Investigation Summary
At FL370 over Iowa, the No. 2 (tail-mounted) General Electric CF6-6 engine suffered an uncontained failure of its fan disk. Fragments of the disk severed all three independent hydraulic systems of the DC-10, leaving the crew with no conventional flight controls — no elevator, aileron, rudder, flap, slat, or wheel-brake authority. Captain Al Haynes, First Officer Bill Records, Flight Engineer Dudley Dvorak, and DC-10 instructor Denny Fitch (a deadheading passenger who volunteered) used asymmetric throttle inputs on the remaining two wing engines to steer and descend the aircraft toward Sioux Gateway Airport. The improvised approach ended in a crash-landing on Runway 22 at 240 knots with a high sink rate; the aircraft cartwheeled and broke apart. 184 of 296 occupants survived. The crew's coordination became a defining case study in Crew Resource Management.
Final Conclusions
The NTSB determined the probable cause was 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 originating from a previously undetected metallurgical defect in a critical area of the No. 2 engine fan disk. The subsequent uncontained failure of that disk severed hydraulic lines for all three redundant systems. The crew's performance — using differential thrust to maintain limited control and bringing the aircraft to a survivable crash-landing — was credited with the survival of 184 of the 296 occupants.
Video Analysis
The Landing No Simulator Could Replicate | United Airlines Flight 232 — How the crew flew a DC-10 with no flight controls using only engine thrust.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
DC-10 Triple-Redundant Hydraulics
Three independent hydraulic systems, all routed through the tail section near the No. 2 engine. The uncontained fan-disk fragments cut all three at the same chokepoint — a single-point failure the design had not anticipated.
CF6-6 Fan Disk
A metallurgical defect (hard alpha inclusion) had been present in the titanium since manufacture and grew into a fatigue crack over 18 years of service. Fluorescent penetrant inspections had failed to detect it.
Differential Thrust Flight Control
Asymmetric throttle inputs on the two wing engines yawed the aircraft; the resulting bank produced turn. Symmetric thrust changes drove the natural phugoid oscillation up or down. There is no rudder or elevator effect from thrust alone — only an indirect, oscillatory response.
Interesting Facts
- 01NTSB simulator crews could not reproduce the landing — no other crew put the aircraft on the runway
- 02Denny Fitch was a deadheading United DC-10 training check airman who volunteered from the cabin
- 03184 of 296 occupants survived a crash widely expected to be unsurvivable
- 04The accident directly led to the development of NASA's Propulsion-Controlled Aircraft (PCA) research
Safety Actions & Advisories
Hydraulic system fuse and isolation review
FAA and manufacturers reviewed triple-hydraulic transport designs (DC-10, MD-11, L-1011, 747) for single-point vulnerabilities. The MD-11 received a hydraulic fuse to isolate a ruptured tail line; the 747 hydraulic routing was reassessed.
Titanium rotating-part inspection (Sioux City AD)
FAA Airworthiness Directives mandated improved ultrasonic and eddy-current inspections of CF6 and similar titanium fan/compressor disks, with stricter records on hard-alpha inclusion screening at the billet stage.
Crew Resource Management (CRM) curriculum
Source ↗The cockpit voice recorder became a foundational CRM training example worldwide. Captain Haynes' use of all available resources — including a passenger-volunteer check airman — reshaped how airlines teach decision-making under extreme abnormals.
NASA PCA research
Source ↗NASA Dryden flew an MD-11 and F-15 demonstrating that throttle-only flight control could be automated, producing survivable landings without flight-control surfaces. The research informed later flight-control backup studies.