A320 Dual Hydraulic Failure: PTU Logic Pilots Misread

✈ A320 Training · AEO Tier 1

A320 Dual Hydraulic Failure: Why the PTU Will Not Save You

Q: What happens when an A320 loses both GREEN and YELLOW hydraulic systems?

The HYD G+Y SYS LO PR alert triggers when both GREEN and YELLOW system pressures fall to or below 1,450 psi. ECAM directs LAND ASAP, PTU OFF, and the affected pumps OFF. Inoperative systems include G+Y HYD, F/CTL PROT, STABILIZER, REVERSER 1+2, SPLR 1+2+4+5, FLAPS, YAW DAMPER, AP 1+2, ANTI SKID, N/W STRG, NORM BRK, AUTO BRK, and L/G RETRACT. SLATS operate at SLOW speed. The BLUE system remains, normally pressurized by the BLUE electric pump if AC power is available.

Q: Does the A320 PTU work in a dual hydraulic failure?

No. The PTU runs automatically when the GREEN/YELLOW differential pressure exceeds 500 psi. It is a power transfer device, not a fluid transfer device. In a dual GREEN and YELLOW low-pressure condition, there is no source side, so the PTU cannot recover the failure. ECAM directs PTU OFF as part of the HYD G+Y SYS LO PR procedure.

Q: Why does ECAM direct PTU OFF in a dual GREEN and YELLOW failure?

With both systems below pressure the PTU has no driving source and cannot restore either system. Hydraulic fluid is not transferred between systems, only power. Leaving the PTU running risks PTU overheat and potential loss of the remaining hydraulic circuit. Selecting PTU OFF is a protective action commanded by the HYD G+Y SYS LO PR procedure.

Q: Which flight controls remain after a dual hydraulic failure on the A320?

There is no fixed memory list — read the F/CTL SD page to confirm actuator and surface status. The HYD G+Y SYS LO PR procedure tells you F/CTL PROT, STABILIZER, SPLR 1+2+4+5, and FLAPS are inoperative, and SLATS operate at SLOW speed. Aileron and elevator availability depends on which BLUE-powered servojacks remain. Ailerons, for example, are powered by GREEN/BLUE servojack pairs, so BLUE keeps them in play.

Q: What is the role of the BLUE hydraulic system in an A320 dual failure?

After GREEN and YELLOW loss, BLUE is the remaining hydraulic system. It is normally pressurized by the BLUE electric pump when AC power is available in flight. If electrical power is also degraded, the RAT drops out automatically and a hydraulic pump coupled to the RAT pressurizes BLUE. Hydraulic power is only one part of flight control authority; computer reconfiguration, law degradation, and surface availability all matter.

Q: When does the RAT deploy on an A320 with dual hydraulic failure?

The RAT does not deploy automatically on a hydraulic failure alone. It deploys automatically when both AC BUS 1 and AC BUS 2 are lost, typically from a dual engine failure or major electrical disruption. A drop-out RAT coupled to a hydraulic pump then pressurizes the BLUE system. The crew can also deploy the RAT manually.

Q: How does flight control law change after a dual hydraulic failure on the A320?

Normal Law and its protections are lost. The aircraft transitions to Alternate Law with reduced protections. Load factor limitation remains similar to Normal Law, but high angle of attack protection is replaced by low speed stability with stall warning, and bank angle protection is not provided. Alpha floor is inoperative. At landing gear extension the aircraft reverts to Direct Law in pitch and roll. The trigger is explicit: WHEN L/G DN, DIRECT LAW.

Q: What pressure does the BLUE system operate at on the electric pump versus the RAT?

Normal hydraulic system operating pressure is 3,000 psi. When the BLUE system is powered by the RAT, system pressure is 2,500 psi. The difference is published in the system description and is the verified number to teach.

Q: What does the F/CTL ECAM page tell you in a dual hydraulic failure?

The F/CTL SD page shows spoiler and speedbrake position, hydraulic pressure to each control, aileron and elevator actuator status, ELAC and SEC computer status, pitch trim, and yaw indications. Use it alongside ECAM STATUS to confirm aircraft state during the HYD G+Y SYS LO PR procedure. It presents the live indications you need to interpret, not a simple hydraulic-source map.

Q: What is the PTU Mirage trap in A320 dual hydraulic failure training?

The PTU Mirage is the assumption that the PTU will transfer pressure between GREEN and YELLOW and restore lost flight controls during a dual failure. It cannot. The PTU requires a healthy source side, and a dual failure leaves it with none. ECAM commands PTU OFF. Crews who wait for PTU recovery burn through situational awareness instead of working ECAM actions and identifying the surviving BLUE-powered surfaces.

