Which of the following is NOT a typical use of APU bleed air?

• A. Environmental control and cabin pressurization
• B. Anti-ice systems
• C. Starting air for engines via cross-bleed
• D. Fuel tank inerting system

Answer: (D)

APU bleed air is a source of compressed air used to power pneumatic systems when the engines aren’t providing bleed air. It’s most commonly used for environmental control and cabin pressurization, keeping cabin temperature and pressure comfortable; it also feeds anti-ice systems to protect surfaces from ice buildup; and it can provide starting air for engines via cross-bleed, allowing one air source to start others when ground equipment isn’t available.

Fuel tank inerting, on the other hand, requires a nitrogen-enriched, inert gas to displace oxygen inside the fuel tanks to suppress flammability. This gas is produced by a dedicated nitrogen generation or air separation system and is designed to be inert, not bring in oxygen. Bleed air from the APU contains oxygen and moisture, so it isn’t suitable for inerting and wouldn’t reliably achieve the required inert conditions. That’s why inerting is not a typical use of APU bleed air.