According to the NTSB, frost the size of a grain of salt, distributed as sparsely as one per square centimeter over a wing's upper surface, can destroy enough lift to prevent a plane from taking off.
Worse yet, the NTSB points out that small patches of ice or frost can result in localized, asymmetrical stalls on the wing, resulting in roll control problems during lift off - not what you want to happen as you're rotating.
I'm Fine, It's Only A Little Frost
How can a little frost cause such big problems? Jagged frost particles cause disruptions to the airflow over the wings and tail, destroying significant amounts of lift. According to the FAA, the mere presence of frost can:
- Reduce a wing's maximum lift by 30 percent or more
- Reduce the angle of attack for maximum lift by several degrees
- Increase drag significantly
- Drastically change the aircraft's handling qualities and performance
As you can see in the diagram below, frost causes major disruptions in airflow over a wing, often preventing the airplane from generating enough lift to take off.
When Does Frost Occur?
On calm nights when the ground is cooler than the overlying air, the ground can lower the air temperature below dew point. When this happens, moisture in the air condenses on the ground, buildings, or your aircraft. If the temperature is above freezing, dew forms - but if the temperature is below freezing, frost appears.
How Do I Remove Frost From My Airplane?
The FAA provides guidance on how to prevent or remove frost from your airplane. Their four recommendations on frost are:
- Use wing covers to prevent frost accumulation on wings
- Wait for frost to melt
- Store the aircraft in a heated hangar
- Deice the wing surface
What About Polishing the Frost?
Many of us have learned to polish frost smooth with a broom - doesn't that make a takeoff OK? The FAA recommends that pilots remove all frost or ice from aircraft wings and control surfaces. Manufacturers and the FAA don't know what level of ice or frost is safe for flight - so they don't know when polishing frost is "good enough."
In fact, large aircraft operators are prohibited under FAR 91 Subpart F from polishing frost smooth - they must remove all of it prior to takeoff. While light GA aircraft don't fall under this rule, it makes sense to comply with it. Our wings are just as susceptible to the reduction in lift and increase in drag caused by ice and frost. And, removing a wing cover on a cold morning is a lot easier than balancing on a ladder trying to scrub off frost.
