Aircraft systems: the bleed valves

Welcome to part two, where we open every valve of the A320 engine bleed. This is where the exam questions live: each statement in the quiz tests one number or one cause and effect, so we will go valve by valve, then summarise every way the bleed can be shut, and finish by answering the 2020 questions.

This deck figure is the map for the whole lecture, so let us follow the air. Air leaves the intermediate port, stage 5, through the intermediate pressure check valve, the IPCV; the high pressure port, stage 9, has its own valve, the high pressure valve, HPV, which regulates to 36 psi. Both join at the pressure regulating valve, the PRV, which regulates to 44 psi and is commanded through a sense line from the temperature limiting thermostat, the TLT; a transferred pressure transmitter, P t, measures before it and a regulated pressure transmitter, P r, after it. Downstream, the overpressure valve, the OPV, which is spring-loaded open, protects against a PRV failure; then the precooler cools the air before it goes to the users, and a branch goes to the engine start valve. On the cold side, fan air passes the fan air valve, the FAV, spring-loaded closed, through the precooler and overboard; the FAV is driven pneumatically by the temperature control thermostat, the TCT. The TLT carries a solenoid commanded by the BMC on CFM engines, or by the engine control on V2500 engines, and a temperature sensor after the precooler, plus the valve position switches, report to the BMC.

Before the details, memorise this table, because half of the true or false statements test it. The intermediate check valve simply lets air flow outwards from stage 5 when its pressure exceeds the downstream pressure. The high pressure valve is spring-loaded closed, regulates to 36 psi plus or minus 4, and shuts completely when stage 5 alone exceeds 36 psi. The pressure regulating valve is also spring-loaded closed; with as little as 8 psi it opens fully, and it regulates to 44 psi plus or minus 4, under the control of the TLT sense line. The overpressure valve is the odd one: spring-loaded open, it starts to close at about 74 psi and is fully closed at 90. The fan air valve is spring-loaded closed and opens only as the temperature control thermostat sends it air, when the bleed exceeds 200 degrees. The crossbleed valve is electric, with two motors, and the APU bleed valve is solenoid controlled; so with no bleed, the HPV, the PRV and the FAV are closed and only the OPV is open.

This plot answers the classic question of which valve regulates at which engine power, using the stage 5 pressure as the variable. At idle, when stage 5 gives less than 36 psi, the higher pressure air from stage 9 holds the IPCV closed, the HPV regulates to 36 psi, and the PRV, below its 44 psi reference, stays fully open, so the system gets 36 psi. Between 36 and 44 psi, stage 5 alone exceeds the HPV reference, so the HPV shuts completely and the air comes from stage 5 through the check valve, still with the PRV fully open: the system gets whatever stage 5 gives. Above 44 psi, typical of cruise, the PRV takes over and regulates to 44 psi plus or minus 4. Above that line are the protections: the BMC closes the PRV electrically if the regulated pressure exceeds 57 psi, and if even that fails, the overpressure valve closes between 74 and 90 psi. And remember the obvious case the notes point out: if even stage 9 cannot reach 36 psi, the valves simply pass the air they receive, since a valve can reduce pressure but never add it.

Let us practise the regimes as an exam would ask; after each question, pause the video and answer before the solution appears. First, idle, with 25 psi at stage 5 and 60 psi at stage 9: what are the check valve, the HPV, the PRV and the overpressure valve doing, and what pressure does the system get? The stage 9 air holds the check valve shut, the HPV regulates to 36 psi, the PRV, below its 44 psi reference, is fully open, and the overpressure valve is open: the system gets 36 psi. Second, climb, with 40 psi at stage 5. The HPV closes completely, the check valve opens, the PRV is still fully open, and the system simply gets about 40 psi. Third, cruise, with 100 psi at stage 5. The HPV stays closed and the PRV regulates to 44 psi plus or minus 4. Fourth, in cruise the PRV's regulating element fails high. The pressure rises, and when the regulated pressure transmitter reads 57 psi, the BMC energises the TLT solenoid, which vents the sense line and closes the PRV, and with it the HPV. Fifth, and the TLT solenoid fails as well. Then the last barrier is the overpressure valve, which starts closing at 74 psi and is fully closed at 90, so downstream pressure never exceeds that band; the method is always: compare stage 5 with 36 and 44 psi, then apply the protections in order.

