McDonnell Douglas DC-10 accident at Tucson, Arizona, 2 May 1972

Accident narrative

On May 2, 1972, a Continental Airlines McDonnell Douglas DC-10 on a crew training flight experienced a massive failure of its No. 2 engine while in a holding pattern near Tucson, Arizona. The aft portion of the engine separated from the aircraft in flight. The crew shut down the engine and landed safely at Tucson International Airport. None of the nine people on board were injured.

### The flight

The flight departed Tucson at 1152 mountain standard time to conduct type ratings for captains and training for second officers. The instructor pilot-in-command had approximately 6,160 total flying hours, with 17 hours in the DC-10. The trainee captain had 9,000 total hours, including 5 hours in the DC-10. The second officer trainee had 2,100 hours as a flight engineer, with 3.5 hours in the DC-10.

### The sequence of events

Approximately one hour after departure, the DC-10 was at an altitude of 5,000 feet mean sea level in the Ryan Radio Beacon holding pattern. The training schedule required one circuit of the pattern before commencing an approach to Tucson. As the aircraft completed the last 45-degree turn from the fix onto the outbound leg, the crew felt a mild vibration.

Shortly after, the second officer instructor reported zero oil quantity and zero oil pressure on the No. 2 engine. Immediately, the engine's low oil pressure and master warning lights illuminated, the reverser unlock lights flickered, and a thumping sensation was felt in the cockpit. The crew executed a standard in-flight engine shutdown and returned to Tucson, landing on Runway 11L at 1306.

### What the investigation found

Ground inspection revealed that the No. 2 engine's low-pressure turbine (LPT) assembly, turbine rear frame, and turbine reverser assembly had separated from the aircraft. The missing components were recovered from the ground near Tucson. The aircraft sustained minor damage from the loss of the engine's aft section.

Teardown examination of the engine showed that a forward stiffener ring on the high-pressure turbine (HPT) pressure tube had separated. The engine manufacturer had previously added three stiffener rings to prevent pressure tube failures. The Board noted that while there was a history of weld cracking on these rings, this was the first known instance of the ring itself cracking. Fracture surfaces indicated fatigue crack propagation under very high-frequency loading.

The Board concluded that after the ring cracked and broke its weld bond, centrifugal force would have operated to spread the ring until it was thrown off the pressure tube. It lodged against the inside of the HPT front hub and the first-stage turbine disk. The Board estimated this created a turbine imbalance of 46 inch-ounces.

This imbalance applied vibrational stress to the "C" sump, inducing fatigue cracks in the support cone. The fracture of the support cone allowed the bearing housings to rotate, which ruptured the bearing oil lines. The No. 5 bearing was found devoid of residual oil, with its inner race cracked and showing evidence of heavy side loading and skidding.

The Board deduced that the released oil formed an air-oil mixture that was ignited by the normally high temperatures in the area, creating an intense fire. This fire overtemperatured the LPT shaft hub section, reducing its strength until it failed. The final structural failure occurred when the bolts attaching the LPT stator case failed in overload, allowing the LPT assembly, rear frame, and reverser to separate and fall from the aircraft.

The Board noted that a massive engine failure of this type has the potential to cause the loss of an aircraft by inflicting damage to adjacent structures or systems.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the incident was "the failure of a stiffener ring on the pressure tube located within the high-pressure turbine shaft of No. 2 engine. This failure resulted in a condition of rotor imbalance which precipitated a sequence of component failures culminating in separation of the aft portion of the engine."