- United DC-10 at Sioux City
- 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
- United DC-10 at Sioux City
Technical Related Lessons:
Fan hub failures of large, high bypass turbofan engines constitute a potentially catastrophic threat to the airplane (Threat Category: Uncontained Engine Failure)
- In this accident, the titanium fan disk fractured and separated into two large pieces and other smaller pieces. The fan debris penetrated the horizontal stabilizer and severed hydraulic lines, causing complete loss of hydraulic fluid and the associated loss of motive power to the entire flight control system. The loss of flight controls left the airplane uncontrollable via conventional means. Only through exceptional skill and coordination was the flight crew able to establish minimal airplane control via the use of asymmetric thrust. The lack of precise control during the landing phase of flight contributed to the loss of control on short final approach and the accident during landing.
A double vacuum arc remelt (VAR) manufacturing process for critical, high energy rotating titanium engine components is inadequate to consistently produce defect-free parts (Threat Category: Uncontained Engine Failure)
- It was discovered by industry that the use of a double VAR process did not consistently produce defect free parts. Industry had elected to transition to a triple VAR process, and had implemented an inspection process to monitor double VAR manufactured parts still remaining in the fleet, until such time as they could be replaced. The #2 fan disk on the accident airplane had been manufactured using the double VAR process and had been inspected several times prior to its failure.
System redundancy and isolation should include assessment of both physical and zonal hazards. (Threat Category: Lack of System Isolation/Segregation)
- In the case of this accident, the airplane's hydraulic systems were physically isolated from one another, but due to space constraints in the tail section of the airplane, were in close proximity to one another. All three systems were therefore vulnerable to fan disk failure. Damage from the fan hub fragments resulted in the loss of all hydraulic fluid and associated loss of the flight control system.
Common Theme Related Lessons:
Reliance on repetitive inspections of critical engine components has inherent reliability limitations (Common Theme: Flawed Assumptions)
- Due to the critical nature of the fan disk, maintenance practices require that it be subjected to regular, detailed inspections to look for various types of damage, or indications that the disk is no longer serviceable. The #2 fan disk on the accident airplane had been inspected several times prior to the accident, and no damage had been discovered. The NTSB estimated that the crack which led to the disk failure had been approximately one-half inch long at the time of failure. It was assumed that the detailed, regular inspections would result in detection of a flaw that would require removal of the disk from service prior to failure.
The type and frequency of non-destructive tests being employed should consider detection reliability, as well as the consequences of failure to detect a flaw. (Common Theme: Human Error)
- The #2 fan disk on the accident airplane had undergone a fluorescent penetrant inspection several times prior to the accident. No cracks or other damage were detected during any of the inspections. The NTSB estimated the crack that caused the disk failure had grown to approximately one-half inch at the time of failure, yet was still undetected. Some non-destructive inspection techniques require varying levels of human interpretation of the inspection results and have varying degrees of reliability for identifying defective parts.