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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 18:12 
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Username Protected wrote:
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.



Correction, if the hydraulic flow light comes on you do not , I repeat do not have to put the gear down right away. That just means you lost one of your hydraulic pumps.

If the HYDRAULIC PRESSURE LIGHT COMES ON, pull the breaker because you could develop other problems.
If you are concerned you could put the gear down before you pull the breaker.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 18:20 
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Location: Central NY, (N03)
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Makes sense! Thank you so much!


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 19:32 
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You have the backup blowdown in the event you lose all your fluid.

I’ve had a hydraulic gear pump fail in flight. It was a non event. When preparing to land the only thing to be aware of is it will take longer for the gear to extend and lock with only one pump operational.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 19:39 
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Location: Central NY, (N03)
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Ok, yes. That was my understanding ~ that if all else failed, I would have the emergency nitrogen bottle available to blow down the gear.

I’m learning a lot from you guys. I really appreciate your experience and inputs!

John


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 19:44 
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Username Protected wrote:
You have the backup blowdown in the event you lose all your fluid.

I’ve had a hydraulic gear pump fail in flight. It was a non event. When preparing to land the only thing to be aware of is it will take longer for the gear to extend and lock with only one pump operational.


Depends on how you lost your fluid. If you broke a hose or sprung a leak the blow won’t work. Blow down only works if the system is intact with no leaks.

Basically you are up $hit creek.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 21:09 
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The blowdown line is a separate system. It is not affected by a hydraulic leak.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 21:19 
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Username Protected wrote:
The blowdown line is a separate system. It is not affected by a hydraulic leak.


Nope that’s not true, you break a line going to any of the three gears and you are screwed.

It’s not a separate line, the nitrogen goes in the same lines as the hydraulic fluid.

Check your diagram in the POH.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 21:28 
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I stand corrected. I thought the blowdown lines were connected directly to the actuators. Learned something new today. Thanks!


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 28 Jul 2021, 21:36 
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It’s a common misconception. Here’s the diagram.


Please login or Register for a free account via the link in the red bar above to download files.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 29 Jul 2021, 08:16 
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Username Protected wrote:
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.



Correction, if the hydraulic flow light comes on you do not , I repeat do not have to put the gear down right away. That just means you lost one of your hydraulic pumps.

If the HYDRAULIC PRESSURE LIGHT COMES ON, pull the breaker because you could develop other problems.
If you are concerned you could put the gear down before you pull the breaker.


Correct.

The issue is if the light is indicating low flow due to low oil caused by a oil leak, not because of pump failure. Once the gear is down, it doesn’t matter the reason (providing there aren’t other complicating factors such as low fuel)

I doubt there would be enough oil to get gear down if you waited for the second light to come on? I’m trying to remember who gave me this tip, but I recall it was a lesson learned from experience.


And I agree that you have to pull the breaker if the hyd pressure light stays/comes on. I recall reading about an instance where the oil got so hot it burnt out the plastic sight tube?

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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 29 Jul 2021, 10:55 
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A hydraulic flow light just means you lost the pump. It does not mean you lost all the oil.

If just “one” hydraulic “FLOW” light comes on I would “not’ put the gear gear down.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 31 Jul 2021, 12:29 
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Check the quality of you fluid and change it if necessary.

I had a problem this winter with water in my hyd fluid. The content of which was enough to cause ice crystals to form in the fluid and block my nose gear actuator.
If you notice your mains always lock first then the nose. There’s a restriction in the body of nose actuator which got blocked with ice causing a nose gear failure.

I blew the emergency bottle with no problems and landed
Once in the warm shop on jacks and powerpac we couldn’t reproduce the problem.

The system was full and the red clear fluid color in the sight tube looked normal. The only option was to pull the actuator and inspect it for damage or worn o rings.
We replace it with an OH’d unit for good measure. ($2400 core!)
It wasn’t until we mistakenly over filled the system and noticed a pink milkshake of oil over flowing on the hanger floor that the culprit was identified.

Postmortem on the actuator revealed some corrosion on the internal portion of the steel shaft possibly due to high moisture content.

In the turbine world this fluid change would be covered in the MM, not so in the piston world.

Just my $.02

AB


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 31 Jul 2021, 22:40 
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Location: Central NY, (N03)
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Great tip Alan! Thank you. I’ll be sure to watch the nose gear light more closely.

It just so happens that I had a hydraulic leak earlier this year where what appeared to be the entire volume was pumped out during and after a landing. Took an entire can of the red stuff once the leaking fitting was changed out. So, I can’t say whether or not the fluid pumped overboard was as you described or not. I can say that the fluid in the reservoir and system is now new due to the above and the leak is no more. System seems to be working as it should. Thankfully!

Great info from all who have responded!

John


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 01 Aug 2021, 17:17 
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I had a similar incident to Alan's years ago. I flew a 402C with the same gear system in scheduled service. On occasion, the right main would not lock down. The first and second time, the bottle would not lock it down either. I had never been taught how little the handle moved and I determined that if I was going to land the airplane with a gear unlocked, I was going to walk into the Chief Pilots office with the handle in my hand. I eventually got the gear locked by maneuvering the airplane. We put it in a local shop's hangar, had them service the gear, and it worked fine.

Several months later, the same thing happen. This time I ferried it stiff legged back to the company shop 150 miles away. Same thing. Worked fine.

The third time, about a year later, same scenario. I just happened to be flying it the day it failed as I switched aircraft/routes. They replaced the actuators.

A couple other crews had the same issue. Eventually, the aircraft landed and the right main collapsed on landing.

At that point they disassembled the entire system and flushed it out. They found water in the system. Hindsight being 20/20, each time that the failure occured, the aircraft had been sitting on a extremely cold ramp overnight (<10F) and had the engine heaters plugged in. We figured out that the water in/near a valve was warmed up long enough to depart, then froze once enroute.


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 Post subject: Re: Cessna 400C series hydraulic pressure mystery
PostPosted: 01 Aug 2021, 17:50 
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Location: Central NY, (N03)
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YIKES Jason!

Sounds like gear failure was becoming old hat for you guys!

Well, I live in Upstate NY where it gets mighty cold for months. I do keep it hangared with engine heaters/insulated covers but, it's not a "heated hangar". My gear system seems to be buttoned up tight now - not losing a drop in the sight glass window for months. But, I guess there's no real way of knowing whether or not water has somehow found its way into the system until its too late..? We'll see what happens this coming winter!

Thanks for the input. I love hearing these stories from you freight/mail/people haulers!

Respectfully,
John


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