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Pressure thrust from a choked exhaust nozzle

Aviation Mechanic Powerplant (AMP) practice question 5 of 8 on Engine Exhaust and Reverser Systems. ACS element AM.III.L.K2 · 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 choked exhaust nozzle has an outlet opening of 250 square inches, and the pressure inside the nozzle is 6 psi higher than ambient. How much pressure thrust is being produced?

  1. A1,500 pounds per square inch
  2. B750 pounds of thrust
  3. C1,500 pounds of thrust
Show the answer and explanation

Correct answer: C. 1,500 pounds of thrust

  • A — incorrect The arithmetic gives the right number, but pounds per square inch is the unit of the pressure differential, not of thrust; multiplying psi by square inches yields pounds of force.
  • B — incorrect This is half of the correct product and does not follow from multiplying the full 6 psi differential by the full 250 square inches of nozzle area.
  • C — correct Pressure thrust is found by multiplying the pressure difference between the inside of the nozzle and ambient air by the area of the nozzle opening: 6 psi x 250 sq in = 1,500 pounds.

BACKGROUND

When the velocity of the exhaust gases at the nozzle opening reaches Mach 1, the flow passes only at that speed and does not increase or decrease. If there is sufficient flow to maintain Mach 1 at the opening plus extra flow being restricted by that opening, the result is a choked nozzle. The extra flow builds up pressure inside the nozzle, and this is sometimes called pressure thrust. A differential in pressure then exists between the inside of the nozzle and the ambient air, and pressure thrust is calculated by multiplying that pressure difference by the area of the nozzle opening. Many engines cannot develop pressure thrust because most of the gas energy is used to drive turbines that turn propellers, large fans, or helicopter rotors.

STUDY TIP

Memorize the relationship as pressure difference times nozzle area, and watch the units: psi times square inches gives pounds of force. Confirm the nozzle is choked before expecting any pressure thrust at all.

KEY POINTS

  • A choked nozzle exists when flow is at Mach 1 at the opening and extra flow is restricted.
  • Pressure thrust equals the nozzle pressure differential multiplied by the nozzle opening area.
  • Engines that spend most gas energy driving turbines usually develop no pressure thrust.

SOURCES

  • sources/amp-powerplant/ch03.txt "Convergent Exhaust Nozzle"
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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.