From FAA land...a few scenarios/conditions to consider...there are others in the document...if one of 2 may be inoperative, it appears the touchdown one must be op for II and III...
(1) Other Than CAT II/III. In all operations other than CAT II/III, (including non-precision, approach procedures with vertical guidance (APV), and CAT I precision
operations), the touchdown RVR VS data is controlling when installed and operational. If the touchdown RVR VS is inoperative, some operators may be authorized, via OpSpecs, MSpecs, or
an LOA, to substitute an operative mid-point RVR VS for visibility minima greater than or equal to 1,800 RVR. Refer to OpSpec/MSpec/LOA C052 (Straight-In Non-Precision, APV, and
Category I Precision Approach and Landing Minima—All Airports) for current RVR operational requirements.
(3) CAT II. Touchdown and rollout RVR VSs are an FAA system requirement for runways that are 8,000 feet or less. Touchdown, mid-point, and rollout RVR VSs are an FAA
system requirement for runways in excess of 8,000 feet, but AFS-400 may approve CAT II operations on a runway with only two RVR sensors (a touchdown zone (TDZ) and either a
mid-point or rollout RVR sensor) on a case-by-case basis. All CAT II operations less than 1,600 RVR require touchdown and a second RVR VS (either mid-point or rollout). The
touchdown RVR data is required and controlling for all CAT II operations. Where installed, mid-point and rollout RVR VSs provide advisory visibility information to pilots. CAT II
runways require associated RVR systems. Refer to OpSpec/MSpec/LOA C059 (Category II Instrument Approach and Landing Operations (Optional): 14 CFR Parts 91, 121, 125, 125M,
135, and 91K Operators) for current RVR operational requirements.
(5) CAT III. All CAT III operations require touchdown, mid-point, and rollout sensors of an RVR reporting system. AFS-400 may approve CAT III operations on a runway with only
two RVR sensors (a TDZ and either a mid-point or rollout RVR sensor) on a case-by-case basis. CAT III runways require associated RVR systems. Refer to OpSpec/MSpec/LOA C060
(Category III Instrument Approach and Landing Operations) for current RVR operational requirements.
https://www.faa.gov/documentLibrary/...r/6560.10C.pdf
Last edited by underfire; 5th Feb 2018 at 21:50.