Which of the following is mandatory for aircraft operating under IFR but not for aircraft operating under VFR?

• A. Radar altimeter
• B. Machmeter
• C. Alternate static source
• D. A turn and slip indicator or turn coordinator

Answer: (C)

For aircraft operating under Instrument Flight Rules (IFR), an alternate static source is mandatory in order to ensure reliable operation of flight instruments in case the primary static source becomes contaminated or fails. This requirement emphasizes the critical need for accurate altitude readings and instrument performance in IFR operations, where pilots rely heavily on these instruments due to the lack of visual references.

While VFR (Visual Flight Rules) flying allows more flexibility and reliance on visual navigation and less stringent equipment requirements, IFR operations are strictly regulated to ensure safety in instrument conditions. If the static source is compromised, the instruments could provide misleading information, which poses a significant safety risk. Therefore, the alternate static source is a key component in IFR operations to maintain situational awareness and ensure safe navigation.

The other options listed are not specifically mandated for IFR. For example, a radar altimeter or a machmeter may be beneficial under certain circumstances, but they are not universally required for IFR operations. Similarly, while a turn and slip indicator or turn coordinator can aid in navigation, they are more often associated with basic flight training and not strictly required under IFR.