Ice and Rain Protection — Page 253, Lesson 314

Let's pick up with the pneumatic de-icing system operation, because that's where we left the boots — inflated, deflated, and held flat. When you switch the system on, pressure is admitted to the boot sections to inflate the tubes. That inflation physically weakens the bond between the ice and the boot surfaces, and the ice breaks away. Then, at the end of the inflation stage of the operating sequence, the air in the tubes is dumped to atmosphere through automatic opening valves, and the tubes are fully deflated by the vacuum supply. So you have a pressure phase to crack the ice, then a vacuum phase to pull the boots flat again. That inflation and deflation cycle repeats for as long as the system is in operation. Now, when you switch the system off, vacuum is supplied continually to all tubes of the boot sections. That holds the sections flat against the wing and tail leading edges, which minimizes aerodynamic drag. So the vacuum isn't just for deflation — it's also the resting state that keeps the boots from flapping in the airflow. The de-icer boots are pulsated in a set cycle, and the frequency of that cycle can be varied by the frequency selector. That's how you cater for light or heavy icing conditions. For cycling purposes, the boots are usually divided into three groups. Group 1 is the port and starboard mainplane outboard boots. Group 2 is the port and starboard mainplane inboard boots. Group 3 is the fin and tailplane boots. So you've got outboard wings, inboard wings, and then the fin and tailplane as separate groups. Here's the key point: the cycle itself takes 34 seconds, irrespective of what you select on the cyclic frequency selector. The selector doesn't change the cycle duration — it merely alters the delay period between cycles. For example, 206 seconds for light icing, and 26 seconds for heavy icing. So in light icing you wait longer between cycles; in heavy icing you cycle much more frequently. Now let's move to thermal anti-icing and de-icing. Hot air systems on modern aircraft are generally engine bleed air, and those are said to be anti-icing. Other methods of obtaining the hot air will be described, and depending on the duration of application and the temperature applied, they may be either de-icing or anti-icing systems. So the distinction isn't just about the heat source — it's about how long you apply the heat and at what temperature. One note before we go further: some large jet transport aircraft are not equipped with tailplane ice protection. Those aircraft have been certified with that limitation, which is an important operational consideration. That figure shows the schematic diagram of the pneumatic de-icing system controls and indicators — you can see how the pressure, vacuum, and valve arrangement ties together with the boot groups. So to summarize where we are: pneumatic boots use pressure to crack ice, vacuum to deflate and hold flat, cycle in three groups over 34 seconds with a variable delay between cycles, and then we're moving into thermal systems where engine bleed air provides anti-icing heat.