Everyone has it wrong about Unusable Fuel

[SpamPilot]

Gentle Mooniacs,

In the face of an upcoming official reweighing of my M20K (after wholesale replacement of my old avionics with newer, lighter, better equipment), I have been pondering the collective wisdom on how one is supposed to determine unusable fuel, both for W&B purposes as well as flight planning.  I'm pretty sure the collective wisdom is wrong.

I read in these pages some who advocate using the fuel pump to drain the tanks, and then weigh the aircraft on that basis.  Even my A&P, who has done this hundreds of times, was going to follow that very same procedure on the basis of "I've never seen it done another way" until I explained why it was wrong, or at least incomplete.

I also read in these pages some who advocate establishing their minimum usable fuel level by running one tank "dry" in cruise flight, landing on the other tank, and then filling the "empty" tank to determine maximum usable fuel volume.  This is more for flight planning purposes than measuring aircraft weight, but it's basically the same approach and result.

The thing is, neither approach respects the intent of "unusable fuel".  It turns out "unusable fuel" is a formal term defined by the regulations that govern the design of certificated aircraft.  The regulation is 14 CFR 23.959, which reads, in relevant part:

"The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank."

My translation of this is: don't think about level flight, think about approaches and go-arounds.  I believe you are at increased risk and not in compliance with regulations if you determine unusable fuel/minimum usable fuel by the common techniques of running a tank "dry" or pumping fuel until the flow stops.  In both of these cases, you are not measuring the "most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank".  The most adverse condition of intended operation is neither level flight, nor sitting on the ground in a near-level-flight attitude.  The most adverse condition is probably a steep descent, but it could also be a steep ascent, and maybe one should consider a smallish amount of uncoordinated flight at the same time.  It would depend on the fuel tank geometry vs. fuel pickup location.  Some aircraft (not Mooneys, except maybe the Mite) use header tanks to manage fuel flow under adverse feed conditions.  Forward slips are also an adverse feed condition on the tank in the downhill wing, but there is an operating limitation warning you not to perform forward slips with low fuel in the downhill wing, so it's not an "intended" operation for the tank in question.  In any event, you should want to ensure you have enough fuel in the tanks so you do not unport the fuel pickup during any normal maneuver (including unplanned maneuvers) you might need to perform in order to ensure the continued safety of flight, which includes go-arounds and steep approaches to landing.

(certification nerds will point out that most Mooneys are largely certfied under CAR 3 and not Part 23; I did not check CAR 3 for similar language, but regardless, I think Part 23 expresses the correct requirement)

The "optional" Mooney weighing procedure, which has you pump the fuel out and then add back in the specified unusable fuel, appears to take this into account.  The standard Mooney weighing procedure, which has you weigh with full tanks and then subtract the weight of the specified amount of unusable fuel, also appears to take this into account, though it has more opportunity for inaccuracy due to the fact that a tank being "full" is a function of at least ground slope, uneven tire pressure, fuel temperature, and filling technique.

Edited November 16, 2021 by SpamPilot

[Hank]

Interesting viewpoint. I found the amount of unusable fuel for my Mooney without doing anything more difficult than downloading the TCDS and reading the right section. 

[David Lloyd]

In 1975 when my airplane was built, the original weight worksheets show the airplane being weighed with full fuel.  The useable fuel weight was then deducted. That weight was based on 52 gallons of avgas at 80 degrees, 288 pounds.  If you want to weigh with full fuel, you have to know how much the fuel weighs at various temperatures.  That method is not shown in the service manual.  

The service manual does say to remove fuel by loosening a fuel line and using the electric fuel pump to remove as much fuel as possible.  Then remove the sump drains and remove the remainder.  Airplane needs to be level to do that properly.  Last year the shop weighed mine and only about a cup of fuel came from either sump drain.  After good and empty, add in the unusable fuel.

