Low fuel warning system / airworthiness issue?

[floridaflyer]

I'm being told the low fuel warning system on my M20K Rocket is not working correctly, it does not flash on the annunciator panel as it should with low fuel. The fuel gauges and FF meter are working fine and I have never flown with less than 2+ hrs reserve so was not aware of the issue. Is this issue sufficient to fail an annual inspection? I am not familiar with MMEL, limitations etc so any insights would be very helpful.

[ArtVandelay]

Did you check the light bulb(s)?

Both sides not working?

[floridaflyer]

The PC board is bad. To clarify, I'm not looking for troubleshooting tips, I'm trying to determine if inop low fuel warning is an A/W issue.

Edited April 21, 2020 by floridaflyer

[Vno]

Start with your POH and look in the limitations section. Kinds of Operation Equipment list. See if that part is on the list. It is not on the J. Now you have a Rocket so there might be an updated list with the Rocket STC. If it is on the list then you have to have it working to be airworthy. If it isn't on the list then you are in the realm of you might be able to fly with it inop. That would depend on your A&P and what responsibility he would be willing to take. If something is  installed in the airplane, then it has to be operable per FAR 91.213. It is easier if you look up the reg but here are some highlights which direct how this works. I have had mixed success with having items deferred (made inop) or having to fix them before airworthy.

(d) Except for operations conducted in accordance with paragraph (a) or (c) of this section, a person may takeoff an aircraft in operations conducted under this part with inoperative instruments and equipment without an approved Minimum Equipment List provided—

(1) The flight operation is conducted in a—

(i) Rotorcraft,non-turbine-powered airplane , glider, lighter-than-air aircraft, powered parachute, or weight-shift-control aircraft, for which a master minimum equipment list has not been developed; or

(2) The inoperative instruments and equipment are not—

(i) Part of the VFR-day type certification instruments and equipment prescribed in the applicable airworthiness regulations under which the aircraft was type certificated;

(ii) Indicated as required on the aircraft's equipment list, or on the Kinds of Operations Equipment List for the kind of flight operation being conducted;

(iii) Required by §91.205 or any other rule of this part for the specific kind of flight operation being conducted; or

(iv) Required to be operational by an airworthiness directive; and

(3) The inoperative instruments and equipment are—

(i) Removed from the aircraft, the cockpit control placarded, and the maintenance recorded in accordance with §43.9 of this chapter; or

(ii) Deactivated and placarded “Inoperative.” If deactivation of the inoperative instrument or equipment involves maintenance, it must be accomplished and recorded in accordance with part 43 of this chapter; and

(4) A determination is made by a pilot, who is certificated and appropriately rated under part 61 of this chapter, or by a person, who is certificated and appropriately rated to perform maintenance on the aircraft, that the inoperative instrument or equipment does not constitute a hazard to the aircraft.

An aircraft with inoperative instruments or equipment as provided in paragraph (d) of this section is considered to be in a properly altered condition acceptable to the Administrator.

(e) Notwithstanding any other provision of this section, an aircraft with inoperable instruments or equipment may be operated under a special flight permit issued in accordance with §§21.197 and 21.199 of this chapter.

Brian

[floridaflyer]

Thanks Vno this is very helpful.

[carusoam]

Briefly...

Each POH has four lists...

Required equipment, VFR, IFR, Day, Night...

These are the tests for AW...

It is highly unlikely that the lights on the annunciator are an AW issue...

But, that is the place to look...  minimum equipment lists... found in the POH.

By the way... to make the low fuel level lights work... you have to press the test button to see all the lights come on the annunciator panel... this actually initiates the low fuel warning system... if you don’t test the system... it won’t work...

Call that a goofy system that never evolved properly... 

Expect to find these details in the POH as well...

PP thoughts only, not a mechanic...

Best regards,

-a-

[floridaflyer]

Thank you carusoam.

[ArtVandelay]

The annunciator should be listed as required equipment, because it indicates landing gear position, I believe that’s the L in “Tomato Flames”.
But since that functionality is working, I think you’re legal.

