You can fly an instrument approach through the clouds perfectly; you can break out from under a cloud deck and catch a glimpse of the runway ahead, but that doesn’t mean you can land on that runway — you still must have the proper visibility.
The regulation that allows a pilot to descend below a Minimum Descent Altitude (MDA) or Decision Altitude (DA) to a landing has multiple stipulations. Last week we discussed what constitutes a ‘runway environment.’ But having the runway in sight is not enough. The runway in sight is just one of the four possible stipulations. In this article we will figure out what the rules say about the visibility at the end of the approach.
HOW FAR CAN YOU SEE?
The rule pertaining to this question is 91.175(c)(2) and it says that a pilot cannot fly any lower than the published MDA or DA unless ‘the flight visibility is not less than the visibility in the standard instrument approach procedure being used.’ There are actually many types of visibility and each could produce a different number at the same time.
FLIGHT VISIBILITY: Use of this term is the most the most important part of this rule. Flight Visibility is the average forward horizontal distance that can be seen from the cockpit. From the cockpit means that Flight Visibility is the visibility judged by the pilot in flight! That is not the same thing as visibility reported by a ground observer, a controller, or an RVR machine. This regulation says that there is a particular visibility amount that must be present in order to land out of an instrument approach and it is the pilot that judges that amount. Put another way, if the approach you are flying requires a visibility of 1 mile, and the ATIS, or AWOS, or even a controller reports the visibility to be only one half mile, you can still fly the approach (under part 91). You could even land out of the approach despite the fact that the reported visibility is below the minimum as long as the pilot determines that the flight visibility is at least one mile. The ATIS, AWOS, or controller is reporting ‘ground visibility’ and that does not apply to this rule.
Note: Now we can point to many FAA regulations that simply don’t reflect good common sense — but this is not one of them! The regulation writer here seems to be right on target here. There is no one in a better position to determine flight visibility than the pilot. Airports can be quite large and spread out. Ground weather observers are very seldom positioned on the approach end of a runway, but it is the approach end of the runway that really counts. There could be fog rolling in over the Eastern edge of the airport reducing the visibility to near zero, but the Western edge still has 1 mile. If the approach being flown comes in from the West, there should be no problem making the landing, even though the reported ground visibility would be less than 1. The pilot makes that call on how far they can see, determines that flight visibility meets the regulation and makes the landing.
The RUNWAY VISIBILITY VALUE and RUNWAY VISUAL RANGE: These are visibility estimates that are derived from a machine called a transmissometer. There is one form of visibility estimate that does look down the runway in the direction of the approach — the RVV or RVR. The machine calculates the visibility in fractions of a mile using a projector and receiver that are mounted along the side of the runway. The transmissometer calculates visibility with great accuracy, but it still is a form of ground visibility not flight visibility. Pilots should use RVR as a guide, but the decision on flight visibility remains in the cockpit.
Inside Information: Once a Part 91 pilot landed after shooting an ILS approach that required one half mile of visibility while the RVR was reporting 2200 feet which is less that one half mile. The FAA questioned the legality of the approach, but the pilot was able to convince the NTSB that in fact one-half mile of flight visibility was present at the time of the landing.
HOW CAN A PILOT KNOW WHAT THE FLIGHT VISIBILITY TRULY IS?
Pilots should never attempt to land when the visibility is in fact too low, so how can you be so sure of yourself that you override an RVR measurement? I use the middle marker beacon, approach lights, and runway as a yardstick.
If the approach has a middle marker, the approach chart will indicate the distance from the middle marker to the runway threshold. For most approaches, this will be approximately one half mile. So if I can see any part of the runway when I hear the middle marker beacon sound off — I know I have at least that half mile of flight visibility and that is all you need for most ILS approaches. This gives me proof that I have the legal visibility requirement and this tells me that I am safe and legal to land. I also use the approach light system as a measuring stick.
ALS systems have different lengths, but that all have one thing in common — they all have a cross bar of steady burning white lights that are located 1,000 feet from the runway threshold. The cross bar is used to help level the wings when first coming out of the clouds.
Meanwhile on the runway there are two broad stripes painted on either side of the centerline that are located 1,000 feet beyond the threshold. These stripes are called the ‘Runway Aiming Point‘ or ‘Fixed Distance Markings.’ While flying in over the approach light system, if I can see the cross bar ahead and then beyond all the way to the Aiming Point stripes than I also have a half mile of flight visibility (distance to the cross bar + 1000 feet to the threshold + another 1000 feet to the Aiming Point markings).
For non-precision approaches without middle markers, and approach light systems, I use the runway itself. Before landing you most often can see all the way down to the far end of the runway. You can judge the flight visibility distance with the total length of the runway. The approach at my home airport requires one-mile visibility, but our runway is less than a mile long. That means that I must be able to see farther that the runway length to land there every time. If I ever get down to MDA and can’t see the far end of the runway from over the runway threshold, I must make a missed approach. At night you can use the runway lights as a measuring device. If you can see that lighted outline of the entire runway then you would have the visibility that is at least as far as the runway length. The runway lights themselves are spaced 200 feet apart. It might not be practical to be counting the lights down the runway as you reach the decision altitude – but one half mile would be 13 runway lights (200 feet between each light x 12 = 2400 feet + the first light on the corner of the runway = 13).
BOTTOM LINE: Landing an airplane through IFR conditions is more complicated that it looks. The pilot must have — in sight — at least one item that constitutes ‘runway environment‘ (last weeks article) and also the required flight visibility as judged by the pilot. But there is at least one (and in some cases two) more very important issue remaining.
Did you know it is possible to see all the runway environment items, and have the required visibility, but still it can be unsafe and illegal to land? We uncover the ‘normal approach‘ next week.
