19 Jul 19898:33 reconstruction2K views

United 232: The DC-10 That Lost All Hydraulic Control

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Timeline
1
19 July 1989 — UA232 departed Denver-Stapleton for Chicago and Philadelphia.
2
About 1 hour 7 minutes into the flight — The No. 2 engine suffered an uncontained fan-disk failure.
3
Immediately afterward — Engine debris penetrated the aircraft's three hydraulic systems.
4
The crew declared an emergency and worked with ATC to select Sioux City as the diversion airport.
5
Approach — The crew used differential thrust to influence pitch and roll because conventional controls were unavailable.
6
16:00 local time — The aircraft struck the runway environment at Sioux Gateway Airport, broke apart and caught fire; 185 occupants survived.

An uncontained engine failure destroyed the hydraulic systems

United Airlines Flight 232 suffered an uncontained failure of its tail-mounted No. 2 engine while cruising from Denver toward Chicago and Philadelphia. Fragments from the failed fan disk penetrated all three hydraulic systems, leaving the DC-10 without normal flight-control authority. The crew and an off-duty DC-10 check airman used differential thrust to control the aircraft as they diverted toward Sioux City. The landing was catastrophic, but 185 people survived.

The failure began inside the tail engine

The NTSB investigation found a fatigue crack in the No. 2 engine's titanium fan disk. The crack originated from a metallurgical defect that had not been detected during previous inspection processes. When the disk failed, fragments escaped at high energy and damaged all three hydraulic systems.

The aircraft became almost uncontrollable

The DC-10 relied on its hydraulic systems to operate its primary flight controls. Once the systems were lost, the crew could not use the aircraft in its normal configuration. They discovered that differential thrust from the two remaining engines could influence the aircraft's trajectory and used that capability to maintain a rough flight path.

Sioux City offered the best available option

The crew initially discussed other airports but selected Sioux Gateway because of its location and suitability for the emergency. An off-duty DC-10 check airman travelling as a passenger joined the cockpit and assisted with throttle control. The final approach remained unstable because the aircraft could not use its normal flaps and control surfaces.

The survival outcome was unusually high for the severity of the crash

The aircraft struck the runway and broke apart, but the response of the crew, airport emergency services and people on the ground contributed to the survival of most occupants. The NTSB's investigation also became a major reference point for maintenance inspection, hydraulic-system redundancy and crew resource management.

The NTSB's conclusion identified: “inadequate consideration given to human factors limitations”

Accident Facts

Date
19 July 1989
Aircraft
McDonnell Douglas DC-10-10
Registration
N1819U
Location
Sioux City, Iowa, USA
Operation
United Airlines Flight 232
Defining event
Uncontained No. 2 engine fan-disk failure with loss of all hydraulic systems
Outcome
185 occupants survived; 111 died
Investigation
NTSB DCA89MA063

Sources

Audio from the original air traffic control recording, sourced from public recordings and released investigation records. The reconstruction, animation and written account are our own.
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