Loss of Elevator Authority on Landing

[kkendall2718]

When landing my RV-10 with full flaps, only 2 people and 60-80 lbs in the luggage compartment, still a very forward CG, I am at full elevator deflection to keep the nose slightly up before touchdown. I have gotten in the habit of using reduced flap settings unless the CG is farther aft in the envelope. A fellow pilot that helped me with Phase 1 testing of my plane, just flew another RV-10 and called to say that he experienced something similar. Are there rigging mistakes that could be the culprit or are other RV-10 pilots experiencing the same behavior?

 

[bjdecker]

What was the indicated airspeed?

 

[Aden Rich]

Are you elevators within the full deflection specs for RV-10's? Meaning, do they have some room left in travel and you can adjust your stops more to get to full deflection? I've seen elevators that did not go all the way up or down when reviewing Van's recommended MAX deflections.

 

[BobTurner]

When landing my RV-10 with full flaps, only 2 people and 60-80 lbs in the luggage compartment, still a very forward CG, I am at full elevator deflection to keep the nose slightly up before touchdown. I have gotten in the habit of using reduced flap settings unless the CG is farther aft in the envelope. A fellow pilot that helped me with Phase 1 testing of my plane, just flew another RV-10 and called to say that he experienced something similar. Are there rigging mistakes that could be the culprit or are other RV-10 pilots experiencing the same behavior?

This is not uncommon although your case seems a bit more extreme (80 lbs in baggage?). I carry 20 lbs of water in the aft baggage area (easy to pour out when the back seats are full). Do you have a light weight battery (eg earthex) in the back? Or an especially heavy prop up front? Is the plane unpainted (paint will move cg aft).
You can check the up elevator max deflection angle, make sure it’s in spec.
Re-check weight and balance measurements.
Have passenger slide his seat back prior to landing.
Do not trim full nose up for landing (this trades more elevator authority for higher control force).

 

[bjdecker]

Following up to my question -- Before we go chasing angles and rigging I'd like to clarify the actual flight dynamics of your "event"

1. Was the stick all the way aft to the stop and the nose wasn't moving up? Or did it "feel" like the nose wasn't moving up (you could pull on the stick but the nose didn't move)
2. During #1, what was the IAS?
3. During #1, what was the VSI showing?
4. During #1, What was the RPM, MAP.
5. Where was the Elevator trim set and how do you normally configure the elevator trim? Do you adjust the trim to null out stick forces, or do you fly with a little bit of back pressure required so you "feel" the airplane?

Dynamics aside, there is a technique thing also at work - As @BobTurner mentioned, do not trim the elevator full nose up for landing as those big trim tabs (2 of them on the -10) will really quash the available "lift" from the elevator.

 

[rocketman1988]

I typically use half flaps. I also carry ballast most of the time to bring the cg into the middle of the range. I am flying in and out of 2800’ with displaced thresholds and performance hasn’t been a problem. During phase 1 testing, I did the forward cg flights and experience the same thing but not quite as extreme.

 

[scsmith]

Do not trim full nose up for landing (this trades more elevator authority for higher control force).

This is true. The RV-10 has unusually large trim tabs. Trimming for full nose up will deflect those large surfaces down. If you leave the trim closer to neutral, you will have to pull harder, but you will get more control authority.

 

[larryMar]

VGs on the underside of the horizontal stab did wonders for at least one of my non RV airplanes. Also some tape to act as a gap seal may help if it doesn't already have one.

 

[justa6ereh]

What you are describing is just normal from virtually every aircraft I’ve owned….with a little power…blowing across the elevator, you get more authoriity out of it.

which is demonstrated aptly by the STOL crowd. RV’s aren’t made for gliding in, rather….establish your attitude with trim…add drag with flaps and power up as necessary to fly the attitude to the ground.

 

[tom_AZ]

I typically use half flaps. I also carry ballast most of the time to bring the cg into the middle of the range. I am flying in and out of 2800’ with displaced thresholds and performance hasn’t been a problem. During phase 1 testing, I did the forward cg flights and experience the same thing but not quite as extreme.

I don't have flap presets, and have 30 degree max deflection. I generally land with 20 degrees, unless I need to make a steep approach for some reason, e.g short runway with obstacles. The difference in my stall speed between 20 and 30 degrees is two knots, not often affecting rollout in a meaningful way. Partial flaps also reduces the forward stick pressure needed on a go-around, and minimizes (or avoids) the time and effort to reduce flaps to 10 degrees.

