Hercules Crash In Baghdad Points To Metal Fatigue In C130's Wing Center

Air Safety Week, Feb 21, 2005

The feedback loop occurs because a gyroscopic moment (produced by a pitch or yaw velocity) is applied in a plane that is 90 degrees to the excitation angular velocity. If the excitation mode couples with a pliant engine-mount or wing resonant frequency, the prop-end of the powerplant can begin to whirl around the static prop centerline, describing a cone of ever-increasing base diameter. The result, if the mode is not checked, is a wildly wobbling gyroscope that eventually begins to transmit its violent motion to a natural outlet: the wing. Think of what happens if you push a spinning top -- it will wobble erratically.

For example, suppose that an aircraft has an engine mounting structure with insufficient STIFFNESS (as differentiated from strength). Suppose that aircraft encounters a violent gust that causes it to pitch-up rapidly. If the prop rotation is clockwise, the prop gyro-moment would try to bend the engine mount structure to the right. If the mount was sufficiently flexible, it would deflect (yaw) rapidly to the right, which would generate an upward gyroscopic moment on the engine mount. The flexible mount deflects upward, causing a yawleft moment. The 90 degrees-out-of-phase excitation continues, and if these excitations and deflections occur at the natural frequency (or harmonic) of the engine/mount system, the deflections can develop into a whirling deflection of the engine structure of increasing amplitude -- until something breaks.

When you consider the engine mount in the C130 context, you are also including the wing (and possibly one with changed frequency response characteristics because of concealed cracking and outer panel beef-ups). That was the Electra problem -- its wing was "flutter" reacting with the outboard engines. What the investigators found, after the second crash, was that the engine mounts weren't strong enough to dampen the whirl mode that originated in the outboard engine nacelles. The oscillation transmitted to the wings caused severe up-and-down vibration, which matched the wing's harmonic and so the wingflutter grew in amplitude until a wing tore off. Lockheed had to beef up that wing and the engine mounts (with diagonal braces). The C130 engines and the Allisons of the Electra are almost identical. Whirl-mode can be hard to detect. According to Don Keller of NASA Langley: "Some flutter modes are elusive. You can't predict them until they happen."

In Another Whirl

In 1999, the Air Line Pilots Association (ALPA) submitted a petition to the National Transportation Safety Board (NTSB) to have the cause of a Dec. 28, 1991, crash of a Beech 1900C reclassified as a whirl-mode breakup. N811BE of Business Express crashed off Rhode Island (NTSB NYC92FA053) and the NTSB had called it a loss of control/disorientation accident -- despite one wing being found far from the main wreckage. Professor R.O. Stearman of the University of Texas later carried out a signal analysis of the CVR (cockpit voice recorder) and identified the structural acoustic signals of whirl-mode and the ensuing aircraft breakup. Evaluation of the Federal Aviation Administration (FAA) SDR (Service Difficulty Report) data revealed that six engine truss designs had been implemented in response to truss cracking over the 10-year history of the aircraft.