On 7 June 1969 YK-9 was assigned logistical resupply missions.  At approximately 1400 hours, five miles northwest of Da Nang , 1stLt. Casciano was lifting from LZ Eagle (Hill 364) when Cpl. James Huebner, who was flying in the chase aircraft as a gunner, recalls this transmission from YK-9, "We've got some hydraulic problems and we're going to take it low and slow."  Cpl. Huebner further recalls, "I could see that the aircraft was noticeably in trouble.  The pilot tried to return to the LZ but did not make it.  I saw it turn 180 degrees, pitch down and crash nose first into the side of the mountain.  It then burst into flames and rolled down the mountain.  The pilot of my aircraft (I don't remember the other crew members names) immediately landed in the LZ.  The crew chief and I, as well as other Grunts from LZ Eagle, started searching for survivors.  We found Jon D. Mills climbing back up the mountain toward the LZ and I escorted him back to my aircraft.  We immediately lifted, leaving our crew chief at the site to continue searching for survivors, and flew J.D. to the hospital at Da Nang.  Afterwards my pilot was directed by Da Nang Direct Air Support Center to return to Marble Mountain as there were other aircraft on scene to provide assistance."

1stLt. Casciano survived the crash sustaining 2nd and 3rd degree burns (extent unknown).  LCpl. Mills suffered lacerations of his left eye and nose, plus 2nd degree burns on his left forearm.  Maj. Archbold and Pfc. Pankuch did not survive the crash.

Lt.Col. Larry W. "Slick" Britton recalls, "I was the Squadron Safety Officer at the time and conducted the investigation of YK-9's crash.  Upon arriving at the crash site, with the assistance of the Grunts from LZ Eagle, we began retrieving the wreckage of YK-9.  One of the Grunts found the Ramp & Hatch manifold, brought it to me and asked if it was important.  The Ramp & Hatch  manifold was installed on the right rear side of the aircraft to allow the Crew Chief to raise and lower the ramp and hatch to embark or offload personnel or supplies.  As soon as I saw the manifold I knew we had something because the safety wire that was attached to either side of the manifold, and went though a plug at the end of the manifold, was in place while the plug was missing.   This screw in plug had failed and was allowing hydraulic fluid to blow out under 1500 psi.  When Pfc. Pankuch informed his pilot of the leak, Lt. Casciano followed the appropriate emergency procedures by activating the Isolation switch which should have isolated the Utility Hydraulic System (ramp & hatch, etc.) from the #2 Boost System.  Pfc. Pankuch told Lt. Casciano the leak had apparently been stopped."

"Further investigation as to why activating the isolation valve failed to stop the leak revealed  that there was a hydraulic line running from the Utility/#2 Boost Reservoir to the Ramp & Hatch manifold which was separate from the rest of the system and, therefore not affected by the Isolation valve.  This line allowed hydraulic fluid under 50 psi. pressure to flow to the Ramp & Hatch manifold to provide "make-up" pressure to move the pistons inside the manifold.  We all know what a mess high pressure hydraulic leaks make in an aircraft, there is hydraulic fluid every where.  Therefore, when Pfc. Pankuch witnessed the reduction of the 1500 psi. blow out, the 50 psi. leak was masked by the abundance of hydraulic fluid already inside the aircraft."

"The statement from Lt. Casciano revealed that he did not intend to attempt a flight back to Marble Mountain.  He realized that LZ Eagle was a one bird zone which would not allow another maintenance crew to land and repair the damage.  His call over the radio that he was going to 'fly low and slow' was to move the aircraft down to an LZ known as the Rock Crusher which was located at the bottom of Hill 364.  As Lt. Casciano lifted he experienced control problems, attempted to return to LZ Eagle but was unable to do so.  For some reason, the Number One Boost System was unable to provide the necessary control response or authority.  With the loss of the Utility and Number Two Boost, the crew of YK-9 ceased to be crew members and were simply passengers from that point on".

"I think that Boeing Vertol (manufacturer of the CH-46)  changed the design, after the crash of YK-9, so the entire Utility/#2 Boost system would be isolated by activating the Isolation Valve."

"The Grunts on LZ Eagle were extremely helpful after the crash, attempting to rescue survivors, securing the area and even rigging ropes down the side of the mountain to the resting spot of the wreckage (it was about a 60 - 70 degree slope).  A few days later, we filled a couple of large parts cans with beer and ice, painted the cans purple and placed the 'Fox' stencil on them and delivered them to the Marines on LZ Eagle.  To our surprise the Grunts immediately started pulling the beer out and dropping it on the ground.  Once the beer was out, they took the ice and put it into their cans of drinking water.  Apparently they had no trouble getting beer on top of Hill 364, but they said they hadn't had ice up there for a year!  After that, we would take a couple of cans of ice up there whenever we could."

Information provided by:
    HMM-364 Command Chronology
    James Huebner, former Cpl. USMC
    Larry W. "Slick" Britton, Lt.Col. USMC(Ret)