You are in IMC at FL310, heavy turbulence, an hour from the alternate. The 𝗘𝗖𝗔𝗠 shows 𝗛𝗬𝗗 𝗚𝗥𝗘𝗘𝗡 and 𝗬𝗘𝗟𝗟𝗢𝗪 reservoir cautions, and both system pressures fall below 1,450 psi. The HYD G+Y SYS LO PR alert triggers. Your hand drifts toward the 𝗣𝗧𝗨 switch, hoping it will bridge the gap. It will not. The ECAM is about to direct 𝗣𝗧𝗨 𝗢𝗙𝗙, 𝗟𝗔𝗡𝗗 𝗔𝗦𝗔𝗣, and a list of inoperative systems that should reshape your next ten minutes. Here is what is happening and what you actually have left to fly the aircraft.

What happens when an A320 loses both GREEN and YELLOW hydraulic systems?

The HYD G+Y SYS LO PR alert triggers when both 𝗚𝗥𝗘𝗘𝗡 and 𝗬𝗘𝗟𝗟𝗢𝗪 system pressures fall to or below 1,450 psi[cite: 30]. ECAM directs 𝗟𝗔𝗡𝗗 𝗔𝗦𝗔𝗣, 𝗣𝗧𝗨 𝗢𝗙𝗙, and the affected pumps OFF[cite: 31].

Inoperative Systems List[cite: 32, 210]:

• G+Y HYD • F/CTL PROT • STABILIZER • REVERSER 1+2 • SPLR 1+2+4+5 • FLAPS • YAW DAMPER • AP 1+2 • ANTI SKID • N/W STRG • NORM BRK • AUTO BRK • L/G RETRACT

𝗦𝗟𝗔𝗧𝗦 operate at 𝗦𝗟𝗢𝗪 speed[cite: 33]. The 𝗕𝗟𝗨𝗘 system remains and is normally pressurized by the BLUE electric pump if AC power is available[cite: 33, 50].

Does the A320 PTU work in a dual hydraulic failure?

No[cite: 35]. The 𝗣𝗧𝗨 runs automatically when the 𝗚𝗥𝗘𝗘𝗡/𝗬𝗘𝗟𝗟𝗢𝗪 differential pressure exceeds 500 psi[cite: 35, 139]. It is a power transfer device, not a fluid transfer device — it requires one system at normal pressure to drive the other[cite: 36, 40]. In a dual 𝗚𝗥𝗘𝗘𝗡 + 𝗬𝗘𝗟𝗟𝗢𝗪 low-pressure condition, there is no source side, so the 𝗣𝗧𝗨 cannot recover the failure[cite: 37, 140].

Why does ECAM direct PTU OFF in a dual GREEN and YELLOW failure?

Two reasons[cite: 40]. First, with both systems below pressure the 𝗣𝗧𝗨 has no driving source and cannot restore either system — hydraulic fluid is not transferred between systems, only power[cite: 40, 148]. Second, leaving the 𝗣𝗧𝗨 running in this condition risks 𝗣𝗧𝗨 overheat and potential loss of the remaining hydraulic circuit[cite: 41, 148]. Selecting 𝗣𝗧𝗨 𝗢𝗙𝗙 is a protective action commanded by the HYD G+Y SYS LO PR procedure[cite: 42].

Which flight controls remain after a dual hydraulic failure on the A320?

There is no fixed memory list of surviving surfaces — read the 𝗙/𝗖𝗧𝗟 𝗦𝗗 page to confirm actual actuator and surface status[cite: 44, 154]. What the HYD G+Y SYS LO PR procedure does tell you is that F/CTL PROT, STABILIZER, SPLR 1+2+4+5, and FLAPS are inoperative, and SLATS operate at SLOW speed[cite: 45, 155]. Aileron and elevator availability depends on which 𝗕𝗟𝗨𝗘-powered servojacks remain — ailerons, for example, are powered by 𝗚𝗥𝗘𝗘𝗡/𝗕𝗟𝗨𝗘 servojack pairs, so 𝗕𝗟𝗨𝗘 keeps them in play[cite: 46, 156].

What is the role of the BLUE hydraulic system in an A320 dual failure?

After 𝗚𝗥𝗘𝗘𝗡 + 𝗬𝗘𝗟𝗟𝗢𝗪 loss, 𝗕𝗟𝗨𝗘 is the remaining hydraulic system[cite: 49]. It is normally pressurized by the 𝗕𝗟𝗨𝗘 electric pump when AC power is available in flight[cite: 50, 102]. If electrical power is also degraded, the 𝗥𝗔𝗧 drops out automatically and a hydraulic pump coupled to the 𝗥𝗔𝗧 pressurizes 𝗕𝗟𝗨𝗘[cite: 51, 162].

When does the RAT deploy on an A320 with dual hydraulic failure?

The 𝗥𝗔𝗧 does not deploy automatically on a hydraulic failure alone[cite: 54]. It deploys automatically when both AC BUS 1 and AC BUS 2 are lost — typically a dual engine failure or major electrical disruption[cite: 55, 163]. A drop-out 𝗥𝗔𝗧 coupled to a hydraulic pump then pressurizes the 𝗕𝗟𝗨𝗘 system[cite: 56, 162]. The crew can also deploy the 𝗥𝗔𝗧 manually if the situation calls for it[cite: 57, 163].

