Max Zero Fuel Weight in a Mooney?

[Jeff_S]

Hey all,

I had cause to dip back into ForeFlight’s W&B module recently to determine if I could take an Angel Flight that was requested. It presented with a fairly robust husband, wife and toddler child plus 50 lbs of their luggage, enough mass that I knew my normal mental W&B calculation would not suffice. And FF confirmed my suspicion, as it would require takeoff with barely enough fuel to meet my personal minimums for fuel on landing. 
 

But the W&B calculations in FF showed a metric I didn’t recognize: Max Zero Fuel Weight.  I scoured my POH and found no such limitation. So perhaps this happened with a FF update and my infrequent use of that module kept it hidden from me. But it got me thinking…how would I determine such a number? I haven’t found a way to remove it from FF, so I put my thinking cap back on to see what logic I could apply.

I settled on the Max Landing Weight of 3200 lbs, figuring that a worst case scenario would have me touching down with just enough gas to taxi to the ramp. That may be fairly okay, or wildly stupid, which leads to my question. In planes like ours, what would be the rationale for a Max Zero Fuel Weight and if there is any, how should it be determined?

To finish the story, I rejected the flight. I do know that the first leg of the trip was completed by another Mooney pilot, so if s/he is here on MS my hats off to you for squeezing that load into your plane. I admit that the factory A/C in my system has significantly reduced my useful load, but here in Florida that’s an easy tradeoff to make!

[blaine beaven]

As far as I’m aware, our Mooney aircraft do not have a max zero fuel weight because there aren’t the same structural concerns that exist in larger aircraft. 
 

Max Zero Fuel weight is the limit of how much weight of equipment, people, baggage etc you can have in the plane that isn’t fuel. This has more to do with the strength of various parts of the structure, and the ability to overstress the structure if you say, put 95% of the weight in the fuselage instead of 80% and the other 20% in fuel in the wing tanks. 

[PT20J]

The wings lift the fuselage. The more weight in the fuselage, the greater the bending moment where the wings attach. Large aircraft with a lot of useful load have a zero fuel weight limitation to avoid loadings that over-stress the wing/fuselage connection. 

[EricJ]

2 hours ago, Jeff_S said:

But the W&B calculations in FF showed a metric I didn’t recognize: Max Zero Fuel Weight.  I scoured my POH and found no such limitation. So perhaps this happened with a FF update and my infrequent use of that module kept it hidden from me. But it got me thinking…how would I determine such a number? I haven’t found a way to remove it from FF, so I put my thinking cap back on to see what logic I could apply.

Max zero fuel weight is used on airliners and large aircraft, generally not GA aircraft.   So, yeah, it won't be in your POH.    Some jets/turbines may have it as well, but it's not a thing for us plebian single-engine piston drivers.

[Jetpilot86]

The Mooney’s don’t have one. But good explanations above. 

[Ragsf15e]

11 hours ago, EricJ said:

Max zero fuel weight is used on airliners and large aircraft, generally not GA aircraft.   So, yeah, it won't be in your POH.    Some jets/turbines may have it as well, but it's not a thing for us plebian single-engine piston drivers.

The Meridian I fly for work (turbine, so fits with your description) has both a zero fuel and a max landing weight.  They are pretty hard to come up against since there’s not much room for fuel left at max zero fuel, but once or twice I’ve had to consider them.  I really like having ff for that!

[PeteMc]

The answer you are looking for is put in your planes Max Weight.  

True for Mooney, Piper, etc. that don't have a Max Zero Fuel Weight value listed anywhere.  Same as Max Landing, Max Takeoff, Max Ramp...

 

Edited November 24, 2023 by PeteMc

[kortopates]

The answer you are looking for is put in your planes Max Weight.  
True for Mooney, Piper, etc. that don't have a Max Zero Fuel Weight value listed anywhere.  Same as Max Landing, Max Takeoff, Max Ramp...
 

