Boeing 720B accident at Albany, New York, 5 Nov 1961 (N-793TW)

Accident narrative

On November 5, 1961, Trans World Airlines Flight 66, a Boeing 720B, experienced an uncontained failure of its No. 1 engine while cruising at 25,000 feet near Albany, New York. Fragments from the disintegrating turbine penetrated the left wing, the No. 2 engine pylon, and the fuselage, causing a ruptured wing fuel cell and a rapid loss of cabin pressurization. The crew initiated an emergency descent and continued to a safe landing in Boston. None of the passengers or the crew of seven were injured.

### The flight

Flight 66 was a regularly scheduled nonstop passenger service from Los Angeles, California, to Boston, Massachusetts. The aircraft, registration N 793TW, departed at a gross weight within maximum allowable limits. Captain Kenneth A. Doherty, who had 13,277 flying hours including 216 in the Boeing, commanded a crew of seven that included Captain S. Tudor Leland, Second Officer Herbert K. Wheeler, and Flight Engineer Eddie Seward, alongside four stewardesses.

The flight was cleared under Instrument Flight Rules to 27,000 feet. After leveling off, the crew noted a slight vibration in the No. 1 engine’s tachometer needles for about 15 minutes. The captain referred to this as a "nervous needle." The vibration then ceased and did not recur. Near Cleveland, Ohio, the flight encountered turbulence and descended to 25,000 feet.

### The emergency

At approximately 1536 Eastern Standard Time, in the vicinity of Albany, the crew felt and heard a muffled explosion, and the aircraft began to yaw to the left. The autopilot was immediately disengaged, and the aircraft was brought back to a normal flight attitude.

The flight engineer advised that cabin pressure was dropping. The crew went on emergency oxygen, activated the seat belt sign, and contacted Air Traffic Control, receiving clearance to descend to and maintain 9,000 feet. The utility hydraulic system and the No. 1 generator failed. The crew shut down the No. 1 engine and initiated an off-airways gear-up descent using only the inboard spoilers.

The second officer then advised that the turbine section of the No. 1 engine had disintegrated and the exhaust section was oscillating severely. The firewall shutoff valve was actuated, and the aircraft's speed and rate of descent were reduced. Passing through 19,000 feet at approximately 200 knots, the cabin pressurization warning horn sounded and the oxygen flow light came on.

An inspection from the cabin revealed severe damage to the left wing and the No. 1 pylon. One turbine fragment had penetrated the left side of the fuselage at head height, crossed the cabin, struck the right wall, and dropped to the floor. The fragment retained enough residual heat to burn a one-inch hole in the floor carpeting.

A stewardess seated in the last row later described looking out the window toward the No. 1 engine just prior to the failure. She witnessed several red bursts of flame or exhaust, and seconds later saw the engine pod burst. She felt the aircraft shudder and then smooth out. During the depressurization, all passengers successfully donned oxygen masks, though several unoccupied mask containers failed to open, and masks in three lavatories failed to drop because they had been stored too tightly. The stewardesses used walkaround oxygen bottles to attend to the passengers.

Because of favorable weather conditions, runway length, and available emergency equipment, the captain elected to continue to Boston. Westover and Pease Air Force Bases had been alerted as a precaution. Emergency gear and flap extensions were made over water, and a normal approach was made to runway 22L. Trim was adequate to compensate for the yaw of the inoperative engine and the total loss of fuel from the damaged outer left wing tank. After touchdown, reverse thrust was applied to the Nos. 2 and 3 engines. Normal braking pressure was depleted, so airbrakes were used to bring the aircraft to a full stop after it cleared the active runway.

### What the investigation found

Ground inspection revealed the No. 1 engine pod was ruptured near the turbine. Approximately 80 percent of the turbine nozzle case was missing. The low-pressure turbine section was largely destroyed; a section of the second stage turbine disc was found imbedded in the left wing, and half of the third stage disc was recovered on the ground near Albany. The fourth stage turbine disc and blades were not recovered. The left wing and fuselage sustained numerous holes, the outer left wing tank was punctured, and the left spar was partially torn through.

Teardown of the No. 1 engine revealed severe damage to multiple bearings and heavy rubbing of the compressor blades. The main oil filter was clogged with foreign material, and the intermediate case "last chance" strainer was completely blocked.

The investigation determined that carbon deposits had formed in the No. 4 and 5 bearing areas due to high temperatures and center tube leakage. These deposits contaminated the oil system, causing the main filter to clog and bypass. The downstream strainers then choked, starving the bearings of oil.

The Board concluded that the No. 2 thrust bearing overheated and yielded, allowing the low-pressure compressor and turbine assemblies to move forward. The compressor blades began rubbing against the stator vanes, producing the red bursts witnessed by the stewardess. As the bearing deteriorated, the steel balls fused to the inner race, grooving and heating the hub journal until it sheared under torsional loading. This uncoupled the low turbine from the compressor. No longer absorbing energy, the low turbine accelerated until centrifugal loading caused the discs to burst through the engine casing. The Board noted this process of self-destruction took approximately 10 seconds.

The engine had been modified at TWA's shops weeks earlier to install heatshields around the No. 4 and 5 bearings. The manufacturer had wired airlines advising them to thoroughly clean the diffuser case and No. 5 support during this modification, but TWA personnel testified the engine was likely already rebuilt beyond that stage when the notice was received. The areas were not cleaned, and the main oil filter was given only a cursory examination prior to the flight. The Board evaluated a theory that two seals had been omitted during this overhaul, which would have allowed bleed air to break down the oil, but concluded it was unlikely, noting the seals were likely destroyed by heat during the failure sequence.

The Board believed that vibration sensing equipment could have warned the crew of the No. 2 bearing failure in time to shut down the engine before the turbine burst, and recommended the Federal Aviation Agency require such instrumentation in commercial turbojets. The Board also noted that the crew's actions during the emergency were in accordance with all safety directives and performed efficiently. Weather was not considered a factor.

### Probable cause

The Board has determined that the probable cause of this accident was oil starvation of the No. 2 bearing which caused its failure. This precipitated the fracture of the low-pressure compressor rear hub and the overspeeding and subsequent disintegration of the low-pressure turbine section.