Aircraft systems: the pneumatic system

Welcome to the first of three lectures for the aircraft systems exam of Vehicles Aeroespacials. The exam covers two systems of the Airbus A320: the whole pneumatic system, ATA chapter 36, and the hydraulic system, ATA chapter 29, up to and including the Green system. This part explains what the pneumatic system is for, where the air comes from, who uses it, and how the crew and the computers control it; part two goes inside every valve, and part three covers the hydraulics.

First, what to expect. The scope agreed for this exam is the whole pneumatic deck and the hydraulic deck up to the Green system, which ends on page 12, so the Yellow and Blue system details are out. The 2020 systems quiz, the closest past paper, is a set of statements to mark as true or false: for example, whether the high pressure valve opens at 36 psi, or which reservoir is filled from which panel. Older papers also ask you to explain how a component works, such as the temperature limiting thermostat, or to list conditions, such as when the pressure regulating valve closes electrically. Some old papers ask about the MD87, which belonged to an earlier version of the course; the current decks only cover the A320, so skip those. The winning skill is precise: for every valve, know its default position without air, the pressures and temperatures that move it, and which element commands it.

The pneumatic system of an aircraft takes compressed air from one of its sources, the engines, the auxiliary power unit or a ground cart, and delivers it to the systems that need it. On the A320 the air is delivered regulated to 44 psi and about 200 degrees Celsius: those two numbers are the reference values of the whole system. The main user is the air conditioning and pressurisation system, chapter 21, then engine starting and the anti-ice of the wings and the engine intakes, chapter 30; the deck's generic list also mentions the vertical stabiliser. Bleed air also pressurises the hydraulic reservoirs, so that the pumps never cavitate, and the potable water tank, so that water reaches the galleys and toilets. Finally, it can provide motive power, for example to actuate thrust reversers or to drive air-driven hydraulic pumps, and on the A320 it also heats the aft cargo hold, which is needed to carry live animals.

There are three sources of compressed air. The engines are the main source in flight: from their high compressor stages the air comes at between 50 and 410 psi and between 180 and 540 degrees Celsius, from idle to cruise, far too hot and too variable to use directly, which is why it must be regulated. The auxiliary power unit, the APU, is a small gas turbine in the tail that supplies electrical and pneumatic power on the ground, for engine starting and air conditioning, and in flight as a backup, up to 20 000 feet. On the ground, a pneumatic cart can be connected through a quick-release access panel under the fuselage, onto a connector with a check valve that stops the air escaping. The notes stress that the APU is on the Minimum Equipment List, the document that says which items may be inoperative and for how long: deferred defects of category B, C and D allow 3, 10 and 120 days. Two drawing conventions: odd numbers are on the left and even numbers on the right, and both the APU and the ground connection feed the left side, side 1, of the crossbleed duct.

This is the generic layout from the deck, and every airliner follows it. Each engine feeds its own side of a common manifold through its bleed valves; pneumatic ducts are drawn dashed. A crossbleed, or isolation, valve in the middle can join or separate the two sides. The APU joins the manifold on one side, through its own bleed valve and a check valve that prevents reverse flow into it. And the users are connected along the manifold: the air conditioning packs, the wing anti-ice, the engine starters and the pressurisation of other systems. The point of the architecture is flexibility with isolation: any source can feed any user, and a failed or leaking side can be cut off.

Now the heart of the system: how the air is taken from the engine. Normally the air comes from an intermediate compressor stage, the fifth stage of the high pressure compressor on the A320, through a check valve, the intermediate pressure check valve, that lets it flow only outwards. At idle or low thrust, stage 5 cannot give enough pressure, so the air is taken from a higher stage, stage 9, through the high pressure valve, the HPV. Both paths join upstream of the pressure regulating valve, the PRV, which delivers 44 psi to the system. The air is then cooled in the precooler, a heat exchanger where cold air bled from the fan flows around the tubes carrying the hot bleed air; the two streams never mix, and the used fan air is dumped overboard; a fan air valve controls how much fan air is used. Pressure and temperature sensors at several points let the bleed computers monitor and control the whole chain.

The deck shows the typical bleed regulating valve, and the A320 valves follow the same pattern. A butterfly in the duct is moved by a pneumatic actuator, while the valve is controlled electrically through a solenoid: electrically controlled, pneumatically actuated. The actuator compares a reference pressure with the pressure downstream of the butterfly: if the downstream pressure is too high it closes the butterfly a little, and if it is too low it opens it, so the butterfly hunts constantly around the reference value. The valve therefore has three functions: to switch the bleed on and off, to regulate the pressure, and to protect the system against high temperature and reverse flow. Position switches send discrete signals which, together with the pressures, are checked by a built-in test to confirm the real position of the butterfly.

