Weighing the Aircraft

[GAHorn]

I rec’d an inquiry from a Member who is having his airplane re-weighed and asks: Should the fuel tanks be empty or full when weighed?

My answer to him explained the differences in how the final result is achieved with either method… either drain the wings ..OR… fill them and do the math to remove the useable fuel for the final calculations… But the official answer is to follow 43.13 which stipulates that if the mfr’r provides a method then that method must be used.

I haven’t found a Cessna procedure specified for the 170, but did find an article which purported to represent Cessna’s instructions for a 182 ..
” Cessna has some specific requirements that must be followed. Preparation details include the following: inflate the tires, drain the fuel (at the wing sumps), drain the oil, crew seats slide all the way forward, seat backs vertical, flaps retracted, and controls in their neutral position, all needed to get repeatable results."

Has anyone seen a Cessna procedure specifically for the 170 or for the fleet? If so, let’s add that to the Mx Library.

Meanwhile, here’s the FAA posting: https://www.faa.gov/sites/faa.gov/files ... k_ch10.pdf

[n2582d]

Your quote of directions seems to be a boilerplate in most, if not all, of Section 6 POH's for more current SE Cessnas. (Google "Cessna 172 Pilot's Operating Handbook"). It's strange to me why it would be found there rather than in Cessna Service Manuals. When's the last time you saw a pilot rather than an A&P weighing an aircraft? I would want to locate the seat where I normally use it rather than full forward as instructed in these POHs. Or at least match what Cessna used at the factory, which -- if I did my math right -- is 35.88". The other thing that needs to be done differently than these POH's describe is to realize that , for the C-170, oil is not part of the empty weight.

[lowNslow]

The other thing that needs to be done differently than these POH's describe is to realize that , for the C-170, oil is not part of the empty weight.

I had seen that and always thought it rather strange - are you ever going to fly it without oil??

[gfeher]

At least for draining the fuel, that Cessna procedure does not make sense to me for the 170 models or any other tailwheel aircraft. For tailwheel aircraft, the plane needs to be in level flying position when draining usable fuel. I support the tail up in level flying position as specified by the manufacturer and drain the fuel by disconnecting the fuel line at the carburetor. Whatever is left in the system is unusable fuel.

[n2582d]

Gene,
Unusable fuel is a given value as specified in the TCDS -- 19 lbs. for the '48, and 30 lbs. for the A and B models. My guess is that one can drain more than that last 5 gallons of "unusable fuel" by draining through the gascolator if the aircraft is leveled longitudinally. CAR 3.437 lays out how a manufacturer is to determine unusable fuel.

§ 3.437 Determination of unusable fuel supply and fuel system operation on low fuel.
(a) The unusable fuel supply for each tank shall be established as not less than the quantity at which the first evidence of malfunctioning occurs under the conditions specified in this section. (See also § 3.440.) In the case of airplanes equipped with more than one fuel tank, any tank which is not required to feed the engine in all of the conditions specified in this section need be investigated only for those flight conditions in which it shall be used and the unusable fuel supply for the particular tank in question shall then be based on the most critical of those conditions which are found to be applicable. In all such cases, information regarding the conditions under which the full amount of usable fuel in the tank can safely be used shall be made available to the operating personnel by means of a suitable placard or instruction in the Airplane Flight Manual.
(b) Upon presentation of the airplane for test, the applicant shall stipulate the quantity of fuel with which he chooses to demonstrate compliance with this provision and shall also indicate which of the following conditions is most critical from the standpoint of establishing the unusable fuel supply. He shall also indicate the order in which the other conditions are critical from this standpoint:
(1) Level flight at maximum continuous power or the power required for level flight at Vc, whichever is less.
(2) Climb at maximum continuous power at the calculated best angle of climb at minimum weight.
(3) Rapid application of power and subsequent transition to best rate of climb following a power-off glide at 1.3 Vso.
(4) Sideslips and skids in level flight, climb, and glide under the conditions specified in subparagraphs (1), (2), and (3) of this paragraph, of the greatest severity likely to be encountered in normal service or in turbulent air.
(c) In the case of utility category airplanes, there shall be no evidence of malfunctioning during the execution of all approved maneuvers included in the Airplane Flight Manual. During this test the quantity of fuel in each tank shall not exceed the quantity established as the unusable fuel supply, in accordance with paragraph (b) of this section, plus 0.03 gallon for each maximum continuous horsepower for which the airplane is certificated.
(d) In the case of acrobatic category airplanes, there shall be no evidence of malfunctioning during the execution of all approved maneuvers included in the Airplane Flight Manual. During this test the quantity of fuel in each tank shall not exceed that specified in paragraph (c) of this section.
(e) If an engine can be supplied with fuel from more than one tank, it shall be possible to regain the full power and fuel pressure of that engine in not more than 10 seconds (for single- engine airplanes) or 20 seconds (for multiengine airplanes) after switching to any full tank after engine malfunctioning becomes apparent due to the depletion of the fuel supply in any tank from which the engine can be fed. Compliance with this provision shall be demonstrated in level flight.
(f) There shall be no evidence of malfunctioning during take-off and climb for 1 minute at the calculated attitude of best angle of climb at take-off power and minimum weight. At the beginning of this test the quantity of fuel in each tank shall not exceed that specified in paragraph (c) of this section.

