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Consider Cessna 172 which has 4 seats, including the pilot, with the seat configuration as the picture below. Say the airplane has no passenger, only the pilot him/her self. I am not sure where it center gravity (CG). For longitudinal (roll), probably we can consider it separate between left and right seats. But for pitch, I don't have any idea.

My question is, if a pilot want to fly his/her airplane, how he/she assign stability control so the airplane will not has tendencies, either regarding the longitudinal (roll) or the pitch. What else control he/she needs to apply to make the airplane safely fly due to the unbalance load?

Cessna 172's seat configuration

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3 Answers 3

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Use the available trim (elevator, and rudder if installed), and then manage the left/right fuel tanks if they are not feeding evenly. Use the autopilot if so equipped, even if it's just a single axis wing leveler.

With just 1 passenger, it is hardly out of balance tho. The POH has a Weight & Balance chart as seen here How are the limits of the center of gravity chart established?, plotting front seat weight and fuel will show the plane to be well within limits.

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  • $\begingroup$ Also, a single pilot would typically sit front & left in a C172 as that is where the primary panel instruments are usually located, along with useful things like the key to start the airplane. $\endgroup$
    – CrossRoads
    Commented Dec 11, 2018 at 15:47
  • $\begingroup$ You will also adjust trim from time to time, as airspeed changes, fuel is burned, &c. The idea is basically to trim so that the aircraft maintains whatever attitude you've set it at with no control input. $\endgroup$
    – jamesqf
    Commented Dec 11, 2018 at 18:58
  • $\begingroup$ Dear @CrossRoads, I quote this "a single pilot would typically sit front & left in a C172 as that is where the primary panel instruments are usually located." But that position is just mirroring the situation of the seat depiction, not fully related to the balancing effort. $\endgroup$
    – AirCraft Lover
    Commented Dec 13, 2018 at 2:01
  • $\begingroup$ Dear @jamesqf, yes, there have to be done. That also must be done to a parachute jump airplane which the load is changed drastically after the passenger have jumped. $\endgroup$
    – AirCraft Lover
    Commented Dec 13, 2018 at 2:03
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He'll have to keep some control input to keep the aircraft flying straight and level depending on how severe the imbalance is.

If there are trim controls, those can be used to help, alleviating the need to provide input manually (the C172 has only elevator trim btw., no rudder or aileron trim).

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  • $\begingroup$ But what I need actually is a specific action to be done when facing such situation. Of course, when a pilot entering his/her plane, or even before entering his/her plane, he/she should have known the load. Thence the pilot decided what to do, even when it still ramp. It required to avoid tendencies. $\endgroup$
    – AirCraft Lover
    Commented Dec 13, 2018 at 2:07
  • $\begingroup$ I don't do any specific thing in my plane. In my plane, the stabilator trim (vs elevator in a 172) is still where it was set from my previous landing. When I take off, I may adjust it for a little more up-trim or down-trim as needed. You can't tell how much until you are climbing out and determine how much you are pulling on the yoke to hold the deck angle you want. If just a short climb to pattern altitude for takeoff/landing practice, maybe very little change. If going up for a longer flight, maybe a different deck angle for faster speed/shallower climb. There is no hard & fast rule. $\endgroup$
    – CrossRoads
    Commented Dec 13, 2018 at 14:17
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For front to rear center of gravity, longitudinal loading in other words, always try to load heaviest forward/lightest aft. If you have a fat slob with you and a small person, put the small person in the back seat and the fatso beside you. Don't load two in the back with the front seat empty unless you work out the CoG calculations and determine you are in limits. If you are in the certified range the airplane will handle fine and is flown normally.

Laterally, not a lot of options and the effects of lateral imbalance are pretty mild if the fuel levels are more or less equal in a 172, so not too big a deal.

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  • $\begingroup$ The 172 has four seats, but the rear ones are only really useful for children or baggage. Three adults plus full fuel can easily put you at/above max weight, regardless of CG. $\endgroup$
    – StephenS
    Commented Dec 11, 2018 at 17:28
  • $\begingroup$ Flying with full fuel all the time is kind of a renter's luxury. I've flown 172s with four adults aboard and half tanks or so and they fit fine. When I flew a 185 on floats in the bush, I was always going with less than half tanks, generally fuel for the trip plus 10 gallons or so, and rarely more than 30-35 imp gallons total, so I could take a full cabin load. In the commercial world any more fuel than necessary to get there and back, with a safety reserve, is ballast. $\endgroup$
    – John K
    Commented Dec 11, 2018 at 18:19
  • $\begingroup$ I can see it's a luxury it you rent wet, but renting dry only works if everyone has to top off at return. The "ballast" only really matters if it puts you over max weight, and I'd look at moving to a bigger plane rather than give up options if something goes wrong in the air. That's the real renter's luxury: using the best plane for each mission. $\endgroup$
    – StephenS
    Commented Dec 12, 2018 at 2:23
  • $\begingroup$ Dear @JohnK, I quote this: For longitudinal loading, always try to load heaviest forward and lightest aft. If you have a fat slob with you and a small person, put the small person in the back seat and the fatso beside you. Is any the more technically explanation? Don't we need to consider its traverse/pitch load? In my opinion, should be heavier aft as we have flap. Of course we have slat, but flab is look longer. $\endgroup$
    – AirCraft Lover
    Commented Dec 13, 2018 at 2:20
  • $\begingroup$ You don't want the center of gravity too far aft. You just load to keep the CofG in the certified range, if you have the luxury of doing calculations. If you observe the light/aft rule, you can be sure you will be in the range if you are unable to do a weight and balance calculation, which happens quite a lot in bush flying. $\endgroup$
    – John K
    Commented Dec 13, 2018 at 2:27

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