Frost
On a recent November morning, around 8:30, I went over to the airport to practice slow flight with our new vortex generators. Since I was there I wiped the dew and bugs off of the Cherokee. We’d had some rain so the dew was pretty thick. While I was wiping the dew off of the pilot side wing, I notice that it wouldn’t come off of the wing root. The root was still in the shade of the fuselage and even though the low temperature was in the high 30’s the previous night there was still a substantial amount of ice on the wing. The sparkling clear night allowed the heat from the airplane to radiate out into space and cooled the airplane to below freezing. As the dew condensed, it turned to ice.
Had I been there an hour earlier, I suspect that both wings would have been covered in clear ice that looked a lot like dew. In colder climates, frost on the wings is regular occurrence in the winter and can be deadly.
AC 135-17 PILOT GUIDE Small Aircraft Ground Deicing
Test data indicate that ice, snow, or frost formations having thickness and surface roughness similar to medium or course sandpaper on the leading edge and upper surfaces of a wing can reduce wing lift by as much as 30 percent and increase drag by 40 percent.
Larger accretions can reduce lift even more and can increase drag by 80 percent or more. AOPA
Handbook of Aeronautical Knowledge p 10-14
DEW AND FROST On cool, calm nights, the temperature of the ground and objects on the surface can cause temperatures of the surrounding air to drop below the dewpoint. When this occurs, the moisture in the air condenses and deposits itself on the ground, buildings, and other objects like cars and aircraft. This moisture is known as dew and sometimes can be seen on grass in the morning. If the temperature is below freezing, the moisture will be deposited in the form of frost. While dew poses no threat to an aircraft, frost poses a definite flight safety hazard. Frost disrupts the flow of air over the wing and can drastically reduce the production of lift. It also increases drag, which, when combined with lowered lift production, can eliminate the ability to take off. An aircraft must be thoroughly cleaned and free of frost prior to beginning a flight.
The Knowledge Tests have several questions on frost.
Private
Q: How will frost on the wings of an airplane affect takeoff performance?
A: Frost will disrupt the smooth flow of air over the wing, adversely affecting its lifting capability.
Q: Which conditions result in the formation of frost?
A: The temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dewpoint is below freezing.
Q: Why is frost considered hazardous to flight?
A: Frost spoils the smooth flow of air over the wings, thereby decreasing lifting capability.
Instrument
Q: Test data indicate that ice, snow, or frost having a thickness and roughness similar to medium or coarse sandpaper on the leading edge and upper surface of an airfoil can
A: reduce lift by as much as 30 percent and increase drag by 40 percent.
Q: Why is frost considered hazardous to flight operation?
A: Frost causes early airflow separation resulting in a loss of lift.
Q: Which conditions result in the formation of frost?
A: Temperature of the collecting surface is below the dewpoint of surrounding air and the dewpoint is colder than freezing.
Commercial
Q: Frost covering the upper surface of an airplane wing usually will cause
A: the airplane to stall at an angle of attack that is lower than normal.