Jump to content

M20F fuel quantity indication adjustment


Recommended Posts

I have the factory original fuel quantity indication system. Is there any adjustments that can be made to dial in the indicator to better match what is really in the tank? When my tanks are full (32 gallons a side) the indicators don't quite make it to the full mark. I know, it is just a nit but I would like for the full indication to more closely match actual. I also realize the best way to know fuel on board is with careful calculation, fuel used, fuel added.

Link to comment
Share on other sites

I've heard (according to the FAA) that the only time the gauge HAS to be accurate is when it's empty, so I would assume there is a way to adjust it, more than likely by bending the rod the float is mounted to?

Link to comment
Share on other sites

Indicating empty would be more important than full! The left tank is full to the top now. I'll check the float to make sure it is at the full limit as well. thanks for the idea. I am not inclined to bend the rod at this point, my luck it will probably break!

Link to comment
Share on other sites

I use a dipstick before takeoff. I've never really trusted the fuel gauges on a GA aircraft but have always used dipsticks, fuel added to refill, and a really big cushion. This thread makes me think it might be a good idea to see how close the fuel gauge comes to predicting when I'll run dry. It's been a while since I had that "suddenly a glider at 5000ft" feeling. Better than two cups of coffee.

  • Like 1
Link to comment
Share on other sites

If you take the case off the indicator you will find a pair of wipers on a wire wound resistor. Adjust at your own risk.

I would suggest taking off the side panels, removing the wire from the sensor and measuring the resistance with a meter at full and empty. Then get two precision multi turn pots and adjust one to the full value and the other to the empty value. Then connect the pots to the sense wire and adjust the wipers until the gauge reads both empty on the empty pot and full on the full pot. Moving both wipers in unison will change the offset, moving them closer or further from each other will change the scale.

If you don't understand the above, then leave them alone.

It will be a lot of work. Remember that there are two float sensors in series. The one you can see through the gas cap is only half of them. The other is down by the wing root and not accessible from the fuel cap.

If you do choose to adjust the gauges, make a pencil mark in the circuit boards of the original wiper positions before you adjust them.

Edited by N201MKTurbo
  • Like 1
Link to comment
Share on other sites

2 hours ago, RLCarter said:

I've heard (according to the FAA) that the only time the gauge HAS to be accurate is when it's empty.

Completely incorrect, please stop propagating this old wives' tale.  The certification standards require fuel gauges to be accurate throughout the complete range of the indicator.  The myth comes from a misunderstanding of the sub-part dealing with usable vs. unusable fuel.  I don't mean to be a jerk about it, but propagation of this OWT is a substantial reason fuel gauges get signed off when they're actually unairworthy.  This just further propagates the myth they're less reliable or less important than other critical indicators.  I'm sure this drives the professionals absolutely nuts.

The OEM fuel gauges in your Mooney and other GA aircraft are designed to be reasonably accurate throughout their range.  They need to be, because gauges are the only mechanism that will detect a developing, in-flight fuel system leak.  Totalizers and timers are of course excellent safety guards against fuel starvation, but they're meant to supplement the gauges, not replace them.  If your gauges are inaccurate, your aircraft isn't in compliance with its type certificate, and it needs maintenance.

Kudos to the OP for wanting his fuel gauges to be accurate, and to businesses like CIES for providing technology that improves accuracy and reliability.

  • Like 3
Link to comment
Share on other sites

Are you talking about the panel gauges or do you have the dials on the tank? Supposedly getting the electronic sending units refurbished won't remain accurate all that long. I have the wing-mounted gauges and they are spot-on down to about 10 gallons where they read zero in addition to an EDM-900 with the fuel totalizer that seems to be accurate within < 0.5 gallons when I take on more than 50 gallons; I attribute the variance to fuel venting and how much the person pumping the fuel milks it. 

Link to comment
Share on other sites

2 hours ago, Vance Harral said:

Completely incorrect, please stop propagating this old wives' tale.  The certification standards require fuel gauges to be accurate throughout the complete range of the indicator.  The myth comes from a misunderstanding of the sub-part dealing with usable vs. unusable fuel.  I don't mean to be a jerk about it, but propagation of this OWT is a substantial reason fuel gauges get signed off when they're actually unairworthy.  This just further propagates the myth they're less reliable or less important than other critical indicators.  I'm sure this drives the professionals absolutely nuts.

