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Here is the scenario:  Family visiting and I want to take my brother-in-law (weighs 220 lbs) and his son (weighs 180 lbs at age 16) and myself (I weigh 180 lbs) on a local scenic flight.

I do the weight and balance for my rental aircraft and discover I can not take one of the passengers due to exceeding the gross weight.  My brother-in-law opts to stay behind and I take his son.  My brother-in-law went about 12 years ago with me when he was somewhat thinner and so was I.

I continue to be bothered by the fact that we all could not go.  Here is the specific information for the aircraft and If someone could look at the numbers I would appreciate feedback on whether I figured this all correct.  It was based on a 1974 Skyhawk with the following:

Basic Empty Weight of 1431.05 with an arm of 39.67 (moment of 56565.45 or 56.84)  NOTE: The weight and balance data for the aircraft states: New Empty Weight Center of Gravity is 39.72 inches ( Is this now the ARM?)

Front Passengers weight of 400 at arm of 37=14800.00 moment (14.8)

Rear Passenger weight of 180 at arm of 73= 13140 moment (13.1)

Fuel weight of 288 (48 gals) at arm of 48.6= 14000 moment (14.0)

A stated moment of -.2 for the 15 lbs of oil.


All weights added equals: 2314.05 (Gross allowed weight is 2300)

CG derived is: 98.54

CG envelope show at max weight of 2300 lbs the range is 88-108.  I could not take all 3.  Considering fuel burn taxiing prior to takeoff would the 14 pounds be acceptable know ing the hour long flight would reduce the weight by roughly 40 pounds?

I know this is a lengthy question but would appreciate help on this.  It seems to me that with full fuel it would be almost impossible to take 4 passengers in a C172 unless they were all very thin!  I also might add that the FBO I rented the aircraft from could not tell me if the aircraft had the 48 useable capacity tanks or the 38 useable tanks.  This is another issue I will address with them because no literature on the aircraft specifcally told me the tank size so I had to consider the higher weight.  With the standard tanks of 38 usable gallons the weight would have been fine however it still seems the 400 lbs would have to be in the rear seats.



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3 Answers

  1. Jim F. on Mar 16, 2013

    They don’t know the size of fuel tanks on their aircraft?!? Sorry, but I’d find another place to rent. What other vital information do they not know or are omitting from the books?

    Is there anyway you can let them (the FBO) know ahead of time to not fuel the plane after the last couple of flights? Then you can do a couple W&Bs for either tank configuration with 1/3 or 1/2 full tanks and make it work for taking both pax.

    While it is technically a 4-place aircraft, as you’re finding out, it’s rarely even a practical 3-place aircraft.

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  2. Sam Dawson on Mar 16, 2013

    1. It is a required placard for a Cessna 172 to have the fuel capacity placarded by the fuel cap for future reference. If it is not there the airplane is not airworthy.
    2. Cessna 172s will not carry four adults and full fuel. Most FBOs and flight schools where I have worked as a policy did not top off the tanks after flights just in case a person wanted to take four people on a flight. I recommend getting a “Fuel Hawk” dipstick to check the fuel in the future.
    Also in the future first figure your zero fuel weight with passengers, then see if you can take enough fuel for the mission. If so, ensure this is the fuel in th tanks- not necessarily full.
    Airplanes are not cars. We do not always fly with full fuel.

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  3. Sam Dawson on Mar 18, 2013

    Just so you understand my answer about the 172 not being airworthy without the fuel quantity being placarded I will clarify my answer. I don’t want you to just take the word of “some dude” on a website.
    In training we often use the mnemonic AROW to memorize the required items in an airplane. One of the things I don’t like about mnemonics is that people often forget the underlying information that is actually represented by a letter- in this case “O”. We like to teach people the “O” stands for “Owner’s Manual/POH/AFM” but that is not what the underlying regulation- 91.9 states. What it says is:

    “91.9 Civil aircraft flight manual, marking, and placard requirements.
    (a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.

    (b) No person may operate a U.S.-registered civil aircraft—

    (1) For which an Airplane or Rotorcraft Flight Manual is required by § 21.5 of this chapter unless there is available in the aircraft a current, approved Airplane or Rotorcraft Flight Manual or the manual provided for in § 121.141(b); and

    (2) For which an Airplane or Rotorcraft Flight Manual is not required by § 21.5 of this chapter, unless there is available in the aircraft a current approved Airplane or Rotorcraft Flight Manual, approved manual material, markings, and placards, or any combination thereof.”

    Several statements are there about placards.

    If you go to chapter 2 of your information manual for the 172 you will see a placard that must be by each fuel tank that shows the capacity of that fuel tank. In addition, there must be a placard on the fuel selector that shows the total fuel capacity as well as the capacity of each tank.

    You will also find these placards on the 172 Type Certificate Data Sheet (TCDS) for older airplanes that do not have a chapter two in the POH.


    And finally the required placards are in the Cessna maintenance manual… if you have one.

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