- Delta Air Lines MD-88 Flight 1288
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Delta Air Lines MD-88 Flight 1288
Technical Related Lessons
Manufacturing processes for critical rotating engine components (e.g., life-limited parts), and changes to those manufacturing processes, must be assessed by robust process validation procedures before being implemented. When these processes are properly validated, it is expected that no manufacturing-induced anomalies will be introduced and that no uncontained engine failures due to manufacturing-induced defects will occur in the life of the engine fleet. (Threat Category: Uncontained Engine Failure)
- Volvo, as a fan hub vendor was manufacturing the JT8D-200 series hub per Pratt & Whitney specifications. Volvo proposed a change to the tie rod hole drilling process (single pass plunge-drilling of the hole vs. the previous multi-step "pecking" process). The change was deemed "insignificant" since the drilling operation is followed by boring and honing process and it was believed that any material defects inadvertently introduced by drilling would be removed by the post-drilling operations. Pratt & Whitney concurred with implementation of the new process. Because it was considered insignificant, the revised drilling process did not undergo robust validation (i.e., no metallurgical evaluation was performed). Ultimately, it was determined during the investigation of this accident that the boring and honing processes were not successful at removing the subsurface microstructure defects. The new process subsequently introduced material defects that became critical cracks, and culminated in pre-mature fan hub failure and the accident.
Drilling of high length-to-diameter ratio holes in critical engine components presents a complex technical manufacturing challenge . The drilling process and inspection procedures must be carefully designed and validated to assure they are robust and meet the design intent. (Threat Category: Uncontained Engine Failure)
- The previous tie rod hole drilling process used a standard drill that was removed every .20 inches during drilling. This was described as a "pecking process," the purpose being to purge chips from the hole. Chip purging was accomplished by repeatedly removing the drill tool and applying high pressure coolant to the work interface. The fundamental reason for using a pecking hole drilling operation for the JT8D-200 series fan hub was due to the disc having a large cross section (over three inches). This resulted in a very high hole length-to-diameter ratio of 6 (3 in / .5 in = 6) which is highly prone to chip congestion. The altered process, using a "plunge" (continuous drilling) process and a coolant channel drill, resulted in a higher-than-expected level of chip congestion, elevated heating at the drill/part interface, and unexpected micro-structural anomalies on the surface of the hole.
Nondestructive evaluation methods of critical engine components must be capable of detecting and quantifying metallurgical flaws resulting from manufacturing-induced anomalies, and must also be capable of detecting fatigue cracks that may form during in-service engine operation. Robust nondestructive evaluation methods should ensure that critical rotating engine components installed in production, overhauled, or repaired engines are free of defects that can result in hazardous rotor burst events. (Threat Category: Uncontained Engine Failure)
- It was determined that the accident fan hub was produced and released into service with a manufacturing-induced defect caused by a drilling operation that resulted in altered microstructure, micro-cracking, and material depletion in a tie rod hole. Following manufacture of the fan hub by Volvo, traces of the altered microstructure were detected by a blue etch anodize inspection; however, the blue etch indications did not match any of the blue etch templates used to identify material anomalies, and therefore was not considered a "blue etch" indication. It was only considered an observation made of the surface condition. These results were passed to visual inspection personnel to alert them to an anomalous surface condition. The fan hub was then dispositioned for a geometric surface condition by the use of a stylus. Investigators concluded that use of the stylus, traditionally utilized to inspect for geometric surface flaws, was not capable of detecting the actual micro-structural condition that existed in the subject tie rod hole caused by the drilling process. The fan hub anomaly was ultimately "cleared" by use of the stylus inspection process. The hub was then sent to Pratt & Whitney for installation into a production engine. Subsequent industry-wide review of the manufacturing and overhaul of critical engine components concluded that the producers of life-limited, high-energy rotating engine parts should employ appropriate nondestructive evaluation technologies that are capable of detecting and quantifying both geometric and metallurgical rotor surface and surface-connected manufacturing-induced material anomalies that can result from finish and semi-finish machining processes.
- In addition, during in-service operation, the material anomalies and micro-cracking that were not detected during the production process began to initiate fatigue cracks that continued to propagate with every engine operating cycle. The Delta Airlines fluorescent penetrant inspection process, while representing state-of-the-art inspection processes for engine overhaul facilities, was ineffective at detecting these fatigue cracks during several inspection opportunities.
Employment of an engine industry, manufacturing-induced anomaly Lessons Learned Database, based on voluntarily submitted data, has led to precursor awareness, improvements in manufacturing process technologies, and a reduction in the risk of uncontained engine failures. (Threat Category: Uncontained Engine Failure)
- It is essential that the gas turbine manufacturers and regulatory authorities continue to populate and maintain the manufacturing-induced anomaly Lessons Learned Database (LLDB). The LLDB is now a central location for all engine industry manufacturing-induced anomaly data, and directly aids in identifying industry trends that could be the precursors to catastrophic failure events. The LLDB has helped to establish "best manufacturing practices" for production of engine rotors as a direct result of lessons learned from the root cause of manufacturing-induced anomalies. Recently, the LLDB was expanded to also include design and maintenance safety data, and as such, has evolved to encompass the overall product life cycle.
Common Theme Related Lessons
Original Equipment Manufacturer (OEM) oversight of vendors and vendor processes related to manufacturing of safety critical parts and systems (e.g., life-limited engine parts, airplane primary structural elements, safety-significant items, etc.), must be sufficiently robust to ensure that the parts meet the design intent, are free from manufacturing-induced defects, and that the level of safety intended by the regulations is maintained. (Common Theme: Organizational Lapses)
- Pratt & Whitney records showed that the company had approved Volvo's request to use a one-pass plunge drilling process with a coolant channel drill rather than a standard twist drill that is removed periodically during drilling to clear chips from the hole. Manufacturing records also noted that the process change was characterized as an "insignificant" change. It was further indicated that generally, in the machining of titanium holes, if greater than ten thousandths of an inch of material was removed in subsequent operations (boring and honing processes in this case), that any defects caused in the initial drilling operations would be removed. Therefore, the process change was classified as "insignificant." Because it was considered "insignificant," the revised drilling process was not required to undergo a rigorous validation (i.e., no metallurgical evaluation was required or performed). In the case of the Pensacola tie rod holes, the total radial depth of material removed after drilling was about .0185 inch. Pratt & Whitney and Volvo both believed that the post-drilling material removal was sufficient to remove any drill-caused defects. The investigation determined that this was not the case.
Oversight of the critical manufacturing process validation function can assure that critical processes are adequately qualified initially and remain robust as process changes occur. This oversight is essential toward achieving the goal that no uncontained engine failures occur in the life of an engine fleet. (Common Theme: Organizational Lapses)
- As a result of this accident, the FAA and engine manufacturers have deployed a multi-tier approach to reducing the risk of uncontained engine failures. One of these strategies involves the recognition that proper safety oversight must include an assessment of the adequacy and effectiveness of process validation procedures. FAA regulations and policy relating to oversight of the critical manufacturing process validation function are necessary elements in any comprehensive safety management system.