What is the nominal delay time for APU shutdown with the APU bleed air in use?

What is the nominal delay time for APU shutdown with the APU bleed air in use?

• A. 30 - 60 Seconds
• B. 60 - 120 Seconds
• C. 120 - 180 Seconds
• D. 180 - 240 Seconds

Answer: (B)

When the APU bleed air is being used, the environmental control system relies on a bleed source to feed the packs and maintain cabin conditions. If you command the APU to shut down, the system requires a short delay to transition smoothly from APU bleed to the next source (or to wind down the APU) without causing a sudden drop or surge in bleed air. This delay allows the APU to wind down and the packs to reconfigure so cabin pressure and temperature remain stable. That’s why the nominal delay is about 60 to 120 seconds. Too short a delay wouldn’t give enough time for a safe, smooth transition; a longer delay isn’t necessary and would keep the APU involved longer than needed.

When the APU bleed air is being used, the environmental control system relies on a bleed source to feed the packs and maintain cabin conditions. If you command the APU to shut down, the system requires a short delay to transition smoothly from APU bleed to the next source (or to wind down the APU) without causing a sudden drop or surge in bleed air. This delay allows the APU to wind down and the packs to reconfigure so cabin pressure and temperature remain stable.

That’s why the nominal delay is about 60 to 120 seconds. Too short a delay wouldn’t give enough time for a safe, smooth transition; a longer delay isn’t necessary and would keep the APU involved longer than needed.