Chalk walkers are required for aircraft towing under which condition?

Chalk walkers are utilized during aircraft towing particularly when the Disabled Tow Mode is engaged and when Brake Rider procedures are not in effect. This situation arises because, without the brake rider, there’s no one in the cockpit to manage the aircraft's braking system, which could lead to control issues during towing. The chalk walkers ensure safe maneuvering by guiding the aircraft’s movement and managing clearance and safety around the aircraft.

The correct choice highlights a specific operational safety protocol that is essential when the normal braking system cannot be relied upon. In contrast, towing with a fully operational brake system does not require additional personnel like chalk walkers, as the pilot can adequately manage the brakes. Low visibility conditions also typically utilize other protocols or procedures depending on the specific operational guidelines. During routine maintenance checks, established procedures might phase out the need for chalk walkers in favor of more controlled and direct guidance, where the circumstances do not align with the scenario outlined in the correct answer.