Technically Max Ramp = Max takeoff + Taxi/run-up fuel, if you want to take on the max fuel you can


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[wombat]

For the max gross STC part of the Rocket mod there is a limitation that all weight above 2,900# must be fuel.   Slightly different in name, but it is effectively a 0 fuel weight limitation.

[Rick Junkin]

5 hours ago, kortopates said:

Technically Max Ramp = Max takeoff + Taxi/run-up fuel, if you want to take on the max fuel you can


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^^^^This^^^^

That's what I put in for Max Ramp.

I calculated my own MZFW this way:

Max Zero Fuel Weight = Max Landing Weight - 63# (10.5g of fuel).

10.5g is 45 minutes at 14gph, which is my cruise LOP fuel flow rounded up to the nearest gph. My normal planning target is a minimum landing fuel of 20g, just because, but the Max Zero Fuel Weight limit serves to provide a warning for maintaining capacity for the legal IFR reserve and still stay under the Max Landing Weight.

Cheers,
Rick

[PeteMc]

6 hours ago, kortopates said:

Technically Max Ramp = Max takeoff + Taxi/run-up fuel, if you want to take on the max fuel you can

"Technically" that maybe true.  But you should not create a number that doesn't exist in the POH or any other Manufacturer Documentation. 

So for the answer to the question asked, all of those Max X fields in FF, G Pilot or any other W&B program get the Max Loading CG Weight for your model (2900 for my K).

 

[Fly Boomer]

On 11/23/2023 at 8:16 AM, Jeff_S said:

But the W&B calculations in FF showed a metric I didn’t recognize: Max Zero Fuel Weight.  I scoured my POH and found no such limitation. So perhaps this happened with a FF update and my infrequent use of that module kept it hidden from me. But it got me thinking…how would I determine such a number? I haven’t found a way to remove it from FF, so I put my thinking cap back on to see what logic I could apply.

I think it's calculated.  Also, Foreflight seems to use 5.82 lbs/gallon for fuel.  Some airplanes (mine for example) has both a max takeoff weight and a max landing weight, so the software is just showing you what your weight will be when you land.  For the Beech crowd, I have heard that, with some loads, some of those airplanes will fly out of CG as they burn fuel.  The tool shows a line from takeoff through landing so you can see where you will be in (or out of) the envelope throughout your flight.

EDIT:  I see that fuel density can be changed in setup.

SECOND EDIT:  On further examination, I see that you can also specify the zero fuel weight in setup.  I saw the zero fuel weight on the right with the loading chart, and thought that is what you were referring to.

[Rick Junkin]

On 11/24/2023 at 2:26 PM, PeteMc said:

"Technically" that maybe true.  But you should not create a number that doesn't exist in the POH or any other Manufacturer Documentation. 

So for the answer to the question asked, all of those Max X fields in FF, G Pilot or any other W&B program get the Max Loading CG Weight for your model (2900 for my K).

 

Interesting. With your approach, you can never take off at Max Gross Weight unless you were to top off the fuel to max gross at the end of the runway with the engine running right before takeoff. Otherwise you take off at Max Gross minus the weight of the fuel you used for start, taxi, and takeoff.

Section 2 of the POH for my M20M states a maximum gross weight of 3,368 pounds and a maximum takeoff weight of 3,368 pounds. It also states a maximum landing weight of 3,200 pounds. The performance charts in Section 5 reference performance at a maximum gross weight of 3,368 pounds. The POH is silent on a maximum taxi weight or any ground handling weight for that matter.

My interpretation of this data is I can take off with the airplane weighing 3,368 pounds. To arrive lined up for take off at 3,368 pounds I need to have had an extra 9 pounds of fuel when I started to account for the gallon and a half of fuel I will burn during start and taxi. So to get there I plan for and put on an extra gallon and a half of fuel. I can call it whatever I want, like "start and taxi fuel", or "ramp fuel", or whatever else I want to call it, to get me to the end of the runway and ready for takeoff at Maximum Takeoff Weight. Foreflight has an entry for Max Ramp Weight, so I choose to call it that and use that line for my personal reference. I'm not changing the POH, I'm not changing the certification, I'm providing myself with a reference to get me to Maximum Takeoff Weight when I'm ready to take off.

