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Vertical extent of exhaust plume turbulence

Remote Pilot (Part 107) practice question 35 of 36 on Airspace / Operational Requirements. ACS element UA.II.B.K4f · difficulty: recall. Want to answer it in a timed quiz? Use the Remote Pilot (Part 107) practice mode.

Question bank updated 2026-09-22

QUESTIONStudies cited in the Aeronautical Information Manual predict that the significant turbulent effects of an exhaust plume from a smoke stack or cooling tower can extend to what height?

  1. AOver 1,000 feet above the top of the stack
  2. BNo more than 500 feet above the top of the stack
  3. CAbout 100 feet above the top of the stack
Show the answer and explanation

Correct answer: A. Over 1,000 feet above the top of the stack

  • A — correct The manual states that some studies predict the significant turbulent effects of an exhaust plume can extend to heights of over 1,000 feet above the height of the top of the stack or cooling tower.
  • B — incorrect No 500 foot ceiling on plume turbulence appears in the guidance. Using that figure would place an aircraft well inside the region where significant turbulence is predicted.
  • C — incorrect This figure is far too low. The predicted effects reach many times that height, which is why pilots are told to steer clear rather than simply overfly the structure.

BACKGROUND

Exhaust plumes are visible or invisible emissions from power plants, industrial production facilities or other industrial systems that release large amounts of vertically directed unstable gases. High temperature plumes can cause significant air disturbances such as turbulence and vertical shear, and other hazards include reduced visibility, oxygen depletion, engine particulate contamination, exposure to gaseous oxides and icing. Effects are most pronounced in calm stable air where the plume is very hot and the surrounding area is still and cold, while any amount of crosswind helps dissipate them. The size of the tower or stack is not a good indicator of the effect, because the major factors are the heat or size of the effluent, the ambient air temperature and the wind speed, and smaller aircraft can expect to feel an effect at a higher altitude than heavier aircraft. When able, a pilot should steer clear by flying on the upwind side of smokestacks or cooling towers, and should remember that a stack with no visible plume may still be in full operation.

STUDY TIP

Fix the 1,000 foot figure in mind and pair it with the upwind avoidance technique. Also remember that an invisible plume is just as hazardous as a visible one.

KEY POINTS

  • Significant plume turbulence can extend over 1,000 feet above the top of a stack or cooling tower.
  • Effects are worst in calm, cold, stable air; crosswind helps dissipate them.
  • Fly on the upwind side of stacks and treat stacks without visible plumes as active.

SOURCES

  • AIM 7-6-16 "Avoid Flight in the Vicinity of Exhaust Plumes (Smoke Stacks and Cooling Towers)"
  • sources/aim/aim_7-6_potential_flight_hazards.txt
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SkyDrill is not affiliated with the FAA. This is an original practice item based on 14 CFR part 107, 14 CFR part 89, and the FAA Remote Pilot – Small UAS Study Guide (FAA-G-8082-22). Regulations and handbooks are revised; confirm details against the current edition.