Learn Farsi: خلبان در فرود مراقب است. - The pilot is watchful during the landing.

You would replace the simple preposition در (in/during) with the formal conditional phrase در صورت (dar surat-e). Because it requires an Ezafe to connect to the abstract noun, you would say: خلبان در صورتِ فرود مراقب است (The pilot is watchful in the event of a landing).

We already know مواظب (movazeb) for "careful." Why use مراقب (moraqeb) here?

Both words mean "careful" or "watchful," but مراقب is slightly more formal and implies active vigilance, monitoring, or keeping a sharp eye out for danger (which perfectly fits a pilot in the cockpit). مواظب is more common in daily conversation, like telling a friend to "be careful" on the stairs.

Why does در mean "during" here instead of its usual meaning of "in"? Could we use در طول (dar tul-e) instead?

You could absolutely use در طول فرود (during the landing), but the simple preposition در is very frequently used right before event nouns like فرود (landing), پرواز (flight), or جلسه (meeting) to mean "in the process of" or "during." It sounds highly natural and concise.

Is the noun فرود (landing) related to a specific verb we should know at this level?

Yes, it is the noun component of the compound verb فرود آمدن (forud amadan), which means "to land" or "to descend." Recognizing these standalone nouns will help you quickly unpack formal compound verbs in news and literature.