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 Post subject: Cessna 400C series hydraulic pressure mystery
PostPosted: 13 Jul 2021, 20:10 
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Joined: 10/24/19
Posts: 131
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Location: Central NY, (N03)
Aircraft: 421C
Hello All,

I recently purchased a 1978 421C, (less than 8 mos ago), so I apologize for the length and especially if this topic has been covered in the past. I have nearly 100 hours on her now. A couple of months ago the hydraulic pressure light on the annunciator panel for the starboard engine started illuminating during gear transition. The gear moves quickly up or down so, it’s only for a few seconds and the light goes back out.

Believing that I was being warned of an impending problem and potential failure, my mechanic and I decided that it would be best to review the logs and determine the age of the hydraulic pump on that engine and perhaps send it out for overhaul. I reviewed the logs all the way back to 78 and found that the hydraulic pump on the port engine(s) had been replaced on 3 different occasions during these past 43 years. I was unable to locate any mention of the pump on any of the starboard engines throughout this period. Ok, so let’s send it out for overhaul. 2 months later it was returned and reinstalled (overhaul shop confirmed that it was “worn”). Anyone guess what happened next?

Yup. Same problem - hydraulic pressure light illuminates during gear transition. Well, ok. We know that the pump is good - perhaps it’s the sending unit..?? Phone call to the guys in Wichita was made, who agreed that it may be the sending unit. I’m told that the system is redundant, simple and even if one pump fails, the other provides enough pressure to operate the gear. Spoke to the folks who have done the past few annuals and they can change it out during annual next month. Part search in Kansas showed that they have them in stock for $1500.00, (no return of course).

Then I came across this SAIB, regarding the hydraulic gear for a 402C which was prompted by an incident where the pump(s) continued to run after takeoff. I am told that the 400C series have identical hydraulic gear systems. Within this bulletin it describes how the hydraulic pressure lights are SUPPOSED to illuminate during gear transition!

https://twincessna.org/pdf/402HYDPRESSSAIB.pdf

So, my question for all you Golden Eagle drivers out there, (or anyone else with this specific knowledge), Is it Lights On or Off??! I have one engine without a light and one with a light during transition.. Both of them used to be out and would only illuminate at idle power until the above described occurred.

Your input is greatly respected & appreciated!
John


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 13 Jul 2021, 21:46 
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Joined: 05/04/11
Posts: 451
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Location: Covington, GA
Aircraft: 421C, 58
Mine come on during gear cycle. One side comes on sooner/brighter/more steady than the other, and always has. They don’t align perfectly when bringing power up from idle either. I’ve replaced RH pump due to failure, but that did not affect annunciator light behavior.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 14 Jul 2021, 00:04 
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Joined: 10/24/19
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Location: Central NY, (N03)
Aircraft: 421C
Username Protected wrote:
Mine come on during gear cycle. One side comes on sooner/brighter/more steady than the other, and always has. They don’t align perfectly when bringing power up from idle either. I’ve replaced RH pump due to failure, but that did not affect annunciator light behavior.



Thanks Steve!

Just so that I know what to look for - how or what enabled you to become aware of the RH pump failure if the annunciations weren’t effected?

John


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 14 Jul 2021, 06:19 
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Joined: 05/04/11
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Location: Covington, GA
Aircraft: 421C, 58
It was an isolated failure. When it failed, the annunciator came on and stayed on. The annunciator behavior was the same prior to failure and after the pump was replaced.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 14 Jul 2021, 08:50 
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Joined: 05/17/11
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Location: KFRG
Aircraft: 421C
My guess is the check valve in the manifold assembly is failing causing back pressure. The only light that illuminates when I cycle my gear is the HYD-PRESS light which is correct. Anything else indicates an issue.

Attachment:
421-hydraulic.jpg


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 14 Jul 2021, 11:00 
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Joined: 04/17/11
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Location: Queen Creek, AZ
Aircraft: Cessna 150M
It would help if you could clarify exactly which light you are seeing. Are you getting a "R HYD FLOW" light or are you seeing the "HYD PRESS" light?

Jeff


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 14 Jul 2021, 21:04 
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Joined: 10/24/19
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Location: Central NY, (N03)
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Username Protected wrote:
It would help if you could clarify exactly which light you are seeing. Are you getting a "R HYD FLOW" light or are you seeing the "HYD PRESS" light?

Jeff


It’s the hydraulic pressure light Jeff.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 14 Jul 2021, 21:09 
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Joined: 10/24/19
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Location: Central NY, (N03)
Aircraft: 421C
Username Protected wrote:
My guess is the check valve in the manifold assembly is failing causing back pressure. The only light that illuminates when I cycle my gear is the HYD-PRESS light which is correct. Anything else indicates an issue.

Attachment:
421-hydraulic.jpg


Thanks Anthony,

That’s what the guys in Wichita said, check valve and flow switch! I incorrectly identified it as a ‘sending unit’ in my first posting.

So, I have one that illuminates during gear transition and the other doesn’t. How concerned should I be, in your opinion??


