Instrument Flight Rules (IFR) Practice Exam 2025 – The All-in-One Guide to Master Your IFR Certification!

Using alternate static air introduces changes in altitude readings due to the pressure difference that can occur in various conditions. When a pilot switches to alternate static air, the static pressure reading relies on air pressure within the aircraft, which may be influenced by the aircraft's cabin pressure.

If the outside static pressure is higher than the cabin pressure, this means that the atmospheric conditions outside are relatively more compressed than what the altimeter is reading due to the alternate static air source. This scenario can result in the altimeter reading a higher altitude than the actual altitude of the aircraft.

In essence, this phenomenon relates directly to the relationship between static pressure and altitude readings. When flying at a lower altitude where atmospheric pressure is greater than what’s being sensed inside the cabin (after using alternate static air), the altimeter will reflect a higher altitude than true conditions.