The Bonanza I owned for many years had 40 gallon tanks, 37 usable.  Every time I ran a tank dry, between 40.1 and 40.4 gallons went back in it.  My Mooney has bladders and 27.4 gallons usable, another 1.25 gallons unusable.  When I have gotten down below 27 gallons from a tank, the fuel pressure gauge starts dropping. A little rudder to move the fuel toward the pickup will return the pressure to normal for another few tenths. The electric fuel pump will also keep the pressure up for a little more.  As long as both pumps can suck up enough fuel to fill the carb bowl, the engine won't know the pump is seeing fumes.  Unlike the Bo, the most fuel I have ever gotten in the Mooney tank was 27.6 gallons, and that involve a lot of rudder to slosh the fuel.

On running a tank low, note that my airplane is carbed.  A fuel injected engine, as soon as fuel pressure drops, quits running.

[PilotX]

Weigh it completely empty. Weigh it with unusable fuel. I lost 74 lbs due to a major avionics upgrade. However the CG shifted so far forward that I had to out 50lbs of sand in the luggage compartment for W&B. If only I had the $$s for th me four blade prop, I could fix that issue. $20K vs $5 in sand. Smoother landing with that earth in the back. 

[Pete M]

34 minutes ago, PilotX said:

Weigh it completely empty. Weigh it with unusable fuel. I lost 74 lbs due to a major avionics upgrade. However the CG shifted so far forward that I had to out 50lbs of sand in the luggage compartment for W&B. If only I had the $$s for th me four blade prop, I could fix that issue. $20K vs $5 in sand. Smoother landing with that earth in the back. 

Seems odd. You removed the heavy radios, put in light radios and your cg moved forward? Mine moved rearward. What was removed in the rear that was so heavy?

[PT20J]

When weighing the Mooney, just follow the procedure in the AFM/POH which is also in the Service and Maintenance Manual. 100LL has a range of densities depending mostly on temperature. Mooney specifies using 5.85 lbs/ gal. 

If you are also determining the CG, it is important to use the Mooney procedure because due to the landing gear design, the position of the gear axels varies.

Skip

[Andy95W]

9 hours ago, SpamPilot said:

Gentle Mooniacs,

In the face of an upcoming official reweighing of my M20K (after wholesale replacement of my old avionics with newer, lighter, better equipment), I have been pondering the collective wisdom on how one is supposed to determine unusable fuel, both for W&B purposes as well as flight planning.  I'm pretty sure the collective wisdom is wrong.

I read in these pages some who advocate using the fuel pump to drain the tanks, and then weigh the aircraft on that basis.  Even my A&P, who has done this hundreds of times, was going to follow that very same procedure on the basis of "I've never seen it done another way" until I explained why it was wrong, or at least incomplete.

I also read in these pages some who advocate establishing their minimum usable fuel level by running one tank "dry" in cruise flight, landing on the other tank, and then filling the "empty" tank to determine maximum usable fuel volume.  This is more for flight planning purposes than measuring aircraft weight, but it's basically the same approach and result.

The thing is, neither approach respects the intent of "unusable fuel".  It turns out "unusable fuel" is a formal term defined by the regulations that govern the design of certificated aircraft.  The regulation is 14 CFR 23.959, which reads, in relevant part:

"The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank."

My translation of this is: don't think about level flight, think about approaches and go-arounds.  I believe you are at increased risk and not in compliance with regulations if you determine unusable fuel/minimum usable fuel by the common techniques of running a tank "dry" or pumping fuel until the flow stops.  In both of these cases, you are not measuring the "most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank".  The most adverse condition of intended operation is neither level flight, nor sitting on the ground in a near-level-flight attitude.  The most adverse condition is probably a steep descent, but it could also be a steep ascent, and maybe one should consider a smallish amount of uncoordinated flight at the same time.  It would depend on the fuel tank geometry vs. fuel pickup location.  Some aircraft (not Mooneys, except maybe the Mite) use header tanks to manage fuel flow under adverse feed conditions.  Forward slips are also an adverse feed condition on the tank in the downhill wing, but there is an operating limitation warning you not to perform forward slips with low fuel in the downhill wing, so it's not an "intended" operation for the tank in question.  In any event, you should want to ensure you have enough fuel in the tanks so you do not unport the fuel pickup during any normal maneuver (including unplanned maneuvers) you might need to perform in order to ensure the continued safety of flight, which includes go-arounds and steep approaches to landing.