I concede I may not be following a course of action that others choose to follow, but it makes sense and puts me in compliance with the POH.

Cheers,
Rick

EDIT: My bad, I was working from memory when I typed this post. With the incorporation of SB 20-248 which increased the maximum takeoff weight to 3,368# the reference to a "maximum gross weight" was removed from Section 2 (Limitations) of the POH and replaced with a maximum weight - takeoff (3,368#) and a maximum weight - landing (3,200#).

[PeteMc]

10 minutes ago, Rick Junkin said:

Interesting. With your approach, you can never take off at Max Gross Weight unless you were to top off the fuel to max gross at the end of the runway with the engine running right before takeoff. Otherwise you take off at Max Gross minus the weight of the fuel you used for start, taxi, and takeoff.

Guessing you didn't read my message....

"...that doesn't exist in the POH or any other Manufacturer Documentation..."  Pretty sure I've said something along those lines in all my posts.  If you have it, obviously use it.  Not all the Mooneys (or other planes) have all of the info. 

 

[Rick Junkin]

5 minutes ago, PeteMc said:

Guessing you didn't read my message....

I read what you posted. Perhaps I misunderstood.

1 hour ago, PeteMc said:

But you should not create a number that doesn't exist in the POH or any other Manufacturer Documentation. 

The numbers I've created are for my own reference in meeting the published POH numbers, and I've chosen to use the Foreflight designations to have them displayed on the W&B chart. Kind of like penciling in your standard loading on the POH charts back in the day. 

So I have a line that shows me how much people and bags I can put in the airplane and still have room for enough fuel to meet my personal minimums (Max Zero Fuel Weight) and a limit that warns me if I've exceeded a fuel load that will allow me to be in position for takeoff at or below maximum takeoff weight (Max Ramp Weight). Neither of those weights are in the POH, but they're in Foreflight and convenient to use as pointers for staying within the POH limits.

We may be saying the same thing, and if so great.

Cheers,
Rick

[kortopates]

Guessing you didn't read my message....
"...that doesn't exist in the POH or any other Manufacturer Documentation..."  Pretty sure I've said something along those lines in all my posts.  If you have it, obviously use it.  Not all the Mooneys (or other planes) have all of the info. 
 

Not a limitation but actually your POH does provide a taxi/runup amount of fuel in the Sect 5 - Time, fuel and Distance to climb table. As an example, for the J the notes show that fuel is based on using 9 lbs. That’s fuel you can add above max gross weight the realize the POH performance numbers.


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[PeteMc]

2 hours ago, kortopates said:

Not a limitation but actually your POH does provide a taxi/runup amount of fuel in the Sect 5 - Time, fuel and Distance to climb table.

I'm not sure what your point is?  But then I also don't know what the 252 POH says.

I do have taxi fuel burn estimates for my 231, but I also have a "Maximum Certified Weights: Gross Weight 2900 Lbs."  So even though I know from the POH and from my JPI what my typical fuel burn is, it really doesn't matter.  I can't add fuel for that taxi, even though I know I'll burn it off, if that puts me above 2900lbs.  And if I'm at or under 2900lbs, all the extra numbers FF is asking about for the W&B do not apply to me, so FF's recommendation is to put in your Gross Weight. 

Not trying to be a pain here, I just don't see any of your points about calculating this or that UNLESS your POH says you can load more on the ground.  If you guys want to push the envelope and go slightly over gross with taxi fuel, go for it.  If you rounded up on your other numbers, it's not going to make a bit of difference.  If you rounded down, it's probably still not going to make a difference.   I'm just answering the original question.  I was under the impression that no Mooney has a Max Zero Fuel Weight and so if my information is correct, then the correct answer is to put in Gross (per FF about their program).