[floridaflyer]

Thanks Tom.

[1980Mooney]

I am puzzled by this comment:

On 4/21/2020 at 10:25 PM, carusoam said:

By the way... to make the low fuel level lights work... you have to press the test button to see all the lights come on the annunciator panel... this actually initiates the low fuel warning system... if you don’t test the system... it won’t work...

Call that a goofy system that never evolved properly... 

I am curious why you think the annunciator panel and specifically the low fuel warning system will not work unless you press the test button (at some unspecified time).  The annunciator panel functions when power is on.  As a safety feature the test button sends power to the lights in order to verify that they will illuminate when they receive a power signal from the appropriate annunciator circuit.  The safe practice as noted in the POH is to test the annunciator panel before start, however there is no mention in J or K POH's that the Low Fuel level annunciator might be compromised if you did not previously press the test button.  Based upon personal experience over 20 years I have experienced the Hi/Lo Vac, Hi/Lo Volt, Gear Unsafe, Gear Down and yes the Low Fuel annunciator lights on.  I have not been vigilant about pressing the test light button before each flight yet the annunciator warning lights have always functioned properly.  J and K Annunciator panels were the same with exception of Ram air light in the place of  Alt air light.  Annunciator panels changed between 1979 and 1980 model years but basically had the same functionality.  Not sure what year model floridaflyer is inquiring about.  

Edited April 24, 2020 by 1980Mooney

[carusoam]

5 hours ago, 1980Mooney said:

I am puzzled by this comment:

I am curious why you think the annunciator panel and specifically the low fuel warning system will not work unless you press the test button (at some unspecified time).  The annunciator panel functions when power is on.  As a safety feature the test button sends power to the lights in order to verify that they will illuminate when they receive a power signal from the appropriate annunciator circuit.  The safe practice as noted in the POH is to test the annunciator panel before start, however there is no mention in J or K POH's that the Low Fuel level annunciator might be compromised if you did not previously press the test button.  Based upon personal experience over 20 years I have experienced the Hi/Lo Vac, Hi/Lo Volt, Gear Unsafe, Gear Down and yes the Low Fuel annunciator lights on.  I have not been vigilant about pressing the test light button before each flight yet the annunciator warning lights have always functioned properly.  J and K Annunciator panels were the same with exception of Ram air light in the place of  Alt air light.  Annunciator panels changed between 1979 and 1980 model years but basically had the same functionality.  Not sure what year model floridaflyer is inquiring about.  

I have the O1 from ‘94...

And I have looked this up a few times in the POH...

Unfortunately, my memory was built in the late 1800s... I may need to look this up again...  

I have an O1 from....

Best regards,

-a-

[floridaflyer]

The Mooney in question is a 1987 model. 

[amillet]

20 hours ago, 1980Mooney said:

I am puzzled by this comment:

I am curious why you think the annunciator panel and specifically the low fuel warning system will not work unless you press the test button (at some unspecified time).  The annunciator panel functions when power is on.  As a safety feature the test button sends power to the lights in order to verify that they will illuminate when they receive a power signal from the appropriate annunciator circuit.  The safe practice as noted in the POH is to test the annunciator panel before start, however there is no mention in J or K POH's that the Low Fuel level annunciator might be compromised if you did not previously press the test button.  Based upon personal experience over 20 years I have experienced the Hi/Lo Vac, Hi/Lo Volt, Gear Unsafe, Gear Down and yes the Low Fuel annunciator lights on.  I have not been vigilant about pressing the test light button before each flight yet the annunciator warning lights have always functioned properly.  J and K Annunciator panels were the same with exception of Ram air light in the place of  Alt air light.  Annunciator panels changed between 1979 and 1980 model years but basically had the same functionality.  Not sure what year model floridaflyer is inquiring about.  

From M20J POH

[PT20J]

45 minutes ago, amillet said:

From M20J POH

I think I see the confusion. The low fuel warning light circuit has some built-in time delay so that the lights don't flicker excessively when the fuel level gets down to the set point. That's why the fuel lights don't come on immediately with the other lights when the test button is pressed, and instead require that you hold the button down for a few seconds to test the lights. This probably could have been written more clearly to make it clear that the circuit does not require arming.