The OEM fuel gauges in your Mooney and other GA aircraft are designed to be reasonably accurate throughout their range.  They need to be, because gauges are the only mechanism that will detect a developing, in-flight fuel system leak.  Totalizers and timers are of course excellent safety guards against fuel starvation, but they're meant to supplement the gauges, not replace them.  If your gauges are inaccurate, your aircraft isn't in compliance with its type certificate, and it needs maintenance.

Kudos to the OP for wanting his fuel gauges to be accurate, and to businesses like CIES for providing technology that improves accuracy and reliability.

Perhaps the misunderstanding, or difference in interpretation may be the wording of 23.1337.  It doesn't specify what accuracy is required throughout the range.  However, 23.1337 does specifically say that the indicators MUST read zero when the usable fuel is zero. The fact that only empty is mentioned specifically could lead to any number of interpretations.  I'm not endorsing flying with non-linear fuel gauges. Are there any other regs that clarify?

Link to comment
Share on other sites

6 minutes ago, Cyril Gibb said:

Perhaps the misunderstanding, or difference in interpretation may be the wording of 23.1337.  It doesn't specify what accuracy is required throughout the range.  However, 23.1337 does specifically say that the indicators MUST read zero when the usable fuel is zero. The fact that only empty is mentioned specifically could lead to any number of interpretations.  I'm not endorsing flying with non-linear fuel gauges. Are there any other regs that clarify?

No other regulation is needed for clarification.  FAR 23.1337 doesn't specify an accuracy constraint, but neither does any other powerplant gauge regulation in Subpart F.  In fact, 23.1337(b)(1) doesn't specify an accuracy constraint for "zero", for that matter.  What 23.1337 does say is, "An indicator calibrated in appropriate units and clearly marked to indicate those units must be used."  No reasonable interpretation of "calibrated" would allow for wild inaccuracies in fuel gauges across their full range.  If you actually read all of 23.1337 in context, it's clear sub-part (b)(1) is written to address the difference between zero absolute fuel vs. zero usable fuel.  My guess is someone made a clever joke about 23.1337(b)(1) a long time ago, and it somehow "went viral" long before the modern interpretation of that phrase.

For anyone tempted to devolve into pedantic parsing of the absolute letter of 23.1337, just look at the big picture.  FAR 23 is filled with regulatory requirements about instruments.  Most use essentially the same type of "calibration" language, and lack any numerically-defined accuracy or precision constraints.  But we don't make silly arguments that oil pressure or CHT or tachometer gauges have absurdly lax accuracy standards.  Grossly inaccurate CHT/tach/etc. gauges certainly wouldn't pass muster with a DER or other certification authority, despite the lack of specific accuracy/precision constraints.  Neither would fuel gauges.

  • Like 1
Link to comment
Share on other sites

You mean they worked and were accurate from the factory in 1965?

What was the standard in 1965?

I wouldnt ever trust any GA fuel guage. 

It would be amazing if mine were accurate all the way across the range. I'll buy off on half, below half, or above half. But that's about it. 

-Matt. 

Link to comment
Share on other sites

3 hours ago, Cyril Gibb said:

However, 23.1337 does specifically say that the indicators MUST read zero when the usable fuel is zero. The fact that only empty is mentioned specifically could lead to any number of interpretations.  I'm not endorsing flying with non-linear fuel gauges. Are there any other regs that clarify?

What could be pointed out is that  Part 25 Transport Aircraft regulation for fuel indication is written identically   25.1337 = 23.1337

Nobody believes it is a good idea that a Boeing 737 should only be accurate @ zero fuel. 

The interpretation by the ACO's (Plural) for all aircraft is that the gauge is to read from empty to full - where empty is the demonstrated zero usable fuel value.  Full fuel is also controlled by regulation (expansion space etc. ) and is also an accurate number posted on all modern Type Certificate Data Sheets, POH's and wing and cockpit placards.  

The actual working interpretation is found in Advisory Circular 23.17C which references a 3% max tolerance band  - and measurement form full to empty - the advisory specifically warns against using fuel totalizers as "DE-FACTO" fuel gauges.  This guidance is for certified aircraft programs - so no thrown vegetables.  

