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  #1  
Old 08-15-2016, 05:34 PM
Chrysopelea Chrysopelea is offline
 
Join Date: Jan 2016
Location: santa rosa CA
Posts: 90
Default Bad sensor???

Today, during my lesson, my plane and engine flew perfectly, but after landing at one airport, on takeoff I glanced at my Skyview and it showed fuel pressure REALLY low, like 1.9, bouncing around there, 1.9 to 2.1, but running fine, and 2 minutes after taking off it was back up to 5.6-5.8.

I've checked, and usually have 5.8 with both pumps running, 5.6 with the electric fuse pulled, and the electric pump alone (plane off) gives about 3.6lbs by itself.

I'm thinking maybe a bad sensor or bad connection to the sensor. This is the second flight after it's annual, and it decides to act up in a couple different ways.

Funny how that is, as I doubt the two issues I had today are in any way due to the annual or anything the mechanic did, heck, I was there and helping for the engine part of the annual!

I'm going to wiggle all the connections (first thing to always do, that, or hit it with a hammer) and hope to never see those low pressures again!

Anyone else experience anything like this?
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  #2  
Old 08-15-2016, 06:17 PM
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Jesse Jesse is offline
 
Join Date: May 2005
Location: X35 - Ocala, FL
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I worked on a -12 that had a low fuel pressure issue and I seem to remember he had the banjo fitting for the fuel return line with a restrictor fitting wrong (I don't remember the details) and that caused a low reading, even though the pressure wasn't actually low. Sorry I don't remember the details.
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  #3  
Old 08-15-2016, 06:37 PM
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Piper J3 Piper J3 is online now
 
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Location: Hinckley, Ohio
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Quote:
Originally Posted by Chrysopelea View Post
Today, during my lesson, my plane and engine flew perfectly, but after landing at one airport, on takeoff I glanced at my Skyview and it showed fuel pressure REALLY low, like 1.9, bouncing around there, 1.9 to 2.1, but running fine, and 2 minutes after taking off it was back up to 5.6-5.8.
Any chance you shut the engine down for some period of time after landing? I'm thinking you may have experienced vapor lock. I learned this lesson on my 12 and now I leave the master on to keep the electric fuel pump running if I anticipate a hot engine restart after sitting for 10 minutes or so.
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  #4  
Old 08-15-2016, 07:18 PM
Slane Slane is offline
 
Join Date: May 2008
Location: Hillsboro, OR
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My buddy and I have both experienced this, each in his own plane. I think Jim may have it right in regard to vapor lock. We both had landed, shut-down, then restart within 10-15 minutes and both had acceptable fuel pressure on restart. However, soon after lift-off, the pressure warning came on and our gauges jumped around just as you describe. At that point in the take-off phase, it was safer to continue and fly the pattern. After a short period of time, fuel pressure indications returned to normal.
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  #5  
Old 08-15-2016, 09:05 PM
Chrysopelea Chrysopelea is offline
 
Join Date: Jan 2016
Location: santa rosa CA
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It was actually one of the cooler days we've had a lesson, and we landed, and taxied around and took back off, never stopping the engine.

It does sound like a possible vapor lock, as the engine never skipped a beat, and pressures returned after a few minutes.

I'm torn between running both pumps all the time while in flight, or using the electric only during takeoffs and landings. It was explained to me that pulling the electric pump fuse during flight will extend the electric fuel pump's life, and also will let me know when one, or the other goes bad. With both running, the electric or mechanical pump could go bad without my knowing, and then if the mechanical one went south, it'd be coitans for me I say, coitans.

My plan is to make a nifty little fuse holder for the pump fuse so it has a place once removed.

At my work we have a high powered laser, and I'm going to laser engrave up a plaque with my plane info, and home airport frequencies, as I seem to have trouble remembering them. It's going to be super cool, made of anodized titanium, probably blue like my plane. If we have any thin zirconium, I could make it black chrome looking, and paint in the engraving with white, that would be cool too.
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  #6  
Old 08-15-2016, 09:40 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by Chrysopelea View Post
It was explained to me that pulling the electric pump fuse during flight will extend the electric fuel pump's life, and also will let me know when one, or the other goes bad. With both running, the electric or mechanical pump could go bad without my knowing, and then if the mechanical one went south, it'd be coitans for me I say, coitans.
The pump is rated for continuous duty operation.
They have been run in RV-12's that way for many years now.

I am not clear on how it is important to know that one of the pumps has failed.

If this logic is valid, then we should fly around with only one ign. system active.
That might extend the life of the second one and then you would know if/when the first one failed.

The likelihood of both pumps failing on any given flight is quite remote. You will likely know if the engine driven pumps fails because the electric pump is unable to keep the system pressure as high. You will know that the electric pump has failed when you go to do an engine start on the next flight.

