Which two D-pressure indicators are not visible for checking?

The correct answer identifies that the two D-pressure indicators associated with the Auxiliary Power Unit (APU) are not visible for checking. Typically, the APU is often situated in a location that's not readily accessible or may be obscured from direct view, making it challenging to monitor those internal indicators during ground operations or maintenance checks.

In contrast, the indicators on the AMAD (Accessory Gearbox Assembly) and the engine are generally positioned in a more accessible area, allowing for easier visual checks during pre-flight inspections or while performing routine maintenance. Additionally, indicators located in the nose area are designed to be visible as part of the aircraft's exterior monitoring process, facilitating easier checks by ground crew.

Understanding the visibility conditions of these pressure indicators is crucial for ensuring proper maintenance protocols are followed and can significantly affect operational readiness and safety checks.