What is the consequence if the time limit written in the AD is not overdue?

The appropriate consequence when the time limit stipulated in the Airworthiness Directive (AD) is not exceeded is that the aircraft must not be operated except for test flights. This is crucial as the AD represents a regulatory mandate designed to ensure the continued airworthiness and safety of the aircraft.

When an AD specifies a time limit for certain maintenance, inspections, or modifications, it signifies that the aircraft can only be operated under strict conditions to mitigate risks associated with potential safety issues. Thus, allowing operation solely for test flights ensures that any necessary corrective actions can be addressed without compromising safety.

The other options highlight serious consequences like the invalidation of the type certificate or the cancellation of the certificate of airworthiness, but these situations would pertain to non-compliance or the operational status of the aircraft in more critical scenarios. The essence of maintaining safety hinges primarily on adhering to the specified time limits, leading to the conclusion that restricting operation—even if within the time frame—represents prudent safety management practices.