W&B uncertainty

[Rich]

Well, now:

I just got my plane back from getting the MTOM/MLM and the rear wing spar carry-through reinforcement done. As part of the former the plane was weighed and a new empty weight and CG documented. Imagine my surprise when I found out that my plane is something like 20 pounds heavier than I have thought for like 12 years.

The plane has actually been weighed (and EW/CG calculated) three times in its history. Here are the numbers therefrom:

1. Original delivery (2002, at the factory): 1701.4 lb., CG 95.9 in.
2. Modified for the large baggage compartment (2004, also at the factory): 1708.8 lb., CG 95.7 in.
3. The aforementioned MTOW/MLM modification (2016, a local Diamond Service Center): 1729 lb., CG 95.4 in.

In between 2 and 3, over the years various minor things have been done that increased the EW to 1710.8 inches, calculated incrementally by math, without reweighing. These were minor things like installing the the XeVision lights, an M20 oil separator, and a transponder swap last year. The MLM mod (thinner MLG) should have decreased the overall weight by 3 lb., according to the documents provided by Diamond.

It gets more strange. In all cases the plane was weighed by measuring the weights on each of the 3 landing gear. Diamond does not actually provide the positional stations of the gear for this procedure, letting the weighing entity calculate these by a process described in the AMM. Here are the stations that the three processes came up with:

1. NLG 37.91, MLG 107.99
2. NLG 36.024, MLG 108.15
3. NLG 37.875, MLG 106.875

Another data point is that the wheelbase DAI publishes for the DA40 is "approximately" 5 ft. 6 in., or 66 inches. Note that the deduced wheelbase from each of the measurements above is, approximately:

1. 70 inches
2. 72 inches
3. 69 inches.

Additional data: There were no farkles added to N40XE between the weighing events of nos. 1 and 2.

So the upshot is I don't know which of these sets of numbers one should believe, if any. My EW lies somewhere between 1710 lb. and 1729 lb., and my empty CG lies somewhere between 95.4 and 96.9 in. - probably. Some obvious questions:

1. How could the installation of the large baggage compartment resulted in a change of CG forward, when all the new components were well behind the original CG and the even the max loaded CG of 102 inches?
2. Did the large baggage compartment really only add a net 7.4 lb. to the empty weight? Of course, some components were removed in the process, such as that rear bulkhead and the "ski-tube" we had. Conversely, It doesn't seem like it would have added 30 lb. to the empty weight. Having removed this collection of stuff back there it doesn't seem that heavy.
3. Why does the process of determining LG station yield such variant results? Even DAI themselves came up with different numbers two years apart.

The good news is that I can withstand a substantial uncertainty in these numbers and still be assured that I'm in the envelope. The bad news is we go to quite a bit of trouble (in BFR's if never again) to calculate W&B within a pound and 1/10 of an inch and this is a largely invalid exercise for my plane.

[Antoine]

Hi Rich

This is really unsettling. Maybe the following are worth exploring?
- How accurate are the scales (if they are within say +/- 1% that's about 17 lbs per measurement)
- Humidity absorption by the composite construction?

The good news is that CG is more forward than you thought. So you've been adding some safety margin for the last 14 years….

[Rich]

Good points, Antoine. The possibility of weight gain due to moisture absorption has occurred to me. The plane was long based in a damp part of the country. Even when in a hangar at PAE it was plenty damp in the winter. If this is really a possibility, it has implications for us all. It has had a several months now to dry out, but...

BTW, I've ruled out metric/US conversion factors. DAI weighs in kg and converts to US. I've confirmed that the conversions are correct.

I suspect I know what causes the variation in LG station measurements. I can do some measuring myself to confirm it, but it may have to do with leveling the A/C for weighing.

[rwtucker]

Just noodling here but my sense of strain gauges used in this context is that you would be doing well to achieve +/- 1% with units that would be affordable by a shop; 1.5% or so might be closer.

Interestingly, Aircraft Spruce offers aircraft weighing scales in the $6K and up, and in the $475 and up categories. Neither mention accuracy. The manual for the $475 single unit specifies an "Accuracy Class 6000e." Perhaps Antoine is familiar with that term. It does not correspond to the ASTM Classification system with which I am familiar. I see an E1 & E2 in European standards but the corresponding accuracy would be on the order of a gram. Perhaps 6000e means something like 6,000 grams or roughly 13 pounds, per weighing (i.e., per wheel) when measuring 500-700 pounds. Even here, we are not given distribution statistics; is it +/- 13 pounds within two standard deviations? Three? One?

Even if you were using highly accurate balance beams, if you were weighing the wheels one at a time and the weighing process involved elevating any wheel materially, the sum of those three weights might be different than the theoretical total weight.

There is a good chance that the mystery is explained by unavoidable errors in measuring.

[rwtucker]

Rich,

Is the M20 approved for our Da40's? I like this product but did not know we could install them?

[Rich]

M20 is approved. Several of us have installed them.

[Rich]

The good news is that CG is more forward than you thought. So you've been adding some safety margin for the last 14 years….

Well, no. Actually, mine is one of the forward CG planes. Taking the new numbers as gospel, I'm out of forward CG if I fill the tanks, put two 180-lb guys in the front seats and have nothing in the baggage area. I have to do some truly goofy loading to have the CG get past the middle of the range toward the rear. This has always been true and now computes even more so.

[Chris B]

M20 is approved. Several of us have installed them.

Hi Rich -

Do you have a link for approval with the DA40? The only thing that I can find is the STC listing on the M-20 Turbos site. This includes the DA20-C1 (M-20 Model 300), but not the DA40: link to STC list (PDF).

Also, do you also route the fuel line drains through the M20? On my airplane it seems like that drain line is a major contributor to belly scum.

Chris

[Rich]

M20 is approved. Several of us have installed them.

Hi Rich -

Do you have a link for approval with the DA40? The only thing that I can find is the STC listing on the M-20 Turbos site. This includes the DA20-C1 (M-20 Model 300), but not the DA40: link to STC list (PDF).

Also, do you also route the fuel line drains through the M20? On my airplane it seems like that drain line is a major contributor to belly scum.

Chris

Hm, in retrospect I don't have a link for approval. But my IA did it with a 337.

Fuel line drains are not routed through it. I don't know how you would do it and the M20 (like all the oil separators) returns fluid to the crankcase. You wouldn't want residual fuel going there.

[rwtucker]

Sorry for continuing with this hijack but where are you guys mounting the M20 and draining into which plug?