Low fuel warning

[whiskytango]

On a flight yesterday something weird happened, and I am hoping that the gurus on MS can give me some guidance.  I was in IMC about 3 hours into a 3.7 hour flight when the right tank Low Fuel light flashed on briefly.  This didn't make sense, since my fuel computer said I had 2.8 hours total of fuel remaining, and I switch tanks every 30 minutes.  The fuel gages on the panel also showed that both tanks had at least 18 gallons each, and the needles on the fuel gages had been moving as would be expected.  As I got closer to my destination, the left tank Low Fuel light also came on briefly (gulp!).  I weighed declaring an emergency but all of the engine parameters were normal (MAP, RPM, fuel flow GPH) and it seemed like an instrumentation problem rather than a low fuel problem.  After landing I checked to see if there was any evidence of a major fuel leak, and didn't see anything.  I have the Monroy aux tanks so my total useable fuel is 104 gallons.  I had topped off all 4 tanks before departing, and had visually confirmed that they were all filled.  Based on my experience with this plane, including the recently overhauled engine, I file for 6 hours fuel capacity.  I know the only way to determine if there was a real low fuel condition is to refill the tanks to see how much they take.  I am holding off on doing that to see if anyone on MS knows of fuel system hardware inspections or tests that would require the tanks to be de-fueled.   

[Hank]

You can always stick the tanks to.see how much is in there. Then you'll know if the alerts were real or not.

[ArtVandelay]

Your sender may need to be overhauled. What happens is as the float arm moves, the resistance is fluctuating. To test this theory, pull the sender, using an ohmmeter watch the readings as you slowly move the arm, they should be consistent. No jumps.



Tom

[carusoam]

Keep in mind the annunciator is responsible for deciding when to light the light...

The fuel level float gauge is responsible for sending the signal continuously from the tanks...

A dirty connection or dirty gauges may be causing some intermittent issues...

PP thoughts only, not a mechanic...

Best regards,

-a-

[whiskytango]

On 10/31/2019 at 10:12 AM, Hank said:

You can always stick the tanks to.see how much is in there. Then you'll know if the alerts were real or not.

Update: I did stick the main tanks and found that there was about 3 inches of fuel in each, so there never was a low fuel condition.  I suspect that there is a problem with the senders, as Tom and Anthony suggested.  The logs show that neither has been overhauled in the last 37 years .

[carusoam]

Ken,

Look into going Ceis senders before spending money on the old system...

If you have a JPI900 already, the Cies is a low(ish) cost add-on, with technical improvements...

PP thoughts only, not a mechanic...

Best regards,

-a-

[whiskytango]

3 minutes ago, carusoam said:

Ken,

Look into going Ceis senders before spending money on the old system...

If you have a JPI900 already, the Cies is a low(ish) cost add-on, with technical improvements...

PP thoughts only, not a mechanic...

Best regards,

-a-

I will do that. Thanks!

[kortopates]

On 10/31/2019 at 7:00 AM, whiskytango said:

The fuel gages on the panel also showed that both tanks had at least 18 gallons each, and the needles on the fuel gages had been moving as would be expected.  

If that statement is accurate, the issue can't possibly be with your fuel senders. You should know if your fuel senders are working properly by the above observations and if your fuel gauges match what you find sticking the tanks. As Anthony said above, its your annunciator that determines when to light up the low fuel warning based on input from the fuel gauge you can read off your fuel gauge. So if you fuel gauge is not low, the annunciator is the problem. There is a procedure to calibrate your annunciators low fuel warning threshold in the maintenance manual, but what you describe sounds more like an intermittent electrical problem:

 

On 10/31/2019 at 7:00 AM, whiskytango said:

 As I got closer to my destination, the left tank Low Fuel light also came on briefly (gulp!).

You don't say, but assuming the fuel gauge did NOT show near empty briefly at the same time, this is really pointing at an intermittent electrical issue that could be in the annunciator or wiring to it. I would begin by reseating the plug in the back of the annunciator. Check the security of the plug connection while re-seating it. I flew with a student once that had to bang the glareshield with his hand every 15 minutes to get an annunciator light to go off because the plug in the back of the annunciator had become too loose over the years. 

Edited November 1, 2019 by kortopates

[whiskytango]

5 minutes ago, kortopates said:

If that statement is accurate, the issue can't possibly be with your fuel senders. You should know if your fuel senders are working properly by the above observations and if your fuel gauges match what you find sticking the tanks. As Anthony said above, its your annunciator that determines when to light up the low fuel warning based on input from the fuel gauge you can read off your fuel gauge. So if you fuel gauge is not low, the annunciator is the problem. There is a procedure to calibrate your annunciators low fuel warning threshold in the maintenance manual, but what you describe sounds more like an intermittent electrical problem:

 

You don't say, but assuming the fuel gauge did NOT show near empty briefly at the same time, this is really pointing at an intermittent electrical issue that could be in the annunciator or wiring to it. I would begin by reseating the plug in the back of the annunciator. Check the security of the plug connection while re-seating it. I flew with a student once that had to bang the glareshield with his hand every 15 minutes to get an annunciator light to go off because the plug in the back of the annunciator had become too loose over the years. 

