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Rough repair surface and boundary layer

Aviation Mechanic General (AMG) practice question 13 of 20 on Physics for Aviation. ACS element AM.I.J.R1 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONAfter a repair, the upper surface of a wing is left rough and uneven. Compared with a smooth, clean surface, what effect does this have on the wing's airflow?

  1. AThe boundary layer stays laminar farther aft, and the wing can operate at a higher angle of attack
  2. BThe boundary layer thickens and the wing stalls at a lower angle of attack
  3. CSkin friction is reduced, so drag decreases at a given speed
Show the answer and explanation

Correct answer: B. The boundary layer thickens and the wing stalls at a lower angle of attack

  • A — incorrect That is the benefit of a smooth, clean surface. Roughness moves the transition to turbulent flow forward and brings on the stall earlier.
  • B — correct As friction between the air and the wing surface increases, the boundary layer thickens, becomes more turbulent, and separates sooner. A smooth clean surface delays the onset of stall and allows a higher angle of attack, so a rough surface does the reverse.
  • C — incorrect Roughness increases friction between the air and the surface; it does not reduce skin friction or drag.

BACKGROUND

The boundary layer is a very thin layer of air that adheres to the wing surface because air has viscosity. It begins as a smooth laminar layer, then thickens and becomes turbulent as it loses speed to skin friction; where it is turbulent, drag due to skin friction is relatively high. If the turbulence becomes great enough the air breaks away from the surface and the wing stalls. One way of controlling the boundary layer is to make the wing's surface as smooth as possible and keep it free of dirt and debris. As friction between the air and the surface increases, the boundary layer thickens, becomes more turbulent, and eventually a wing stall occurs. With a smooth and clean wing surface the onset of a stall is delayed and the wing can operate at a higher angle of attack, which is why ice, or even splattered bugs on a high-speed airplane, can be a serious problem.

STUDY TIP

Treat any surface roughness left by a repair as the same kind of problem as ice or debris on the wing: more friction, thicker boundary layer, earlier stall.

KEY POINTS

  • Surface roughness increases friction and thickens the boundary layer
  • A turbulent boundary layer separates sooner, causing an earlier stall
  • A smooth, clean surface delays stall and permits a higher angle of attack

SOURCES

  • FAA-H-8083-30B ch05 "Boundary Layer Airflow"
  • FAA-H-8083-30B ch05 "Boundary Layer Control"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.