Frank Dillon, Tom Fleming, Jim Hoag, Bruce Powell, Don Borey
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Google: The UH-1D "Huey" uses a two-channel hydraulic system with a total capacity of 10 pints (5.3 pints per reservoir), operating at approximately 1000 psi. It uses MIL-H-5606 red hydraulic fluid to power the servos, which assist the pilot with the flight controls (cyclic, collective, and tail rotor) to reduce workload. Each system has a pump that operates at different speeds, and a backup pump can activate if a primary pump fails
Frank Dillon noticed this study of Army helicopter servocylinders but did not remember any helicopter hydraulic issues from his Centaur tour. Note with this study: Although the AH-1G is not specifically a part of this report the information rendered is directly applicable"
Jim Hoag: If “O” rings failed it’s quite possible due to not flying the helo (extensive maintenance?) and their drying out rendering them brittle. In my 16 years crewing UH-1s, OH-58s & AH-1s, I never had hydraulic failures.
Tom Fleming: When I was Service Platoop leader and aircraft maintenance officer, our only Hydraulic failures were enemy fire caused. Fortunately most of our gunship we had were 1B conversion to C models. The 1st new C models had a Hydraulic boost emergency that gave you about 3 cyclic pulls (so they said)) luckly we never had use them
Bruce Powell: I was in R&D (Aviation Research & Development) after Nam but had not heard of this study. Very few incidents related to Hydraulic failures in our UH-1H, AH-1G and OH6 aircraft have been mentioned on our website, that I know of, and those were probably related more to enemy fire than defective Hydraulic systems.
I made a mention of this subject in the Newsletter just in case someone remembers.
Don Borey: I had a hydraulics failure while on a Recon mission a little North and West of Lai Khe in either September or early October 1971. we had just started the Recon. My LOH was on the deck and I was orbiting overhead. I remember the Master Caution coming on and when I looked, the HYD Failure light was on. I called the scout to come up to altitude, and we headed back to Lai Khe. I think Bob Jones was my front seat. I remembered the caution in the manual that you only had 2 full strokes on the Collective before it froze. I set up a long final to the runway and began a slow descent. I was setting up for a running landing to the dirt on the right side of the runway. Low and behold, the airfield crash rescue fire truck pulled onto the dirt on the right side of the runway. So much for my plan. I continued but shifted over to the runway. I had Bob get on the collective with me which was getting a bit stiff. We touched down and skidded quite a long way down the runway before coming to a stop. When we got out of the Cobra, the Hydraulic fluid was still dripping out under the ship. Jeff Griffin, an IP, chewed my butt about landing on the runway instead of the dirt. He said I could have hooked a skid shoe and turned over. I didn't, but I did wear the skid shoes out on both skids.
Centaur Website links related to Hydraulics:
https://www.centaursinvietnam.org/WarStories/WarEssays/E_KeepEmFlying.html
https://www.centaursinvietnam.org/WarStories/WarEssays/E_YearOfTheRat.html
https://www.centaursinvietnam.org/WarStories/WarEssays/E_EasterOffensiveMick.html
https://www.centaursinvietnam.org/WarStories/WarEssays/E_CamodianMedivac.html
https://centaursinvietnam.org/WarStories/WarDiscussions/D_SkilledHardLanding.html
https://www.centaursinvietnam.org/WarStories/WarDiscussions/D_CentaurAircraftResearch.html
https://centaursinvietnam.org/WarStories/WarDiscussions/D_TrustYourCrewChief.html
https://www.centaursinvietnam.org/WarStories/WarDiscussions/D_Ordnance.html