We have a PA28-235 with 25 gal main fuel tanks and 17 gal aux tanks. The POH says we should use the mains first without giving a reason. It makes sense to me to use aux tanks as soon as possible after reaching cruise altitude to make it easier for autopilot to control along the longitudinal axis. Why should aux tanks not be used until the mains have been used first?
@Robert Coulter
Pure guess – burning the inboards first reduces the amount of lateral moment change making it easier to stay in trim laterally, especially in turbulence.
@Ronald Levy
Good guess, Ron (probably an educated guess). The copy of the PA-28-235B POH I have states “In order to keep the airplane in best lateral trim during cruising flight, the fuel should be used alternatively from each main tank, and when they are exhausted, from each tip tank.”
They use confusing terminology in the POH, referring to the outboard tanks as Aux tanks in one place and Tip tanks in other (most) places. Normally, if they are truly Aux tanks, takeoff and landing would not be allowed with them selected. The TCDS for the PA-28-235 refers to “Tip tanks”.
By using the fuel from the inboard tanks, you also spread out the weight distribution, which eases the load on the airframe. If you used the tips first, most of the weight would be located near the longitudinal line and create more of a bending moment on the wing attach points during turbulence.
“By using the fuel from the inboard tanks, you also spread out the weight distribution, which eases the load on the airframe. If you used the tips first, most of the weight would be located near the longitudinal line and create more of a bending moment on the wing attach points during turbulence.”
The closer any weight is to the CG the less the bending moment and thus strain on the air frame.
@James Jopson
I suggest you read this article that was specifically written to address wing bending moments in the Cherokee 235.