Ep 42 United Flight 232

Engineering News – Houston Astrodome (1:45)

This week's engineering failure is the United Flight 232 (9:20). The DC-10 aircraft (12:30) was a three engine aircraft. About an hour after take off, the tail engine catastrophically exploded and the plane had to make an emergency landing (22:05). A full investigation (40:25) was done to determine what went wrong and how it could be prevented from ever happening again.


Engineering News

United Flight 232

Episode Summary

Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole

And I’m Brian. And we’re both from Calgary, AB.

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This week in interesting things in engineering, the Houston Astrodome.

  • This isn’t really new news, in fact it’s old news. But we thought it was interesting and wanted to share.

  • The Houston Astrodome was one of the first major sports venues to install artificial turf or Astroturf.

  • The dome opened in 1965 with Tifway 419 Bermuda Grass bred for indoor use as the playing surface. For you grass people, I’m sure that means something, for the rest of us, this is grass.

  • The domed ceiling of the stadium had several semi-transparent Lucite panels that allowed sunlight in to feed the grass. But soon players were complaining of glare. To combat this, two sections of the panels were painted white, but then the grass died from lack of sunlight.

  • For most of the 1965 season, the Astros (baseball) played on green painted dirt and dead grass.

  • And so, they installed a new type of artificial grass, marketed at ChemGrass, but eventually became known as Astroturf.

  • Because they could only get a small amount of this new product, the infield and foul territory were covered for the start of the 1966 season at a cost of $2/ft. With the rest of the field being completed by mid July.

  • Groundskeepers dressed as Astronauts cleaned the turf with vacuum cleaners between innings which would certainly be a sight to see.

  • The original product was a short pile synthetic turf and transitioned to a more tall pile polyethylene turf with infill since the early 2000s. The pile length is the length of the turf that stands up. So a short pile is like a thin carpet, and a tall pipe is more like shag carpet, except it stands straight up.

  • Fun fact, the stadium was named first as the Astrodome, which led to the artificial turf being called Astroturf.

  • Allegiant Stadium - rolling grass

  • University of Phoenix

  • If you want to read more about the origins of Astroturf, check out the sources on the webpage for this episode at Failurology.ca.

Now on to this week’s engineering failure; United flight 232

  • July 19, 1989

  • United Airlines flight from Denver to Chicago, continuing on to Philadelphia.

  • Crash landed in Sioux City Iowa after catastrophic failure of its tail mounted engine due to an unnoticed manufacturing defect in the engine’s fan disk, leading to loss of many flight controls.

  • There were 296 passengers and crew on board; 112 died and 184 survived


  • McDonnell Douglas DC-10-10 operated by United since 1971.

  • By the time of the crash, the plane had operated 43,401 hours and 16,997 cycles

  • The plane had 3 engines. One on each of the wings and one on the tail. Each of the three engines also powered three independent hydraulic systems to power the flight controls.

  • If an engine was lost or the hydraulic pump failed, a ram air turbine could provide emergency electric power to auxiliary pumps.

    • The Gimli glider also had a ram air turbine that was used to power some of the controls when it ran out of gas, but as it started to slow down, even the ram air turbine didn’t have enough airflow to keep everything operational.

    • Check out our mini fails on our patreon for our episode on the Gimli Glider, it's a good one.

  • The engine and hydraulic system were intended to be redundant. If two hydraulic systems were rendered inoperable, the one remaining hydraulic system would allow for full operation and control of the plane.

  • But, at least one hydraulic system has to be functional to control the plane. It was not designed to revert to manual control if the entire hydraulic system failed. Seems like a huge oversight to me. We’ve said this before, a plane can’t exactly pull over to the side of the road if there is an issue.

  • The crew on the flight included:

    • Captain - Aldred Clair (Al) Haynes, 57, hired by United Airlines in 1956, he had almost 30,000 hours of total flight time with over 7,000 hours in a DC-10

    • Co-Pilot - William Roy (Bill) Records, 48, hired by United Airlines in 1985, he had approximately 20,000 hours of total flight time with 665 hours in a DC-10.

    • Flight Engineer - Dudley Joseph Dvorak, 51, hired by United Airlines in 1986, he had 15,000 hours of total flying time, with 1,900 as a flight engineer and 33 hours in a DC-10

    • Dennis Edwared (Denny) Fitch, 46, was a training-check airman flying as a passenger on 232. Fitch has learned of the 1985 crash of Japan Airlines Flight 123 that crashed after total loss of the hydraulic systems. He has practiced these conditions on a simulator.

    • 8 flight attendants


  • Took off from Denver at 1:09 pm local/mountain time, headed for a stop in Chicago on its way to Philly.

  • At 2:16pm mountain, with DC-10 at FL 370 or 11,000m altitude, the tail engine exploded and completely disintegrated, taking with it parts of the No 2 hydraulic system and severing the No 1 and 3 hydraulic system lines serving the horizontal stabilizer on the tail section.

  • The plane jolted, autopilot disengaged, and the First Officer took hold of the control column while the captain tried to wrangle the tail engine, whose instruments indicated malfunction. The Flight engineer recommended they cut off fuel to the tail engine. This took place over 14 seconds.

