Air Intake — Flap Closed When Off, Diverter Ram Recovery, the 15-Minute Open

The ECB commands the inlet just as it commands fuel and ignition. The air intake feeds external air to the APU's inlet plenum chamber — and its flap, diverter and ducting hide several pilot-relevant behaviours: the flap is closed whenever the APU is off, the diverter recovers ram air in flight, and after a protective shutdown the flap stays open for 15 minutes.

The air intake system supplies external air to the inlet plenum chamber of the APU... The air intake has an air intake flap which is in the closed position when the APU does not operate. The air intake flap-actuator 516KB moves the air intake flap to the appropriate position. The flap actuator receives the command signals from the ECB 59KD and sends the flap position signals to the ECB. — AMM 49-16-00

1. Three components, ahead of the firewall

Per AMM 49-16-00, the intake has three primary components — the air intake diverter, the air intake (with its flap, nose and actuator), and the air intake duct. The diverter and intake sit in the lower fuselage ahead of the forward APU firewall (FR95), between FR92 and FR95; air flows aft through the duct into the inlet plenum chamber.

The location of the air intake prevents the ingestion of the main-engine exhaust gases or APU exhaust gases through the APU... The walls of the air intake and the air intake duct are fireproof. Their inner surfaces have a layer of sound absorbent material to decrease the noise.

Two structural facts matter to the crew: the intake is placed so the APU never re-ingests engine or APU exhaust, and the intake/duct walls are fireproof and sound-absorbent (the duct itself is a resonator-type muffler — part of why the APU is acoustically tolerable on the ground).

2. The diverter — ram recovery and fluid rejection

The air intake diverter has two tasks: (a) During the flight, the diverter increases the ram air recovery as it diverts the low energy part of the boundary layer. (b) On the ground and in the flight, the diverter (together with the fluid gutters) diverts that the fluids that flow along the fuselage, so that they do not go into the air intake.

Per AMM 49-16-00, the diverter does two jobs:

• In flight — it sheds the low-energy boundary layer so the APU gets higher ram-pressure recovery (clean, high-energy air).
• Ground and flight — together with the fluid gutters it deflects fluids running along the fuselage away from the inlet.

[!note]- Why the diverter matters for the in-flight backup role (integrative synthesis) The APU is the in-flight backup electrical/pneumatic source (00, ATA-70-31). Shedding the boundary layer (AMM 49-16-00) gives it usable ram recovery at altitude — feeding the same demanding high-altitude start the ignition and starting systems are sized for. Fluid rejection protects against water/fuel/oil ingestion in any phase.

3. The flap — closed when off, ECB-driven

Per AMM 49-16-00, the air intake flap closes the inlet whenever the APU is not operating — a flush, sealed door. The flap-actuator 516KB (an electro-mechanical linear actuator) opens and holds it; the ECB supplies it 24.5 V DC, controls the power, and monitors the movement and timing. A manual override lets maintenance work the flap by hand on the ground.

Open logic: pushing the MASTER SW on energises the ECB, which drives the flap open before the start proceeds.

Close logic — three triggers (AMM 49-16-00):

• after MASTER SW to reset with the APU not running;
• when APU speed drops below 7 % during a normal, automatic or emergency shutdown;
• it stays open for 15 minutes after a protective shutdown (not an emergency S/D), once speed is below 7 % — and if the protective S/D happened on the ground (Flight Phase 1–4), the 15 minutes does not start counting until the aircraft is airborne.

[!warning]- After a protective shutdown the flap stays open 15 minutes — a cooling/drain window A normal or emergency shutdown closes the flap as soon as the APU passes below 7 %. But a protective auto-shutdown leaves the inlet open for 15 minutes (AMM 49-16-00) — letting residual airflow cool and drain the compartment before sealing. Contrast this with the emergency shutdown, which gives no cooling (12) and closes the flap immediately. The flap state is part of how the two shutdown classes differ.

4. Indicating and the start-time check

Per AMM 49-16-00, when the flap reaches open, the ECB sends EIS a signal and you read a green FLAP OPEN on the APU system page. If the ECB detects a flap-actuator fault during the start sequence, it commands an APU auto shutdown — amber FAULT on the MASTER SW, APU FAULT / AUTO SHUT DOWN / MASTER SW … OFF on the EWD, MASTER CAUT, single chime, APU in INOP SYS on the SD.

[!note]- The inlet must be confirmed open before the start commits (integrative synthesis) A flap fault during start is not tolerated — it forces an auto shutdown (AMM 49-16-00). The ECB times the flap travel and will not commit fuel/ignition into a closed or uncertain inlet. This is the inlet's piece of the ECB protection logic.

5. Counterintuitive points

[!warning]- The flap is CLOSED when the APU is off Default state is closed/sealed (AMM 49-16-00) — reducing drag and keeping fluids/FOD out; it opens only on a MASTER SW-on command.

[!warning]- Flap behaviour distinguishes shutdown types Normal/emergency S/D → flap closes below 7 %. Protective S/D → flap stays open 15 min (cooling/drain), counted only once airborne if it tripped on the ground (AMM 49-16-00).

Self-test

[!note]- Q1. What position is the flap in when the APU is off, and what drives it? Closed. The flap-actuator 516KB (24.5 V DC from the ECB) moves and holds it; the ECB monitors position and travel time.

[!note]- Q2. The diverter's two tasks? In flight, increase ram recovery by shedding the low-energy boundary layer; ground and flight, reject fluids (with the gutters) from the inlet.

[!note]- Q3. When does the flap close — and the exception? Below 7 % during normal/auto/emergency shutdown; but after a protective S/D it stays open 15 min (counted only once airborne if it tripped on the ground).

[!note]- Q4. What happens if a flap-actuator fault is found during start? An APU auto shutdown (amber FAULT, APU FAULT / AUTO SHUT DOWN / MASTER SW OFF) — the inlet must be confirmed open.

Key takeaways

References

• AMM 49-16-00 (Air Intake System — Description and Operation) — three components (diverter / air intake / duct) ahead of FR95; placed to prevent exhaust re-ingestion; fireproof + sound-absorbent walls, resonator-type duct muffler; diverter two tasks (ram recovery by shedding boundary layer / fluid rejection with gutters); flap closed when APU off, actuator 516KB driven at 24.5 V DC by ECB with manual override; open on MASTER SW; close below 7 % on normal/auto/emergency S/D, stays open 15 min after protective S/D (counted airborne if it tripped on ground); green FLAP OPEN on APU page; flap-actuator fault during start → APU auto shutdown.
• FCOM DSC-49 — APU air intake flap indication on the APU system page (overview).

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.