Practice test
Unit 11 · Performance and Weight & Balance · Lesson 3 of 3

Weight and balance

11 min readLesson 48 of 593D lab

By the end of this lesson you'll be able to:
  • Explain datum, arm, moment and CG
  • Compute a loaded CG
  • Solve a weight-shift problem

The terms

  • Datum: an imaginary vertical line chosen by the manufacturer, from which all distances are measured. Often the firewall or the front of the spinner.
  • Arm: the distance from the datum to an item, in inches. Positive is aft of the datum.
  • Moment: weight × arm (often shown divided by 1,000 to keep numbers small).
  • Center of gravity (CG): total moment ÷ total weight.

Useful weights: avgas 6 lb/gal, oil 7.5 lb/gal, water 8.35 lb/gal.

Computing the CG

ItemWeight (lb)Arm (in)Moment (lb-in)
Empty airplane (incl. oil)1,50038.057,000
Pilot and front passenger34037.012,580
Rear passenger15073.010,950
Fuel (40 gal × 6)24048.011,520
Baggage3095.02,850
Total2,26094,900

CG = 94,900 ÷ 2,260 = about 42.0 inches aft of the datum. Now check the POH: is 2,260 lb at or below max gross weight, and is a 42.0-inch CG inside the envelope at that weight? Also check that it stays in limits as fuel burns off.

Shifting weight

If you move weight, the CG moves by a predictable amount:

weight moved ÷ total weight = CG change ÷ distance moved

Example: moving a 30 lb bag 50 inches forward in a 2,260 lb airplane. CG change = 30 × 50 ÷ 2,260 ≈ 0.66 inches forward.

Use the lab. The weight and balance lab lets you load people, bags and fuel and watch the CG move on the envelope in real time.
Try it in 3D: Weight & balance in 3DLoad people, bags and fuel and watch the CG move on the envelope.

Key takeaways

  • Moment = weight × arm; CG = total moment ÷ total weight.
  • Avgas 6 lb/gal, oil 7.5 lb/gal.
  • Weight shift: weight moved ÷ total weight = CG change ÷ distance moved.

What the examiner may ask

Practice answering out loud before you open each answer.

Is the airplane within weight and balance limits for today's flight?
Show your computation for takeoff and landing (fuel burned), and compare with the POH envelope.
What happens to the CG as you burn fuel?
It moves forward or aft depending on where the fuel tanks are relative to the CG. Check landing CG too.
What's the difference between basic empty weight and useful load?
Basic empty weight includes unusable fuel and full oil; useful load is max gross weight minus basic empty weight, covering people, baggage and usable fuel.

Common mistakes

  • Using standard weights for passengers instead of real weights.
  • Forgetting to check the CG at landing as fuel burns off.
  • Exceeding the baggage compartment limit even when total weight is fine.

Read it in the FAA sources

PHAK · Chapter 10Weight and BalanceOpens the PHAK at this chapter ↗

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

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