On Instruments: How low can you go?

Here’s hoping that the weather is on your side. If the reported visibility at the airport is below published minimums, you may not be able to descend below the decision altitude (DA), decision height (DH), or minimum descent altitude (MDA) for the approach you’re attempting. I’m saying may because there are some pretty big exceptions to that rule. Exceptions that deal with flight visibility—the visibility as perceived from the cockpit—not ceiling or cloud-base altitudes. Acceptable visibility is what most of us are counting on to make it to the runway. This visibility assumes you’re on an approach path that puts you continuously in a position from which you can descend to the runway at a normal rate, and land within the touchdown zone. We’re obviously not talking about a situation where you’re below an approach’s vertical profile, dragging the airplane up to the runway, facing the possibility of terrain or obstacles, and hoping against hope that the flight visibility is better at low altitude.

First and foremost, it’s the pilot who makes the call on visibility. After all, the visibility in a METAR—or from sources such as an automated surface observation system (ASOS), automated weather observation system (AWOS), or automatic terminal information service (ATIS) installations—are ground observations. It’s reported by automated sensors that sample a small volume of air hugging the surface—or, more precisely, about six feet above the surface in the case of ASOS equipment. This is all very helpful, but what really matters is what we can see on approach. If we have one mile visibility on final approach and an AWOS says it’s one-half mile—well, the regulations say we may be able to safely and legally continue the approach; descend below DA, DH, or MDA; and land.

There’s that conditional “may” again. It all depends on what you see ahead. Ideally, the visibility is good enough to see the entire airport environment (which includes the runway itself, plus any approach lighting systems). But in fog, heavy rain, snow—especially at night—you may not see the runway or its threshold. The only things visible may be the brightest lights of an approach lighting system.

Nothing is more comforting than spotting the sequenced flashing lights—commonly called the “rabbit”—after a taxing approach in low instrument conditions. But don’t get too carried away.

The regulations become more specific about how many elements of the runway environment must be identifiable. In FAR 91.175 (c), the rules state that in order to legally continue a Category I approach (those with typical ceilings and visibilities no less than 200 feet and one-half mile) you must be able to see at least one of the following visual references:

• The approach light system
• The runway threshold
• Runway threshold markings
• Runway threshold lights
• Runway end identifier lights
• A visual approach slope indicator
• Touchdown zone, touchdown zone markings, or touchdown zone lights
• Runway or runway markings
• Runway lights

Nothing is more comforting than spotting the sequenced flashing lights—commonly called the “rabbit”—after a taxing approach in low instrument conditions. But don’t get too carried away. FAR 91.175 also says that you can’t descend below 100 feet above the runway’s touchdown zone elevation unless you can see either the red side row of lighted bars or the last row of red lights before the runway. So seeing a few of the rabbit’s balls of light alone isn’t good enough to let you continue your descent to a landing. For all practical purposes, you have to be able to see right up to the runway threshold.

This means you need to keep not one, but two altitudes in mind when it comes to managing your descent profile: the ceiling minimums published on the instrument approach plate, and the altitude signifying 100 feet above the touchdown zone. Bear in mind that we’re talking about Category I approaches in this article. Category II and III approaches, as well as approaches using enhanced vision systems (such as head-up displays portraying an infrared view of the scene ahead) use rules that permit landings in much worse weather than Category I’s 200-and-a-half minimums.

Just remember the rules leave it up to your interpretation of the view ahead. Yes, visibility minimums are increased if certain ALS elements are inoperative, and nonprecision approaches won’t have the long, colorful lighting arrays that approaches to runways served by precision approaches have. And, yes, you should keep this firmly in mind as you brief yourself for the approach.

But what if you do see the lights and other indicators you’re supposed to? Are you certain? Then proceed with the approach and land. Any doubts about what you’re seeing? Could that row of lights possibly be along a highway, or is it part of an ALS? If there’s any doubt, execute a missed approach. Short final is no time for dwelling on visual illusions, or turning toward a runway you think is there.

Here’s another thought worth remembering: Keep any wind correction angles in mind. If you have a strong crosswind, you’ll be in a crab to stay on the final approach course. This means the nose of the airplane is pointed away from the runway. Scan straight ahead and you may well miss seeing the approach lights and other cues, because they’re way off to your right or left. It’s tempting to continue the approach, even though you’ll never see the runway environment if you don’t widen your scan. It’ll be a guaranteed missed approach unless your peripheral vision happens to catch sight of any lights in time.

If you have the chance to fly a simulator, ask your instructor to give you a night approach at minimums with a big crosswind component. You’ll see what I’m talking about.

Email [email protected]