What is the maximum allowed frost thickness on the underside of the wing due to cold-soaked fuel?

• A. 1/8 inch
• B. 1/4 inch
• C. 3/16 inch
• D. 1/2 inch

Answer: (A)

Frost on the underside of a wing changes the surface that the air flows over, roughening it and altering the wing’s effective shape. This disrupts the smooth airflow needed to generate lift, raising the stall speed and reducing available lift during takeoff. Because of these safety concerns, there is a strict maximum frost thickness allowed for frost formed from cold-soaked fuel. The limit is 1/8 inch. Keeping frost within this small thickness helps ensure the wing will perform as designed during rotation and initial climb. If frost is thicker than that, it needs to be removed before flight, since thicker frosting would meaningfully degrade lift and controllability. The larger thickness options would exceed safe margins and are not permitted.