Solo, or with just a front seat PAX and nearly full fuel, I definitely need to bring the stick all the way back to avoid landing flat, but it's doable even with some up elevator trim. I do keep a go bag of tools and "stuff" (~10 lb), towbar, tiedowns, etc... in the baggage area, nowhere near what the OP is carrying. Since changing to an EarthX battery, I keep the tool bag against the aft bulkhead. Like others, something seems off--be it elevator deflection, W&B calculations, or ???.

 

[Carl Froehlich]

I’d guess you did not install the big battery or equivalent aft of the baggage compartment bulkhead (in the per plans location). Did I guess right? It would be helpful if you provided your W&B data.

I mount two PC-625 batteries aft and never had a nose heavy issue as you discribe.
Carl

 

[Kyle Boatright]

When landing my RV-10 with full flaps, only 2 people and 60-80 lbs in the luggage compartment, still a very forward CG, I am at full elevator deflection to keep the nose slightly up before touchdown.

Are you sure the elevator is banging against the stop (and not the stick hitting the seat cushion or your stomach)? I'm not a big guy and I find that with a forward CG, it takes a hefty pull on the stick to flare, but I don't think I've ever hit the stop.

 

[eatonair]

Had similar incident recently on the Rv6 but for very different reasons. My son had inadvertently placed a full bottle of water between the stick and seat cushion, I didn’t catch it . Made for a very exciting landing!
My preflight briefings have been amended.

 

[Richard Connell]

I guess it is possible you are out of the front of the W&B envelope? Do you have loading and WB data?
It’s been a while but I seem to remember from testing that at the very front of the envelope I couldn’t trim any slower than maybe about 72-75kts but I still had plenty of elevator authority to flare.
I can’t load out the front of the envelope unless solo with a midget pic

 

[scsmith]

Here is a good flight test to do. Do a 1-g power off stall (at a safe altitude so you can recover). Approach the stall as gradually as you can. It is more important to do it power-off than to worry about maintaining constant altitude as you approach the stall -- you are trying to simulate a landing flare flight condition. Now, here is the test: If you get to full back stick and it doesn't stall, then you don't have adequate elevator control authority. You should measure your control deflection and compare to the specs, and verify your c.g. is not ahead of the forward limit.

Having said that, my RV-8 with solo c.g. won't give a sharp nose drop either. At back stick it shutters a little and stays nose up, but the sink rate is over 2500 ft/min. The low aspect ratio wing just does not want to fully stall. But the RV-10 wing is higher aspect ratio. I did work hard to design in a gentle stall break, but it should break.

Some glider trainers, the G-103 in particular, are marginal on elevator power and won't do a full stall when approached this way, it will buck and shutter, but won't give a clean nose drop. So that level of control authority is apparently considered acceptable by certifying authorities, but I would prefer to see enough elevator power to get a nose-drop.

Please report back your results!

 

[kkendall2718]

What was the indicated airspeed?

I am flying about 70 Kts IAS over the runway threshold, I do not know what my exact airspeed is at touchdown but it is probably faster than it could be if I had more elevator to use.

 

[kkendall2718]

Are you elevators within the full deflection specs for RV-10's? Meaning, do they have some room left in travel and you can adjust your stops more to get to full deflection? I've seen elevators that did not go all the way up or down when reviewing Van's recommended MAX deflections.

Elevator travel was checked before the DAR visit. I have been planning to recheck and compare to some other RV-10s.

 

[kkendall2718]

This is not uncommon although your case seems a bit more extreme (80 lbs in baggage?). I carry 20 lbs of water in the aft baggage area (easy to pour out when the back seats are full). Do you have a light weight battery (eg earthex) in the back? Or an especially heavy prop up front? Is the plane unpainted (paint will move cg aft).
You can check the up elevator max deflection angle, make sure it’s in spec.
Re-check weight and balance measurements.
Have passenger slide his seat back prior to landing.
Do not trim full nose up for landing (this trades more elevator authority for higher control force).

Passenger slide the seat back before landing...that is an interesting idea.
I have heard from another pilot that they avoid trimming all of the control force out. When I add nose up trim in the pattern, and acutely aware of the stick moving aft to a new neutral position.

Thanks.