Now the PRV from the inside, as the notes describe the deck figure. The butterfly is linked through a rod and a crank to a piston that separates two chambers: chamber C, the opening and reference chamber, and chamber D, the closing chamber, which also holds a spring. With as little as 8 psi upstream, air pressurises chamber C and pushes the piston up, opening the butterfly completely; part of that air also reaches chamber E and the sense line to the TLT. A calibrated regulator element keeps the pressure in chamber C at the 44 psi reference. A tap downstream of the butterfly feeds a small funnel valve: while the downstream pressure is below 44 psi it stays seated, but above 44 it lifts, vents to atmosphere and fills chamber D, so the piston moves down and the butterfly partly closes; the pressure falls, the funnel seats again, and the butterfly reopens. So the butterfly oscillates quickly around the 44 psi position, which is why the specification is 44 plus or minus 4 psi. To close the PRV deliberately, the sense line to the TLT is depressurised: chamber E then lets a small ball uncover an orifice that vents chamber C, and the spring closes the butterfly; as a last thermal protection, a fuse melts at 450 degrees and vents the same line.

The high pressure valve works on the same principle as the PRV, with a reference of 36 psi. It is spring-loaded closed, opens fully once more than 8 psi reaches it, and regulates when the stage 9 pressure exceeds 36 psi, again hunting to 36 plus or minus 4. When stage 5 delivers more than 36 psi on its own, the check valve opens, the pressure downstream of the HPV butterfly exceeds 36, fills its closing chamber, and the HPV closes completely: stage 9 is no longer needed. The key link: chamber C of the PRV is piped to chamber E of the HPV, so when the PRV is closed by depressurising its sense line, the HPV loses its opening pressure and closes too. But not the other way round: in cruise the HPV is closed while the PRV regulates, so a closed HPV says nothing about the PRV.

Because the PRV and the HPV close together, the exam loves to ask when the PRV closes, so here is the complete map. Manually, the crew closes it by setting the ENG BLEED pushbutton to OFF, or by pushing the engine fire pushbutton; both energise the TLT solenoid, which vents the sense line. Automatically, the BMC energises the same solenoid on five conditions: more than 57 psi after the PRV, read by the regulated pressure transmitter; more than 257 degrees after the precooler, read by the temperature sensor; a leak detected by the loops; the start sequence of that engine; and APU bleed selected, which is how the APU gets priority. Pneumatically, inside the TLT, with no electrical signal: a reverse flow, detected by the pressure difference across the PRV, closes it, while an over-temperature of about 235 to 245 degrees only bleeds the sense line partly, so the PRV throttles back rather than shutting. And the PRV itself closes with no upstream pressure, below 8 psi, because its spring closes it, and as a last resort its thermal fuse melts at 450 degrees.

Temperature protection works as a ladder of independent devices, each a step further from normal, and an exam question can start at any step. In normal operation the TCT and the fan air valve hold the air leaving the precooler at 200 degrees plus or minus 15, continuously and pneumatically. If the bleed heats up anyway, for example because the TCT has failed, around 235 to 245 degrees the TLT probe partly bleeds the PRV sense line and the PRV throttles back, limiting the temperature. At 257 degrees the temperature sensor tells the BMC, which energises the TLT solenoid: the PRV, and with it the HPV, close completely, and an ECAM caution appears. And if everything else fails, at 450 degrees the PRV's thermal fuse melts and vents the sense line, closing the valve as a last resort. So for a failed TCT the answer is: TLT limitation first, then BMC closure at 257, then the fuse at 450.

The TLT is the element that commands the PRV, and its operation is a classic open question. It has a regulator body and a thermostatic probe inserted in the duct after the precooler. The probe is an outer tube, made of a metal that expands more and faster, welded at its tip to an inner rod; when the air heats up, the tube lengthens and pulls the rod, which opens a small vent valve. Between about 235 and 245 degrees this vent bleeds the PRV sense line and the PRV throttles back, limiting the temperature. At 257 degrees the BMC energises the solenoid on top of the TLT, whose magnetic force lifts its valve, vents the sense line completely, and closes the PRV. Reverse flow is detected without electricity: a diaphragm separates a tap from before the PRV and a tap from after the precooler; normally the upstream pressure wins, but if the downstream pressure is higher the diaphragm moves, lets air into the solenoid valve chamber and pushes that valve open, venting the sense line and closing the PRV.

Downstream of the PRV comes the overpressure valve, the last pressure protection. Its butterfly is spring-loaded open, the opposite of the other valves, and it only closes when the PRV has failed to regulate. Upstream air passes a filter and acts on two spring-loaded pistons: at about 74 psi the springs start to yield and the butterfly begins to close, and at 90 psi it is fully closed; the deck quotes 73.8 to 89.9 psi. The higher the pressure, the faster it closes, and when the pressure recovers it opens again. Then comes the precooler, installed in the pylon: a bundle of metal tubes carrying the hot bleed air, with cold fan air flowing around them and then dumped overboard, so the two streams exchange heat but never mass.

