If the airlines didn’t fly every time the weather was less than ideal, they’d never fly. As a result, technology and rules are in place to maximize efficiency and opportunity while minimizing risks. One example lies in getting airplanes off the gate when the weather at the destination is forecast to be below minimums.
Like everyone else, the standard IFR 1-2-3 rule applies: If the weather at the destination from one hour before to one hour after the ETA is forecast to be less than 2,000 feet and three miles, an alternate is required. This is no big deal, obviously, and many of us have left with the weather forecast to be right at the minimums for the approach.
However, sometimes the forecast calls for a possibility of weather that is going to be temporarily below the landing minimums. In FAA weather lingo, we call this “conditional phrases,” and they consist of BCMG, PROB, and TEMPO. For example, the main body of the forecast may have the visibility at one-half mile, but a TEMPO phrase may show a possible drop to on-quarter mile at the ETA.
When this happens, the airlines that have been granted approval to do so can use what is called Exemption 3585. Under the terms of the exemption, the flight will be required to have not one, but two alternates. Further, the method used to determine the alternates is changed as well.
Remember, the airlines do not use the 600-2 and 800-2 rules that GA use for determining the suitability of an alternate; the rules for determining a Part 121 alternate are beyond the scope of this post, but suffice it to say, it’s possible that an airport could be an alternate as long as the forecast is calling for weather of at least 400 feet and one mile.
Under Exemption 3585, the forecast (again, we can use conditional phrases) at the ETA for the first alternate must call for a forecast of no worse than one-half the visibility and ceiling required for the approach. In our example of a 400 and one, the weather at the first alternate can’t be forecast to be less than 200 and one-half.
Looking ahead to the second alternate, the FAA has a pretty simple criteria: This one must be essentially a sure thing. The forecast for the second alternate can also utilize conditional phrases. However, this time, the forecast must call for weather—even with conditional phrases—that equal the ceiling and visibility that can be used for the approach. No reductions are allowed. In essence, if the conditional phrases must have such good weather, it stands to reason that the main body is going to be for nearly VFR conditions.
There is one other option: Category 2 approaches. CAT II approaches can be flown with a runway visibility range (RVR) reading of 1,200 feet—that is, one-quarter mile of visibility. Such approaches are a pretty hair-raising experience. However, CAT II approaches are a significant investment because of the maintenance requirements for the airplanes, and if the airline does not have a great deal of diversions in a calendar year caused by low visibility, CAT II isn’t worth the cost. Exemption 3585 does the trick.
This is a fairly simple explanation, and the variety of possibilities can get complex and tricky, but Exemption 3585—sort of a poor man’s CAT II that was originally put together for People Express—is an indispensible tool, and if you should ever be hired by a regional, you will spend a lot of time in training dissecting Exemption 3585.
The sad thing is that while you while you will spend hours learning 3585, you will rarely use it. In 16 years of airline flying, I have taken full advantage of 3585 fewer than a dozen times. Category II on the other hand….—Chip Wright