This is consistent with Transport Aircraft, which universally relies on TSO'd components 

The TSO C55a lists the maximum tolerance to be 3% of total volume - and if you are really good you can be rated for 0.75% of total volume 

Ask the guy who certifies them 

--  One interesting note -  when you just STC  fuel system indication, the FAA is very picky, they would not certify a combination of our sensor and an analog Stewart Warner gauge as the assortment of analog gauges, used, rebuilt,  NOS i used to present , not one satisfied the FAA for accuracy.   Hysteresis could be as high as a 1/4 tank - which is consistent with Matts pirep above. 

  • Like 1
Link to comment
Share on other sites

2 hours ago, MB65E said:

You mean they worked and were accurate from the factory in 1965?

Yes, that's my understanding.  I suppose it's possible Mooney and the FAA pencil-whipped the certification process, I can't prove they didn't.  But it's unlikely.

2 hours ago, MB65E said:

What was the standard in 1965?

CAR 3, specifically CAR 3.672 which isn't much different from FAR 23.1337.

2 hours ago, MB65E said:

I wouldnt ever trust any GA fuel guage. 

My point isn't a question of "trust", it's just about what the certification standards require.  That said, ignoring fuel gauges because you think they're so inaccurate as to be worthless removes a valuable tool from your fuel management strategy.  It's akin to saying you don't trust your attitude indicator, but it's acceptable because you have needle, ball, and airspeed.  Or that you don't trust your nav radio but it's acceptable because you have an iPad.  There are certainly pilots that do such things and I'm not going to be holier-than-thou about it.  But such aircraft aren't in compliance with their basis for certification, and are less safe than aircraft with all redundant systems functioning as designed.

2 hours ago, MB65E said:

It would be amazing if mine were accurate all the way across the range. I'll buy off on half, below half, or above half. But that's about it.

If your fuel gauges are that far off, the gauges and/or the senders need servicing.  They certainly were not designed to such lax standards.  You seem to have the impression it's not possible to maintain the fuel gauges to reasonably accurate standards because they were never designed to be accurate in the first place.  That's just not the case.  Sure, the wiper-arm potentiometers on the sender wear out and need to be serviced or replaced, the springs on the gauges themselves may wear over time, etc.  But those are long-term maintenance requirements, not design limitations.  Servicing your fuel indicating system is no different than replacing shock disks, resealing fuel tanks, replacing trim jack screws, or any other PITA issue we'd all like to avoid, but which are in fact the standard of care for airworthiness.

  • Like 1
Link to comment
Share on other sites

§ 3.672 Fuel quantity indicator. Means shall be provided to indicate to the flight personnel the quantity of fuel in each tank during flight. Tanks, the outlets and air spaces of which are interconnected, may be considered as one tank and need not be provided with separate indicators. Exposed sight gauges shall be so installed and guarded as to preclude the possibility of breakage or damage. Fuel quantity indicators shall be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply as defined by § 3.437. 

Link to comment
Share on other sites

Makes sense Vance. I agree. 

I replaced my guages and sending units a few years ago. They improved the system substantially. Mine just never indicate full, however are accurate the last 10gal on each tank. I'm satisfied with them. The only way to have them perfect is CAP type probes. 

-Matt

Link to comment
Share on other sites

5 hours ago, Vance Harral said:

 In fact, 23.1337(b)(1) doesn't specify an accuracy constraint for "zero", for that matter. 

Yes it does. I can read. This is the text taken from the FAA 23.1337.

(b)(1) Each fuel quantity indicator must be calibrated to read ‘‘zero’’ during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under 23.959(a)

You are saying that the reg doesn't specify an accuracy constraint for "zero" ?  Seems clear to me.

Link to comment
Share on other sites

16 minutes ago, Cyril Gibb said:

You are saying that the reg doesn't specify an accuracy constraint for "zero" ?  Seems clear to me.

With respect - and I mean that honestly - I don't think you understand the meaning of the word "accuracy" in the context of measuring devices.  The part of the regulation you quoted says "zero".  It doesn't say "zero plus or minus one gallon", or "zero plus or minus one percent of the total fuel quantity", or anything else of the sort.  Those would be accuracy constraints.  The point of subpart (b)(1) is clearly to define what zero means, not to specify the accuracy of the measuring device which indicates it.  You're grasping for a definition which fits the funny joke about the fuel gauges only having to be accurate at zero, but that's neither the letter nor the intent of the regulation.

