The term "restrictive flight envelope" refers to a set of limitations and boundaries that impose restrictions on an aircraft's performance and handling. An alternative synonym for this phrase could be "constrained flight parameters", which highlights the limitations placed on the plane's operation. Other similar terms include "limited operating range" and "restricted operational capabilities". These synonyms imply that there are certain conditions or situations in which the aircraft may not be able to perform to its maximum potential. Whichever phrase is used, it's important to understand the significance of these limitations for the safety and effective operation of the aircraft.