One of the joys of instrument flying is bursting above a cloud layer to find pristine blue skies, or breaking out of an overcast 200 ft above the ground and finding the runway perfectly lined up with your glide path. But flying in and out of clouds means that you are much more likely to encounter one of aviation's most fatal hazards: structural icing.
What is Structural Icing?
Structural icing is the accumulation of ice on the exterior of the aircraft. It can happen on any surface of the aircraft, but small and narrow objects will typically ice up the fastest. As an IFR pilot, you have to be familiar with all the kinds of icing, and the effect it has on your aircraft.
For icing to occur, you need visible moisture and close to freezing or sub freezing temperatures. Visible moisture basically means a cloud or liquid precipitation. As an instrument rated pilot, you will be operating in the clouds under rain or even when it snows! Structural icing is much more of a risk for you than for your VFR counterparts.
Types of Icing
Structural icing comes in three flavors, depending on the temperature and the shape of the water droplets: clear ice, rime ice and mixed ice. Clear ice is usually smooth, transparent and hard. It’s typically formed between 0° C and -10° C. Rime ice is rough, milky and opaque and is formed between -10° C and -20° C. Mixed ice is a mix of clear and rime ice.
With rime ice the deposit doesn’t have a lot of weight but its danger lies in the aerodynamic alteration of the wing camber. Rime ice is usually brittle and can usually be broken off easily by anti-icing equipment. This is not to say that it’s not a hazard. Icing of any kind is always a hazard, and if you encounter icing in flight your best move is always to get out of the icing conditions.
Icing Encounters
Most aircraft have a nose down pitching moment because the center of gravity is ahead of the center of lift. The role of the tail of your aircraft is to counteract this moment by providing downward lift. Actions that move the wing away from a stall can increase the negative angle of attack of the tail. The tail being usually much thinner than the wings it’s more susceptible to icing. If ice builds up on the tail, it may stall. This is known to happen right after extending the flaps. The pilot may feel a sudden nose-down pitch and the aircraft can become uncontrollable.
Imagine you’re flying an ILS down to your home airport. You are in a cloud and the temperatures are close to freezing. Right after you extend your flaps for the landing, you feel a sudden nose down pitch. What should be your first action?
Don't try to pull back on the yoke. If you experience a tail stall, you most likely won’t have a lot of elevator control, and pulling on the nose won’t fix the tail stall anyways. The best thing to do is to go back to your previous flap configuration. If you suspect that ice is causing a tail stall, you should retract flaps to the previous setting immediately. Don’t forget to adjust your speed to the lower flap setting, and if you have any de-icing equipment, use it!
To Summarize:
For structural icing to occur you need two things: temperatures close to or below freezing and visible moisture.
There are three different types of structural icing:
Clear Ice:
Glossy and transparent.
Most likely between 0° C and -10° C
Rime Ice:
Opaque and milky
Rough
Most likely between -10° C and -20° C
Mixed Ice:
Mix of rime and clear
Cloudy
The tail of your aircraft is particularly susceptible to icing, which can cause a tail stall. This will most likely occur after you extend the flaps, so your first action should be to retract the flaps if you encounter it.