Funny annunciator story: The museum DC-3 has a bunch of warning lights and a press to test function. The before starting checklist calls out testing the annunciators. There is a similar delay in the engine fire alarms. Pilots had been flying this for two years before I was checking out as copilot and noticed that no one ever held down the test button long enough to test the fire warning. I also noticed an annunciator labeled "Kamborian" that no one had ever noticed. (Why test things if you don't care about the results?) No one knew what a kamborian was so I did some research and found out that the airplane had once been a corporate transport for a company that had a CEO named Kamborian. It was a call light for when he needed coffee.

Skip

[carusoam]

Holy cow, Skip!

I think I missed their point...  

Best regards,

-a-

[1980Mooney]

On 4/25/2020 at 12:34 PM, PT20J said:

I think I see the confusion. The low fuel warning light circuit has some built-in time delay so that the lights don't flicker excessively when the fuel level gets down to the set point. That's why the fuel lights don't come on immediately with the other lights when the test button is pressed, and instead require that you hold the button down for a few seconds to test the lights. This probably could have been written more clearly to make it clear that the circuit does not require arming.

Skip

On 4/25/2020 at 12:41 PM, carusoam said:

Holy cow, Skip!

I think I missed their point...  

Best regards,

-a-

Exactly.  Look at page 17 of Chapter 24 of the Service and Maintenance Manual - M20J regarding Left and Right Low Fuel calibration.  At the end of the paragraph it notes that there is an approx. 2 second delay when pressing the Test button once the annunciator system is placed back into normal mode.

http://mooney.free.fr/Manuels%20M20J/M20J/Mooney%20Service%20Manuel%20M20J%20Vol.%201%20of%202.pdf

[carusoam]

http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 1 of 2.pdf

 

Thank you 1980!

Briefly...

1) There is an adjustable set point for when the light comes on... for the J it is 2.5 gallons... (not very much)

2) There is a hysteresis in the sensor output.... this allows the light to stay on after it is first triggered...when the fuel level is sloshing in the bumps...

3) The hysteresis is what is being tested while holding the test button down...

4) To see no hysteresis... put the annunciator in the calibration mode...

 

5) So...

What we learned today...

Many sensors have a hysteresis designed into them... so they trigger, and stay on for a while... while the level drops below a consistently low level...

If they get falsely triggered... they will reset after the hysteresis period...

 

The POH probably isn’t going to convey the engineering of the circuits very well...

 

6) If your low level light starts to flash on and off... this is a sign that your annunciator’s hysteresis circuit wants adjustment... Ceis has done some work on tuning the hysteresis they use by averaging data in some cases.... JPI probably uses programming in their displays to handle the hysteresis on their end...  

7) In other places... we learned that the vernatherm uses a mechanical hysteresis curve to open and close the valve to the oil cooler... this naturally cycles the valve a bit slower than it would otherwise...   so it works well, and doesn’t wear out as fast...

8) For the OP... @floridaflyer check out this handy dandy piece of info from the maintenance manual for your J... things to test on your annunciator  to see what is working or not... could be a broken wire or a simple setting that isn’t right...

9) Thanks a bunch to 1980 for posting the exact page... just finding the chapter and page was challenging enough...  

10) reminder... Most of us don’t need to use the low level sensor, but... the day you see the light come on... is the day you recognize how simply elevated stress can mess with your MKI rev.0 timing function...

PP thoughts only, not a mechanic...

Best regards,

-a-

[flyboy0681]

1 hour ago, carusoam said:

2) There is a hysteresis in the sensor output.... this allows the light to stay on after it is first triggered...when the fuel level is sloshing in the bumps...

3) The hysteresis is what is being tested while holding the test button down...

4) To see no hysteresis... put the annunciator in the calibration mode...

 

 

Not hard to see where the word hysteresis comes from. If that warning came on in flight I'd become hysterical.