Thanks Paul.  I will try reseating the plug into the annunciator and check the response.  

The reason I suspect the senders is that I am assuming they are potentiometers connected to a float linkage.  My experience with potentiometers over the years has been that over time they will randomly develop very small regions of high resistance due to dirt accumulation and wear.  This can cause intermittent false resistance readings when the wiper passes over the high resistance region.  My thought was that at varying pitch and roll attitudes the senders were intermittently sending false resistance to the annunciator circuit, but the fuel gage needle was too slow to respond to these momentary conditions.

[ArtVandelay]

You don't say, but assuming the fuel gauge did NOT show near empty briefly at the same time, this is really pointing at an intermittent electrical issue that could be in the annunciator or wiring to it. I would begin by reseating the plug in the back of the annunciator. Check the security of the plug connection while re-seating it. I flew with a student once that had to bang the glareshield with his hand every 15 minutes to get an annunciator light to go off because the plug in the back of the annunciator had become too loose over the years. 

The problem is the analog fuel gauges are immune to transient changes, without knowing how the annunciator works, it might not be.
I had similar problem, it turned out to be the sender.

Cies I’m sure are nice, but if you have dual senders, that’s 2+ AMUs with labor, not exactly inexpensive and will certainly get you kicked out of the CB club.


Tom

[kortopates]

22 minutes ago, whiskytango said:

Thanks Paul.  I will try reseating the plug into the annunciator and check the response.  

The reason I suspect the senders is that I am assuming they are potentiometers connected to a float linkage.  My experience with potentiometers over the years has been that over time they will randomly develop very small regions of high resistance due to dirt accumulation and wear.  This can cause intermittent false resistance readings when the wiper passes over the high resistance region.  My thought was that at varying pitch and roll attitudes the senders were intermittently sending false resistance to the annunciator circuit, but the fuel gage needle was too slow to respond to these momentary conditions.

Very possible, yet I would expect to see a fluctuations in the fuel gauge, yet I haven't seen this specific problem and can't be sure. But the annunciator is still well supported by the OEM who is International Avionics in Texas, you can reach the guy that repairs them (who's name escapes me) at 972-417-2820. I'd bet he can tell you specifically if he thinks this can be caused by a sender issue or not or likely be able to help you isolate it.

Incidentally, the CIES senders are about $400 each, (I think I paid $395 but not sure about current pricing), but compare that to the cost for http://www.airpartsoflockhaven.com to overhaul your fuel senders - its not a whole lot cheaper.

[DonMuncy]

Could one change to CIES  senders "one at a time", as the originals malfunction, or must they be all or none?

[whiskytango]

17 minutes ago, kortopates said:

Very possible, yet I would expect to see a fluctuations in the fuel gauge, yet I haven't seen this specific problem and can't be sure. But the annunciator is still well supported by the OEM who is International Avionics in Texas, you can reach the guy that repairs them (who's name escapes me) at 972-417-2820. I'd bet he can tell you specifically if he thinks this can be caused by a sender issue or not or likely be able to help you isolate it.

Incidentally, the CIES senders are about $400 each, (I think I paid $395 but not sure about current pricing), but compare that to the cost for http://www.airpartsoflockhaven.com to overhaul your fuel senders - its not a whole lot cheaper.

Thanks Paul.  It is good to hear that the OEM is still in business and providing support!

[kortopates]

13 minutes ago, DonMuncy said:

Could one change to CIES  senders "one at a time", as the originals malfunction, or must they be all or none?

No, although CIES can operate in a resistive analog mode they still won't be compatible with the Mooney fuel gauge. Their FAQ page explains what they support (https://ciescorp.net/documentation/frequently-asked-questions/) but they also make mention of some future mods to interface with a OEM gauge such as through a stepper motor - doesn't sound like a CB solution though but its on the same web page.

[j3gq]

On 10/31/2019 at 8:43 PM, carusoam said:

PP thoughts only, not a mechanic...

Could you change signature line to "PP observations only, not a mechanic ..." ?

[j3gq]

@whiskytango

take a good look at this :
http://www.knr-inc.com/shoptalk-articles/25-shoptalk/85-201702
some interesting details about fuel gauges in the second half of the doc

[carusoam]

6 hours ago, j3gq said:

Could you change signature line to "PP observations only, not a mechanic ..." ?

Some of my writing doesn’t even qualify for personal observations.  
 

I do my best 100% of the time... full effort.

Best regards,

 

-a-

[j3gq]

I can see this in every post !!

:=)