(certification nerds will point out that most Mooneys are largely certfied under CAR 3 and not Part 23; I did not check CAR 3 for similar language, but regardless, I think Part 23 expresses the correct requirement)

The "optional" Mooney weighing procedure, which has you pump the fuel out and then add back in the specified unusable fuel, appears to take this into account.  The standard Mooney weighing procedure, which has you weigh with full tanks and then subtract the weight of the specified amount of unusable fuel, also appears to take this into account, though it has more opportunity for inaccuracy due to the fact that a tank being "full" is a function of at least ground slope, uneven tire pressure, fuel temperature, and filling technique.

Please change your font.  I got halfway through and decided I wasn’t in the mood for eye strain this morning.

[PilotX]

2 hours ago, Pete M said:

Seems odd. You removed the heavy radios, put in light radios and your cg moved forward? Mine moved rearward. What was removed in the rear that was so heavy?

[cliffy]

And of course every owner/A&P will know instinctively what the unlabeled fuse holder next to the green box was for when it blew?  :-) :-) :-)

That is sure hidden. Kind of like the placement of our Amp meter fuses on top of the inner firewall.  

If one drops plumb bobs down on the center of the axles and the nose gear pivot tube one "may" find that the distances between axles and Datum vary from airplane to airplane. The last guy to reweigh the airplane may not have used the correct distances. Always pays to recheck everything.

[Aerodon]

That was a border line rant showing your lack of knowledge, nothing new to me.  The unusable fuel is set by the manufacturer during certification.  The only thing we need to do for a reweighing is to figure out how to empty the tanks completely and then put back the unusable fuel quantity.  Not difficult.

Knowing where the unusable fuel is and why it's unusable might help you use it in flight.  A Super Cub has a front and back header connected to different tanks, knowing which one to run dry first must gain a gallon.  I assume the unusable fuel is split between the tanks.  Emptying one tank in flight might give you as much as half of the unusable fuel?

And on the other side of the equation, knowing your tanks and filler necks etc. might give you an extra couple of gallons of useable fuel.

Aerodon

[Aerodon]

3 hours ago, PilotX said:

Weigh it completely empty. Weigh it with unusable fuel. I lost 74 lbs due to a major avionics upgrade. However the CG shifted so far forward that I had to out 50lbs of sand in the luggage compartment for W&B. If only I had the $$s for th me four blade prop, I could fix that issue. $20K vs $5 in sand. Smoother landing with that earth in the back. 

It's very hard to lose 74 lbs in an upgrade, care to share what you took out and put back in?

Seems like you need to do some research on 'charlie weights', you can put roughly 7,11 or 18 lbs in the tail, better than anything loose in the baggage compartment.

The 252 I am working on has no charlie weights, a fairly full rear avionics shelf, and I'm filling it with remote equipment - GTX345R, GDL69A, WX500, KN63, GAD43e, GRS77, GDC74, ELT.  Removing the standby vacuum. 

Aerodon

[EricJ]

W&B is not the precision process that we sometimes think it is, as evidenced by the regulatory allowance to rework the initial factory W&B by analysis ad infinitum with error accumulation for the life of the airplane.    You can go forever and never re-weigh it, just keep assuming the previous 50 years of modifications were correct and continue adding reworks on top of it.

Scales aren't as accurate as we sometimes think, either.    My scales weigh both airplanes and race cars, and in most racing rules if you get weighed for compliance and are found underweight, it is essentially standard procedure to roll the car off the scales, re-zero the scales and roll it back on.    Very often that eliminates a potential penalty.   Often you don't even need to re-zero the scales (sometimes we forget).   I've seen the same when weighing airplanes, it's just not that precise.

The unusable fuel quantity to use is found in the TCDS Note 1, listed with quantity and arm for each model.   Given the general imprecision inherent in the process I don't see a problem using the method of filling the tanks and subtracting the weight of everything but the unusable quantity listed in the TCDS.  

