Intelligence
Is reading and understanding really so difficult?
Regulations state that TDZ RVR may be replaced by MID RVR if TDZ RVR is U/S.
Regulations also state the minimum required values for TDZ, MID and Stopend RVR.
Let's assume the real RVR values at a runway are 200, 150 and 75 metres.
Today, the TDZ RVR transmissometer is U/S, so there is no way that the pilots can obtain the knowledge that the TDZ RVR is really 200 m.
The (approach category D) aircraft wanting to start approach is Cat 3A capable, so needs 200 m TDZ RVR value.
Substituting TDZ with the value of the MID RVR means that the pilots have to regard available RVR's as 150, 150, 75. This means that known situation is BELOW LIMITS and approach is not allowed.
Don't need law references for this explanation, it is just a matter of knowing what the meaning of SUBSTITUTION is, go ask your English language teacher.