 

[kkendall2718]

I guess it is possible you are out of the front of the W&B envelope? Do you have loading and WB data?
It’s been a while but I seem to remember from testing that at the very front of the envelope I couldn’t trim any slower than maybe about 72-75kts but I still had plenty of elevator authority to flare.
I can’t load out the front of the envelope unless solo with a midget pic

I have checked the WB data extensively. I have noticed that when I run through theoretical load scenarios, fuel does not shift the CG AFT significantly, this is a nose heavy airplane and it is sounding like I need to use different landing configs depending on the loading.

 

[kkendall2718]

Here is a good flight test to do. Do a 1-g power off stall (at a safe altitude so you can recover). Approach the stall as gradually as you can. It is more important to do it power-off than to worry about maintaining constant altitude as you approach the stall -- you are trying to simulate a landing flare flight condition. Now, here is the test: If you get to full back stick and it doesn't stall, then you don't have adequate elevator control authority. You should measure your control deflection and compare to the specs, and verify your c.g. is not ahead of the forward limit.

Having said that, my RV-8 with solo c.g. won't give a sharp nose drop either. At back stick it shutters a little and stays nose up, but the sink rate is over 2500 ft/min. The low aspect ratio wing just does not want to fully stall. But the RV-10 wing is higher aspect ratio. I did work hard to design in a gentle stall break, but it should break.

Some glider trainers, the G-103 in particular, are marginal on elevator power and won't do a full stall when approached this way, it will buck and shutter, but won't give a clean nose drop. So that level of control authority is apparently considered acceptable by certifying authorities, but I would prefer to see enough elevator power to get a nose-drop.

Please report back your results!

I did several power off stalls during Phase 1 testing. There was no problem getting the aircraft to stall. Still, I will recheck my elevator deflection.

 

[BobTurner]

I have checked the WB data extensively. I have noticed that when I run through theoretical load scenarios, fuel does not shift the CG AFT significantly, this is a nose heavy airplane and it is sounding like I need to use different landing configs depending on the loading.

If you ran various scenarios then you should have observed the somewhat unusual behavior that if you takeoff with the CG close to a limit (either limit-forward or aft), then, as you burn fuel, the CG moves closer to (or past) that limit. The effect is larger near the aft limit (I once made a gross weight flight where I knew that I had to land with 20 gal or more of fuel remaining, or be out of aft cg.). But if you take off right at the forward CG limit, you’ll be landing with the cg too far forward. I would hope that Vans padded the numbers a bit to allow for cg measurement errors, but I don’t know.

 

[lr172]

This is true. The RV-10 has unusually large trim tabs. Trimming for full nose up will deflect those large surfaces down. If you leave the trim closer to neutral, you will have to pull harder, but you will get more control authority.

in addition, this results in far less drama on a go around. i do not trim for neutral on final and it is still a hard pull if you firewall the throttle before the trim is out.

 

[BobTurner]

in addition, this results in far less drama on a go around. i do not trim for neutral on final and it is still a hard pull if you firewall the throttle before the trim is out.

I think you meant a hard ‘push’ on a go around.

 

[lr172]

I think you meant a hard ‘push’ on a go around.

yes, thank you.

 

[Richard Connell]

I have checked the WB data extensively. I have noticed that when I run through theoretical load scenarios, fuel does not shift the CG AFT significantly, this is a nose heavy airplane and it is sounding like I need to use different landing configs depending on the loading.

So when you see this behavior where is the CG? On the forward limit? Forward of the forward limit?
Bob is correct in that if you are close to the forward limit on takeoff then you can be through the forward limit after burning off fuel. The CG moves forward with burn not aft. It’s subtle but there.
It’s no big deal though. If anything it likely gives you more loading flexibility. Most 10 owners fly around solo or 2 up with modest ballast in the baggage area. I always have 30lb or so.

 

[bjdecker]

So when you see this behavior where is the CG? On the forward limit? Forward of the forward limit?
Bob is correct in that if you are close to the forward limit on takeoff then you can be through the forward limit after burning off fuel. The CG moves forward with burn not aft. It’s subtle but there.
It’s no big deal though. If anything it likely gives you more loading flexibility. Most 10 owners fly around solo or 2 up with modest ballast in the baggage area. I always have 30lb or so.

Uhm --- is that a -10 thing?

In my -7 the CG moves aft when fuel is burned -- here's the math.