  • With all three hydraulic systems compromised, the plane was not responding to any controls and was banking to the right with the nose dropping, the same position it was when the engine exploded. By reducing the left wing engine to an idle and max power to the right engineer, they were able to level out the plane.

  • The crew tried to activate the ram air turbine, but with no success.

  • They radioed United maintenance personnel, but were told that since the loss of all hydraulics was “virtually impossible” they didn’t have a procedure. Houston, we have a problem, well, actually San Francisco.

    • I would be pissed! You’re in the air, you’ve just lost an engine and all control of the plane and the ground personnel has the audacity to say “oh we didn’t plan for this, good luck though”

  • A plane with control loss goes into phugoid cycles where it pitches up and climbs then pitches down and descends, slowing down and speeding up as it does so. With every cycle, flight 232 lost about 460m (1,500 ft) of elevation.

  • Fitch arrived in the cockpit, ready to assist around 2:29pm. Through use of the throttles, he was able to reduce the phugoid cycle and make some rough steering adjustments.

  • Air Traffic Control made room for an emergency landing at Sioux Gateway Airport in Sioux City Iowa, about halfway between Denver and Chicago.

  • So they are about to make an emergency landing in Sioux City, do they belly land or try to put down the landing gear? With the hydraulic system compromised, the landing gear mechanism wasn’t operative, but there was a level in the cockpit that would cause the landing gear to fall into position. They went with option 2 and the lever worked.

  • They had planned to land on runway 31, but with very limited ability to control the plane, it decided it wanted to land on runway 22. The fire trucks that were waiting on runway 22 had to quickly move out of the way.

  • A safe landing requires a speed of 260 kph (140 knots) and a sink rate of 1.3 m/s (300 fpm). Flight 232 was going about 410 kph (220 knots) at a sink rate of 9.4 m/s (1,850 fpm) when it landed around 3pm mountain; just under 2 hrs after takeoff.

  • The plane started to roll as it came in for landing, and Fitch was not able to get it under control.

  • The right wing touched down first, spilling fuel that ignited immediately.

  • The tail section broke off on impact, the plane bounced several times, shredding landing gear and breaking the fuselage into several large pieces. The right wing was torn off and the plane skidded sideways, rolling over on its back and coming to a stop in a corn field to the right of runway 22.

  • The majority of passengers and crew with fatal and serious injury were located in first class and the rear section which was impacted by smoke from the fuel tank fires. Miraculously, everyone in the cockpit survived, despite serious injury. And most of the 184 survivors were in the middle section with only minor or no injury; at least no physical injury, I'm sure they were never the same after this. United Airlines had a “children’s day” promotion that caused 52 children to be on board, many of them traveling alone.


  • The National Transportation Safety Board determined the cause of the accident was

    • Inadequate consideration given to human factors

    • Limitations in the inspection and quality of control procedures used by United’s engine overhaul facility

  • The engine had a titanium alloy stage 1 fan disc with a previously undetected metallurgical defect in a critical area that then caused a fatigue crack which ultimately led to catastrophic failure of the engine. The investigation found penetrating fluorescent dye on the crack, indicating that they were checking for cracks, but still missed this one.

    • Because titanium reacts with air when melted, impurities can form and cause cracks. GE used a double vacuum process to reduce this risk, but impurities still formed. They later added a 3rd vacuum stage.

    • These engines were used to power many civilian and military aircraft at the time. A large number of fan discs were examined by ultrasound and at least two other engines were found to have similar defects.

  • The failure was uncontrolled and knocked out what was left of the hydraulic system. A scenario that doesn’t appear anyone had planned for, included the plane’s design team and manufacturer, which caused a complete loss of flight controls.

  • Despite all of this, the crew were able to use throttles to maintain some semblance of control, but they still landed too hot and the plane broke apart and caught fire upon landing.

    • Ok this was by no means a perfect landing, but it could have been so much worse.

  • Three factors were said to contribute to the survival rate of those on board

    • The accident occurred during daylight in good weather

    • It occurred during a shift change at the Sioux City regional trauma center and burn unit, allowing for more medical personnel on staff

    • And it occurred when the Iowa Air National Guard was on duty at Sioux Gateway Airport, allowing 285 trained personnel to assist with triage and evacuation.

  • Reconstructions of the accident on flight simulators were attempted but even expert pilots were unable to produce a survivable landing.

So there you have it, the catastrophic failure of a fan blade on the tail engine of a DC-10 on the way from Denver to Chicago. Through the actions of the flight crew, they were able to bring the DC-10 to Sioux City Iowa and make a semi-controlled landing on the runway that led to the survival of 184 people, in a situation that many people considered impossible to survive due to the lack of controllability of the aircraft.

For photos, sources and an episode summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to Failurology, so more people can find it. If you want to chat with us, our Twitter handle is @failurology, you can email us thefailurologypodcast@gmail.com, or you can connect with us on Linked In. Check out the show notes for links to all of these. Thanks, everyone for listening. And tune in to the next episode where we will tell you about the Harbor Cay condo collapse; a structure so problematic, it collapsed before it was even complete.

Bye everyone, talk soon!