United Airlines flight 232 was a scheduled domestic service that originated at the old Stapleton International Airport in Denver, Colorado. Its destination was Philadelphia International Airport in Pennsylvania, which it served via Chicago O’Hare in Illinois.
On July 19th, 1989, the flight was operated by a McDonnell Douglas DC-10-10. It was almost completely full, with 285 passengers. In terms of crew, the flight had two pilots, a flight engineer, and eight flight attendants.
A catastrophic engine failure
United Airlines flight 232 departed Denver’s Stapleton International Airport and took to the Colorado skies at 14:09 local time. For the first hour and seven minutes of its flight, it continued towards its first stop in Chicago as planned. Then, disaster struck.
While making a right turn and cruising at an altitude of 37,000 feet (11,300 meters), an uncontained engine failure rocked the DC-10‘s tail-mounted engine. This occurred at 15:16, and came about after the engine’s fan disk disintegrated inflight. The errant disk was ejected from the affected engine by the force of its explosive disintegration.
What made this all the more dangerous was the fact that, in doing so, it tore through the engine’s hydraulics and their supply hoses. The aircraft then received more hydraulic damage when flying debris severed further hydraulic system lines as they penetrated the DC-10’s tail area, having been violently ejected from the affected engine.
An unprecedented hydraulic power loss
14 seconds after the initial failure, the flight’s crew elected to cut the fuel supply to the faulty engine. Meanwhile, they had experienced a jolt from the explosion, and the DC-10’s autopilot had turned off. Owing to the hydraulic damage, the pilots found that, when attempting to maneuver the aircraft, their flight controls did not respond.
Even when both pilots pushed their control columns simultaneously, the aircraft failed to respond. This was despite their inputs being stronger than those typically used in an average DC-10 flight. Meanwhile, the plane was pulling down and to the right.
Only when the left engine was idled and the right engine subjected to maximum thrust did it level out.
With the DC-10 now in level flight, the crew were finally able to notice that its hydraulic fluid and pressure gauges were reading as empty. Even the use of auxiliary pumps was unable to restore hydraulic power.
Meanwhile, the unprecedented nature of the total loss of hydraulics meant that United’s maintenance teams also couldn’t help.
Flying without hydraulics
Although the aircraft’s altitude was fluctuating owing to the lack of control surface command, it was generally losing height. Indeed, as it continued to pull to the right, every clockwise circle that the DC-10 made cause it to lose around 1,500 feet of altitude.
At 15:29, a DC-10 instructor with United named Dennis Fitch entered the cockpit to assist the crew. Fitch was flying as a passenger, and was also a qualified captain on the DC-10.
He helped by looking out of the windows to monitor the plane’s ailerons (rendered unmovable by the loss of hydraulics). Fitch also controlled the stricken aircraft’s throttles.
Several phugoid cycles later, the crew were able to arrange an emergency landing at the nearby Sioux Gateway Airport, situated just outside Sioux City, Iowa. The airport cleared the flight to land on any of its runways, and the crew were cautious to stay away from the city itself. Of course, to crash in an urban area could lead to extensive casualties.
The crew intended to make their emergency landing on runway 31 at Sioux Gateway. Before doing so, they elected to make a series of right turns to burn some of the DC-10’s excess fuel.
Such maneuvers came more naturally to the stricken airways, as it had been pulling in this direction since the initial uncontained engine failure.
However, upon exiting the last of these turns, the plane was instead lined up with runway 22. This was shorter than the planned runway 31, as well as being closed. This is where the airport’s fire service had positioned its trucks in anticipation of a runway 31 landing.
However, they were able to vacate the 2,100-meter long strip before UA232 crash-landed.
The loss of hydraulic power meant that the DC-10’s flaps were unable to extend. As a result, it made its approach at a higher speed (220 knots / 410 km/h) and sink rate (9.4 meters per second) than usual. Just before touchdown, at 16:00, the plane began to roll right once again. This caused the aircraft’s right wingtip to hit the ground first.
The force of this impact caused a fuel spill, which quickly caught fire as the DC-10 bounced along the runway. The plane’s tail section broke free at first, with the rest of the fuselage later breaking into several smaller pieces. The right wing was also torn off.
Tragically, the crash of flight 232 resulted in 112 fatalities, either from the crash impact or smoke inhalation.
The accident has also become a frequently-cited case study when it comes to crew resource management (CRM). The NTSB credited United’s CRM training, which encourages captains to work with their co-pilots, rather than as a final authority, as an important factor in the effective way in which flight 232’s crew dealt with the emergency situation.