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Lorn wrote: <<None of the above solutions would have stopped me from having a gear up landing. …….
Wolfgang's pressure switch closer would have saved my bacon.>>

Lorn,

I would be careful with that conclusion.

The failure to extend, or more accurately, the failure to fully extend, on the 320/360 gear is one mode that hasn’t really been discussed in any great detail on the LML. It is certainly at least as important of the hydraulic lock that has seen a lot of attention.

When retracted, the landing gear stores a lot of potential energy. Gravity, and in the case of the Outback Gear, aerodynamics, are trying to pull the gear out of the wells. This stored energy becomes very significant during the extension cycle.

When ‘gear down’ is selected, the pump pushes the spool against the up-side poppet to open the ‘up’ circuit. This unlocks the high side circuit and allows the gear to start down. Initially the gear isn’t driven down by the pump. Instead it is pulled down by gravity. The initial volume flow rate generated by falling gear is more than the return side of the pump can absorb. This in-rush and backing up of fluid generates a pressure spike that impinges on the face of spool. If high enough, it will begin to push the spool back and start to close off the poppet valve. As this occurs, back pressure is transmitted though the high-side circuit, through the cylinders, all the way back to the low side circuit and corresponding pressure switch. If the pressure spike climbs to the low side pressure switch setting, the pump will shut down. This is a very fast and dynamic event. Several key factors come into play: The pressure generated by the inrush of fluid from the descending gear decays very quickly; The pump is transfer fluid at a near constant rate; The pressure spike takes a finite time to propagate; And finally the pump has inertia. Even if the pressure spike momentarily reaches the low side pressure switch setting and power is cut to the pump, the spool down of the motor combined with the rapidly decreasing push from the gear can enable the spike to drop back below the pressure switch setting. This can be heard as a hesitation in the pump shortly after gear down is selected. It can also be observed when monitoring system pressure during the event. If the spike is long enough in duration, the pump can be shut down completely with the system remaining in a stable condition. In that case:

1. Both high and low sides will be pressurized at a value higher than the low side pressure switch (550 psi in a nominal case)

2. The spool will be centered

3. The gear down pressure switch is open (pressure just above set-point)

4. The gear up pressure switch is closed (pressure between high and low set points)

In this state, the gear can still be retracted, but not further extended. The system is in a state where it thinks it is fully extended. This condition will unfortunately not be corrected with Wolfgang’s module.

So how does one resolve the issue long tem? The solution is actually quite simple; Increase the pressure setting of the low side pressure switch. As the pressure switch setting is increased, the pressure spike has less influence and the duration of the pump interruption decreases. Once the setting is higher than the pressure spike, there is no longer any interruption in pump operation and the gear always extends normally.

Chris Zavatson

N91CZ

360std

www.N91CZ.net

From: Lorn H Olsen <lorn@dynacomm.us>
To: lml@lancaironline.net
Sent: Monday, February 20, 2012 4:58 AM
Subject: [LML] Re: 320/360 Hydraulic Pressure Switch

Angier,

If you are flying and the gear doesn't come down, opening the dump valve for a fraction of a second and then closing it, fixes the problem.

On a climb out your switch would work but the problem only occurs when there is at least a 50°F temperature raise. Why not just land and open a fitting? To see if you are going to have a problem on the ground, just open and then close the dump valve. The pump should run. If it doesn't, you will have to open a fitting to relieve the pressure. You could also install pressure gauges and just look at them.

The pressure gauges are probably the 1st extra item that should be installed.

The problem with all of the above solutions is that they can require a pilot's time at the worst possible moment.

None of the above solutions would have stopped me from having a gear up landing. At the OM, on an IFR approach, I put the gear lever down. It was very turbulent. I was busy keeping the dirty side down. At 500' I was crabbed 45°. The wind was 6kts on the ground. I just didn't notice that both switches were open.

Wolfgang's pressure switch closer would have saved my bacon.

Lorn

> From: "Greenbacks, UnLtd." <N4ZQ@VERIZON.NET>
> Date: February 17, 2012 1:07:36 PM EST
>
> Seems to me the simplest and least expensive option would be to install a momentary on switch in the cockpit to override and close the open pressure switches.
>
> Just my 2 cents.
>
> Angier Ames
> N3ZQ

--
Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES, Comm, Inst
DynaComm, Corp., 248-345-0500, mailto:lorn@dynacomm.us
LNC2, FB90/92, O-320-D1F, 1,800 hrs, N31161, Y47, SE Michigan

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