Cross-sectional area of a 5-inch cylinder bore

Aviation Mechanic General (AMG) practice question 12 of 25 on Mathematics. ACS element AM.I.H.K1 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONThe bore (inside diameter) of an aircraft engine cylinder is 5 inches. Using 3.1416 for pi, what is the cross-sectional area of the cylinder?

AApproximately 78.54 square inches
B19.635 square inches
C15.708 square inches

Show the answer and explanation

Correct answer: B. 19.635 square inches

A — incorrect This results from squaring the 5-inch diameter instead of the 2.5-inch radius; the area formula uses the radius.
B — correct Radius = 5 ÷ 2 = 2.5 inches; area = 3.1416 × (2.5)² = 3.1416 × 6.25 = 19.635 square inches.
C — incorrect This is the circumference of a 5-inch circle (5 × 3.1416), a linear measure, not the area.

BACKGROUND

The area of a circle is A = π × r², where π is 3.1416 and r is the radius. The radius is always one-half of the diameter, so a bore of 5 inches has a radius of 2.5 inches. Substituting gives 3.1416 × (2.5 inches)² = 3.1416 × 6.25 square inches = 19.635 square inches. This is the handbook's own cylinder cross-section example. The same area calculation is the first step in finding piston displacement.

STUDY TIP

Always halve the diameter before squaring; using the diameter in the area formula gives an answer four times too large.

KEY POINTS

Circle area = π × r²; radius = diameter ÷ 2.
5-inch bore: radius 2.5 in, area 19.635 sq in.
Circumference (d × π) is a length, not an area.

SOURCES

FAA-H-8083-30B ch03 "Circle" (area of the cylinder cross section example)

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