[Aerodon]

15 minutes ago, EricJ said:

The unusable fuel quantity to use is found in the TCDS Note 1, listed with quantity and arm for each model.   Given the general imprecision inherent in the process I don't see a problem using the method of filling the tanks and subtracting the weight of everything but the unusable quantity listed in the TCDS.  

Because you don't know where the 'top of tank' is, there are Mooneys that can take a few more gallons than the book says by careful 'overfilling'.  When i do my Cies gauge calibration, I plan on doing empty, unusable fuel, the 3 or four steps, 'factory full quantity' and then see how much more I can get in.

Just for kicks, I weighed a C172 in the non level position, the nose wheel was about 100 lbs heavier than the level position.  So my instructor was correct, use the elevator to keep the weight off the nose wheel wherever possible.

Aerodon

 

[cliffy]

If the reweigh is done in a shop or business "reviewed" by the FAA every year you can bet that the scales are certified as accurate every year just like their torque wrenches are every year. 

I know for a fact that my D model fuel tanks hold EXACTLY 26 gallons from dry empty. Not a tenth more will go in. 

A W&B report "should" use the unusable fuel and arm listed on the TCDS (or follow on STC). 

That all being said when I reweighed many years ago (my first on this Mooney) we found a 90 pound error on the first W&B done by the factory!!!

It was in their math!  Just because the factory did it  doesn't mean it was correct

Decades of 337s just compounded on that error

What's the old saying?   Trust but Verify? 

[Hank]

11 minutes ago, cliffy said:

What's the old saying?   Trust but Verify? 

The Quality Manager at a previous job had a sign on the office wall: 

In God we trust. All others bring data.

[PilotX]

58 minutes ago, Aerodon said:

It's very hard to lose 74 lbs in an upgrade, care to share what you took out and put back in?

Seems like you need to do some research on 'charlie weights', you can put roughly 7,11 or 18 lbs in the tail, better than anything loose in the baggage compartment.

The 252 I am working on has no charlie weights, a fairly full rear avionics shelf, and I'm filling it with remote equipment - GTX345R, GDL69A, WX500, KN63, GAD43e, GRS77, GDC74, ELT.  Removing the standby vacuum. 

Aerodon

[Shadrach]

32 minutes ago, EricJ said:

W&B is not the precision process that we sometimes think it is, as evidenced by the regulatory allowance to rework the initial factory W&B by analysis ad infinitum with error accumulation for the life of the airplane.    You can go forever and never re-weigh it, just keep assuming the previous 50 years of modifications were correct and continue adding reworks on top of it.

Scales aren't as accurate as we sometimes think, either.    My scales weigh both airplanes and race cars, and in most racing rules if you get weighed for compliance and are found underweight, it is essentially standard procedure to roll the car off the scales, re-zero the scales and roll it back on.    Very often that eliminates a potential penalty.   Often you don't even need to re-zero the scales (sometimes we forget).   I've seen the same when weighing airplanes, it's just not that precise.

The unusable fuel quantity to use is found in the TCDS Note 1, listed with quantity and arm for each model.   Given the general imprecision inherent in the process I don't see a problem using the method of filling the tanks and subtracting the weight of everything but the unusable quantity listed in the TCDS.  

It’s always been a “fuzzy number” when compared to reality.  I don’t think the manufacturers were required to weigh every aircraft coming off the line under CAR. So the estimations or “errors” as some might call them may well have started at the end of the production line. People like to split hairs on weight and balance calculations. I actually think that’s good practice because I think the process should be precise even if the calculated or weighed results don’t perfectly match reality. If done precisely, results should be well within the margin of safety.

[jaylw314]

21 minutes ago, Shadrach said:

It’s always been a “fuzzy number” when compared to reality.  I don’t think the manufacturers were required to weigh every aircraft coming off the line under CAR. So the estimations or “errors” as some might call them may well have started at the end of the production line. People like to split hairs on weight and balance calculations. I actually think that’s good practice because I think the process should be precise even if the calculated or weighed results don’t perfectly match reality. If done precisely, results should be well within the margin of safety.

precision =/= accuracy though.  Whatever happened to significant digits?