How does flight control law change after a dual hydraulic failure on the A320?

Normal Law and its protections are lost[cite: 59]. The aircraft transitions to 𝗔𝗹𝘁𝗲𝗿𝗻𝗮𝘁𝗲 𝗟𝗮𝘄 with reduced protections — load factor limitation remains similar to 𝗡𝗼𝗿𝗺𝗮𝗹 𝗟𝗮𝘄, but high angle of attack protection is replaced by low speed stability with stall warning, and bank angle protection is not provided[cite: 59, 168, 169]. Alpha floor is inoperative[cite: 60, 170]. At landing gear extension the aircraft reverts to 𝗗𝗶𝗿𝗲𝗰𝘁 𝗟𝗮𝘄 in pitch and roll[cite: 60, 176]. The trigger is explicit: 𝗪𝗛𝗘𝗡 𝗟/𝗚 𝗗𝗡, 𝗗𝗜𝗥𝗘𝗖𝗧 𝗟𝗔𝗪[cite: 61, 177].

What pressure does the BLUE system operate at on the electric pump versus the RAT?

Normal hydraulic system operating pressure is 3,000 psi[cite: 63, 183]. When the 𝗕𝗟𝗨𝗘 system is powered by the 𝗥𝗔𝗧, system pressure is 2,500 psi[cite: 63, 184]. The difference is published in the system description and is the verified number to teach[cite: 64].

What does the F/CTL ECAM page tell you in a dual hydraulic failure?

The 𝗙/𝗖𝗧𝗟 𝗦𝗗 page shows flight control surface and status indications: spoiler and speedbrake position, hydraulic pressure to each control, aileron and elevator actuator status, ELAC and SEC computer status, pitch trim, and yaw indications[cite: 68, 205]. Use it alongside ECAM STATUS to confirm aircraft state during the HYD G+Y SYS LO PR procedure[cite: 69, 217].

What is the PTU Mirage trap in A320 dual hydraulic failure training?

The PTU Mirage is the assumption that the 𝗣𝗧𝗨 will somehow transfer pressure between 𝗚𝗥𝗘𝗘𝗡 and 𝗬𝗘𝗟𝗟𝗢𝗪 and restore lost flight controls during a dual failure[cite: 72]. It cannot[cite: 73]. The 𝗣𝗧𝗨 requires a healthy source side, and a dual failure leaves it with none[cite: 73, 111]. Recognizing the mirage early — and completing the published ECAM procedure — is the first job[cite: 75]. For more detail, read our full A320 dual hydraulic failure breakdown[cite: 115, 301].

Performance

Braking After Dual GREEN + YELLOW Loss

For HYD G+Y SYS LO PR, ECAM STATUS shows 𝗕𝗥𝗞 𝗬 𝗔𝗖𝗖𝗨 𝗣𝗥 𝗢𝗡𝗟𝗬[cite: 77, 190]. Maximum brake pressure is 1,000 psi and seven full brake applications are available[cite: 78, 191]. ANTI SKID, NORM BRK, and AUTO BRK are inoperative[cite: 79, 191]. Loss of 𝗚𝗥𝗘𝗘𝗡 alone removes normal braking; loss of 𝗬𝗘𝗟𝗟𝗢𝗪 alone removes alternate braking; loss of both leaves the yellow accumulator only, with parking brake as the final fallback if accumulator pressure is exhausted[cite: 79, 80]. Understanding configuration is vital—see our guide on related A320 system traps[cite: 116, 302].

FCOM Documentation

Manual References

[FCOM] §PRO-ABN-HYD, HYD G+Y SYS LO PR, p.55–70/74, Rev 25 NOV 24 [cite: 82, 222]
[FCOM] §DSC-29-10-10, Hydraulic General, p.1/2, Rev 22 MAR 16 [cite: 83, 222]
[FCOM] §DSC-29-10-20, Hydraulic Generation, p.1–2/6, Rev 22 MAR 16 [cite: 84, 222]
[FCOM] §DSC-29-20, Controls and Indicators, p.1–2/10, Rev 25 NOV 24 [cite: 85, 222]
[FCOM] §DSC-27-20-20, Law Reconfiguration and ALTN Protections, p.3/14 [cite: 86, 222]
[FCOM] §DSC-27-20-30, ECAM F/CTL Page Indications, p.11–15/18, Rev 15 OCT 24 [cite: 87, 222]
[FCOM] §PRO-ABN-F_CTL, Status Notes (WHEN L/G DN: DIRECT LAW), p.14/72 [cite: 88, 222]
[FCOM] §PRO-ABN-BRAKES, NORM + ALTN Fault and Y ACCU PR ONLY, p.4/18 [cite: 89, 222]
[FCTM] §PR-AEP-HYD, Dual Hydraulic Failures, p.1–2/4, Rev 28 NOV 23 [cite: 90, 222]
[FCOM] §DSC-32-10-10, Landing Gear Description, p.4–5/6, Rev 22 MAR 16 [cite: 91, 222]