42 Gallons @ 80" aft of the datum - 20160 moment
5 Gallons @ 80" aft of the datum - 2400 moment

Above examples plugged into the envelope show the CG @ 85.08" aft of the datum (42 gallons) and 85.74" (5 gallons) aft of the datum -- the CG moves aft as fuel is burned.

 

[Richard Connell]

Uhm --- is that a -10 thing?

In my -7 the CG moves aft when fuel is burned -- here's the math.

42 Gallons @ 80" aft of the datum - 20160 moment
5 Gallons @ 80" aft of the datum - 2400 moment

Above examples plugged into the envelope show the CG @ 85.08" aft of the datum (42 gallons) and 85.74" (5 gallons) aft of the datum -- the CG moves aft as fuel is burned.

Yes it’s a 10 thing…the OP flys a 10

 

[bjdecker]

Yes it’s a 10 thing…the OP flys a 10

Hmmm, the math doesn't support that observation.

Using the sample numbers from the -10 manual as an example shows the CG moving from 114.88" aft (60 gallons) to 115.72" aft (5 gallons) -- the CG moves AFT as the fuel burns.

 

[dmattmul]

My 10 moves a little over an inch aft going from 60 gals to 5 gals.

The 14 moves a little under and inch aft.

Hopefully I didn't mess something up

 

[BobTurner]

Okay, everyone, please re-read post #21, wrt the -10.
If If you takeoff with the cg near the aft limit, the cg moves further aft as you burn gas.
If you take off with the CG close to the forward limit, the cg moves forward (slowly) as you burn gas.
There are quite a few -10’s out there whose empty cg is far enough aft that they never encounter the forward case limit.

 

[Richard Connell]

Hmmm, the math doesn't support that observation.

Using the sample numbers from the -10 manual as an example shows the CG moving from 114.88" aft (60 gallons) to 115.72" aft (5 gallons) -- the CG moves AFT as the fuel burns.

I’m confident my CG calcs are correct.

For the OP.
I assume that you are getting close to 30degrees up on the elevator? Mine came out at 29+. If you were at the minimum of 25 that may be worth looking at first.
If the throws are good, then it probably only leaves CG as the variable here.

 

[dmattmul]

I am flying about 70 Kts IAS over the runway threshold, I do not know what my exact airspeed is at touchdown but it is probably faster than it could be if I had more elevator to use.

FlySto has some great analytics to feed data from your EFIS into it. It will give you your landing profile and IAS at touchdown (or at any altitude during the approach) The dashed line is 1.3 Vso. This was my flight from today.

 

[dmattmul]

Okay, everyone, please re-read post #21, wrt the -10.
If If you takeoff with the cg near the aft limit, the cg moves further aft as you burn gas.
If you take off with the CG close to the forward limit, the cg moves forward (slowly) as you burn gas.
There are quite a few -10’s out there whose empty cg is far enough aft that they never encounter the forward case limit.

True, mine would never be able to get that far forward even with my Earth X batteries in the tailcone but possible with lots of heavy stuff FWF. (metal prop, pad mount heavy back-up alternator, compressor, etc.) One needs to be in the forward 13% of the CG range for this to occur but agreed theoretically possible. (I've flew mine in the forward 1/3 of the range and felt very nose heavy, so I fly with salt bags in the baggage area)

If the CG was that far forward (In the forward 13% of the CG range) not sure if one would have enough elevator authority especially with max trim, hey wait a minute.

 

[scsmith]

I did several power off stalls during Phase 1 testing. There was no problem getting the aircraft to stall. Still, I will recheck my elevator deflection.

Sure. But you may or may not have done them the way I described. Typical stalls are done with more deceleration, more 'dynamic' change in attitude. This is more likely to produce a good stall break. Try again next time you fly to sneak up on it very gradually. You might find you can get the stick all the way back without a break. I can.

 

[lr172]

True, mine would never be able to get that far forward even with my Earth X batteries in the tailcone but possible with lots of heavy stuff FWF. (metal prop, pad mount heavy back-up alternator, compressor, etc.) One needs to be in the forward 13% of the CG range for this to occur but agreed theoretically possible. (I've flew mine in the forward 1/3 of the range and felt very nose heavy, so I fly with salt bags in the baggage area)

If the CG was that far forward (In the forward 13% of the CG range) not sure if one would have enough elevator authority especially with max trim, hey wait a minute.

When solo, i am outside the fwd limit and need 20-30 pounds in baggage to get it back in. Did that on purpose to maximize load capacity. Very few scenarios where i cannot load the full gross weight and not blow the afr cg.

