How Freezing Rain Forms
According to the FAA, freezing rain is created when snow falls through a warm layer, then through a sub-zero layer of air. This temperature profile is known as a temperature inversion. Looking at the diagram below, there are three basic layers of air in this scenario. A below-freezing layer aloft, an above-freezing layer in the middle, and another below-freezing layer of air at the surface.
As the melted drops exit the middle (above-freezing) layer and fall through the lower-freezing layer, they move so quickly that they don't have time to completely freeze before they get to the ground.
So what happens to these cold water drops? They become supercooled, creating a significant icing risk to aircraft. What is supercooling though?
Supercooling is a state where a liquid is below freezing but isn't a solid (in this case, ice), meaning that as the droplets fall through the atmosphere they can't crystalize. But when supercooled droplets impact the surface of your aircraft, they stick and freeze.
Why Are Supercooled Large Droplets Dangerous?
Large drops, like you'd expect with freezing rain, can form a heavy glaze on your airframe (clear ice) that can be difficult to remove, especially if ice forms aft of de-icing equipment. Clear ice can also be hard to see because of its smooth, transparent appearance.
ATC Recording: Inadvertent Icing Encounter In Freezing Rain
In 2020, an accident occurred on approach to Lubbock, Texas, from what was likely freezing rain.
Before we dive into the NTSB's preliminary report, listen to the ATC audio and radar imagery in the video below.
The NTSB Preliminary Report
The following report was published by the NTSB shortly after this accident.
On October 26, 2020, at 1558 central daylight time, a Cessna 210 airplane, N9622T, was destroyed when it was involved in an accident near Lubbock, Texas. The private pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
The airplane was operated on an instrument flight rules (IFR) flight plan from Belen Regional Airport (BRG), Belen, New Mexico, to Corsicana Municipal Airport (CRS), Corsicana, Texas, but had diverted to Lubbock Preston Smith International Airport (LBB), Lubbock, Texas.
A review of the air traffic control recordings and ADS-B data revealed the airplane was in instrument meteorological conditions (IMC) during the descent toward LBB and the pilot reported that he had been in IMC "for quite a while." The pilot was instructed to set up for the RNAV (GPS) Y instrument approach to runway 35L. During the approach, the pilot was unsure of the instrument approach to expect and was not in position to intercept the final approach course, so the controller vectored him to the east to set up for the same approach with a different initial approach fix (IAF).
When queried by the controller, the pilot reported that he was experiencing structural icing and was in "freezing rain." After the airplane crossed the intermediate fix, ZOVOC, and turned inbound, the groundspeed (gs) gradually decreased from about 80 kts to about 50 kts. After crossing the final approach fix, UFACI, about 4,700 ft mean sea level (msl) and 48 kts gs, the airplane made a left turn toward south-southeast and descended. The pilot reported to the controller that the airplane experienced an autopilot issue, so the controller provided new vectors to the pilot. The flight track showed that the airplane continued to descend, then made a sharp left turn before the data ended. The controller reported that radar contact was lost and there were no further communications from the pilot. Figure 1 shows the ADS-B flight track overlaid onto Google Earth with the approach fixes and the accident site labeled.
The responding Federal Aviation Administration (FAA) inspectors reported that the airplane impacted a residential area about 200 yards from the final recorded ADS-B point and about 6 miles south of LBB. A post-impact fire consumed most of the fuselage and the inboard sections of each wing. The inspectors found numerous chunks of ice in the wreckage near the wings, and pieces still attached to some of the airplane's leading-edge surfaces. The ice chunks were concave-shaped and featured a smooth surface on the inside of the curve (likely the shape of the wing's leading edge). The ice ranged from 1 to 2 inches thick.
What Can You Do?
Specific to freezing rain, the warm layer aloft is a well-known phenomenon. If you know there's warmer air above you, climbing may be an option. However, that may only a real option for jet with a lot of excess thrust. If you don't have the performance to climb out of the conditions, turning around and flying back to non-icing weather is another option.
Freezing rain can overwhelm most anti/de-icing equipment quickly and entirely coat non-protected surfaces. Regardless of your decision to climb, turn around, or take another action, you should always exit freezing rain conditions as quickly as possible.
Have you encountered freezing rain on the ground or in the air? Tell us about your icing experiences in the comments below.
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