Aircraft Pneumatic Systems — Page 218, Lesson 276

Let's start with the cabin air distribution system. I want you to picture the inside of a passenger cabin. The goal is to keep the passengers comfortable, and that means more than just blowing air into the middle of the room. Most passenger transport aircraft supply warm air to the cabin walls through floor and wall passages. This keeps the interior surfaces at cabin temperature. Why does that matter? Because if the walls are cold, you get draughts and direct heat losses from the passengers' bodies to the cold surface. By keeping the walls warm, the entering air temperature can be closer to the cabin temperature, so the air you're delivering doesn't have to be extra hot to compensate for cold walls. Now, the ducting is split into two distinct sections. One section carries cold air, and the other carries heated air. The cold, conditioned air goes to the passengers through what's called the gasper air system. That's the little adjustable nozzles you see above each seat. Conditioned air is also supplied to the flight deck, to the crew stations, where the crew can adjust the flow direction and quantity. And it's also supplied to the flight deck windows for de-misting, to keep the windscreen clear. Let me show you the layout. Now, let's look at the gasper air more closely. Gasper air is tapped from one of the zone supply ducts, and importantly, it's tapped upstream of where trim air is added. So it's taken before the trim air mixes in. The gasper fan provides a positive supply of conditioned air to all zones through individually controlled outlets, which are called punkah louvres. So each passenger has their own little louvre they can open or close. Now, trim air. This is the key to zone temperature control. To avoid large temperature gradients between the extremities of the cabin, the cabin is divided into sections, and each section is dealt with as a separate distribution problem. That's what we call zone trim. Here's the important part: the temperature delivered by the packs is determined by the zone requiring the coolest air input. So the packs cool the air down to satisfy the coldest zone. Then, to satisfy the individual zone requirements, hot trim air is added to the output of the packs. So the packs provide the base cool air, and trim air warms it up zone by zone. The pressure and quantity of trim air depends on inputs from the cockpit and cabin temperature control systems. And the pressure of the trim air is controlled by pressure regulating valves. So the system senses what temperature each zone needs, and it regulates how much hot trim air is mixed in, with the pressure regulating valves keeping that trim air at the right pressure. So to sum up the flow: cold conditioned air from the packs goes to the zones, the gasper fan pulls some of that air for the passenger outlets, and hot trim air is added to each zone's supply to fine-tune the temperature. The whole thing is managed by the cockpit and cabin temperature control systems, with pressure regulating valves keeping the trim air pressure in check.