 

[lr172]

Sure. But you may or may not have done them the way I described. Typical stalls are done with more deceleration, more 'dynamic' change in attitude. This is more likely to produce a good stall break. Try again next time you fly to sneak up on it very gradually. You might find you can get the stick all the way back without a break. I can.

On my 10, i have cases where i do not get a stall break. It just mushes and sinks like a stone. I noted you said i should get a break. These would be at the very fwd end of the cg envelope. Do you think my calcs are off and i am actually fwd of the envelope? Was pret Sure I confirmed my eleventh travel was at the spec.

Total shocker for me, as the 6 has a very pronounced break

 

[scsmith]

On my 10, i have cases where i do not get a stall break. It just mushes and sinks like a stone. I noted you said i should get a break. These would be at the very fwd end of the cg envelope. Do you think my calcs are off and i am actually fwd of the envelope? Was pret Sure I confirmed my eleventh travel was at the spec.

Total shocker for me, as the 6 has a very pronounced break

At the forward limit, I am not surprised that you can not always get a break if you approach it smoothly. That characteristic may well be the criterion that actually sets the forward limit in this case.

 

[RNB]

For this discussion, I think some absolute specifics would better flavor things.
I'd repeat the ask for your weight and balance sheet.
Then share what you calculated your CG was
My example flight planning for OSH attached. Note the totals row is my empty totals.
CG for this was 112 point something.

 

[lr172]

At the forward limit, I am not surprised that you can not always get a break if you approach it smoothly. That characteristic may well be the criterion that actually sets the forward limit in this case.

Is it dangerous that I do not get a break, assuming I still have authority to flare? I typically carry 30# of water in the baggage area. My W&B calcs told me that when solo with 5 gallons of fuel, I am a few tenths out of the FWD limt. The ballast puts it almost an inch inside the limit. I have flown a few times without the ballast and maybe those times were when I experienced that.

 

[FORANE]

When landing my RV-10 with full flaps, only 2 people and 60-80 lbs in the luggage compartment, still a very forward CG, I am at full elevator deflection to keep the nose slightly up before touchdown. I have gotten in the habit of using reduced flap settings unless the CG is farther aft in the envelope.

I don't have 10 knowledge but full elevator deflection to keep the nose slightly up before touchdown sounds like it needs to be addressed (as you are...). I flew my old Lancair for 1600 hours or so with CG very far forward. It had no elevator travel problem keeping the nose up prior to touchdown, but once the mains touched the nose was coming down. There was nothing you could do to keep the nose off once the mains touched. It made landings challenging. Just as you have gotten in the habit of using reduced flap settings, I noticed increased elevator authority when flaps were not present. It was significant enough that I eventually learned to land it by flying into ground effect then retracting flaps when airborne a few feet off the runway. That would result in a flare on command and smoother landings. That unconventional technique became my standard technique for every landing for over 1000 hours in that plane.

 

[dmattmul]

When solo, i am outside the fwd limit and need 20-30 pounds in baggage to get it back in. Did that on purpose to maximize load capacity. Very few scenarios where i cannot load the full gross weight and not blow the afr cg.

I would be interested seeing your W&B. I went with a composite prop and Earth X batteries just to keep the gross weight as low as possible. I did have a full interior done which makes my 10 a little fat. Add to that built in O2 and a B&C pad mount alternator. I did not want to exceed the maximum design gross weight as some have done as this reduces safety margines. Attached is an example of my 10.

 

[RV8JD]

I would be interested seeing your W&B. I went with a composite prop and Earth X batteries just to keep the gross weight as low as possible. I did have a full interior done which makes my 10 a little fat. Add to that built in O2 and a B&C pad mount alternator. I did not want to exceed the maximum design gross weight as some have done as this reduces safety margines. Attached is an example of my 10.


View attachment 120827View attachment 120826

I note that you use “Utility” and “Max Util” in a few places in your W&B. Does that refer to Utility Category Load Limits (+4.4/-1.75 G)? If so, Van’s states that the RV-10 is designed only for Standard Category Load Limits (+3.8/-1.5 G) at their recommended gross weight of 2700 pounds.

From Van’s:

"The design operational stress limit for the RV-10 is standard category (+3.8/-1.5 G)."​

​

 

[dmattmul]

I note that you use “Utility” and “Max Util” in a few places in your W&B. Does that refer to Utility Category Load Limits? If so, Van’s states that the RV-10 is designed only for Standard Category Load Limits at their recommended gross weight of 2700 pounds.