Let us look at each user in turn. Wing anti-ice sends hot bleed air through a valve on each wing into a perforated duct along the leading edge, which bathes its inside surface; telescopic ducts with flexible joints carry the air to the slats, from slat 3 outwards, and over-pressure and over-temperature sensors let the valve cut the supply if something goes wrong. Engine anti-ice is separate: one or more valves on the fan case send hot air from the engine itself to the intake cowl, and a pressure switch tells the crew whether it is working. For engine start, air from the APU, the ground cart or the other engine reaches the starter valve, which opens and drives the air starter on the accessory gearbox: the starter turns the high pressure spool, N2, which drags the fan, N1, and fuel is injected once N2 has reached the right speed; turning the engine without fuel is called cranking. The hydraulic reservoirs are pressurised to 50 psi, normally from stage 8 of the high pressure compressor of engine 1; the professor insists that the deck drawing, which shows stage 9, is wrong. Finally the potable water tank, the aft cargo heating and, above all, the air conditioning packs, which belong to chapter 21.

Here is the A320 layout with its users. Air is obtained from the two engines, the APU, or a ground cart, selected with the ENG 1 BLEED, ENG 2 BLEED and APU BLEED pushbuttons. A crossbleed valve in the transverse duct separates the two sides or joins them so that a single source feeds every user; when it is closed, each engine feeds its own air conditioning pack. On the deck's drawing, engine 1 also feeds the heating of the aft cargo compartment. The system is designed so that the APU bleed has priority: as soon as APU BLEED is selected, the APU bleed valve opens and both engine bleed valves close automatically, so the APU supplies everything. And remember that the APU and the ground connection both enter on the left side, side 1, of the crossbleed duct.

The crew controls the system from the overhead panel 21VU, which the air conditioning and the pneumatic systems share. Each engine bleed pushbutton has two legends: with the bleed requested it is dark, a white OFF appears when the crew deselects it, and an amber FAULT, together with an ECAM message, signals a failure on that side. The APU BLEED pushbutton is normally dark, shows a blue ON when selected, and an amber FAULT on failure. The crossbleed valve is commanded by a rotary selector with three positions, AUTO, OPEN and SHUT, and the valve has two electric motors, one used by the automatic logic and the other by the manual positions. In AUTO the crossbleed valve always follows the APU bleed valve: it opens only when air is taken from the APU, while OPEN and SHUT force it regardless. This is the Airbus dark cockpit: in normal operation the panel is dark, and any light means an abnormal condition or a non-standard selection that must eventually be cancelled.

Information reaches the crew through the ECAM, the two central screens of the main instrument panel. The upper one, the Engine and Warning display, permanently shows the main engine parameters, N1, N2, exhaust gas temperature and fuel flow, plus the fuel on board and the high-lift surfaces, with the warning and caution messages at the bottom. Class 1 alerts are red warnings that require immediate crew action, and class 2 are amber cautions that do not. Class 3 failures are not shown to the crew at all: maintenance reads them on the multipurpose control and display unit, the MCDU. The lower screen, the System Display, shows synoptic pages, chosen automatically by flight phase or with the pushbuttons of the ECAM control panel; the BLEED pushbutton brings up this page. It shows the position of every bleed valve, the pressures and temperatures, and a GND indication that appears only when the aircraft is on the ground, detected by the landing gear proximity sensors.

Now an exercise with the BLEED page: for each situation, picture the page before the answer appears. One: normal cruise, both engine bleeds on and the APU off. Both PRVs are open, drawn in line with the duct; the HPVs are closed, because stage 5 alone is enough; the crossbleed and the APU valve are closed; and each side reads about 44 psi and about 200 degrees. Two: on the ground before engine start, with APU BLEED on and the crossbleed selector in AUTO. The APU bleed valve is open, the crossbleed valve has followed it and is open, both PRVs are closed because the APU has priority, and the GND indication is shown. Three: an engine 2 bleed fault in cruise. PRV 2 is closed, ENG 2 BLEED shows an amber FAULT with an ECAM caution, and the crossbleed stays closed until the crew opens it. Four: one side shows a bleed temperature of 250 degrees. That is well above the 200 plus or minus 15 degree loop, so the TCT and fan air valve are not coping and the TLT probe is already throttling the PRV; if it reaches 257 degrees, the BMC closes that PRV.

