The Curse of the Cargomaster
Readied to transport the first U.S. ICBMs, the Douglas C-133 had a peculiar habit. It kept crashing.
- By John Sotham
- Air & Space magazine, September 2010
(Page 4 of 6)
“We had some of the best minds in aeronautics working on the problems of the C-133,” says Sandstrom. “I took an airplane down to Warner Robins and while it was down there, they tore it completely apart. They put it all back together, and I was sent down there to preflight it. It flew just like it did when we flew it down there. They didn’t figure out anything.”
Roy Isaacs was a young structures engineer at Douglas when the first C-133 rolled off the line in 1956. He remembers one Air Force requirement stating that the airplane, when fully loaded, would be able to clear a 50-foot obstacle at the end of a 10,000-foot runway. “We had to redesign all the sections of the airplane three times,” Isaacs recalls, “and we had to lighten [the airframe] and get the weight down to accommodate the engines. Consequently, it made the airplane have a bunch of problems. That engine is the downfall of the C-133.”
That engine was the Pratt & Whitney T34, the only turboprop available at the time that could get an airplane as heavy as a fully loaded Cargomaster off the ground. During cruise, the engines ran at a constant speed. To vary the thrust, a governor inside the propeller’s nose case (located aft of the propeller blades) changed the blade pitch. When the pilot pushed the throttles forward to increase power, the governor angled the propeller blades to take a bigger bite of air. The mechanism in the nose case was in turn governed by a complex, electrically controlled system that synchronized blade pitch among the four engines.
“You could have a prop malfunction, and a number of things could cause it,” says Ken Kozlowski, a former C-133 crew chief who served as chief mechanic and flight engineer on a privately owned Cargomaster that flew until 2008. Through monastic devotion to understanding every system on the C-133 and by developing his own maintenance procedures, Kozlowski kept the civilian Cargomaster flying as a bush airplane—and slamming onto remote Alaska gravel runways—nearly 40 years after the Air Force let it go.
Minute changes in engine performance, coupled with changes in altitude, airflow, or synchronization, kept the nose case mechanism constantly working to maintain optimum blade angle. At higher altitudes, the props had to increase pitch to move the same amount of air; at lower altitudes, the pitch needed to be reduced. Thomas Kaye, who was an Air Force hydraulic mechanic stationed on Midway, remembers seeing Cargomasters arriving from Japan that had been four-engine transports on takeoff but had only three engines operating by the time they landed. Once, a C-133 showed up with only two engines. It had been able to stay aloft only because the flight engineer injected a water-alcohol mixture (a standard takeoff booster) when the airplane was threatening to head toward the waves.
“The nose case [governor] would go,” says Kaye. “The pitch was constantly changing to keep the [propellers] phased, and it was constantly loading and unloading the gearing in the nose. What they did later on was put a little time-delay relay and slowed the pitch changes down, which relieved the excessive load on the nose cases.”
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.