[cessnut]

Took the words right out of my mouth. Unusable fuel is a given weight in the TCDS, not something to be arbitrarily determined in the field. As far as weighing, the most accurate method, in the absence of published procedures, is to remove all the fuel you possibly can, then add unusable. This errs on the side of conservative and removes variables like fuel density and tanks that hold more or less fuel than advertised.( Think bladders)

[GAHorn]

". The other thing that needs to be done differently than these POH's describe is to realize that , for the C-170, oil is not part of the empty weight.

Can you clarify what you meant by that? What “oil”..? Useable oil perhaps? ( I suspect you are referring to the different Empty Weight definitions in CAR 3 vs FAR 23 …(?)

CAR 3
3.73 Empty weight. The empty weight and corresponding center of gravity location shall include all fixed ballast, the unusable fuel supply (see § 3.437), undrainable oil, full engine coolant, and hydraulic fluid. The weight and location of items of equipment installed when the airplane is weighed shall be noted in the Airplane Flight Manual. (re: cessnut comment,… pls note: this refers to the AFM…not the TCDS, although it is also stated there. Similar to the IPC, the TCDS has a history of containing errors…)

Part 23
Sec. 23.29 — Empty weight and corresponding center of gravity.

(a) The empty weight and corresponding center of gravity must be determined by weighing the airplane with—
(1) Fixed ballast;

(2) Unusable fuel determined under §23.959; and

(3) Full operating fluids, including—

(i) Oil;

(ii) Hydraulic fluid; and

(iii) Other fluids required for normal operation of airplane systems, except potable water, lavatory precharge water, and water intended for injection in the engines.

(b) The condition of the airplane at the time of determining empty weight must be one that is well defined and can be easily repeated.

[gfeher]

Hmm…. Gary and Cessnut, I see your point. I was confusing residual fuel with unusable fuel. AC 43.13-1B, at 10-15.f in the Weighing Procedures chapter says:

“f. Drain the fuel system until the quantity indicator reads zero or until the tanks are empty with the aircraft in level flight attitude, unless otherwise noted in the TCDS or Aircraft Specifications. The amount of fuel remaining in the tank, lines, and engine is termed residual fuel and is to be included in the empty weight. In special cases, the aircraft may be weighed with full fuel in tanks provided a definite means of determining the exact weight of the fuel is available.”

The C-170 TCDS says in Note 1:

“The certificated empty weight and corresponding center of gravity location must include unusable fuel at 19 lb. at (+53) for Model 170 and 30 lb. at (+46) for Model 170A and Model 170B (above values are included in total fuel capacity).”

So it seems to me that the weighing procedure would be to fully drain the fuel with the aircraft in level flight attitude, weight it, and then in the weight and balance calculation add in the unusable fuel at the weight and datum location for your particular model.

[cessna170bdriver]

I’ve always had mine weighed full of fuel and subtract 225 lb at whatever the arm is to determine empty weight and CG. I also leave the headsets, handheld and portable avionics, and whatever usually lives in the glovebox and seat pockets in the airplane because it always flies with that stuff.. Legalities notwithstanding, it just seems to my old OCD instrumentation engineer’s mind that this method would have the best chance for a weight & balance being accurate at full gross weight, which seems to me where it would matter the most. (I also weigh it with oil and fly it with oil, so that doesn’t even enter into the weight and balance calculations.)

[GAHorn]

I’ve always had mine weighed full of fuel and subtract 225 lb at whatever the arm is to determine empty weight and CG. I also leave the headsets, handheld and portable avionics, and whatever usually lives in the glovebox and seat pockets in the airplane because it always flies with that stuff.. Legalities notwithstanding, it just seems to my old OCD instrumentation engineer’s mind that this method would have the best chance for a weight & balance being accurate at full gross weight, which seems to me where it would matter the most. (I also weigh it with oil and fly it with oil, so that doesn’t even enter into the weight and balance calculations.)

Miles, in other words…. you prefer to determine BOW…. “Basic Operating Weight”.

Basic Operating Weight includes Crew, crew-baggage, manuals, food, water, etc etc. but does not include trip fuel, pax, or cargo.

You are certainly aware (this is a rhetorical comment for new forum visitors) this penalizes the numbers for the airplane with regard to capability, i.e., Useful Load calculations…. for example, if the airplane be compared to others in the fleet or market.

If a person is considering purchase of an aircraft, it is worth re-considering whether or not that seaplane kit, ski-fittings, extra sound-proofing, obsolete ADF/radios or extraneous equipment is really desirable… or not.

[n2582d]

Can you clarify what you meant by that? What “oil”..? Useable oil perhaps? ( I suspect you are referring to the different Empty Weight definitions in CAR 3 vs FAR 23 …(?)

Yes, I was recalling this thread. Why the change in CAR 3 vs. FAR 23 empty weight procedure for the weight of engine oil? I suspect that back when CAR 3 rules were formed engines - such as large radials - used a lot more oil. (Kind of like going to Texaco in my ‘72 Vega — “check the fuel and add two quarts of 30 wt.”) Taking off with 1/2 a tank of oil vs. a full tank would’ve been a significant amount of weight.

[GAHorn]

I suspect it also has to do with the proliferation of turbine equipment which typically does not consume oil in measurable quantities that affect individual flights… so a universal method to weigh all types in similar methodology became preferable.