The system is supervised by two Bleed Monitoring Computers, BMC 1 and BMC 2, identical and interchangeable, one for each side, permanently connected to each other by data buses. Each receives electrical signals from the pushbuttons, the crossbleed valve, the engine and APU fire pushbuttons, the positions of the bleed valves, the two pressure transmitters, transferred pressure before the PRV and regulated pressure after it, the temperature sensor and the leak detection loops of its side, and some of the other side. With them it supervises the bleed, shuts off its side if it detects a failure, and informs the crew through the ECAM and maintenance through the fault system. Around them, the System Data Acquisition Concentrators, the SDACs, collect the data for the Flight Warning Computers, which generate the aural and visual alerts, and for the three display computers, the DMCs, two working and one in standby; the CFDIU centralises the faults of every system; the Engine Interface Units pass on the data that each engine's own control unit collects; and the APU is controlled by its Electronic Control Box, the ECB. Finally, leak detection loops run alongside the ducts, the pylons and the APU duct; when one detects hot air, the BMC isolates the leak by closing the corresponding bleed valve before nearby components are damaged.

The APU bleed has its own logic, shown in this diagram. The APU supplies air directly to the left side of the crossbleed duct, on the ground and in flight up to 20 000 feet. Its bleed valve is operated by a pneumatic actuator and controlled by a solenoid, and a check valve downstream prevents air flowing back into the APU. When the crew presses APU BLEED, the blue ON light comes on and the BMC checks that the APU fire pushbutton has not been operated and that no leak is detected in the APU duct; on the ground a leak is tolerated until the engines have started, after which the APU has to be shut down. If those conditions are met, the BMC signals the APU's Electronic Control Box, which opens the bleed valve provided there is no APU shut-down signal and the APU speed is stabilised at 95 %. The ground cart, finally, connects through a quick-release access panel on the lower left fuselage to a connector whose check valve has two spring-loaded flaps that open only inwards.

Starting the engines puts all of this together, and it is the subject of the first question of the 2020 quiz. With the engines stopped there is no engine bleed, so the air comes from the APU or a ground cart. Selecting APU BLEED opens the APU bleed valve and, with the crossbleed selector in AUTO, the crossbleed valve follows it and opens, so APU air reaches both sides. The air then flows in the opposite direction to normal, from the manifold towards the engine, up to the start valve, which opens and feeds the air starter, which turns N2 and then N1. During the start sequence of an engine, its BMC keeps that engine's pressure regulating valve closed electrically, so the starting engine does not bleed. Once the engine is running, the start valve closes; when the crew deselects APU BLEED, the engine pressure regulating valves reopen and regulate again. And if a leak is detected during the start sequence, the crossbleed valve stays open until the sequence ends, so the start is not interrupted; so yes, both engines can be started with APU bleed with the selector in AUTO.

Time to practise in the exam format: after each statement, pause the video, decide true or false, and then listen. One: the APU can supply bleed air in flight at any altitude. False: its bleed is limited to 20 000 feet. Two: the APU and the ground cart feed the right side of the crossbleed duct. False: both feed the left side, side 1. Three: selecting APU BLEED closes the engine bleed valves automatically. True: that is the APU priority. Four: in AUTO, the crossbleed valve opens whenever an engine bleed fails. False: in AUTO it only follows the APU bleed valve, and the crew must select OPEN to cross-feed from the other engine. Five: the hydraulic reservoirs are pressurised from stage 9 of engine 2. False: from stage 8 of engine 1, with the pneumatic manifold as a backup. Six: class 3 failures appear in amber on the upper ECAM. False: amber is class 2, and class 3 is only for maintenance, on the MCDU. Seven: each BMC supervises its own side and also receives some signals from the other side. True. Eight: the engine anti-ice takes its air from the wing anti-ice duct. False: each engine has its own anti-ice valves on the fan case, feeding its intake cowl with its own bleed.

To summarise part one: air comes from stage 5 of the engines, or stage 9 at idle, from the APU up to 20 000 feet, or from a ground cart, and the APU and the cart feed the left side. It goes to the packs, the engine starters, wing and engine anti-ice, the hydraulic reservoirs, pressurised from stage 8 of engine 1, the water tank and the aft cargo heating. On its way it passes the check valve or the high pressure valve, the pressure regulating valve, the overpressure valve and the precooler with its fan air valve and temperature control thermostat. The APU bleed has priority over the engines, and the crossbleed valve in AUTO follows the APU bleed valve. Two BMCs supervise and isolate, the ECAM shows the state, and the cockpit stays dark when all is well; in part two we open every valve and learn its exact logic and numbers.