What is the only MSP code a Plane Captain can call to bring a jet down?

The code needed to bring a jet down due to safety concerns is A30, which corresponds to "Fire bottles low." This is a critical situation that indicates that the fire suppression system, which is vital for ensuring the safety of the aircraft and crew, is not fully operational. If the fire bottles, which contain extinguishing agents, are low, it may jeopardize the aircraft's ability to respond effectively to a fire emergency. Thus, a Plane Captain would utilize this code to initiate necessary actions and ensure the aircraft is brought down safely to address this potentially dangerous issue.

The other codes, while indicating issues that need attention, do not have the same immediate implications for safety. A10, A20, and A40 refer to fuel malfunctions, engine failures, and equipment failures, respectively; though serious, these situations may not necessarily require an immediate landing as directly as a low fire bottle indication would.