= = = = = LNC2 landing gear hydraulic lock up problem. = = = = =

This problem has been lamented on for quite a while by my friend Lorn Olsen and recently I finally got into the details involved.

It appears that when the system is heat soaked, both sides end up at pressures above the pressure switch settings and consequently the pump can not run in either direction rendering the gear cycle inoperable.  This can happen after a long cruise at very low temperatures and a  descent to warmer conditions. The only thing left at that point is to  open the dump valve equalizing the HI and LO side pressures so gravity  and springs can lower the gear. Additionally, this could prevent gear  retraction after takeoff if the pressures are still high.

The pump used is the OilDyne 108 series http://www.parker.com/literature/Literature%20Files/euro_cylinder/v4/108_1301-uk.pdf - see pg 9 for internals.
The Lancair part number is 6422666 and the OilDyne number is 108AMS32- CZZ-3V-14-08

I have talked with the project manager of the 108 series at OilDyne  and found that the only non standard part is the ZZ which indicates a modified LB circuit where the back pressure valve has been modified to  be bi-directional and it's restrictor has been removed. The check  valves are standard and still have a 1:5 actuation pressure ratio.

Heat soak causes the hydraulic volume to increase noticeably  increasing the line pressure. This pressure will build up because the  check valves in the pump prevent any fluid flow back into the pump  when there is no power to the pump. There is a shuttle valve in the  pump that opens the non-action side check valve to open by pressure  from the action side but that pressure is from the pump side of the  check valve. This shuttle valve is spring loaded to center so without  the pump running, neither check valve is open.

A heat soak of as little of 40 degrees can cause this problem.

So, the fix is to prevent the non-action side pressure from increasing above the pressure switch setting due to heat soak and / or bring it  down if it does.

This can be accomplished in two ways.

1) Replace both pressure switches  with SPDT and rewire the gear  system so the pump will run action side even though it's pressure  switch is satisfied AND the non-action side switch is also satisfied  (and it shouldn't be). This will build up pressure  upstream (pump  side) of the action side check valve causing the shuttle to move off  center and the non-action side check valve to open releasing the non- action side pressure.

2) Add an accumulator to each HI and LO side to absorb the temperature induced volume/pressure increase.

I began to like #2. I initially thought 5 cu.in. units (Parker # AD007B25T9A1) with a precharge of about 100-200 psi would work even though they were larger than needed.

Upon further examination, I considered a length of flexible  (expandable) hose added to the system since that would be much easier  to install. I just needed to figure out the required length. After  rounding up expansion numbers from AeroQuip on their 303-5 and 303-8  hose and other details;

303-5 ID=1/4"
303-5 hose expansion = 0.094cc / in at 1000 psi
303-5 hose expansion = 0.123cc / in at 2000 psi
303-5 hose expansion = 0.149cc / in at 3000 psi

303-8 ID = 13/32"
303-8 hose expansion = 0.100cc / in at 600 psi
303-8 hose expansion = 0.137cc / in at 1250 psi
303-8 hose expansion = 0.171cc / in at 2000 psi

Hydraulic fluid volume in the LNC2 is about 27 cu in ;  6.4 cu in in  the lines, ; 20.6 cu in the actuators with door actuators
Hydraulic fluid thermal expansion coefficient = .0005 / deg.F
Aluminum thermal expansion coefficient = .0000123 / deg.F (negligible  for this purpose)
Modulus of elasticity of hydraulic fluid = 250,000 (compression under pressure - psi per percent reduction in volume - negligible for this purpose)

I calculated that about 8ft of 303-5 with AN4 fittings would provide  the desired volume expansion from heat soak.

Fluid in the non-action side of the system = 3.2 cu in.
8ft of 303-5 line = 4.71 cu in
Fluid in the non-action side of the system with added line = 7.9 cu in (ignoring volume of removed line segment)
Extrapolated volume increase in the added line at 300 psi = 0.065cc / in
Volume increase = 0.065 / 2.54^3 x 96 = 0.381 cu in
Delta T allowance = 0.381 / (7.9 x 0.0005) = 96.5 degF with 300 psi increase

Replace an existing piece of line with a 3 loop coil under each seat  pan, one plumbed into the HI side and the other plumbed into the LO  side and bleed the system.

There are other details but these are the basics.
These are initial calculations and have not yet been tested.

CAUTION
If there is a leak in a cylinder or the dump valve, there will still  be a problem that can render the gear inoperative leaving only  emergency dump valve use.
The action side can leak into the non-action side, the pump will turn  on to bring the action side pressure back up, the leak will continue  untill both sides are above the pressure switch setting. Should heat  soak come into play after that, the pressures can continue up to 1500  psi, the thermostatic relief valve setting of the pump.

My first proposed electrical solution can overcome the leak problem  but is a more involved installation.

If anyone wants more details, feel free to email me Wolfgang@MiCom.net

Wolfgang Franke

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Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES, Comm, Inst
DynaComm, Corp., 248-345-0500, mailto:lorn@dynacomm.us
LNC2, FB90/92, O-320-D1F, 1,635 hrs, N31161, Y47, SE Michigan


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