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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.
The Importance of the Temperature / Dew Point Spread
The temperature at which the relative humidity will reach 100 percent is called the dew point. When the temperature and dew point are the same, visible fog or mist may start to form because at 100 percent relative humidity, the air is saturated and can't hold any more water.
That convergence point is also where ice or frost may start building up on your drone, especially on the props, even if you don't see any visible moisture from where you're standing on the ground. That's because the air temperature usually decreases with altitude and brings the temperature and dew point closer; and also because the spinning props cool the air.
Because the dew point provided by weather information sources takes all of the weather conditions at a particular place and time into consideration, it's a very useful measure of the likelihood that your drone will experience icing or frost on a given flight. It's a very important weather factor that should always be part of flight planning on a winter day. The narrower the spread between the temperature and dew point, the more likely it is that your drone will experience icing or frost.
Personally, I consider a temperature / dew point spread of less than 5°F (3°C), to be high-risk for icing in cold weather. That's because the temperature typically drops with altitude, so the temperature / dew point spread at flight altitude usually will be closer at flight altitude than at launch altitude.
That's also why if I decide to fly on a day when the spread is exactly 5°F (3°C), I land frequently to check for icing and frost. I can't tell the temperature at altitude by looking at the sky.
Icing and Your Drone
On copter-type drones, the biggest risk areas for icing are the leading edges and lower cambers of the props, the air intakes and exhaust ports for the cooling fan, the camera gimbal, 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. The picture at the top of this page shows actual icing on the prop of a drone.
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 in weather near or below freezing 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
Photo courtesy of Reddit user u/uaspandion.
I also start thinking about frost on cold days when the spread between the temperature and dew point is 5°F (3°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, frost occurs when atmospheric moisture goes from the gas phase (water vapor) directly to the solid phase (ice) without becoming a liquid. It can be easily recognized by its rougher, crystalline texture.
Unlike icing, which usually forms on the props of a copter drone or the wings and tail 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 formation.
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 20, 2023.