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Minor|
|Departure point||Melbourne, VIC.|