Icing and Frost and How they Affect Your Drone
Icing is a buildup of ice on your drone. It can happen any time the temperature at your drone's altitude is a few degrees above to anywhere below the freezing point of water(32°F or 0°C). The risk of icing increases with the relative humidity.
Keep in mind that because temperature usually decreases with altitude, the temperature where your drone is flying is probably lower than the temperature where you're standing.
The biggest risk of icing occurs when there's also visible moisture in the air at or near the place where you're flying. For example, the risk of icing will be very high on a cold day when mist is rising from a lake that you're flying over, even if the air looks clear at your drone's altitude.
High humidity also increases the risk of your drone's propellers icing up, however, even if there's no visible moisture. The low-pressure area on the upper camber of the propellers, and the vortices produced by their tips, can cause moisture in humid air to condense into liquid water, which can accumulate on the propellers' surfaces even if the OAT (outside air temperature) is a few degrees above freezing.
Personally, I consider relative humidity higher than 50 percent, or a temperature / dew point spread of 5°F (9°C) or less, to be high-risk for icing in cold weather. That doesn't necessarily mean that I don't fly; but I land more often to check for ice, and I monitor the drone more carefully for signs of icing during flight.
On copter-type drones, the biggest risk areas for icing are the leading edges of the props, the air intake and exhaust ports for the cooling fan, and the motor hubs. On fixed-wing drones, all of those areas plus the leading edges of the wings and stabilizers are at high risk for ice buildup.
Icing can also cause the gimbal to lock up. This can damage the gimbal control servos because they're constantly making minor corrections to stabilize the picture. A frozen gimbal is a reason to terminate a mission and land as soon as safely possible.
Some early warning signs that your drone may be icing up include:
- The drone seems to be struggling to ascend, maintain altitude, or maneuver.
- Uncommanded yaw movements, especially when taking off or landing.
- The gimbal stops responding or you get a gimbal warning.
- Uneven responsiveness to control inputs (for example, the drone flies slower in one direction than another).
- Rapid battery discharge or battery heat warnings. Note that battery discharge usually will be more rapid on cold days, but battery overheating on a cold day is usually a sign of icing. It's caused by the increased work the motors and batteries have to do to compensate for the icing's effects on the airfoil.
If you notice any of these symptoms while flying on a cold day, land as soon as possible and inspect for icing. Icing can literally cause your drone to fall out of the sky and crash. It can also cause it to slowly descend into an area from which it can't be retrieved. So be careful.
You can reduce the risk of icing by:
- Never flying when there is visible moisture (fog, clouds, mist, rain, or snow).
- Checking the weather reports before flying and taking note of the temperature, dew point, and relative humidity.
- Either not flying, or exercising extreme caution while flying, on cold, humid days, even with clear skies.
- Being especially cautious when flying near bodies of water, even if the reported relative humidity is low. The relative humidity of the air over and around lakes and other bodies of water is usually higher than the reported relative humidity.
- Landing frequently to inspect your drone (especially its props) for icing any time you fly in cold weather.
If you do find icing on your drone, it's time to stop flying until the weather improves. You dodged one bullet. Don't push your luck.
Frost and Your Drone
I also start thinking about frost on cold days when the spread between the temperature and dew point is 5°F (9°C) or less, especially if the spread is narrowing. Under those conditions, there is a high risk of frost, even before the temperature and dew point converge.
Frost is a deposit of frozen water vapor that is more granular or crystalline than ice. It occurs when atmospheric water vapor touches a surface that is below the freezing point, and freezes without going through the liquid phase. In other words, the moisture goes from the gas phase (water vapor) directly to the solid phase (ice) without becoming a liquid.
Unlike icing, which usually forms on the props of a copter drone or the wings and empennage of a fixed-wing drone, icing can form on any surface whose temperature is below the freezing point. Also unlike icing, you don't necessarily need visible moisture for frost to form. A combination of high humidity and low temperature is enough to trigger frost building.
As with icing, finding frost on your sUAS is a good reason to call it a day. This is true even if the props aren't affected. Because of its rough surface, frost can accumulate rapidly as new frost adheres to the existing frost layer. Frost on the outside of your drone also means that it's possible that frost will form on the inside of your drone, where the electronics are. These are all good reasons to stop flying for the day if you find frost on your drone.
Revised May 19, 2022.