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Analysis

Summary

Blade Failure

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.

Cabin safety

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.

 
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