The temperature loop is the other favourite question, and it has two parts: the thermostat that senses and the valve that acts. The temperature control thermostat, the TCT, has a probe in the duct after the precooler, built like the TLT probe: an outer tube that expands and drags an inner rod. Below 200 degrees the probe's vent is closed, the air that enters the TCT fills its own chamber A, the diaphragm compresses the spring and closes the needle valve, so no air goes to the fan air valve. At 200 degrees or more, the rod opens the air vent, chamber A empties, the needle valve opens, and regulated air flows down the signal line to the fan air valve; the further the rod moves, the more air it sends, so the action is proportional, not on and off.

Now the valve that the thermostat drives, the fan air valve, shown on this deck page. It is a butterfly spring-loaded closed; the signal air from the TCT pressurises the chamber of its actuator, overcomes the spring and opens it, while a small bleed hole lets some air escape so that it can close again when the signal falls. The hotter the bleed, the more air the TCT sends, the wider the valve opens, the more fan air cools the precooler, and the loop settles at 200 degrees plus or minus 15. Notice who is in the loop: the TCT, the FAV, the sense line and the precooler, and nobody else; the BMC reads the temperature but does not regulate it.

Last valve: the crossbleed, with its two electric motors. In AUTO the rule to remember is that the crossbleed valve has the same position as the APU bleed valve. More precisely, when either BMC receives the signal that the APU bleed valve is open, with no leak detected in the pneumatic ducts and no engine fire pushbutton operated, it commands motor number 1 and the valve opens. If a leak is detected during an engine start sequence, the valve stays open until the sequence ends, so the start can be completed. When the BMCs receive the signal that the APU bleed valve has closed, they command motor 1 to close the crossbleed valve. In OPEN or SHUT, motor number 2 drives the valve to that fixed position, regardless of the APU bleed valve; and remember that with APU air and the crossbleed open, APU air flows towards the engines, the engine PRVs are closed pneumatically by their TLTs, which sense the reverse flow, and electrically by the BMCs, because APU bleed is selected, so the APU has priority twice over.

Here are five scenarios to test the whole logic; pause after each question and predict the answer. One: an engine 2 bleed fault in cruise, with the selector in AUTO and the APU off; what happens to pack 2? The crossbleed valve follows the closed APU valve and stays shut, so pack 2 loses its air until the crew selects the crossbleed OPEN, and engine 1 then feeds both sides. Two: the crew pushes the engine 1 fire pushbutton. That energises the TLT solenoid, which closes PRV 1 and, through the link, HPV 1, and in AUTO the crossbleed valve cannot open because a fire pushbutton has been operated. Three: a leak is detected on the left side. The leak loops alert BMC 1, which closes PRV 1, and in AUTO the crossbleed stays shut so that the leak is isolated from the right side. Four: both engine bleeds are lost below 20 000 feet. The crew selects APU BLEED, the APU bleed valve opens and, in AUTO, the crossbleed valve follows it, so both sides are fed from the APU. Five: starting engine 2 with the APU. The crossbleed is open because it follows the APU valve, start valve 2 is open, and the start sequence keeps PRV 2 closed until the engine is running.

Now the 2020 quiz, starting with question 1. The worry is that the APU feeds only the left side, while engine 2's starter is on the right side. But in AUTO the crossbleed valve follows the APU bleed valve, so as soon as the APU bleed valve opens, the crossbleed opens too. APU air therefore reaches both start valves, and during each start sequence the BMC keeps that engine's PRV closed. So the answer is c, yes; option a states a true fact, that the crossbleed follows the APU valve, but draws the wrong conclusion from it, which is exactly the kind of trap these questions set.

Question 2 asks for the false statements about the high pressure valve. Statement a is false: once stage 5 alone exceeds 36 psi the HPV is closed, so it is certainly not opening at 44. Statement b is false for the same reason: stage 5 reaching 36 psi is what closes the HPV. Statement c is true, through the link we saw: reverse flow makes the TLT vent the PRV sense line, the PRV closes, and the PRV's chamber C is what keeps the HPV open, so the HPV closes too. Statement d is false: at idle with stage 5 below 36 psi the HPV is the valve regulating, so it is open; it is closed whenever stage 5 alone exceeds its 36 psi reference, whether the PRV is then fully open or regulating. So the false statements are a, b and d.

