Oh, Say, Can You See?

April 1, 2007

It's the visibility that counts

From the time we learn to fly, we learn some basic weather rules. As a student, much is made of cloud clearance requirements and the minimum visibility required in given airspace. This education continues through the instrument rating.

Although it is true that the purpose of the instrument rating is to allow you to fly in clouds and in less-than-VFR conditions at the surface, it is still necessary to have prescribed limitations on what the ceiling and the visibility must be to conduct safe operations.

Instrument students learn pretty quickly that standard nonprecision approaches — an approach flown using a VOR, GPS, NDB, or localizer for primary navigation, with no vertical electronic guidance — have a visibility requirement of around one mile (improvements in GPS technology are beginning to allow lower GPS approach visibility requirements).

Some might require substantially more than a mile. For turboprops and jets, the requirements might be greater because of higher approach speeds and the decreased reaction time of flying at 140 knots versus 90 knots.

For an instrument landing system (ILS) approach, which offers both vertical (usually 3.0 degrees) and lateral guidance, the standard requirement is for visibility of at least one-half mile. An LPV approach, which is an approach with lateral and vertical guidance using the Wide Area Augmentation System (WAAS), may also feature one-half-mile visibility minimums. Again, some approaches might require more than that, especially if the approach is slightly offset from the centerline of the runway or if obstructions require a steeper-than-normal glideslope; these, though, are exceptions to the rule.

But what about the ceiling? For a nonprecision approach, ceiling requirements of 400 to 500 feet are typical, and for an ILS, 200 feet is the norm, although again, both can vary based on local conditions and the particular aircraft being flown. Combined, the ceiling and visibility are often referred to as minimums.

Dirty little secret

But there is a dirty little secret that gets lost in the details: Ceiling, in the end, doesn't count nearly as much. The controlling factor is the visibility.

During a nonprecision approach, there is an assumption that you will have to level off at the minimum descent altitude until reaching the missed approach point. If you level off, and still find yourself in a layer of clouds, it means the ceiling is lower than forecast, and you will be executing a missed approach.

Likewise, if you level off and find yourself below the clouds, but unable to see the runway because of poor visibility (caused by fog, rain, mist, or snow), you still will be executing a missed approach. Being able to descend below the ceiling won't help you at all in this case if you can't see the runway.

On an ILS, there is no leveling off. The descent is continuous, all the way to the decision height, when the pilot must either land or execute a go-around.

Again, ceiling really doesn't count. If you reach 200 feet agl and find yourself in the clouds, it's time to go missed. Likewise, if you reach 200 feet, and have some look-down visibility with the ground, but don't see the runway environment as defined in FAR 91.175, you must execute a missed approach. You may land only if you have the required flight visibility as prescribed on the instrument approach chart.

Measuring visibility

Visibility may be measured in more than one way. The most intuitive is in parts of one mile: one-half, three-quarters, one mile. We all can relate to that, as one mile is roughly the same as the length of a 5,000 to 6,000-foot runway. One mile also can be measured using what is called RVR, or runway visual range. RVR is measured in hundreds of feet using machines called transmissometers placed along the runway. Smaller airports that don't have a lot of traffic generally have a hard time justifying the cost of RVR equipment.

Larger airports with a lot of airline or business traffic, on other hand, will have RVR, especially if the geographical area is prone to low visibility, such as that caused by fog on winter mornings in Florida or coastal fog in California. If the RVR equipment fails, the visibility is converted into ground visibility using the table in 91.175.

RVR is usually read at three points along the runway: touchdown, midpoint, and rollout. When RVR readings are available, they will be broadcast on the automatic terminal information service. You also will receive them from the controllers when you first check in on the approach frequency, and if necessary, when you switch over to the control tower.

Sometimes, one end of the airport will have a higher elevation than the other end, especially if the airport is near a valley. Cincinnati/Northern Kentucky International Airport, which sits on a plateau in Kentucky just south of the Ohio River, is a frequent recipient of morning fog in the wintertime. It isn't unusual for the north end of the field to have RVR readings of less than 1,000 feet while the south end has readings of more than 2,000 feet.

Takeoffs also are dictated by visibility. General aviation pilots operating under Part 91 have the freedom to conduct a takeoff when the visibility is zero. This practice, although allowed, might be considered the invisibility takeoff, and is certainly inadvisable. The airlines and other commercial operators, however, must meet certain minimum visibility levels for takeoff as prescribed by their operations specifications. My airline allows us to depart with RVR readings as low as 600 feet, provided we have been trained, the runway centerline lights are visible, the centerline markings are visible, and the Jeppesen airport diagram states that such takeoffs are allowed. Takeoff minimums are also found in the front of the NACO booklets.

Think about how short 600 feet is. That is the length of two football fields, and as you accelerate down the runway, there is very little reaction time if a runway obstruction appears, such as an animal or an airplane with a confused pilot. Furthermore, those lights and markings can be hard to see if the airport is covered in blowing snow or rain. Keep in mind too that in such lousy conditions, your forward visibility usually ends in a wall of cloud, so it's like being in a murky lake in that when you look forward, you realize that at some point you see nothing.

One other very important consideration when taking off in low visibility is the possibility that a return to the airport might be necessary. If an in-flight emergency develops, and you must land immediately, and you are a general aviation pilot, you have the right to fly an approach and take a look. If you just took off from a runway where the visibility is below the minimum required for the approach to that runway — and if you do manage to land safely — you might get a little face time with an FAA representative who will be — sorry, I can't resist — visibly upset.

For an airliner to depart, the rules stipulate that a takeoff alternate be listed on the flight plan, and that the airport be no more than one hour away, in still air, at normal cruise speed with one engine shut down. That's good practice to follow. In a single-engine airplane, consider carefully how you will handle a possible emergency after takeoff.

Day-to-day operations

So, how should you apply all of this to your day-to-day operations? Are ceiling values to be ignored? Of course not. If you should hear that the ceilings are below the minimum for an approach in question, then you should consider alternatives, especially if the controllers are telling you that the weather has not changed over several hours.

But if the ceiling is close to the minimum, then it is important to remember that you must be clear of the clouds to have a decent view of the runway environment. If you are in the clear, then the visibility becomes the most important consideration, because it is at that point the last obstacle to the runway.

How important is it? To draw one more example from the airlines, it is so important that an airline crew is not allowed to even initiate an approach if the visibility is less than the required value. There is no let's-take-a-look clause. However, the crew can continue an approach once inside the final approach fix even if the controller announces that the visibility has dropped. The assumption at this point is that the airplane is stabilized, and in all probability being flown with an autopilot. The crewmembers then have the option of flying the remainder of the approach, but it is not unusual for them to terminate the approach and either wait for the weather to improve or go to their alternate.

What does all this mean for the VFR pilot? Plenty. VFR operations stipulate various ceiling and visibility requirements, the simplest of which is to remain clear of clouds. But it can be argued that visibility is still the driver here as well. Have you ever flown on a hazy day that was technically cloud free, but with lousy visibility? If you have, you know what I mean. The weather may technically call for clear skies, but you might be buzzing along at 2,000 feet and realize you don't have more than one mile of visibility (adding the one mile behind you to the one mile ahead of you, for a total of two miles of visibility, does not count). You aren't flying in clouds, per se, but you don't have the required visibility for the operation at hand.

Again, visibility becomes the overall controlling factor. Clouds are generally easy to see. And except in real scud, it's easy to determine when you are out of the cloud. It's what your visibility is when you break out (or when VFR) that becomes the dictating factor in what you can do next.

See what I mean?

Chip Wright, of Hebron, Kentucky, is a Canadair Regional Jet pilot for a commercial airline.