It’s when the ceiling is at or below 500 feet and the visibility at or below one mile. Yes, visibility is the governing variable, because even though a ceiling may be uncomfortably low you may still be able to see the runway environment through cloud bases well enough to land. But that’s a technicality. If the weather is that bad, you’d better be on your toes.
So, how low can you go on a Category I instrument approach in low IFR conditions? The first place to check is the airport’s instrument approach plate. The lowest permitted altitude and visibility will be published at the bottom of the page. These will vary from airport to airport, depending on the type of the approach, nearby terrain, obstacles, and lighting systems. It’s also important to check the notes at the top of the page. There may be situations where your approach minimums may be different than those published—an airport without weather reporting on site will have higher minimums, for example. Or a note may say that the approach is not authorized (NA) at night.
One thing’s for sure, if the weather is that low: You’ll be using an approach with vertical guidance, such as an ILS or an RNAV (GPS) approach with localizer performance with vertical guidance (LPV). A precision approach such as an ILS can give you minimums as low as 200 feet and a half-mile. An LPV can offer the same minimums, but because an LPV approach isn’t officially considered a precision approach there is a drawback. When you name an alternate, you can’t use precision alternate minimums (600 feet and two statute miles) for airports that only have LPV approaches.
But back to the question. How low can you go? To minimums for sure, but what about landing? You’ll have to descend below minimums for that. Fortunately, FAR 91.175 lays out the details. To descend below decision height (DH, on ILS approaches), decision altitude (DA, on approaches with vertical guidance), or minimum descent altitude (MDA, for nonprecision approaches) there are four major requirements. Check FAR 91.175 for the specific language, but almost all the requirements relate to visibility. Let’s hit the high points.
One is that you must be continuously in a position to make a normal landing, using normal maneuvers, and at a normal descent rate. In other words, a stabilized approach. No wild banking to get on the centerline, and no diving for the runway.
The rules state that if you fly the approach and can't see those red bars or lights, you cannot descend below 100 feet above the touchdown zone elevation.Another requirement is that the flight visibility be no less than the published minimum value. Here we get into a subtlety. Flight visibility is the forward horizontal view from the cockpit in flight. In the context of an instrument approach, another practical consideration is slant range visibility—the view from the cockpit down the approach path to the runway ahead. In solid cloud, visibility is nil, but you can see through thin clouds. Ground fog, though, may be thin and patchy, and not give a clear view of the runway. Of course, checking metars while preparing for the approach can give you a heads-up regarding ground visibility if they report a runway visual range (RVR)—the horizontal visibility down the runway. But bottom line, it’s the pilot’s call when it comes to the visibility ahead.
The approach light system can be a big help in making the landing runway “distinctly visible and identifiable,” as the regulations put it. (Category II and III approaches allow qualified pilots and airplanes to land at lower minimums.) Of course, lighting becomes of more central importance at night, or in situations involving dense overcasts and fogs.
The best-equipped approach lighting system is the ALSF (approach light system with sequenced flasher lights) configuration. There are ALSF-1 and -2 variants, with the -1 a Category I configuration and the -2 for Category II operations. Both have flashing lead-in lights—better known as “the rabbit”—that give you blue roll-alignment bars, a set of red side row bars, and red “terminating bars” positioned just before the runway threshold.
If you see the threshold itself, threshold markings, threshold lights, runway end identifier lights (REILs), visual glideslope indicators, the touchdown zone or its markings, runway or runway markings, or the runway lights, then you’re well on your way to a low-vis landing.
Remember those red side row and terminating bars? The rules state that if you fly the approach and can’t see those red bars or lights, you cannot descend below 100 feet above the touchdown zone (TDZ) elevation. You did check the approach plate for the TDZ elevation, right?
Let’s say you flew a perfect approach, flew the rabbit all the way down to the roll alignment bars, only to discover that a fog bank had suddenly become a factor and prevented you from seeing the red bars or lights. You passed decision height, but shortly thereafter lost sight of them. What to do? If you were flying under Category II or III you might be able to legally proceed to the runway and land, but since we’re addressing most GA instrument pilots, we’re talking about Category I rules in this article. Nor are we addressing Garmin’s Autoland capabilities, which are meant to be used in case of pilot incapacitation or other emergencies.
Yes, FAR Part 91 does give us a “look-see” privilege that lets us shoot an approach even if the reported visibility is below minimums. Look-see might work if you can see through thin clouds or fog. But in this example, we had our look-see—and didn’t see those red bars and lights. That means it’s time for a miss. No more descending on this approach.
However, the rules do give you some leeway during the initial part of the missed approach procedure. A transient sink is allowed, because it’s well known that configuration changes can cause an airplane to momentarily lose a bit of altitude before it enters a climb. You still have to perform the missed approach—even if you catch sight of the runway while you begin the climb—but technically speaking you will have gone below the DH, DA, or MDA. You hear stories about airplanes momentarily touching down on the runway while still in IMC, but in no way is this permission to continue with a landing.