If the ambient temperature exceeds a certain limit, what precaution must be taken?

When ambient temperatures exceed certain limits, engaging external cooling systems is necessary to maintain the aircraft's operational integrity and prevent overheating of critical components. Excessive heat can adversely affect both electronic systems and structural components, leading to potential malfunctions or damage.

Using external cooling systems helps to lower the temperature within the aircraft, ensuring safe operation and protecting sensitive equipment from heat stress. This is particularly important for modern aircraft like the F-35, which are equipped with advanced technology that can be sensitive to temperature variations.

The other options, while they may seem relevant, do not effectively mitigate the risks posed by high ambient temperatures. Parking the aircraft in the sun or using a canopy cover might even exacerbate the situation, whereas reducing towing speed does not address the core issue of overheating.