Differences in pressurisation systems and operational and technical difficulties.
Thread Starter
Joined: Nov 2024
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From: Hongkong
Differences in pressurisation systems and operational and technical difficulties.
Hi I'm newbie and a dispatch trainee. Having some questions. What are the differences between passenger and transport aircraft pressurisation systems and what are the operational and technical difficulties?
Joined: Jan 2008
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From: Denver
I'm not certain why you distinguish between "passenger" and "transport" aircraft, because passenger airliners are transport-category aircraft.
However, here is a rough outline of the different type of pressurization systems in current use.
1. Bleed-air pressurization is the most common system in use today. It is used in turbine-powered aircraft (turbojets/turbofan jets, and turboprop propeller aircraft). Because by their nature, turbine engines require a compression stage simply to function and produce thrust and forward movement at all, the pressure is "already there" and can be used to pressurize the aircraft interior as well.
In a bleed-air system, outside ambient-pressure air is sucked in through the main engine inlet. It then passes through the compressor stage to provide high-pressure air to mix with fuel and burn to drive the power turbines.
BEFORE that compressed air is contaminated with nasty stuff in the fuel-and-burning stage, but when already highly compressed, part of it is "bled off" through an engine nozzle/valve and ducted to the aircraft fuselage as pressurized breathing air for passengers and crew. Because the compression process itself heats the air to very high temperatures (~ 200°C) even without combustion, that air then goes through a cooling system (air-conditioners, or "packs") that can also add some humidity. From the "packs" the now-room-temperature-air passes into the interior of the aircraft to the crew and passengers, distributed as needed by additional ducts and outlets, not that different from a house or building's "central air conditioning" system.
That flow of breathable compressed air is continuous, so the pressure inside the aicraft is kept fairly constant (generally automatically) by out-flow valves that "leak" the used air in the aircraft, just enough to keep the cabin/cockpit air pressure constant, allow room for the new incoming pressurized air, and remove the CO2 breathed out by the people inside. Without the out-flow valves, the aircraft cabin could - theoretically - inflate like a balloon, and **pop.**
Operation/Technical difficulties - An occasional one is that, while the engine-compressed air is free of nasty engine-exhaust gasses/fumes, it can become contaminated by lubricating oil from the compressor, if the oil-seals are damaged or worn. Or by externally-applied de-icing fluids that pool in the engine intake when preparing the aircraft for flight in winter conditions. Or by leaks of the conditioning "packs" own lubricants. etc. etc.
But realistically, Mother Nature can come up with all kinds or failure modes, any time there are moving parts, and seals, and so on. Constant maintenance checks are the real solution to minimizing operational and technical difficulties.
Of note, most airliners and other "transport" aircraft often have a small Auxiliary Power Unit (APU) - a tiny jet engine using the same fuel as the aircraft to produce "spinning power" - that can provide some air pressure (along with electrical and hydraulic power) when the main aicraft engines are not running (i.e. at the airport gate). This feeds to the same conditioning packs for internal breathable air, and also to the engines themselves, to spin them up at main-engine start,
2. Turbocharger Pressurization is used for piston-engine aircraft, (usually smaller personal aircraft) where desired. As with an automobile turbocharger, it consists of two fans or turbines connected by a sealed-off shaft. The drive turbine sits right in the engine exhaust system, and is spun by the passing high-speed (and poisonous) engine-exhaust gasses, like a water-wheel. The other turbine, sealed off from the exhaust gases, is spun by the shaft connected to the drive turbine, and sucks in clean outside air and sends that through a pipe or duct to the aircraft interior. As with the bleed-air system, the air pressure in the aircraft is generally controlled by out-flow valves, that leak internal air just enough to keep the pressure inside the aircraft life-sustainable.
3. Electrical Pressurization systems have been used on and off over the years - and are currently used in the Boeing 787 and perhaps others. They are air pumps powered by an electric motor, using the overall aircraft electrical system
4. Hydraulic Pressurization systems are used in the Airbus A350 (and perhaps other aircraft). As with an electrical system, they are not driven by the engines directly, but by the hydraulic-fluid system that provides pressure for moving thing in the aircraft (flight controls, landing gear - and can also drive an air pump).
Either those last two can be powered by an APU (if present, and equipped with the correct accessories) as well as the main engine electrical generators or hydraulic pumps.
Coreections. amendments and additions welcomed.
However, here is a rough outline of the different type of pressurization systems in current use.
1. Bleed-air pressurization is the most common system in use today. It is used in turbine-powered aircraft (turbojets/turbofan jets, and turboprop propeller aircraft). Because by their nature, turbine engines require a compression stage simply to function and produce thrust and forward movement at all, the pressure is "already there" and can be used to pressurize the aircraft interior as well.
