Slow gear cycle?

Jhogan0101 · #1 12-13-20, 04:00 PM

Ive notice my gear takes about twice as long to cycle lately(337B).

I put a white zip tie down the filler neck and I noticed the hydraulic fluid was about 5 inches low from the top

The fluid looked nice and red, i topped off.

I flew around the pattern and cycled a few times, same thing.

I have the optional gear pump on rear engine.

Any ideas?

Skymaster337B · #2 12-13-20, 04:17 PM

Check the filter.

wslade2 · #3 12-14-20, 06:08 AM

Don’t forget, with two pumps you have 2 filters. Also a good time to be sure both pumps working, not just one. If your fluid was low, you have a leak somewhere. With low fluid, may have air in the system.

JAG · #4 12-14-20, 10:47 AM

I would agree wslade2, you could have air in the system. I had air in my system and it causes major gear problems. The book has procedures to purge air, but to ensure the pumps are fully primed, I have used my brake bleeding pressure pump to fill the reservoir. I capped the overflow, pulled the pump off the engine (leave lines hooked up), and undo the pressure line from the firewall and put in a container. once the reservoir fills up from the pressure pot, it will push through the suction line to the pump (turning it slowing like a motor) and out the pressure line. I did that until I had no bubbles. Hook everything back up and you will have eliminated the largest of the trapped air - that can collect in the suction line. Remember, these gear pumps do not like air, and are not really self purging. Then, follow the gear swing / trapped air procedure in the manual.
Yes - this is a bit of work, but take it from experience, the worst thing you can have happen is a full reservoir but a gear that hangs up mid-cycle due to air in the suctions lines.....it is a bit stressful.

Jeff

Jhogan0101 · #5 12-14-20, 12:27 PM

Thanks guys, i thought these systems were self purging?

I cleaned the screens and they looked good.

Anyway, ill take to my a&p. I dont have the jacks or mule to test this and i definitely dont want to leave anything to chance with the gear.
I appreciate all of the info.

n86121 · #6 12-14-20, 05:44 PM

Being a nerd, years ago I actually studied the hydraulic schematics in the service manual. BTW, it is NOT a great topic of discussion with fiends.

Anyway, my pre takeoff procedure is to taxi the gear handle DOWN while taxiing (or during runup).

This pressurizes and cycles the whole system.

How long the handle takes to pop back up from 'gear cycle down' to 'gear is down and locked' position indicates fluid level.

In the summer its about 4-5 secs. In winter a bit longer.

If it takes longer, it tells you the system low.

AND, if there is a leak and it doesn't pop back up because it can't hold 1200-1500 PSI (as I recall), or takes really too long, then you find out BEFORE you takeoff (Which is much better than AFTER).

n86121 · #7 12-14-20, 05:45 PM

I do think the system will self purge, but can take a lot of full cycles to do so.

So even after 'topping' the tank, it will continue to burble air up, and need to be re-topped a few more times before it stays full and happy.

Jhogan0101 · #8 12-14-20, 06:00 PM

Thanks David, where does air usually get introduced?

Also, is there a way to force the system to purge the air?

Sounds to me from what you were saying earlier maybe holding down the gear handle?

JAG · #9 12-16-20, 09:15 AM

I think the system can generally handle a bit of air and would eventually purge through the tank...over time. However, as I highlighted below, air above the reservoir - specifically the suction line to the pump, is not self purging. I learnt this the hard way. Cessna even recognized this and highlights this in the Service Manual. Ensuring you have no air between the reservoir and the pump is essential, and it is simple to remove. It may take a little time to prime that line, but only needs to be rarely.

Jeff

mshac · #10 12-16-20, 12:56 PM

Here's a thought - What about manually cycling the gear doors several times on the ground, with the hand pump? Would that help purge any air in the system?

wslade2 · #11 12-16-20, 07:37 PM

I suspect it would purge air from gear door actuators but not the actual landing gear actuators. So that would be some. Of course the big concern is getting your landing gear legs to push into lock position.

Just got finished doing the front actuators, jacking it up and cycle gear thing.

BTW, reminder, don't have to have a mule. Power drill and splined socket from harbor freight will power the pump after removal from the engine to cycle your gear.

mshac · #12 12-16-20, 07:39 PM

Quote:

Originally Posted by wslade2

BTW, reminder, don't have to have a mule. Power drill and splined socket from harbor freight will power the pump after removal from the engine to cycle your gear.

Unless you have a late-model Skymaster that doesn't use engine-driven pumps.

Jhogan0101 · #13 12-17-20, 10:03 PM

I found this in the service manual.
Thought it was interesting.

https://ibb.co/fnVYmzv

SkyKing · #14 12-23-20, 12:44 AM

I was JUST going to say... "HAVE YOU READ THE SERVICE MANUAL?" These systems are specific and different from model year to model year, so a Cessna Service Manual for YOUR particular species is a MUST HAVE ITEM so you can understand the system - and from there go through the checklists provided - until you reach the cure. None of you should even OWN as SKymaster without the Cessna BIBLE!