14.3 Forward and Aft CG Effects
Key Takeaways
- Forward CG: more longitudinally stable, higher stall speed, more elevator to rotate and flare, longer takeoff and landing, easier stall recovery (PHAK Chapter 10; FAA-H-8083-1).
- Aft CG: less stable, lower stall speed, less elevator needed to pitch up, poorer stall and spin recovery, possibly a slightly faster cruise from less tail download.
- Weight and CG are separate. High weight hurts runway, climb, and stall speed. An out-of-limit CG hurts controllability even when the airplane is light.
- Fuel-burn CG travel depends on where the tanks sit relative to the current CG. Compute landing CG. Do not assume it stays put — even though many trainer wing tanks sit near the CG and the inch barely moves.
ACS PA.I.F.K2e and the stall-and-spin knowledge behind PA.VII ask what happens when the same airplane is loaded on the forward side of the envelope versus the aft side. PHAK Chapter 10 and FAA-H-8083-1 treat that as a controllability story first and a performance story second. The envelope exists because the horizontal tail has a limited moment it can make.
Why the tail cares where the pounds sit
On a conventional airplane the center of pressure (center of lift) of the wing sits aft of a legal CG. Wing lift, acting behind the mass, produces a nose-down couple. The horizontal tail flies at a downward lift (a download) to hold the nose where you want it. The pilot and the trim tab set how much download.
Move the CG forward and that nose-down couple grows: a longer lever arm between mass and wing lift. The tail must download harder. Move the CG aft and the couple shrinks. Near the aft limit the tail may be nearly unloaded. Past the aft limit the wing-lift couple can go nose-up, and the tail may not be able to push the nose back down — which is exactly when you needed it in a stall.
The tail download is not free. The wing must lift airplane weight plus the tail download. More download means the wing works at a higher angle of attack (or a higher speed) to hold level flight. That is the seed of every “forward CG raises stall speed” item.
Forward CG — stable, slow to rotate, honest in a stall
| Effect | Why |
|---|---|
| More longitudinal stability | A larger nose-down couple; the airplane wants to return to the trimmed AOA |
| Higher stall speed | Extra tail download means the wing must make more lift, so it reaches CL-max at a higher speed |
| More elevator to rotate and to flare | The tail is already working hard; remaining nose-up authority is smaller |
| Longer takeoff and landing | Rotation comes late or not at all; approach must be faster; flare may not raise the nose |
| Easier stall recovery | The airplane already wants to pitch down when the wing quits |
| More induced drag in cruise | Higher AOA plus a working tail download; cruise is a bit slower / thirstier |
A CG forward of the forward limit is how pilots write off nosewheels and run off the far end. The elevator hits the stop and the nosewheel is still planted. In the flare the same stop leaves you flat. The airplane is “stable” in the worst way: it will not pitch when you ask.
Aft CG — lighter on the stick, meaner at the stall
| Effect | Why |
|---|---|
| Less longitudinal stability | Smaller restoring couple; the airplane is more willing to keep a pitch disturbance |
| Lower stall speed | Less tail download, so the wing supports something closer to true weight |
| Less elevator to flare — and less to recover | Pitch-up is easy; pitch-down after a stall may not be available |
| Possibly slightly faster cruise | Less download, less induced drag |
| Poorer stall and spin recovery | PHAK: exceeding the aft limit is the critical CG violation in a stall |
An aft-CG airplane feels “nice” on a first takeoff — it rotates early and cruises a knot or two faster. That niceness is the same reduced stability that will not damp a phugoid and will not help you in a departure stall with the ball off-center. PA.VII.D spin awareness and this section are the same sentence: do not take an aft-limit bust into slow flight.
Cruise being “a little faster” at an aft CG is a PHAK/handbook observation, not a license to load to the aft stop for a cross-country. Stay inside. The extra knot is not a procedure.
Weight versus CG — two different failures
Students mash the two ideas together: “we’re heavy, so we’ll be tail-heavy.” False. You can be
- Heavy and centered — legal CG, illegal or barely legal weight. Takeoff distance, climb gradient, and stall speed all suffer. Controllability is still normal.
- Light and aft — a solo pilot, three bags in the rear, tanks low. Weight is no problem. The elevator may already be near its nose-down stop.
- Heavy and forward — four adults, empty baggage, full fuel in a nose-forward tank. Rotation and landing flare are the problems, on top of the performance hit.
Example 2 in 14.2 was heavy with a still-legal 46.3-in CG. Example 1 after the bag came out was light at 39.7 in. Those are different airplanes to fly. The ACS skill is to name which limit you are near and what that limit does to you.
High weight always raises stall speed (square root of weight) and always lengthens the runway numbers. CG position changes how much elevator you have left and how the stall behaves. A 2,548-lb airplane at 45.1 in and a 2,548-lb airplane at 46.2 in (the two 60-lb offloads in 14.2) have almost the same Chapter 13 performance and not the same pitch feel.
Fuel burn — compute, do not assume
As usable fuel leaves a tank, you subtract that tank’s weight and its moment. The CG walks toward the opposite side of the tank: if the tank is forward of the current CG, burning fuel moves CG aft; if the tank is aft of the current CG, burning fuel moves CG forward; if the tank sits on the current CG, the inch does not move.
Common PAR trainers keep the main tanks in the wings near the CG. The landing CG is often within a few tenths of an inch of the takeoff CG, the way Example 2 walked only from 46.2 in to 46.1 in after 30 gallons. That is a typical trainer fact, not a law. Tip tanks, a fuselage header, a baggage-compartment ferry tank, or a twin with fuel in the engine nacelles will walk the CG for real.
The knowledge-test trap is the absolute sentence: “fuel burn always moves CG forward” or “always aft.” The handbook sentence is: find the tank arm, subtract the burned fuel, divide again. Compute takeoff and landing (and any intermediate landing). Do not skip the landing line because “Cessnas don’t move.”
If landing CG would be out of limits after a long burn, you do not “hope the last hour is fine.” You leave fuel in a different tank, leave bags off, or do not go.
Scenario: Priya’s aft-limit solo
Priya is solo in the same 1,500-lb BEW trainer. She puts 80 lb of camping gear at station 95 and fills the tanks. Weight is easy. CG sits at 46.8 in — still inside 47.3, but she is now flying the aft airplane: light stick, early rotation, a stall that does not drop the nose the way last week’s two-up lesson did. She treats slow flight and power-on stalls with extra respect, and she does not add more bags “because we were under gross.” Under gross was never the only question.
Compared with a mid-envelope loading, an aft CG typically produces which combination?
Why does a forward CG raise stall speed and make the landing flare harder?
A four-place trainer burns 30 gallons from wing tanks whose arm is close to the takeoff CG. What should the pilot do about landing CG?