From Van’s:

"The design operational stress limit for the RV-10 is standard category (+3.8/-1.5 G)."​

​

I used the same xls spreadsheet from my 14 which has separate criteria for weight and CG ranges for aerobatic flights. It means non aerobatic CG and weight profile would be used for my 10 as well as my 14. For my 10 it's a moot point for me if it's rated at standard or utility G limits.

Vans also says for all RV's "No RV should ever be operated above its design gross weight limit."

 

[RV8JD]

I used the same xls spreadsheet from my 14 which has separate criteria for weight and CG ranges for aerobatic flights. It means non aerobatic CG and weight profile would be used for my 10 as well as my 14. For my 10 it's a moot point for me if it's rated at standard or utility G limits.

I figured something like that. But didn’t want others to think the RV-10 was stressed for the higher Utility Category Load Limits.

 

[lr172]

I would be interested seeing your W&B. I went with a composite prop and Earth X batteries just to keep the gross weight as low as possible. I did have a full interior done which makes my 10 a little fat. Add to that built in O2 and a B&C pad mount alternator. I did not want to exceed the maximum design gross weight as some have done as this reduces safety margines. Attached is an example of my 10.


View attachment 120827View attachment 120826

I will post a copy for you tomorrow. The heavy hartzell is a help here. I am pretty stock and use 2 pc680 type batteries. My O2 system is built in, but the tank is not. Even have a sensor tied to efis to monitor fill. I use E bottles that get placed in a permanent cradle that sit directly behind the rear seat backs. I was unwilling to spend major money on an FG bottle and would not consider putting AL bottles behind the bulkhead for CG reasons. I like that i can remove the bottles, as i refill at home, not my hanger. I also often carry two bottles on Colorado trips. I can install a bottle in under a minute or two. The O2 bottle is only 6 lbs and at the location i picked, does not create a real hit on CG.

I have nice seats, but otherwise no real interior. Wasn’t looking for an airborne BMW, just a low cost four place airplane. I am a bit frugal in that regard. I was also unwilling to spend $4k on seats, so i did my own upholstery. Lot of work making the templates, but they came out very nice and rival pics i have seen of the expensive stuff. Actually better than some as there is zero voids or puckering that i have seen in pics. They are ultra leather, which is tough as nails and a fraction of the weight of leather. I find them more comfortable than leather, but that is a personal preference.

My empty weight, ready to fly, with tool bag and fire extinguisher(mounted way fwd on top of the tunnel cover) is 1640. So, with full tanks, i am right at 2000. I set gross at 2800 , but have only gone over 2700 once or twice. Personal risk tolerance here. I just believe there is plenty of margin and willing to eat into it a couple of percentage points.

All of these choices keep weight behind the c of g low and net the fwd bias. The primary benefit is getting to gross, even the 2800, without busting the aft limit in most loading scenarios. While i am willing to accept what I consider a very small risk going to 2800, i will NOT exceed the aft cg, which i consider to be a far more serious risk and didn’t want to be tempted, as I suspect some are, trying to get 4 people in the porkier planes. No interest in self teaching myself how to get out of a flat spin. My son once threw a bag much heavier than he declared in the baggage area of my 6. Did NOT enjoy the control feel. Literally felt like i had to push the stick to flare. I now put a hand on everything that goes in the back of any plane.

All about balancing priorities. Would love AC on those August flights droning along under the bravo, but i tell my passengers one of you wouldn’t be able to make this flight if i had AC.

I could go lithium to lower the weight, but so far have not needed more capacity than i have and that would require more ballast for solo flights. Built to plans, the 10 is well set in regards to cg imo. Once you start adding things, it moves in the wrong direction if 4 person travel is a goal.

 

[lr172]

Vans also says for all RV's "No RV should ever be operated above its design gross weight limit."

And vans also says to radically reduce gross weight when extended range tanks are used, so there is that political component. I don’t think that is in play in your above quote, but i do believe there is a conservative cushion involved to account for wide variability in build quality. There is a small percentage of these planes that are poorly built and don’t self destruct at gross. When dealing with this scenario, i believe folks like vans have no choice but to build in margin to account for that. I trust that i am not one of those poorly built planes and therefore take back a bit of that margin. My rationale for my choice. Not a recommendation for others to do the same.