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Fritz

Uncommanded Reverse

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We had an incident where one of the propellers on an aircraft went into an uncommanded reverse. Has anyone experienced this? It is the first time in (our) history that something like this has happened. It is still under investigation and is of huge concern to us because this incident happened on a pre-take-off run. Any suggestions/help will be appreciated!

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Sounds like the old throttle/condition lever cable failure deal.

The tension regulator could take the linkage to reverse, or look for a broken pully that would allow the tensioner to do the same thing.

RZ Hill

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This once happened at Langely, (C130-B) 1963-65 ?. Having just had an alternator (generator change) it was found the alternator was wired out of phase.

IE: A-B-C phase was indeed wired A-C-B phase. This caused all 3-phase equipment on the/that Buss to run in reverse. Maint-control didn't want to believe me, but a properly wired and tested alternator corrected the problem.

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My memory might be bad, but I seem to remember, in the mid-70's, they had birds

going into reverse, inflight, they grounded the whole fleet. I think the cause was

pullies being to small for the cable.

glenn

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The old throttle cable tension regulators were spring loaded and if a cable broke it would go to full reverse or full throttle right now. They lost several aircraft on count of the old style regulators. They put a new ones on that it took so many seconds to go either to full power or full reverse so you had time to take action. To check the new ones out you had to fully depress the regulator and hold it I think 30 second and then release it and time it back to the preset tension. I had to change many out to the new style.

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Late '69 early '70 we had this issue with E models out of CCK. At least one lost between CCK & Taipei...I do remember wondering if they would ground all of the E's until they found the cause...I don't believe that anybody had figured out that the props were going into reverse inflight(at least one engine) causing loss of the airplane... If I remember correctly, a crew caught it in time, while climbing out of CCK, and shut it down before it took them in..Anybody else remember this? It sure was a big deal, at the time! I do remember that it caused for one heck of a lot of discussion at RMK on Herky Hill. I seem to remember that more than one aircraft was lost before the LUCKY crew caught it as it went into reverse (wonder how that'll work with only four eyeballs on the flight deck?)

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We have checked all cables directly after incident, no damaged/broken ones found and all tensions seemed to be 100%. Aircraft was ground runned after incident (without any settings made to aircraft or prop) and technical crew reported that everything was OK. Prop went into reverse only when commanded too.

The strange thing here is that the flying crew thought initially (when taxing) that the Pitch Lock Regular has hung up and therefore pushed the throttle levers from Ground Idle to Flight Idle. As they did that the engine oil temperature and the prop revs went up. Torque stuck at 4,500 inch pounds. They looked outside and saw that the prop (#2) was in reverse.

Flying crew refused to accept the aircraft even after the ground runs and rigging checks have been done and everything confirmed as serviceable. A propeller was exchanged and the aircraft took-off. To date that same aircraft has been checked, twice for cable tensions and rigging, and has been run on ground profusely to establish if the "snag" don't recur. No signs yet. (Aircraft has done about 6 hours of ground runs before the snag occurred the first time during the pre-take-off taxi.)

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When you say “a Prop was exchanged†do you mean the Prop and Prop Control assembly (Valve Housing and Pump Housing) were changed or just the propeller?

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The prop assy was changed (prop, valve housing and pump housing). I interigated the flying crew once again, and they are 100% that the prop was in reverse thrust.

Prop tests done on the static test rig showed that the Pitch Lock Regulator and the Low Pitch Stop are both serviceable and that the hydraulic pressures readings are exactly as those mentioned in the Hamilton manual. We'll start with the valve and pump housing tests today.

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Almost too crazy to be true...did you change the dome assy?? Sounds a little like pilot error, im not accusing anyone but I have had a pilot blame the aircraft to CYA before...just a thought

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Check the wiring at the feather override buttons. Valve Housing issues or Low pitch stop adjustment. Had a situation years ago during flight, engine shut downs, Control commanded # 3 to feather, 3 went to shutdown and prop feathered, and #4 shut down, prop windmilled. Found Crossed wires in the Feather override Buttons.

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The Prop only does what its told to do...by the Valve Housing... The Valve Housing commands movement based on throttle input Alpha Shaft or blade angle reported by the Beta Shaft.... So..... if the Shafts or cams move without operator input the prop will do what it thinks you want it to do.....

The only way to go into reverse is to rotate the Alpha Shaft (Backup valve cam) (throttle movement) to open the back up valve to allow the higher pressure required to release the LPS.

The Backup valve is actuated by both the Alpha and Beta Shaft only via different inputs...Alpha Shaft input via Throttle movement (ground range LPS retraction) and Beta Shaft via blade movement (higher blade angles and to break Feather latches).

