What triggers the CKPT ALT annunciator to light up?

The CKPT ALT annunciator is designed to alert the pilot when the cockpit altitude exceeds 19,000 feet Mean Sea Level (MSL). This function is crucial for maintaining the safety of the crew by ensuring they are aware of potential altitude-related issues, such as insufficient cabin pressure or the risks of hypoxia due to the lack of supplemental oxygen.

When the cockpit altitude surpasses this threshold, the annunciator activates to provide immediate visual feedback, prompting the pilot to take necessary actions, which may include descent to a lower altitude or the use of supplemental oxygen if required. This safety feature is a critical aspect of the T-6's instrumentation and helps to ensure that the pilot remains informed of their operating environment, particularly in high-altitude flight scenarios.

Other options, while relevant to flight safety, do not specifically activate the CKPT ALT annunciator. For instance, loss of cabin pressure could lead to altitude concerns, and low oxygen levels might also be critical, but they do not directly trigger this particular warning light. Likewise, exceeding an airspeed of 300 KIAS does not relate to cockpit altitude warnings. Thus, the correct answer highlights the specific condition under which the CKPT ALT annunciator is actuated.