In a bleed-air system, outside ambient-pressure air is sucked in through the main engine inlet. It then passes through the compressor stage to provide high-pressure air to mix with fuel and burn to drive the power turbines.
BEFORE that compressed air is contaminated with nasty stuff in the fuel-and-burning stage, but when already highly compressed, part of it is "bled off" through an engine nozzle/valve and ducted to the aircraft fuselage as pressurized breathing air for passengers and crew. Because the compression process itself heats the air to very high temperatures (~ 200°C) even without combustion, that air then goes through a cooling system (air-conditioners, or "packs") that can also add some humidity. From the "packs" the now-room-temperature-air passes into the interior of the aircraft to the crew and passengers, distributed as needed by additional ducts and outlets, not that different from a house or building's "central air conditioning" system.
That flow of breathable compressed air is continuous, so the pressure inside the aicraft is kept fairly constant (generally automatically) by out-flow valves that "leak" the used air in the aircraft, just enough to keep the cabin/cockpit air pressure constant, allow room for the new incoming pressurized air, and remove the CO2 breathed out by the people inside. Without the out-flow valves, the aircraft cabin could - theoretically - inflate like a balloon, and **pop.**
Operation/Technical difficulties - An occasional one is that, while the engine-compressed air is free of nasty engine-exhaust gasses/fumes, it can become contaminated by lubricating oil from the compressor, if the oil-seals are damaged or worn. Or by externally-applied de-icing fluids that pool in the engine intake when preparing the aircraft for flight in winter conditions. Or by leaks of the conditioning "packs" own lubricants. etc. etc.
But realistically, Mother Nature can come up with all kinds or failure modes, any time there are moving parts, and seals, and so on. Constant maintenance checks are the real solution to minimizing operational and technical difficulties.
Of note, most airliners and other "transport" aircraft often have a small Auxiliary Power Unit (APU) - a tiny jet engine using the same fuel as the aircraft to produce "spinning power" - that can provide some air pressure (along with electrical and hydraulic power) when the main aicraft engines are not running (i.e. at the airport gate). This feeds to the same conditioning packs for internal breathable air, and also to the engines themselves, to spin them up at main-engine start,
2. Turbocharger Pressurization is used for piston-engine aircraft, (usually smaller personal aircraft) where desired. As with an automobile turbocharger, it consists of two fans or turbines connected by a sealed-off shaft. The drive turbine sits right in the engine exhaust system, and is spun by the passing high-speed (and poisonous) engine-exhaust gasses, like a water-wheel. The other turbine, sealed off from the exhaust gases, is spun by the shaft connected to the drive turbine, and sucks in clean outside air and sends that through a pipe or duct to the aircraft interior. As with the bleed-air system, the air pressure in the aircraft is generally controlled by out-flow valves, that leak internal air just enough to keep the pressure inside the aircraft life-sustainable.
3. Electrical Pressurization systems have been used on and off over the years - and are currently used in the Boeing 787 and perhaps others. They are air pumps powered by an electric motor, using the overall aircraft electrical system
4. Hydraulic Pressurization systems are used in the Airbus A350 (and perhaps other aircraft). As with an electrical system, they are not driven by the engines directly, but by the hydraulic-fluid system that provides pressure for moving thing in the aircraft (flight controls, landing gear - and can also drive an air pump).
Either those last two can be powered by an APU (if present, and equipped with the correct accessories) as well as the main engine electrical generators or hydraulic pumps.
Coreections. amendments and additions welcomed.
Thread Starter
Joined: Nov 2024
Posts: 9
Likes: 1
From: Hongkong
Thank you. I meant to say the difference between military and civilian transport aircraft.
The translation was not correct.
I wanted to study the JL123 case personally, and because a colleague asked me about the pressurisation system on military transport aircraft.
The translation was not correct.
I wanted to study the JL123 case personally, and because a colleague asked me about the pressurisation system on military transport aircraft.
Joined: Jan 2004
Posts: 3,246
Likes: 202
From: Here and there
What type of military transport aircraft? A lot of military aircraft are just the same as passenger aircraft.
Joined: Jan 2008
Posts: 1,269
Likes: 79
From: Denver
In effect, "the balloon popped" because the "balloon" itself was faulty - due to an incompetent repair years earlier. Even normal, correct and functional pressurization was too much for it.
It would have been a survivable event if it has been only a faulty pressurization system. But the containment dome that blew out also broke all the hydraulic-power pipes, and a tail-fin, that allowed control of the aircraft. And the pilots were ultimately (after a heroic 45 minutes keeping the 747 airborne even with virtually no control except engine thrust variations) unable to avoid flying into a mountain.
As to military aircraft my knowledge is limited. They will generally use the same systems, but in different ways.
Aircraft that will be subject to combat are likely to lose any pressurization containment due to enemy fire (bullet and shrapnel holes in the aircraft), so may depend less on it (even if they have it) than civilian aircraft.
