On the ground prior to takeoff, what cabin altitude would you expect to see?

• A. Slightly lower than field elevation
• B. Equal to field elevation
• C. Slightly higher than field elevation
• D. Much higher than field elevation

Answer: (A)

Cabin altitude is the altitude that corresponds to the pressure inside the cabin. On the ground, the pressurization system maintains a small positive pressure relative to the outside air, so the cabin is kept a bit higher in pressure than ambient. That translates to a cabin altitude that’s slightly lower than the field elevation. This prepares the airplane for a smooth, quicker pressurization once you begin climbing. If the cabin altitude were equal to the field elevation, there’d be effectively no differential on the ground, which isn’t how the system is typically set up. A much higher cabin altitude would imply a lower cabin pressure than outside, which isn’t how the ground preflight state is configured.