The Half-Inch Crack the Inspector Missed — United 232

The stage 1 fan disk of a DC-10's tail-mounted CF6-6 engine carried a fatigue crack growing from a metallurgical defect — a hard alpha inclusion hidden in the titanium since manufacture. The disk went through six fluorescent penetrant inspections (FPI) in its life, the last during a shop visit that began in February 1988 at United's CF6 Overhaul Shop in San Francisco. By then the crack was almost half an inch long on the surface of the disk bore — long enough that FPI fluid physically entered the crack and left chemical traces later recovered from the fracture face. The inspector did not see the indication; the disk was stamped serviceable and returned to the fleet. On 19 July 1989, 760 flight cycles later, the disk burst in cruise. The uncontained debris severed the lines of all three hydraulic systems that powered the DC-10's flight controls, and the crew flew the airplane for 44 minutes on engine throttles alone before the crash landing at Sioux City.

The NTSB's probable cause is aimed squarely at the inspection floor: "the inadequate consideration given to human factors limitations in the inspection and quality control procedures used by United Airlines' engine overhaul facility which resulted in the failure to detect a fatigue crack" (NTSB AAR-90-06 §3.2). One inspector, one part, one missed indication — and no process layer behind him designed to catch a human miss on a life-limited rotating part whose failure cannot be contained. Sioux City is the industry's defining lesson that NDI of critical rotating parts must be engineered around inspector fallibility, not around the assumption of a perfect inspection.

1. Prepare the part exactly per the process specification — cleaning, drying, penetrant dwell and developer are each capable of hiding a crack if shortcut.
2. Position and manipulate the part so every inspection surface can actually be viewed — a disk hanging on a cable that is not rotated is not fully inspected.
3. Inspect the full surface systematically under correct lighting; the bore and other high-stress areas of a life-limited disk get deliberate, unhurried attention.
4. Treat every indication as a reject until positively dispositioned — a critical rotating part carries no benefit of the doubt.
5. Build a second layer: independent re-inspection or process redundancy for parts whose uncontained failure exceeds airframe protection.

Frequency of a missed FPI indication on a critical rotating part is not quantified in public sources — but the NTSB wrote that it "is concerned that NDI inspections in general, and FPI in particular, may not be given the detailed attention that such a critical process warrants" (NTSB AAR-90-06 §2.5).

Catastrophic separation of the stage 1 fan disk at 37,000 feet; debris cut the lines of all three hydraulic flight-control systems — all three reservoirs were later found empty. The crew, steering with asymmetric thrust on the two remaining engines, reached Sioux Gateway Airport; the airplane broke up and burned on landing. Of 296 aboard, 111 died — one flight attendant and 110 passengers; 185 survived (NTSB AAR-90-06, Executive Summary and §1.2).