Sounds like you will find one of the Command inputs (shafts AFU)

The cable break provides a false throttle input thus "telling" the Alpha shaft to rotate commanding a blade angle change... It (VH) thinks the throttle is in the ground range so....

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I've seen this problem before on a transient -130 from Dobbins several years ago. The plane came to Little Rock for a modification. Engine shop got dispatched to ops check an anti-ice valve and I noticed all 4 props looked like they were at flight idle. I moved all of the throttles to reverse, hit air start, moved the throttles to ground idle and hit air start again. One of the props stayed at reverse. I moved the throttle all over the place with the aux pump running, but it wouldn't move unless I feathered it. I left the throttle in flight idle and hit air start and the prop went straight to reverse. I checked all the rigging up to the valve housing and everything was fine. We changed the valve housing and everything was perfect.

The valve housing was overhauled. What was found inside it was a retaining pin for I believe was the shuttle valve had become dislodged when I took the prop to reverse and made the shuttle valve hang up, causing fluid to be ported directly to the decrease pitch side of the dome, regardless of alpha or beta shaft inputs. Feathering operation bypasses that valve, so it all makes sense. As for the retaining pin, the valve is supposed to be safetywired now instead of using the failure-prone retaining pin to avoid just this situation.

I'm not saying that's what your cause is. I guess my point is that anything is possible inside the valve housing, most things just highly improbable.

I've also had a prop go full reverse on me during an engine run before. We changed the valve housing and everything was perfect again. Nothing notable was found on the valve housing rebuild that time though. It just got all new seals, thoroughly tested and kicked back out into the system.

Edited by Lkuest

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Just herd of one (recent).... happened during static power check prior to takeoff. Found a bolt on the coordinator worn that allowed the pulleys to rotate freely.....

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"Just herd of one (recent).... happened during static power check prior to takeoff. Found a bolt on the coordinator worn that allowed the pulleys to rotate freely..... "

I'm not one to play the blame game too hastily, but I would speculate that the extended time between HCS's and ISO's have something to do with this. I think it's about 260 or 280 days between each instead of the usual 180. I remember doing an ISO extension inspection to keep some planes flying until we could get around to flying'em to ISO. The most important part of that inspection was checking the integrity of the rigging, something that gets checked through thoroughly during an HSC or ISO.

I think the purpose for this extended time between HSC's and ISO's was to lessen the load on manpower and "do more with less." It just seems to me we're doing less with less, which makes for a lower quality product while still refusing to acknowledge a large difficiency in manpower.

Just my 2 cents.

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Had a problem i have never resolved. Engine will not start with TD. Amp switch in Auto.

Switch to Null and the engine will start and get you home. On shutdown, no need, the engineer justs put the switch back to auto and the engine will shutdown. TD. Amp change cured the problem. Any ideas?

Trev

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Had this one back in the 80's. TD amp is sensing anything off the thermocouples as too hot, and commanding the TD valve to reduce fuel ...... to the point where there is insufficient fuel to keep the flame(s) alive

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I recall seeing some pics of a tear down of the VH -- I believe FOD from a very old style potentiometer was determined to be causal in this incident.

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I know it's been almost a full decade, but did anyone ever figure out specifically what was happening in the prop assembly to allow them to go into reverse during flight? I'm trying to find a HAZREP I remember hearing about involving a 130, a prop going into uncommanded reverse in flight, and then departing the aircraft rupturing the hyds lines and I believe killing the entire crew? I think the explanation I got was it had something to do with faulty gaskets on the speed servo. I knew a pilot that refused to take a plane under the grounds of that HAZREP after MX statically feathered and unfeathered it and called it good to go. Does anyone remember anything like that HAZREP happening? Thanks in advance!!

-P-3C Dude

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As a maintainer, I've seen this a few times, both with engine running and during static pitch changes. Also, with the throttle in any position, as well as reversing with the condition lever in feather. I've never had an aircraft from my unit reverse in flight during the 200,000+ flight hours worth of malfunctions I've either worked or got turnover for, but I do believe this eventuality is covered in simulator training.

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I just wanted to throw in my $.02 on an old topic:

I've seen this twice, once in Al Udeid in 2009, and again at Hurlburt in 2014. In Qatar, it was a propeller that came in to the prop shop, and at Hurlburt, I found it on a flameout inspection. In both instances, the bearing in the differential gear in the pump housing failed, allowing the spring tension on the beta shaft to slam to reverse, and in the process snapping off the beta feedback gear. This was different than the case described, however, because both instances resulted in a flameout.

 

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