I was a Canadian Army RCASC (Royal Canadian Army Service Corps) Lt on an Exchange Posting to the US Army Aviation Test Board from Aug 1963 to Aug 1965. The RCASC was similar to the US Army Transportation Branch. Test aircraft were given the call sign prefix “Bearcat “which we used whenever an emergency occurred or if you needed priority handling by the Air Traffic Controller.
At the time the Logistics Branch of the Test Board was deeply involved in raising the MTBO (Mean Time Between Overhaul), as well as helping to establish MTBF (Mean Time Between Failure) of all dynamic components of the CH-47. The flight program was designed to install fleet components that had reached their MTBO on Test Board hels and flown (usually in 100hr increments) beyond the authorized MTBO at which time the component was removed, subjected to a Boeing analytical inspection and, if deemed serviceable, reassembled and flown for another 100-hr increment. When sufficient samples of a component were flown to the next 100-hr increment, a Board was held to either approve or nor not approve raising the fleet MTBO for that specific component.
Chinooks of the 11 Air Assault Test Division in Ft Benning Ga were the source of most components.
The test flight was a routine 3 hr mission. The flight crew were: Pilot- Mr. Lorry McCullough, (Lorry, civilian CS 14 was a former 82 AB Div D Day vet); Pilot- Lt Robert (Bud) Hill; and Crew Chief - Spec ? Ralph Pigg.
Logistic flights were usually 3 hr duration. Each Chinook was usually scheduled to fly four 3hr profiles each work day. Initially the object was to fly each Chinook 100-150 hrs per month.
On this particular flight, 31 Jul 1964, I had completed my part of the profile and landed Chinook 03451 at County Line stage field, (off the NE edge of Cairns Army Airfield) and passed aircraft control to Lorry McCullough. Lorry was just completing take-off when he asked if I had changed the stick position. I replied No and we both noted the stick in an abnormal almost full forward position. Lorry quickly decided to land and started into descent. I made the radio call “Cairns Bearcat 451 immediate landing” (Note -no request to land was made, just notification of impending action.) By this time, we were off the north end of Cairns, in descent, with about a 20 deg nose high attitude and the stick fully forward. As Lorry pulled in power to arrest our rate of descent, catastrophic failure started. At about 20 ft above ground the aft rotor shaft sheared in two, then the forward and aft rotor blades became unsynchronized cutting into each other and into the drive train tunnel.
The sheared end of the aft shaft and rotor head with attached blades separated from the hel. The aft end of the Chinook rotated down with such force that both aft main landing gear were buried to the belly. The cockpit end then rotated down like a hammer stall half burying the forward wheels. With the forward rotor blades now chopped, the centrifugal forces were no longer in balance causing the forward transmission to be ripped out of its mounting. The forward transmission, with rotor blades still attached, ripped out the cockpit overhead panel (missing both Lorry and me) and flung itself out of the helicopter. When all of the shaking stopped Lorry and I checked to see if each other and the crew chief were OK. We had no serious injuries. At that time, I noticed that the outside fuselage aft of my side was on fire so we bailed out the emergency exit on Lorry’s side. Almost at the same time the fire trucks and Flatiron (Medevac/Rescue hel) arrived. Lorry our crew chief Ralph and myself were loaded into Flatiron and were in the Post hospital within 20 mins.
Subsequent investigation revealed that the aft vertical shaft bearing failed because of spalling. The spalling generated intense heat (1200+o) causing the shaft to stretch (the cause of the abnormal stick position) and ultimately break off. Also burnt paint chips found on the ground track indicated the aft pylon was on fire while we were in flight but we did not know it as there wasn’t any fire detection system in that area.
Further testing of other aft vertical shaft bearings on the whirl tower at the Boeing factory showed that they also failed within a 10hr time span of the time our bearing failure. The tests also revealed that within 2 mins from the start of incipient spalling catastrophic bearing failure would occur.
Front view of CH-47 Chinook 03451 (US Army Photo).
Side view of CH-47 Chinook 03451 (US Army Photo).
Close up view of CH-47 Chinook 03451 (Maj B Hill Photo).
View from above my seat (Maj B Hill Photo).
Aft pylon of CH-47 Chinook 03451 (Maj B Hill Photo).