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Weight and Balance Calculations

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PilotjohnS

Well Known Member
As i write my POH, I noticed everyone seems to use 8 quarts of oil as their basis for the Center of Gravity measurement. But it seems nobody actually fly with 8 quarts, rather anywhere to 4 to 6 quarts. And since the lower oil qty pushes the CG aft, one question:
Why don't we do the aircraft weighing with minimum oil quantity rather than full oil?

I was thinking of doing my weighing with 5 quarts. Comments?
 
Oil is a consumable therefore it should not be part of weight empty. Aircraft should be weighed with oil drained or subtracted out of weight and moment. Oil then should be a line on your weight and balance sheet to be filed in when checking the oil before flight just like fuel.

That said, I am not sure many of us do it that way or even if certified aircraft all do it that way. My weight empty has 7 quarts of oil, 6 in crankcase and one in oil filter. I distinguished it by calling it “operating” weight empty similar to what we do on bigger jets that includes all fluids (brake, hydraulic, water, etc) required for flight except fuel.
 
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Vans specifies oil and unusable fuel as part of the empty weight in the RV8. section 14 W&B, page 14-1.

That being said I calculated the change in my 8’s CG with 4 quarts vs 8. It changed the CG 0.2” out of the 10” CG range. The same as if I had an error in the weight of a passenger by 5 pounds.
 
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plehrke said:
Oil is a consumable therefore it should not be part of weight empty.
Click to expand...
I don't believe this is correct, while I agree with consumable notion but oil does not fall in that category as your engine always have the oil if/when flying.

To the Op, it is probably more accurate W&B if you know you will always have 5 or 6 qt of oil but in practicality, it will probably have very little impact due to location and little weight.
 
From the Pilot Handbook of Aeronautical Knowledge:

Standard empty weight (GAMA)—aircraft weight
that consists of the airframe, engines, and all items of
operating equipment that have fixed locations and are
permanently installed in the aircraft, including fixed
ballast, hydraulic fluid, unusable fuel, and full engine
oil.
 
For some older aircraft there was also ‘certificated empty weight’ (?) that did not include the weight of oil, you had to add that back in. But for sure, if you used full oil in your original measurement and now you run 2 quarts less, it’s fair to reduce your empty weight by 4 lbs for that flight. And calculate any cg correction, too.
Just slightly off-topic: How many calculate CG for both takeoff and landing? The RV10 has a somewhat unfortunate characteristic: if you takeoff with the CG close to a limit, the cg tends to move toward that limit, or even past it, as you burn gas. I have a spreadsheet that automatically does a zero fuel calculation for CG, and, I’ve made at least one aft-loaded flight where I had to land with more than 15 gal of gas still on board, or be out of aft CG.
 
Bavafa said:
I don't believe this is correct, while I agree with consumable notion but oil does not fall in that category as your engine always have the oil if/when flying.
Click to expand...

It always has some oil but could differ flight to flight just like fuel.

But I stand corrected.
Maybe I needed to read whole paragraph. Too much memory from my C-140

FAA-H-8083 dated 2016
Aircraft Weight and Balance Handbook
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/faa-h-8083-1.pdf

Page 3-4

Oil
The empty weight for aircraft certificated under the Civilian Air Regulations (CAR) part 3 does not include the engine lubricating oil. The oil must either be drained before the aircraft is weighed, or its weight must be subtracted from the scale readings to determine the empty weight. To weigh an aircraft that does not include the engine lubricating oil as part of the empty weight, place it in level flight attitude, then open the drain valves and allow the oil to drain out. Any remaining is undrainable oil and is part of the empty weight. Aircraft certificated under Title 14 of the Code of Federal Regulations (14 CFR) parts 23 and 25 include full oil as part of the empty weight. If it is impractical to drain the oil, the reservoir can be filled to the specified level and the weight of the oil computed at 7.5 pounds per gallon. Then, its weight and moment are subtracted from the weight and moment of the aircraft as weighed. The amount and arm of the undrainable oil are found in Note 1 of the TCDS, and this must be added to the empty weight.
 
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Let’s not overcomplicate this folks….

A quart of oil is what - two pounds? Two quarts…four? I bet you don;t know what’s in your cockpit plus or minus four pounds!

I have worked more W&B problems than I can remember - several every time I have gotten a pilot rating, many, many times when building the vvarious airplanes I have , and I’ll wager that a quarter of the exam problems for my A&P were working W&B. And I can honestly say I don’t remember many (if any) that had a line item for “engine oil”. Pilot, passengers, various baggage locations, fuel (in various locations), yes….but no line item for oil.

I just checked a handful of certified light plane flight manuals I have on the shelf here - none of them has a line item for oil in the W&B examples or worksheets for the pilot.

And I KNOW there is no line item for engine oil in the airplane I just had inspected (the electric-motor-powered Xenos….) :)

I’d do the math if I had the time - maybe someone else will do it. Figure the arm for the sump on a typical RV, then show the difference in CG location with a typical flight load for six quarts versus eight. I’d honestly love to see if it makes a significant difference!

Paul
 
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Ironflight said:
I’d do the math if I had the time - maybe someone else will do it. Figure the arm for the sump on a typical RV, then show the difference in CG location with a typical flight load for six quarts versus eight. I’d honestly love to see if it makes a significant difference!

Paul
Click to expand...

.05" per qt in my RV-7

83.25" - (144089 lb-in + 2lb*45.5" ) / (1731 lb +2 lb) = 0.05"
Initial CG ______________New CG____________________Delta

Just for giggles: Moving my seat back angle between extremes will change the CG .1" per person and moving the seat back mount between extremes will change it .3" per person so we can get near a full inch of CG shift with two 150lb people hitting the extremes on seat positions. More if you are bigger. That's a CG shift that is 16 times the effect of a qt of oil and I would guess most folks don't account for it.

That qt of oil in the baggage area is a much bigger change to CG than the qt in the engine....but still not meaningful when compared to leaning forward to retrieve your dropped pen. We aviators just love to pursue 'optimal anxiety' yes?
"Not everything that counts can be counted, and not everything that can be counted counts" Einstein or Cameron depending on who you ask
 
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The only time I have heard of oil being considered for W&B is old big piston radials. The type of plane where range is limited by oil consumption, not fuel consumption. When a barrel of oil is burned per flight, they decided it was important enough to monitor.

To Paul's point, did we weigh the water bottle? What if we went to the bathroom before takeoff? Cold day, and we're wearing a heavy coat? The tires and brakes are worn out and lots of rubber isn't there anymore. How about a 737 when all the flight attendants are at the aft of the cabin?
 
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