But don't take my word for it.  The FAA's guidance to people who certify airplanes to part 23 regulations is laid out in AC 23-17C.  Here's a particularly telling paragraph from that publication, from page 269:

Fuel quantity indicators are also governed by § 23.1301, as are all 14 CFR, part 23 Subpart F appliances.  This regulation requires the installed indicators function as designed and not create a hazard in their operation. This precludes indicators that read higher than the actual fuel level since this would constitute a hazard.

The AC also makes reference to any changes in fuel quantity indicators needing to meet the standards of TSO-C55a, as fuellevel mentioned above.  That TSO details the required accuracy for various classes of fuel and oil quantity indicators.  It doesn't contain the word "zero" at all.  The accuracy requirements it specifies apply across the full scale of the indicator.

I grant that those publications didn't exist in 1965.  But you have to really grasp at straws to claim the FAA's position even in the CAR 3 days allowed fuel gauges to be wildly in error at any level above zero usable fuel.  That was never the intent or implementation, and aircraft with grossly inaccurate fuel gauges have never been legally airworthy.

Link to comment
Share on other sites

I learned these fuel level details as a PP.  it came with a lot of interpretation.  The focus was on the minimum knowledge needed to pass the test.  I don't get the feeling that the average CFI (of that day) was as knowledgeable as the average Mooney owner.

Then I worked in an industry that took their instruments and calibration schedules very seriously. FDA/Pharma industry.  FDA and FAA must share the same wordsmiths in the legal departments....

The same words make the regulations.  It is how we use the words that makes the difference.

If the official maintenance manual supplies a procedure...  that is where we go to get the procedure to calibrate the instrument system.

Feel free to toss out the OWT and its interpretations...

The newest digital float gauges are more accurate from zero to full up.  Combined with a digital display they deliver better information throughout the entire range.  If the tank has nonstandard shape or volume, the float data can be adjusted to match.

A good FF/totalizer can be verified in flight to match the fuel used in the tanks.  If fuel used on both systems doesn't match, either a leak has developed or a manual reset of the totalizer has not occurred....

It is important to follow the rules.

But, more important to have a fuel level system that is giving the pilot reliable info.

I am still using the four independent analog systems that came with my plane, and the digital FF/totalizer.

Looking forward to going all digital one day...

Best regards,

-a-

Edited by carusoam
Link to comment
Share on other sites

Geez louise Vance, I just wanted clarification of the regulatory requirements.  You've come down pretty hard on anyone suggesting that the fuel gauge regs were vague or open to interpretation.

I never mind being proven wrong when I can learn something. You clearly have more knowledge of the regulations than I do.

So the question is:  What are the regulatory requirements for fuel gauge accuracy throughout it's range in our Mooneys to remain within the type certificate, hence airworthy?  +/- 3% ? +/- 1 USgal ?

Link to comment
Share on other sites

FWIW, my factory fuel gauges work pretty good.

One thing to consider when working on fuel tanks and sensors is to make sure the arms hit top and bottom of their swing at the right place. The senders are calibrated to go from stop to stop. If the float hits the stop before hitting the ceiling or floor of the tank or if it hits the floor or ceiling before hitting the stop your gauges will not read to their limits. They need to hit the metal at the same time they hit the stop. Also make sure the floats spin freely on the wire. They are oblong and if they get stuck they will not go up or down as far as they should.

  • Like 1
Link to comment
Share on other sites

The Treasure of Sierra Madre (1927) clearly predates the FARs:

"Gauges, to god-damned hell with gauges! We have no gauges. In fact, we don't need gauges. I don't have to show you any stinking gauges, you god-damned cabrón and chinga tu madre!"

  • Like 1
Link to comment
Share on other sites

22 minutes ago, jkhirsch said:

The Treasure of Sierra Madre (1927) clearly predates the FARs:

"Gauges, to god-damned hell with gauges! We have no gauges. In fact, we don't need gauges. I don't have to show you any stinking gauges, you god-damned cabrón and chinga tu madre!"

1948 ok the book was in 27

Edited by N201MKTurbo
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.