BTW, I think you have an S-LSA?
If so it is not legal for you to modify the aircraft systems (add a switch to turn the aux pump on and off) without manufacturer approval, or operate the systems in a manner contrary to the POH.
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  #7  
Old 08-15-2016, 09:59 PM
DHeal DHeal is offline
 
Join Date: Sep 2006
Location: Windsor, California
Posts: 693
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If you remove the Fuel Pump fuse to disable the electric fuel pump, you also disable the two electric fans that provide cooling to the avionics compartment -- the fuel pump and fans are on the same circuit.

The "full-time" electric fuel pump serves several purposes:

1) Being ON full-time, the pilot is unlikely to forget to use it.
2) The immediate availability of a full-time electric pump may prove timely in a sudden mechanical pump failure at low altitude.
3) The full-time electric pump assists in reducing and clearing fuel vapor formation.

You can verify how each pump is functioning (on the ground and in the air) by momentarily disabling the electric pump and noting the resultant fuel pressure indications. Also, be sure to listen for the clicking of the electric fuel pump as part of your "pre-start" and "shut-down" checklists.

Many ROTAX operators have reported in this forum low fuel pressure indications similar to yours on take-off and climb-out.

You have an SLSA I recall. You will need factory authorization to install any kind of switch on your electric fuel pump. I suspect that such an approval will not be forthcoming.
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Last edited by DHeal : 08-15-2016 at 10:08 PM.
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  #8  
Old 08-16-2016, 03:55 AM
Chrysopelea Chrysopelea is offline
 
Join Date: Jan 2016
Location: santa rosa CA
Posts: 90
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Thanks for the advice. I will rethink this act.

Having the fuel pressure drop on takeoff and climbout is unnerving I must say.

I have checked pressures with and without the electric pump running. 5.6 without and 5.8 with both pumps running, so I lose .2lbs of pressure running on just the mechanical pump.

More than one person has suggested and told me they remove the fuse and use their electric pump only during takeoffs and landings, but after what I've read here I think I'll leave it in.

Also, I was talking about removing the fuse, not modifying the plane and adding a switch, but after hearing that doing so disables other fans and such, I think I'll just fly it with both pumps on all the time and enjoy!

Thanks!
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  #9  
Old 08-16-2016, 07:57 AM
PilotBrent PilotBrent is offline
 
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Location: Hackettstown, NJ
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The underlying issue of occasional low fuel pressure readings on takeoff/climb out and sometimes on decent as well is something that many of us continue to scratch our heads at. Anecdotally, I've spoken to several RV-12 owners who admit to seeing this.

On climb out, my pressure readings fairly often drop into the yellow range for a minute or two and then recover. Engine performance never seems to be an issue and because of that I may be more complacent about these readings than I should be. Rarely today the Skyview audio warning calls out which really gets your passenger's attention. Occasionally on a long decent, particularly from a higher altitude (ie >6,500ft) similar low pressure readings in the yellow range will occur. It is unfortunately inconsistent between flights and I’ve not identified specific differences between the events to better figure this out. The throttle is reduced for these decents, but of course its full on climb out.

Last year there were times I would more often see erratic pressure readings all the way down to the "red range" and the audio Skyview warning would call out. I went ahead and replaced the pressure sensor and while it may have improved I would still see this problem from time to time.

Despite the fire sleeve insulation, at one point I was suspicious that the fuel line that comes off the bango bolt over to the pressure sensor on the firewall was too close to the radiator hose (I had them zip tied together) possibly leading to a warmer temperature in the fuel line affecting the measurement. Long after engine shutdown those hoses are still warm. Yes the line is well insulated, but I was searching for ideas. After discussing this with an A&P friend, he suggested adding a small a standoff and even some aluminum reflector tape to the line to eliminate this as a possible cause. Since then, while I still see these low readings from time to time, they “may” be less common and I rarely drop into the red range any longer. I don't believe this is necessarily resolved, but I'm not seeing the very low readings that I once did.



Again, engine appears to operate flawlessly during these low readings, but I would really like to understand the root cause.
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  #10  
Old 08-16-2016, 08:11 AM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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I have been reading these RV-12 forums since the beginning. Based on the experiences of other that have posted here, the most common reason for low fuel pressure is (in order of likelihood) :
1. Faulty fuel pressure sensor or its electrical connections.
2. Failing engine driven fuel pump. (most noticeable during takeoff)
3. Vapor lock.
I can not recall anyone posting about a failed electric fuel pump, although a few have had the electric pump get stuck for an unknown reason when first turned on. They fixed it with a good rap or tap.
Although I have installed a fuel pump switch in my E-LSA, I never turn it off during flight. My pre-startup checklist includes making sure that every electrical switch is turned on. It is easy to ensure that both fuel pumps are working. Turn on the master switch and observe the fuel pressure. Start the engine and look for an increase in fuel pressure. It will not be much.
A fuse should never be replaced in flight or when the master switch is on. Doing so will distract the pilot from flying the plane and looking for traffic. And there is a good possibility that the fuse will blow from accidentally touching the instrument panel.
The best advice that I can give is to listen to Scott's advice.
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