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At KSLC (Salt Lake City International Airport) the ILS minimum RVR for runway 34L is 1800 ft. If the RVR equipment is inoperative and RVR is not reported how do you determine the minimum visibility for that approach?

Can I interpolate between 1600 rvr visibility (1/4 mi) and 2400 rvr visibility (1/2 mi) and use 3/8 mi as the minimum?

FAR 91.175 (h) has a conversion table but it does not show a convertible value for 1800 RVR.

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According to the AIM, you should not interpolate these values. You should use the next higher RVR instead:

5-4-20 Approach and Landing Minimums

a. Landing Minimums. The rules applicable to landing minimums are contained in 14 CFR Section 91.175. TBL 5-4-1 may be used to convert RVR to ground or flight visibility. For converting RVR values that fall between listed values, use the next higher RVR value; do not interpolate. For example, when converting 1800 RVR, use 2400 RVR with the resultant visibility of 1/2 mile.

(US AIM 5-4-20)

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  • $\begingroup$ Thanks for the info. Interestingly, if the RVR equipment is inop and "prevailing visibility" is being used, the approach end of the rwy could be in the fog with "0" vis but the metar could be reporting 1/2 mi or better. Guess that's why for Part 91 ops "flight visibility" (determined by the pilot) is the determining factor whether or not you can actually land. $\endgroup$
    – RTO
    Dec 4, 2021 at 22:02
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Use the minimum visibility for the next entry in the table above your RVR.

In your case, 1/2 mile.

This is the safe rule, and in aviation the safest choice is usually the best. It also has the advantage that you can't be accused of breaching the regulations, which might happen if you use any value less than 1/2.

The only alternative is to attempt to find an interpolated value between the two RVR values in the table, on the assumption that the table is based on a formula. But the table does not say it is based on a formula, so that's not a safe assumption. It's certainly not a simple formula, and even if it exists you might be guessing the formula wrong.

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  • $\begingroup$ No doubt that would be legal, but if the visibility was being reported as 3/8 mi diverting to an alternate might be a big penalty if getting into SLC was important. Just wondering if there was any official guidance regarding using interpolated values. $\endgroup$
    – RTO
    Dec 4, 2021 at 17:12
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    $\begingroup$ "if the visibility was being reported as 3/8 mi", it would not, either 1/4 or 1/2, due to the accuracy of the eye evaluation. $\endgroup$
    – mins
    Dec 4, 2021 at 18:43
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    $\begingroup$ @mins Interestingly, if the RVR equipment is inop and "prevailing visibility" is being used, the approach end of the rwy could be in the fog with "0" vis but the metar could be reporting 1/2 mi or better. Guess that's why "flight visibility" is the determining factor whether or not you can actually land. See FAR 91.175 (c) and (d). $\endgroup$
    – RTO
    Dec 4, 2021 at 20:38

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