Which of the following actions is part of the procedure when a critical defect light is on?

When a critical defect light is activated, it indicates a serious issue that could affect the safety or performance of the system. The most prudent action in this situation is to declare disable if necessary. This means that the operator should recognize the gravity of the defect and take immediate measures to ensure safety, which might include halting operations or transitioning to a safe mode. By declaring disable, it allows the operator to assess the situation further, involve maintenance personnel, and prevent worsening the situation.

In contrast, increasing speed to prevent further damage is generally not advisable when a critical defect light is on, as it could exacerbate the problem and pose risks. Continuing to the full destination could lead to unsafe conditions or further damage to the equipment. Detraining all guests immediately, while potentially necessary in some extreme scenarios, is typically not the first step unless there is an immediate safety threat to individuals on board. Thus, declaring disable serves as an essential first action to manage the situation appropriately.