 

1 hour ago, cliffy said:

And of course every owner/A&P will know instinctively what the unlabeled fuse holder next to the green box was for when it blew?  :-) :-) :-)

That is sure hidden. Kind of like the placement of our Amp meter fuses on top of the inner firewall.  

I have that fuse holder too and have wondered what it's for.  It's disconnected, so I assumed it was for the interior cabin lights or the instrument clock?

[EricJ]

42 minutes ago, cliffy said:

If the reweigh is done in a shop or business "reviewed" by the FAA every year you can bet that the scales are certified as accurate every year just like their torque wrenches are every year. 

And we've mentioned before that the requirement to calibrate a torque wrench for people who actually need to do such a thing for GA (most don't) is to a standard, with the standard not being specified.   You can make up your own standard and be compliant to it and meet the FAA requirements.  Maybe those on approved maintenance programs do just that.

Likewise most new state-of-the-art scales of the size typically used for our application claim accuracy to about 0.5-0.25%, or about 6-12 lbs on a 2400 lb aircraft weight, or a gallon or two of fuel.   There are further accuracy issues created by simple things like required tare items (e.g., tail stand, chocks, whatever), or even the attitude of the aircraft changing a little during the weighing process due to gear settling or whatever.   You can weigh the same race car twice and get two different weights on the same brand-new calibrated scales, likewise for airplanes.  I usually check tare weight before and after and it sometimes doesn't match exactly.  Doing the whole thing over again doesn't matter.

There are just inherent inaccuracies in the process that are not practical to avoid.   Trying to squeeze accuracy or precision out of it that just isn't there is a waste of effort, especially when the regulations suggest that that level of accuracy really isn't very important.

Edited November 16, 2021 by EricJ

[0TreeLemur]

My A&P/IA Dave, who has since left this world, once told me in a strong voice: "Never weigh your airplane, nothing good can come from it".  I did it anyway.  Then I learned what he meant.

[philiplane]

18 hours ago, SpamPilot said:

Gentle Mooniacs,

In the face of an upcoming official reweighing of my M20K (after wholesale replacement of my old avionics with newer, lighter, better equipment), I have been pondering the collective wisdom on how one is supposed to determine unusable fuel, both for W&B purposes as well as flight planning.  I'm pretty sure the collective wisdom is wrong.ied under CAR 3 and not Part 23; I did not check CAR 3 for similar language, but regardless, I think Part 23 expresses the correct requirement)

 

Un-useable fuel in any plane is a specific amount called out in the maintenance manual and or TCDS. Cut and dry, nothing to ponder.

Empty the plane, add in the specified amount of fuel. Done. 

The main accomplishment of re-weighing any plane is getting the CG correct. That is often far off, farther off than the actual weight might be over the years.

Planes gain weight after paint & interior work, more than anything else.  For example, the current fad of using high density foam insulation in place of lightweight fiberglass, can easily add 20 pounds to a small plane. Add in heavier carpet, conforming foam seats, and a nice paint job, and you're 60 pounds heavier than the factory.

[cliffy]

8 hours ago, EricJ said:

And we've mentioned before that the requirement to calibrate a torque wrench for people who actually need to do such a thing for GA (most don't) is to a standard, with the standard not being specified.   You can make up your own standard and be compliant to it and meet the FAA requirements.  Maybe those on approved maintenance programs do just that.

So sending your torque wrench to a calibration lab for certification doesn't compare it to a US Standard for "foot pounds of torque" ?

1 LB - 1 foot equals 1 foot pound? 

Not quite following the "make up your own standard" conversation.

 

[carusoam]

Fantastic!

A spam post by spam pilot… 

Clearly not everyone has it wrong…
 

And not everyone can read the type face.

That’s pretty funny…

Hope there wasn’t something important in there…

Best regards,

-a-

 

[EricJ]

1 hour ago, cliffy said:

So sending your torque wrench to a calibration lab for certification doesn't compare it to a US Standard for "foot pounds of torque" ?

