This post is about a phenomenon we all have encountered in our lives, especially for those of us living in countries where we have, WINTER! You know it, it's ice! The stuff that makes for great skating rinks, embarrassing falls, and frustrating car cleanings! It is found on this side of aviation, and at times can become quite a nuisance for us, too. But while ice has been around since the very beginning, the tools to fight them have only been developed as we learned more and more from the major accidents since. This post will explore more of this phenomena, and the tools we use in professional flying to deal with it!
The Various Forms of Ice
We have all learned in our basic science class that materials found in our natural world come in 3 different states of matter; solid, liquid, and gas. We experience the liquid form all the time, as this is the water that surrounds our environment. When the sun heats the water close to surface, it turns into vapour (gaseous form) and rises into the atmosphere. However, when the temperature drops enough, the same water molecules have less potential energy and change their internal structure to form a solid, which we experience in our natural world as ice. Depending upon the specifics of how the water cooled, different types of ice can form. In aviation, we identify ice in three different forms; clear, rime, and mixed.
Clear ice forms most commonly when we fly close to clouds that consist of a high concentration of water content. Only a small portion of the water droplet is supercooled. When some of the drop freezes, it releases heat energy onto our airplane wing, which causes it to spread backwards and continue freezing further down the wing. This is the hardest type of ice to detect, because it is characteristically transparent, so it is invisible to the untrained eye. It is also the most dangerous kind, as it can quickly spread beyond the ice protection capabilities of the aircraft.
Rime ice forms in stratus type clouds, which are more stable in nature. They consist of much smaller droplets of water. This type of ice will accumulate on the very tip of the wing, and form in layers. It often looks grainy to our eye and unlike clear ice, is very easily detectable. Rime ice is characteristically brittle, and can usually be taken apart in chunks with de-icing equipment or physical force. But due to this quality, it can quickly change the lift characteristics on our wings and block carburetors inside the engines of smaller, piston-driven aircraft. Thus, it is also dangerous in it's own right. As a side note, mixed ice is just a combination of the two, and in realistic flying we often encounter ice as a combination of the two.
Tools to Combat Ice
On the Pilatus, we have a multitude of tools at our disposal to manage this issue. Proper use of each tool is critical in winter ops, and during the summer months too. Yes, ice can form in the summer since the altitudes at which we fly at are much colder than surface temperatures. The pictures in this section displays a day in my recent flying where I found myself using every weapon in our arsenal to combat ice.
PROBES HEATING
Every aircraft has specific probes it uses to collect air data information such as airspeed, static pressure, and more to get instrument indications in the cockpit. Major accidents have occurred with loss of life simply from probes being blocked by ice. So it is absolutely imperative that the pilot maintain optimal performance of the probes.
On the Pilatus, probes consist of the pitot tubes and static ports, along with the AOA (angle of attack) vanes. All these instruments combined help collect data which a computer processes for various indications. The probe heating helps to prevent ice formation by heating the element electrically. This type of system is known as anti-icing, as it tries to avoid ice from forming at all. Ice or not, we always keep the probes heating on, since it is absolutely essential for flight.
WINDSHIELD HEATING
Just like our cars, windshield heating simply helps to avoid ice or frost from forming over the windshield. On the Pilatus, we have two modes of windshield heat. The light mode covers the entire window, separately for the captain and first officer. The heavy mode covers just the middle portion, which is most commonly used for sight. Ice or not, we keep windshield heat on at all times as it also avoids the window from fogging.
Here's a fun-fact about the windshield heat on the Pilatus! During our training, in emergency situations with engine failure, we rely solely on battery power to maintain electrical heat for these components. But because battery power is limited, we are taught to use heavy heating if possible. It might seem counter-intuitive, but heavy heating only heats the middle portion of the window, thereby using less energy as it covers a much smaller area. This helps preserve the energy in the battery, prolonging it's use in the most desperate times. I have to hand it to the engineers, it is a genius move. Like most things European engineered, the Pilatus is absolutely over-engineered, to our benefit of course!
DE-ICE BOOTS
De-ice boots are just a special composition of rubber that covers the front leading edge of both wings, and both parts of the tail. It is a unique system in that it is pneumatically (air pressure) driven. When we hit the switch in the cockpit, air is transferred inside the tubing of the boots, which causes it to inflate like a balloon. There is a specific sequence in which the boots inflate and deflate; starting with the tail and ending at the wings. The inflation of the boots causes ice that has already formed to break off by destroying it's structure. This keeps the leading edge of the wing and tail clean from ice. Because the boots work best with a decent accumulation of ice, it is called a a de-icing device.
PROP DE-ICE
This system is unique in that they look like the black boots we have on the wing and tail.
However unlike the boots, prop deice uses a system of paired heated elements to prevent ice build-up on the propeller itself. Ice on the prop can be detrimental to the flight as it rapidly changes the aerodynamic properties of the prop. Worse off, it can affect the stability of the flight and engine performance.
Another fun fact! The propeller on the Pilatus is 4 bladed, with some models getting 5 or even 6 blades. To avoid an imbalance in the aerodynamic qualities of the prop, the heating system is designed to only heat a pair of prop (2 opposing blades) at any one time. There is an internal timer which schedules the heating of the props. Sometimes in flight we can feel vibration on the control yoke as the ice is being shed off the prop.
INERTIAL SEPERATOR (INERT SEP)
This system is a little technical as it has to do with the interior operation of the engine. Put simply, it is a electrically driven flap at the air intake point just behind the propeller. We open the inertial separator in order to give the air an alternative route to the compressor turbines. This alternative route is U-shaped, in that the air has to make a complete U-turn horizontally (like a backwards C) on its way to the compressor. This U-shape allows heavier-than-air objects (ice) to fall through and not be able to make the sharp turn up to the engine. It does rob the aircraft of some power, but the difference is minimal and the advantage is avoiding ice buildup in the intake which can be detrimental.
The combined use of the Inert Sep with Prop Deice allows the aircraft to maintain a special mode called Pusher Ice Mode. It is a internal computer mode which reduces the angles of attack at which the aircraft will stall. In fact, it is reduced by 8 degrees, which is good because it helps the aircraft react to loss of lift a lot quicker than otherwise. This helps us detect and avoid stalls a lot earlier, since ice dramatically increases stall speed, reduces lift, and increases drag. It is a genius system designed as a proactive measure, keeping us safe in every flying condition.
CLOSING
So there you have it! I bet you never thought so much about ice, but the next time you see ice in the forecast you will have a new appreciation for all that goes into it's hidden complex nature. Ice on the ground is just something that happens which we take for granted, but as aviation professionals we invest a lot of time into studying it. To speak to it's importance, NASA even has an independent facility where they conduct research solely on the effects of ice on aerodynamic surfaces! They regularly publish their findings, often leading to significant improvements industry-wide. Next time you are flying, look out the window and realize all the little things that go into keeping your giant metal tube, flying in the air! Ice ice baby!
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