M20F fuel quantity indication adjustment

[jkhirsch]

I like the vulgarity from the book better I have seen Blazing Saddles several times though.

Never had read the book, but now that I am a "reader" I may put it on the list.

Edited March 29, 2017 by jkhirsch

[Vance Harral]

9 hours ago, Cyril Gibb said:

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 ?

My knowledge of fuel indicator regulations doesn't come from vast professional experience, just an hour or so of actually reading the regulations and associated ACs, motivated by a "that can't be right" skepticism of the myth.  I'm not inclined to apologize for holding other pilots to that same standard, but it's not my intent to be a jerk and/or a know-it all.  I do apologize for any brusqueness, and freely admit plenty of personal ignorance on other subjects.

Your specific question is an opportunity to admit ignorance myself.  TSO-C55a specifies accuracy tolerances of 0.75%, 2%, and 3% for class 1/2/3 indicators, respectively.  I don't know what constitutes a class 1 vs. 2 vs. 3 sensor, that appears to be defined in an SAE publication I haven't read.  Maybe fuellevel can explain.

Even three percent is a pretty tight constraint.  That's less than one gallon in the 26-gallon tanks in each of the wings on my airplane.  I'd be unlikely to notice or care if my gauges read half tanks when there were really 14 gallons in the tank instead of 13, even though that's technically beyond airworthy tolerances.  But if the gauges showed 3/4 tanks when the dipstick showed 10 gallons, I'd consider it a malfunction to be fixed immediately.

 

 

[fuellevel]

[quote post="338799" timestamp="1490830729" Maybe fuellevel can explain.
Even three percent is a pretty tight constraint.  That's less than one gallon in the 26-gallon tanks in each of the wings on my airplane. 
 
 


So yes the maximum design constraint is 3%. The specification was written for capacitive where temperature, density and fuel quality impact accuracy.

The requirement for your aircraft is in the maintenance manual. Mooneys are quite lax and so is Cessna. Beech is better Pipers requirement for its aircraft is consistent with the 3% concept and surprises all Piper owners.

I can show definitive improvement in Safety having accurate fuel level





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[Vance Harral]

3 hours ago, fuellevel said:

The requirement for your aircraft is in the maintenance manual.

That's interesting.  The maintenance manual for my 1976 F model lacks detail on this, but your comment moved me to go look in the more detailed maintenance manual for the 1977 J model - which I believe uses exactly the same senders and gauges as my airplane.  That manual does have a calibration procedure for the gauges.  It says the gauge must match actual fuel level within 1/2 needle width at empty (zero usable fuel), 1/4, 1/2, 3/4, and full.

I'm not sitting in front of the airplane, but my recollection is there's roughly 1.5 inches of span between the E and F marks on the gauge, and the needle is about 1/16" (0.0625") wide.  A 1/2 needle width error would therefore be about 0.03125/1.5 = 0.021, or about 2%.

The catch is, the fill procedure allows for -1/+2 gallons of error in the 8 gallon increments used to fill the tank to the 1/2/3/4-quarter levels.  So at the quarter tank mark, for example, you could have as much as 10 gallons instead of 8, which is 10/32=0.3125 of a full tank.  The needle could be 1/2 needle width below the 1/4 mark and still meet spec.  In that case, the needle would indicate ((1.5/4)-.03125)/1.5 = 0.2292 of a full tank.  That's an error of 0.3125-0.2292=0.0833 of a tank, or about 8%.

I won't argue with the characterization of 8% as "lax" compared with the 3% spec in TSO-C55a.  But we're still talking about error in the range of a needle width or so.  That's a lot more accurate than I think most pilots would guess is required of the fuel gauges.

[carusoam]

The needle width gets used a lot when learning to fly with analog gauges.

I get the feeling the gauge designers/builders selected the width of the needle and the width of the markings, and the width of the spaces between the markings, when laying out the gauge faces.

Fortunately the FAA doesn't require this level of knowledge to fly a plane, at first...

Best regards,

-a-