Which of the following is NOT part of the Electronic flight instrument system?

The head-up display (HUD) is not considered a part of the Electronic Flight Instrument System (EFIS) because it serves a different function compared to the other instruments listed. While the EFIS primarily focuses on displaying critical flight information directly to the pilots through electronic panels, the HUD projects this information onto a transparent screen in front of the pilot, allowing them to maintain situational awareness without looking down at the instruments.

The EFIS includes instruments such as the electronic horizontal situation indicator, which provides situational awareness regarding the aircraft's position and navigation; the electronic attitude directional indicator, which shows the aircraft's orientation; and the electronic flight director, which helps guide the pilot during various phases of flight. Each of these instruments is designed to convey specific flight data vital for safe and efficient flying.

In contrast, the HUD supplements the electronic flight instruments by providing critical data in a way that requires less head movement, which enhances the pilot's ability to focus on the external environment. This distinction clarifies the role of the HUD as separate from the EFIS.