Special Operational Procedures and Hazards — Page 152, Lesson 232

I want to walk you through a new chapter: Special Operational Procedures and Hazards. We're starting with the MEL — the Minimum Equipment List — and the operator's responsibility. The operator — that's the airline or company running the flight — is required to establish an MEL for each type of aeroplane they use in operation. This MEL must be approved by the Authority, which in our context means the national aviation authority like the DGCA or EASA. The MEL is based on the MMEL — the Master Minimum Equipment List — which has already been accepted by the Authority. The key rule here is that the MEL must not be less restrictive than the MMEL. In other words, the operator's list can be stricter, but it can never allow more unserviceabilities than the master list permits. Unless the Authority grants specific permission, aeroplanes are only permitted to be operated in accordance with the MEL. And even if such permission is granted, it will never allow operations outside the constraints of the MMEL. So the MMEL is the absolute ceiling — you cannot go beyond it. Now, the commander's responsibility. The commander — the captain — is required to make a decision whether or not to accept an aeroplane with unserviceabilities that are allowed by either the Configuration Deviation List, or CDL, or the MEL. So even if the MEL says a certain item can be unserviceable, the captain still has the final say: do I accept this aeroplane or not? Let's move to ground de-icing and anti-icing procedures. Ice will form on the airframe if three conditions are present simultaneously. First, there must be water in a liquid state. Second, the ambient air temperature must be below 0°C. Third, the airframe temperature must be below 0°C. All three need to be true for ice to form. The book then gives us a set of weather condition definitions that are required knowledge. Let's go through each one carefully. First, drizzle. Drizzle is fairly uniform precipitation composed exclusively of fine drops with a diameter less than 0.5 mm, and these drops are very close together. Drizzle appears to float while following air currents, but unlike fog, drizzle actually falls to the ground. Next, fog and ground fog. This is a visible aggregate of minute water particles — droplets — in the air, reducing the horizontal visibility at the Earth's surface to less than 1 km. Freezing fog is a fog formed by supercooled water droplets. Supercooled means the water is still liquid even though its temperature is below 0°C. These droplets freeze upon contact with exposed objects and form a coating of rime ice or clear ice. Freezing rain and freezing drizzle are rain or drizzle in the form of supercooled water droplets which freeze upon impact with the surface. So the key difference from freezing fog is the precipitation type — it's rain or drizzle, not fog. Frost, also referred to as hoar frost, is a deposit of ice having a crystalline appearance, assuming the form of scales, needles, or fans. Frost is formed by sublimation — that's when water vapour is deposited directly onto a surface whose temperature is below zero, without passing through the liquid phase first. Rain is liquid precipitation in the form of drops more than 0.5 mm in diameter, or smaller drops which, in contrast to drizzle, are more widely separated. Rime is a deposit of ice formed by the freezing of supercooled water droplets on objects at a temperature below or slightly above freezing point. It is composed of grains separated by air, sometimes adorned by crystalline branches. Slush is water-saturated snow. The test given is that if you do a heel-and-toe slap-down motion against the ground, the slush will be displaced with a splatter. Finally, snow is precipitation of snow crystals, mostly branched in the form of six-pointed stars. The crystals can be isolated or agglomerated — clumped together — to form snowflakes. These definitions are foundational for understanding when and why de-icing and anti-icing procedures are necessary. We'll build on them as we go further into the chapter.