Chris,

 

While waiting for Wolfgang's answer, some 200/300 series pilots are becoming concerned that they can't get the gear down when needed.  First, Lorn's secondary problem is that he didn't look for the "gear down and locked" lights before landing.  While the gear down pressure switch saw enough pressure to not allow the pump to start, the failure to recognize that the gear was not down contributed to the belly in landing.

 

To allow people to be more comfortable about the hydro-electric system they should be reminded of this

 

1. The failure to RETRACT the gear because of high pressure on both sides is a problem because of the safe design.  Opening the dump valve doesn't resolve the problem because the system is in a static state with the gear down and locked.  There are no physical forces able to change the system condition - no G-maneuvers, no slips or skids, nada.

 

2. The failure to EXTEND the gear because of high pressure on both sides is different.  Opening the dump valve (allowing down side fluid to flow to the upside) will allow the gear to "emergency" extend because of gravity on the mains and the nose gear gas spring.  OK, maybe the nose gear might not extend all the way at 120 KIAS (max gear extension speed), but the pressure lock has been broken and the pump will start if the switch is down and the pump breaker is closed.  Close the dump valve to complete the process.  As a matter of fact, assuming everything is normal and the gear down switch is selected but nothing happens, it may merely require momentarily cracking the dump valve to relieve the hydraulic constipation.

 

Failing to retract is a nuisance.  Failing to extend is, uh, more serious, but resolvable.

 

Scott Krueger AKA Grayhawk

 

Wolfgang,

I seem to recall you dismissed the idea of a three-way valve previously.

<<That's a sound and positive fix.

- - However it requires changing the dump valve, running a return line and pump modification (replace the reservoir).

...................................

Wolfgang>>

<<..........Automatic action by my module can prevent these problems for both up and down gear operation.

Wolfgang>>

 

Could you please explain how your module will prevent the gear extension lock-up failures.  Recall Lorn's photo after his extension failure and gear up landing.  Both pressure gauges showed around 500 psi.  The system is hydraulically locked with one switch open and one closed.

 

Chris Zavatson

N91CZ

360std, 1,400 hrs

http://www.n91cz.net/

 

From: Wolfgang <Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Saturday, January 26, 2013 11:24 AM
Subject: [LML] Re: Thermal influence on Hydraulic Pressure

This has been covered at great length before. Some take the position that "real pilots" don't need anything but the dump valve.

I have proposed two answers,

1) replace the dump valve that only opens a port between the HI and LO sides with a dump valve that dumps back to the pump reservoir.

2) install a small electric module I make across the pressure switches that runs the pump to relieve the over pressure in the "wrong" line automatically.

 

Wolfgang

----- Original Message -----

From: Charles Brown

To: lml@lancaironline.net

Sent: Saturday, January 26, 2013 2:07 AM

Subject: Re: [LML] Re: Thermal influence on Hydraulic Pressure

This has happened to me even at the beginning of a flight, if the temps in the hangar have gone from cold on a previous day, to warm on flight day.  I guess cold temps shrink the fluid and suck more from the reservoir into the lines, and when it warms, the pressure builds on both sides of the system, so that the "up" and "down" pressure switches are both open and the gear won't move until I pop the bleed valve.

Nasty when the gear won't retract just after blasting off into a low overcast.

Charley Brown

Legacy #299  200 hr

On Jan 25, 2013, at 7:23 AM, randy snarr wrote:

. I dont like having to open the dump valve but it is necessary once in a while especially when it is cold. The pressures on both sides of the system climb due to the engine heat and the gear sometimes will not move without dumping the pressure.

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