Question 3 mixes the two temperature devices, and each statement is a trap. Statement a is wrong as a command: the BMC never drives the TCT; 257 degrees is the threshold at which the BMC closes the PRV through the TLT solenoid; read as a time sequence it is defensible, since once the PRV closes the air cools and the probe eventually closes, so a is debatable. Statement b is wrong as a cause and effect: the TCT probe follows only the temperature, and 57 psi read by the regulated pressure transmitter is one of the PRV closure conditions; the probe could happen to be open at that moment if the air is above 200 degrees, but not because of the 57 psi, so b is at best ambiguous. Statement c is wrong: when the probe is closed, below 200 degrees, chamber A stays full, the needle valve is shut, the TCT sends no air, and the fan air valve is held closed by its spring. Statement d is wrong: reverse flow is detected by the TLT and acts on the PRV; nothing in that logic commands the fan air valve fully open, which keeps being modulated by the TCT according to the temperature, and with the hot bleed stopped the temperature tends to fall and the valve to close. So, on the strict cause-and-effect reading, none of the four statements is correct, with a and b debatable if read as sequences in time; there is no official key, so justify each answer exactly like this, which is what earns the marks.

The older papers also contain open questions on the A320, and you now have every answer. For the conditions of the electrical closure of the PRV, list the two manual ones, the bleed pushbutton off and the fire pushbutton, and the five automatic ones commanded by the BMC: 57 psi, 257 degrees, a leak, the engine start sequence and APU bleed selection. To explain the TLT under reverse flow, describe the diaphragm between the tap before the PRV and the tap after the precooler, and how a higher downstream pressure pushes it and opens the solenoid valve pneumatically, venting the sense line. To explain the TCT, describe the probe after the precooler, the vent that opens at 200 degrees, chamber A and the needle valve, the signal line to the fan air valve, and the 200 plus or minus 15 degree result. A 2011 question gives you AUTO, no engine fire pushbutton and no leak, and asks for the four remaining conditions: they are the APU side of the chain, APU BLEED selected, the APU fire pushbutton not operated, no APU shut-down signal, and the APU speed stabilised at 95 %, so that the APU bleed valve opens and the crossbleed valve, which follows it in AUTO, opens too. And whenever you can, add a quick sketch of the circuit with its valves in order: it shows the examiner you understand the whole chain.

The 2012 to 2013 paper has a multiple-choice question on the A320 crossbleed valve that is fully in scope. Statement a is the deck's own AUTO rule, the crossbleed valve has the same position as the APU bleed valve, so in that context it is correct, although in OPEN or SHUT it is not. Statement b is incorrect: the BMCs act through motor number 1, while motor number 2 is the one used by the manual OPEN and SHUT positions. Statement c is incorrect: the automatic opening conditions are the APU bleed valve open, no engine fire pushbutton and no leak, with leaks tolerated during any engine start; the statement's exception for the left engine start is invented, and the APU shut-down signal acts on the APU bleed valve through the ECB, not directly on the crossbleed. Statement d is incorrect: the TLT commands the PRV, never the crossbleed valve. So the intended answer is option 3, b, c and d incorrect; only if you read always as including the manual positions would you choose option 5, and in that case say so in one line.

Fire protection connects both systems of this exam, so it deserves one slide. On the overhead panel there are three fire pushbuttons, ENG 1 FIRE, ENG 2 FIRE and APU FIRE, each protected by a guard. When a fire is detected, the corresponding pushbutton lights red and a chime sounds; the crew lifts the guard and pushes the button, which silences the aural warning while the light stays on. Then they discharge the extinguishing agent: the APU has one bottle that floods its compartment, and each engine has two agents, discharged one after the other for redundancy. Pushing an engine fire pushbutton energises that engine's TLT solenoid, so its PRV, and with it its HPV, close, and in AUTO the crossbleed valve cannot open. On the hydraulic side, the same pushbutton closes the fire valve of the system driven by that engine, Green for engine 1 and Yellow for engine 2. And the APU FIRE pushbutton is one of the conditions the BMC checks before allowing APU bleed, so the APU bleed valve stays closed.

Here is the whole of part two as one table of numbers, worth copying from memory before the exam. 36 psi for the high pressure valve and 44 psi for the pressure regulating valve, both plus or minus 4; 57 psi and 257 degrees for the BMC closure, 235 to 245 degrees for the TLT probe and 450 degrees for the PRV fuse. 74 to 90 psi for the overpressure valve, 200 plus or minus 15 degrees for the temperature loop, 50 psi for the hydraulic reservoirs and 20 000 feet for the APU bleed. In part three we move to the hydraulic system, which uses some of this bleed air to pressurise its reservoirs.