Propeller-system failures, along with the still-worrisome stall characteristics, were key suspects in two more crashes. After the second accident, the Air Force again grounded all the Cargomasters. Between April and August 1965, additional C-133 flight testing was conducted at Edwards. With Douglas engineer Roy Isaacs aboard many of the flights, the dangerous stall characteristics were confirmed, and investigators focused on how to prevent the Cargomaster’s now-infamous right-wing rolloff. Cameras trained on tufts installed on the wings clearly showed the right wing stalling before the left—in fact, the left wing usually didn’t stall at all.
The way to keep the Cargomaster from rolling onto its right wing during a stall turned out to be depressingly simple. “What we ultimately came up with was a triangular piece of sheet metal that we put on the leading edge of the left wing between number-one and number-two engines,” says Isaacs. Called a stall strip, the modification disrupted airflow over the left wing and caused it to stall when the right wing did. The modification was made at Dover and Travis to all aircraft in the fleet. Test pilot “Skip” Johnson test flew every airframe to ensure the strips—which were first temporarily attached to the left wing—were in the correct position so that when the aircraft was stalled during a test, the left and right wings stalled simultaneously. As if to demonstrate that the Cargomaster would never reveal all its secrets, one Dover C-133 snapped into a violent left roll during a test flight after its stall strip was attached. Of the 42 C-133s left in the fleet, it became the only one to sport a stall strip on its right wing.
Senior MATS officers then took the sum total of knowledge about the Cargomaster’s tendencies on the road. “They had a mandatory briefing for all aircrews in the base theater at Travis and at Dover,” Cal Taylor says. By the late 1960s, the stall strip and better aircrew training improved the airplane’s survivability. But because the propeller control system was still prone to fail, and airframes began to fatigue—a problem that would persist until the aircraft was retired in 1971—two more Cargomasters crashed.
Herbert Nakagawa remembers what his aircraft was carrying on April 30, 1967, and it hardly seemed worth his life. “The cargo was basically garbage—old drop tanks, miscellaneous old junk,” he says. “We spent the night on Okinawa and the next day we were going to Midway.”
The weather was fine—scattered clouds. After takeoff and climb to 12,450 feet, the number four propeller began to malfunction. The pilots shut the engine down, feathered the prop, and turned back toward Okinawa’s Kadena Air Base. One of the flight engineers attempted a last-ditch fix. “Master Sergeant Ray Wetzel went behind the engineer’s panel to jiggle with the propeller control system,” says Nakagawa. All four propellers received electrical power through a single circuit that also controlled the pitch regulator. The circuit sparked and failed, and the props were locked at an angle too high for lower altitudes. Wetzel ran to strap in to one of the airline-style seats in back—what some consider the most survivable area in a crash. “At 2,500 feet, all three [remaining] engines flamed out,” Nakagawa says. “Fortunately, we still had airspeed, and we still had control of the airplane. When we hit, I was amazed I had survived. The airplane broke in half right in front of the wings. We had vests on, and we gathered together by the floating nose wheels. The copilot had bought all these cheap Japanese golf balls and while we were bobbing there, all these golf balls came floating up around us.”
Once all nine crewmen had been rescued, they formed an exclusive club: the only airmen to survive a Cargomaster crash.
After Nakagawa’s crash, one more Cargomaster fell. On February 6, 1970, a C-133B left Travis to deliver a Vietnam-scarred CH-47 Chinook to Harrisburg, Pennsylvania. After cruising at 21,000 feet and with clearance granted for a climb to 23,000, Cargomaster 90530 broke up in flight. The majority of the debris fell on a field outside Palisade, Nebraska.
Roy Isaacs flew to Nebraska to help with the investigation. One day, standing on the stage of the town’s National Guard armory and looking at the wreckage that had been assembled there, he noticed something. “You could see all the jagged pieces, but here was a straight line by the side cargo door,” he says. Isaacs used a jeweler’s loupe to examine the edge of a long split about a foot above the side cargo door. Clearly, the metal had fatigued and failed. The crack had blown out a section of fuselage, which entered the arc of the number-two propeller.
C-133 maintainers found fatigue a continuing challenge, especially because air coming off the near-supersonic tips of the propellers produced vibration. To prevent further airframe stress failures during the last 17 months of the Cargomaster’s service, ground crews attached 16 “belly bands,” four-inch metal straps, around the exterior of the fuselage.
“The fact that we tried to compromise the airplane by reducing the skin gauges and the gauges of the longerons—we had an airplane that was too flexible,” Isaacs says today. “I feel the company would be rather cautious in admitting it then, but now, in retrospect, it’s hard to come up with anything different. Unfortunately, the airplane’s reputation suffered, but all in all, the airplane met the 10,000-hour service life requirement. They did a tremendous job for the Air Force.”