What precaution is essential when using air brakes on a downhill grade?

When operating air brakes on a downhill grade, it is crucial to adjust the braking pressure periodically. This practice is important due to the dynamics of weight distribution and inertia that come into play when descending a slope. As the train descends, the gravitational force pulling the train downwards increases, potentially leading to a loss of control if the brakes are not managed carefully.

By adjusting the braking pressure periodically, the engineer can maintain optimal stopping power and control over the train's speed. This helps prevent the train from accelerating too quickly, which could lead to overheating the brakes or even brake failure. Properly managing the braking pressure ensures that the air brakes remain effective throughout the descent, allowing for safer operation on inclines.

Utilizing just emergency brakes may not provide the necessary control and could lead to abrupt stops, while turning off the air brake system would eliminate any braking capability altogether, creating a hazardous situation. Increasing speed while braking is counterproductive and would compromise safety. Thus, regularly adjusting the braking pressure is the vital precaution necessary for safe air brake operation on downhill grades.