 

ADDED: Technically it is not the Gross Weight, but Max Takeoff Weight... But again, all the same in most (all?) up through the 231.  You'll have to fill me in if some of the newer bigger Mooneys have differences in Gross, Takeoff, etc. 

Per Foreflight: If the aircraft's flight manual does not define this limitation, set MZFW to match Max Takeoff Weight.

Edited November 25, 2023 by PeteMc

[GeeBee]

It's a CAR 3 airplane folks, don't overthink it.

[PeteMc]

17 minutes ago, GeeBee said:

It's a CAR 3 airplane folks, don't overthink it.

YES!

[Rick Junkin]

To the original poster, @Jeff_S

On 11/23/2023 at 8:16 AM, Jeff_S said:

But the W&B calculations in FF showed a metric I didn’t recognize: Max Zero Fuel Weight.  I scoured my POH and found no such limitation. So perhaps this happened with a FF update and my infrequent use of that module kept it hidden from me. But it got me thinking…how would I determine such a number? I haven’t found a way to remove it from FF, so I put my thinking cap back on to see what logic I could apply.

The ForeFlight Weight & Balance Guide, page 14, gives ForeFlight's definition of all the weights it uses. As @PeteMc stated it says to set MZFW to Max Takeoff Weight if your flight manual doesn't specify a MZFW. This effectively eliminates the MZFW line, so answers your question about how to remove it.

If you want to repurpose the MZFW weight/line to make it useful to you as a reference, you can enter any value you want, and use it in a way that makes sense to you. There's nothing that keeps you from adding your own notes to your planning materials. You're not changing anything that affects the W&B envelope, just adding a reference within the envelope. You've chosen to use Max Landing Weight, but that line is already on the W&B chart. I chose max landing weight minus legal IFR reserve fuel, and that gives me a reference weight/line that means something to me and is what I recommend (or MLW minus your personal minimum landing fuel) if you're going to go that route. So that gives you my answer to a logic path leading to a value to enter into the MZFW that isn't stated in the POH but has meaning and usefulness as a reference for your load planning. In reality you would be accomplishing this exact same calculation to determine fuel stops with your available cargo load and minimum fuel for your mission, so entering a MZFW just puts the weight/line on your ForeFlight W&B chart as a pre-calculated reference.

The Max Ramp Weight horse has been beat to death already. My take-away is, understand your POH and do what makes sense to you.

Cheers,
Rick

[wombat]

If you are operating your aircraft at all (even taxi) above the max gross weight you are in violation of the limitations in the POH.     It's unlikely to be a big deal.  If anyone asked me (nobody has) I'd recommend against it in the weakest possible terms.

 

But because I'm a problem solver..... @Rick Junkin If you really wanted to take off at max gross weight and not operate outside of the POH's limitations, you could have a passenger walk down to the end of the runway, and when you are lined up to take off, you will have burned off your 9# of fuel and the passenger can get in.  If anyone asked me (nobody has) I'd recommend relatively strongly against having people get in and out of an airplane with the engine(s) running.  So this method is more legal but in my opinion, less safe.

[Pinecone]

12 hours ago, GeeBee said:

It's a CAR 3 airplane folks, don't overthink it.

And is your aircraft empty weight actually THAT accurate????

[Rick Junkin]

4 hours ago, Pinecone said:

And is your aircraft empty weight actually THAT accurate????

LOL - Yes, we are definitely into the realm of "measure it with a micrometer, mark it with a grease pencil, cut it with an axe" from a practical standpoint. The 1.5 gallons of start/taxi/run-up fuel amounts to 0.26% of my maximum takeoff weight. Definitely in the noise level and within the margin of error of weighing everything else that goes into the airplane, especially the bodies. We're splitting hairs over the precise legality, which is always a fun exercise. If you want some more perspective look at the weight waivers available in Alaska, and the weight increase allowed for STCs before a new set of airframe testing data is required (it's 5%). By no means am I suggesting operating above max weight, just providing information on what's already in formal guidance.

