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Value of N in the PLANK formula

Aviation Mechanic Powerplant (AMP) practice question 10 of 13 on Reciprocating Engines. ACS element AM.III.A.K7 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA four-stroke cycle engine is running at 2,400 rpm. What value is used for N in the PLANK indicated horsepower formula?

  1. A1,200, or one-half the rpm
  2. B2,400, the same as the engine rpm
  3. C4,800, or twice the engine rpm
Show the answer and explanation

Correct answer: A. 1,200, or one-half the rpm

  • A — correct N is the number of power strokes per minute, found by multiplying rpm by one-half, so 2,400 rpm gives 1,200.
  • B — incorrect A four-stroke cylinder fires only once every two revolutions, so power strokes per minute are half the rpm.
  • C — incorrect Doubling the rpm moves in the wrong direction; the four-stroke cycle produces fewer power strokes than revolutions.

BACKGROUND

Indicated horsepower is the power developed in the combustion chambers without reference to friction losses inside the engine. For a four-stroke cycle engine it is calculated with a formula whose numerator symbols spell PLANK: P is indicated mean effective pressure in psi, L is the length of the stroke in feet, A is the piston head area in square inches, N is the number of power strokes per minute, and K is the number of cylinders. The product is divided by 33,000 because one horsepower is work done at the rate of 33,000 foot-pounds per minute. N is found by multiplying rpm by one-half, and the handbook's own example uses N equal to one-half of 3,000, or 1,500.

STUDY TIP

Write out what each PLANK letter stands for before plugging in numbers, and halve the rpm for N every time on a four-stroke engine.

KEY POINTS

  • PLANK stands for pressure, length, area, power strokes per minute, and cylinders.
  • N equals one-half the rpm on a four-stroke cycle engine.
  • The product of PLANK is divided by 33,000 foot-pounds per minute.

SOURCES

  • FAA-H-8083-32B ch01 "Indicated Horsepower"
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MORE ON RECIPROCATING ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.