Icing conditions can rapidly deteriorate your performance in a piston aircraft. Here's what happened to one pilot in an IMC flight over mountainous terrain...
Report: Close Call In Icing Conditions Over Mountainous Terrain
We found the following NASA ASRS Report written by an ATC Center Controller...
Aircraft X was overflying the ZZZZZ mountain range and ZZZ2 which is the IAF (Initial Approach Fix) for the active runway at ZZZ1. He was picking up ice so I tried to climb him out of the conditions. He came back and said that he couldn't continue the climb to 17000 ft. and would need to go back down to 15000 ft. He then had said he needed lower and I told him I had opposite direction traffic at 14000 ft., so if he needed lower I would need to turn him to the south.
He said okay so I turned him to a 160 heading and gave him 14000 ft. He then said "I stalled" and I noticed he began a rapid descent. I still had other aircraft and did a quick traffic scan and thought he was in a dive to regain airspeed to pull out of the stall. I thought the most helpful transmission I could give was to tell him what my MIA (Minimum IFR Altitude) in the area was so if he was IMC he would know roughly what altitude he would impact the ground so he could plan his stall recovery dive accordingly. I decided in that moment to say only that and not use radio time/overwhelm him with the low altitude alert. I also wanted to leave the frequency open so he could make any requests like is there a road nearby to crash on or ask for a pilots assistance or recommendations.
I thought he had seconds to a minute to live and felt like I needed to leave the frequency open. I also looked around for a topographical map of the ZZZ1 area which has disappeared or our atlas which is hard to locate so if he made a request of where the lowest possible terrain area or roads were. He pulled out of the dive, came back up on frequency, mentioned he didn't want to climb back into the clouds so I instructed him that I couldn't leave home IFR under the MIA. I asked if he was VMC, he said yes so I quickly gave him the option of canceling IFR and switching to VFR flight following so he could stay below the clouds and remain clear of ice.
Handling An Inadvertent Icing Encounter
Ice accumulation can happen rapidly, even with an altitude change of only 1,000'. In most cases, you know where there's a chance of ice, but at times, it can occur when you don't necessarily expect it.
So what should you do if you experience an inadvertent icing encounter?
After ensuring your pitot heat is on, and your de-ice/anti-ice system is on (assuming your aircraft is equipped with it) your primary objective should be flying towards safety. The idea is simple, but the execution can be a little more complex. By descending, you (in most cases) get yourself into warmer and less icy air. If descending isn't an option, turning around might be one. Fly back to where you came from, and get back to ice-free air.
But the problem is this: if you're on an IFR flight plan, you can't just start turning around in the sky without talking to anyone.
The first thing you should do is tell ATC what is going on, and request a lower altitude or turn (climbing through ice in a piston aircraft can be difficult because of lack of excess power).
If ATC is unable to give you a turn or descent and your situation is urgent, don't wait: declare an emergency, tell ATC your intentions, and fly the plane to safer air. It might result in some paperwork after the flight, but it's much better than becoming an NTSB accident report because your airplane iced up.
Minimum IFR Altitude
This controller mentioned how the pilot recovered well below the MIA in the area, meaning they no longer had obstacle or terrain protection. That's bad news, no matter where you are. But the key in this situation is knowing what the minimum IFR altitude is.
In most cases in icing-equipped piston aircraft, ice accumulation isn't so rapid that you need to immediately change your altitude. But you do need to change altitude in a matter of minutes. That's where asking ATC for the minimum IFR altitude plays to your advantage. You'll know how far you can descend, and hopefully exit icing conditions.
Most icing bands are less than 3,000' thick, and in many cases, changing altitude as little as 1,000' can make a significant difference when it comes to ice accumulation.
Icing Problem #1: You Stall At A Lower Angle-Of-Attack
Ice adversely affects your wing's lift coefficient. According to the FAA's Flight In Icing Guide, the "Maximum Coefficient of Lift (CLmax) is significantly reduced by the ice, and the Angle of Attack (AOA) at which a stall occurs (the stall angle) is much lower with ice than without ice.
You can see in the diagram below how significant the difference is between a clean airfoil and an airfoil with ice.
So what does it really mean? When you have ice on your wing, you'll stall at a lower angle-of-attack, and at a higher-than-normal indicated airspeed.
Icing Problem #2: Drag Significantly Increases
Even at very low AOAs, ice has a significant impact on the drag coefficient of an airfoil.
As ice accumulates on your wing, drag steadily increases. An airfoil drag increase of 100 percent is not unusual, and for large-horn ice accretions, the increase can be 200 percent or even higher.
It's easy to see the problem here. With increased drag, your plane requires significantly more power to maintain airspeed and altitude. And with an increased stall speed from the ice, a reduction in airspeed (from the drag) can lead you to an early stall.
Effects Of Icing On Roll Control
Ice on your wings forward of the ailerons can dramatically affect roll control as well.
Why? Most wingtips are thinner than the root of the wing, making them more efficient at collecting ice. This can lead to a partial stall of the wings at the tips, which affects roll control. As airflow is separated around the wing, control surfaces lose effectiveness because they're no longer flying in undisturbed airflow.
Similarly, pitch and yaw control may be affected by significant ice buildup on the horizontal/vertical stabilizer of your aircraft.
What Do You Think?
Have you ever experienced an inadvertent icing encounter? How do you think this pilot and controller handled this situation? Tell us in the comments below.
Become a better pilot.
Subscribe to get the latest videos, articles, and quizzes that make you a smarter, safer pilot.
