No, the returns from a FMP are not guaranteed but they are relatively more predictable in their returns.

As investments do not flow in or out during the tenure of the scheme it allows the fund manager of the FMP to lock into a  fixed income instrument and hold on to it till the expiry of the instrument.

In other words, the maturity profile of the underlying instruments is less than or equal to the scheme maturity. Therefore, the returns of the scheme closely correspond to the returns generated by the underlying instruments less expenses.