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 15 Jul 2021, 00:49 
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Location: Queen Creek, AZ
Aircraft: Cessna 150M
If I understand the system correctly, please correct me if I'm wrong (seems very much like a 525 series hydraulic system)...

Hydraulic pressure only operates at pressure (1800psi?) while the gear is in use...the rest of the time it just circulates without building pressure. When the system is under pressure the HYD PRESS annunciator is illuminated.

Each pump has an inline flow switch to indicate an abnormal condition with it's respective pump (lack of sufficient flow).

During normal operation, gear retraction for example, the gear handle is placed in the up position, the manifold valve moves to build hydraulic pressure to the retract side of the actuators, illuminating the HYD PRESS annunciator. The gear move up until on the unlock hooks completing the cycle, manifold valve moves to de-pressurize the system and allowing the hydraulic fluid to circulate, and the loss of the built up pressure turns off the HYD PRESS annunciation.

So as Anthony mentioned, the only annunciator light you should see is the HYD PRESS light during gear operation. If you see a L HYD FLOW or R HYD FLOW annunciator, it means the pump is not moving the fluid enough to make the respective flow switch.

Attachment:
Screen Shot 2021-07-14 at 9.43.51 PM.png

I believe this is your annunciator panel? There is only 1 Hydraulic pressure annunciator, not one for each engine.

It sounds like it is working normally now, but might not have been prior when you weren't getting any annunciation.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 15 Jul 2021, 21:58 
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Joined: 11/30/10
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Company: Flagstaff-Williams Dev. LLC
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Its curious that Cessna would use the same hydraulic logic Scheme in the 500 series AC as the early 210 and 182RG series. Not the same components, but same hydraulic logic.

The downside of this is that if for some reason the "unload" valve fails, the engine (or motor) will continue to put power into pressurizing the fluid until it overheats. It high temps seals fail and pressurized and hot hydraulic fluid escapes.

Its for this reason that some aircraft use Skydrol instead of 5606; which is flammable.

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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 15 Jul 2021, 22:40 
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Location: Concord , CA (KCCR)
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The hydraulic pressure light is supposed to come when you move the gear handle up or down and yes it’s on the right hand side.

The other two hydraulic flow lights come on if you have lost a engine driven pump, otherwise they stay off.

An intermittent pressure switch could cause either hydraulic flow light to come on or flicker in flight.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 15 Jul 2021, 23:31 
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Joined: 10/24/19
Posts: 131
Post Likes: +35
Location: Central NY, (N03)
Aircraft: 421C
Username Protected wrote:
If I understand the system correctly, please correct me if I'm wrong (seems very much like a 525 series hydraulic system)...

Hydraulic pressure only operates at pressure (1800psi?) while the gear is in use...the rest of the time it just circulates without building pressure. When the system is under pressure the HYD PRESS annunciator is illuminated.

Each pump has an inline flow switch to indicate an abnormal condition with it's respective pump (lack of sufficient flow).

During normal operation, gear retraction for example, the gear handle is placed in the up position, the manifold valve moves to build hydraulic pressure to the retract side of the actuators, illuminating the HYD PRESS annunciator. The gear move up until on the unlock hooks completing the cycle, manifold valve moves to de-pressurize the system and allowing the hydraulic fluid to circulate, and the loss of the built up pressure turns off the HYD PRESS annunciation.

So as Anthony mentioned, the only annunciator light you should see is the HYD PRESS light during gear operation. If you see a L HYD FLOW or R HYD FLOW annunciator, it means the pump is not moving the fluid enough to make the respective flow switch.

Attachment:
Screen Shot 2021-07-14 at 9.43.51 PM.png

I believe this is your annunciator panel? There is only 1 Hydraulic pressure annunciator, not one for each engine.

It sounds like it is working normally now, but might not have been prior when you weren't getting any annunciation.


Jeff,
I think you may have solved my mystery. I’ll check my annunciator panel tomorrow evening and get back.
Respectfully,
John


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 17 Jul 2021, 20:14 
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Joined: 10/24/19
Posts: 131
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Location: Central NY, (N03)
Aircraft: 421C
I wish to thank all of you for your help in solving my little mystery! Thanks to you guys, I have a much clearer understanding of how my hydraulic system works.

Bottom line: everything is working as it should now - which is a good thing!

Best!
John


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 12:00 
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Joined: 01/29/14
Posts: 207
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A tip I was given when I first got my 421c is that if you ever see a hydraulic flow light come on in cruise, immediately select gear down.

As discussed, the two pumps are constantly in operation, so if a hose gets a leak somewhere, all the oil will be pumped overboard. A hydraulic light will give warning of reduced flow, so get the gear down while there is still some oil in the system.

Note the emergency blow down uses the same hoses, so it will also pump overboard if the leak is bad enough.

Immediately selecting gear down gives you the best chance of getting the gear down and locked. After all, it’s no real drama if it just turns out to be an indication problem.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 13:12 
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Joined: 10/24/19
Posts: 131
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Location: Central NY, (N03)
Aircraft: 421C
That’s a great tip, Brad! I will certainly remember that. Thank you very much!
John


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