Not sure how you got that out of it.   For those required to do so, e.g., Part 145, 135, or 121, the FAA says to use a calibration standard.  They don't specify what standard, though, so presumably you can make up your own or use any old relevant standard that you pick to put in your GMM or GPP for approval.  For Part 91 there's just guidance in AC 43.13 in 7-40 to "Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy."   Again, there is no guidance on what calibration standard to use and no requirement for a calibration certificate.   

1 hour ago, cliffy said:

1 LB - 1 foot equals 1 foot pound? 

Not quite following the "make up your own standard" conversation.

 

You may be confusing measurement units with a calibration standard, I can't tell.  Tool calibration standard examples are published by SAE, ISO, NIST, etc.   The FAA doesn't specify which to use, just that one must be used when one is required to do so, like for Part 145 Repair Stations or 135 or 121 operators.  It is not unusual for a company or industry to make up their own, and I wouldn't be at all surprised if some of the airlines or other maintenance facilities that develop their own maintenance procedures for approval do so, too, especially since the regs seem to imply that you can.  Otherwise referencing any of the many existing standards would seem to comply as well. 

FAR 43.13 says that if the pertinent Maintenance Manual or ICA specifies that a calibrated tool must be used, then whatever is used must conform to the relevant manual or ICA.   The only mention in the SMM for my J model of calibrated tool use is "a calibrated, certified torque application devise must be used to install the fuel quantity transmitters".   It doesn't say what type of certification is required (SAE, ISO, NIST, etc., etc.) or what it is calibrated to, or even that it has to be done more than once during the life of the tool.   So, for Part 91 operation of a J model Mooney there does not appear to be a requirement to use a tool with a calibration certificate other than for installation of the fuel senders.   If you have a torque tool with an SAE or ISO or any other standard certificate, it qualifies for use for that job, and it doesn't appear to matter when that certificate was issued.  For any other task there is only AC guidance to calibrate the tool once a year, to whatever standard you may choose, and that is not a regulatory requirement.

Even though I am not required to for my meager Part 91 operation, I have developed a process that I use every year and create traceability to established standards that are most convenient to me.   I think that more than satisfies the AC guidance as written, even though I am not required to do so.   

Just as an example of an actual calibration standard, here are a couple of sites that go into a bit more detail in the context of a change in the ISO torque tool calibration standard that happened in 2017-2018:

https://calibrationselect.co.uk/torque/torque-cal-iso-6789-2003-vs-2017/

https://www.misterworker.com/en/blog/new-regulations-for-the-calibration-of-torque-control-tools-n166

Note that one result of this change is that the ISO standard now makes a distinction between a 'calibration certificate' and a 'declaration of conformance'.   It hasn't been unusual for people to hand their wrench to the snap-on guy every year who tests it against his reference (which is often just another torque tool) and hands it back.   If you want a new ISO 'calibration certificate', that's not going to be sufficient, and it may not even be sufficient for an ISO 'declaration of conformance' if it isn't done in accordance with the steps as described in the relevant ISO standard document.   It is,  however, potentially sufficient to satisfy 'calibrate the torque wrench' per AC 43.13.   And we haven't even touched on SAE, NIST, ASME, military standards, or the other numerous global standards that could potentially be used instead of ISO.   You could even use the pre-2017 ISO standard or the post-2018.   But for Part 91 the AC doesn't even say you have to use a standard, just that you "calibrate the torque wrench".   The FAA doesn't seem to have a definition for "calibrate" or even "standard" in this context.   The snap-on truck method of comparing it to another "calibrated" tool is one method that is arguably "accepted industry practice".

[Aerodon]

So after I criticized weighing the plane with full fuel and subtracting - I went and looked at my own paperwork and thats exactly what the factory did. (at least they weighed it). In general I find the Mooney factor W&B poor, it is not very detailed.  Piper and Cessna list every single option along with a weight and arm, so it is pretty easy to do a desktop revision.  And the factory typically did a desktop calculation for the plane before they sold it.

Aerodon