But the discussion prompted me to do some research into the certification documents, both CAR 3 and Part 23, to see what they say about maximum weight. As you might expect the wording is sufficiently broad. Interesting to note is the term maximum gross weight isn't used, maximum weight is.

CAR 3.44 defines Maximum Weight:

Maximum weight: The maximum weight at which the airplane may operate in accordance with the 
airworthiness requirements

CAR 3.74 has this to say about Maximum Weight:

§ 3.74 Maximum weight. 

(a) The maximum weight shall not exceed any of the following:

(1) The weight selected by the applicant.

(2) The design weight for which the structure has been proven, except as provided in Sec. 3.242 for multiengine airplanes.

(3) The maximum weight at which compliance with all of the applicable flight requirements has been demonstrated.

So per 3.74(a)(1) the applicant chose a maximum weight equivalent to the maximum takeoff weight and that is the legal number to use. The only possible fuzz on that is if that stated weight is intended just for flight, since CAR 3 appears to be silent on weights specific to ground operations. But I haven't digested the whole document so it may be in there somewhere and I haven't got to it yet. I'm sure someone here is versed on the intent of what's in CAR 3.

Cheers,
Rick

P.S. I grew up in Tactical Air Command in the USAF. In TAC the approach to regulations was, "If it doesn't explicitly say you can't, you can." In our sister command Strategic Air Command, the approach was, "If it doesn't explicitly say you can, you can't." Unfortunately for me I still carry that TAC mentality and it doesn't fit too well with the often vague nature of FAA regulations. While common sense won't always lead me to the legal answer, it has not failed to lead me to a safe answer.

[Rick Junkin]

6 hours ago, wombat said:

If you are operating your aircraft at all (even taxi) above the max gross weight you are in violation of the limitations in the POH.     It's unlikely to be a big deal.  If anyone asked me (nobody has) I'd recommend against it in the weakest possible terms.

 

But because I'm a problem solver..... @Rick Junkin If you really wanted to take off at max gross weight and not operate outside of the POH's limitations, you could have a passenger walk down to the end of the runway, and when you are lined up to take off, you will have burned off your 9# of fuel and the passenger can get in.  If anyone asked me (nobody has) I'd recommend relatively strongly against having people get in and out of an airplane with the engine(s) running.  So this method is more legal but in my opinion, less safe.

Yeah, I don't think I want to try that.

My POH (M20M) lists two separate maximum weights; a Maximum Takeoff Weight, and a Maximum Landing Weight. There is no "Max Gross Weight" stated. I don't have a POH for the OP's M20R but I did look at the TCDS and the R also only states a MTW and a MLW. So from that there is no limit in the TCDS or POH restricting us, M and R, from ground operations in a state that allows us to take off at or below the MTW. From my last post, "It doesn't say I can't, so I can."

The TCDS for the K model states a singular maximum weight, which is a little stickier. Yes, that's a technical term . With it stated as a straight maximum weight, that means something different to me, as in the most the airplane can weigh at any time. So I get that. I think what it REALLY means is the MTW and the MLW are the same for the K, but that isn't how it's stated. So yeah, with it stated that way, I can't successfully argue for adding anything above that weight. I didn't look up your Rocket but I'm guessing it's got a similar limitation statement.

Cheers,
Rick

[PeteMc]

6 minutes ago, Rick Junkin said:

My POH (M20M) lists two separate maximum weights; a Maximum Takeoff Weight, and a Maximum Landing Weight.

What year M?  The 1990 POH shows "Gross Weight" and Max. Landing Weight.  I don't see a Takeoff Weight.