AIM 5-4-25 kicks off with them: "Pilots operating in accordance with an IFR flight plan, provided they are clear of clouds and have at least one mile flight visibility
and can reasonably expect to continue to the destination airport in those conditions, may request ATC authorization for a contact approach."
If you know your approach lighting system, you can determine your flight visibility
from the numbers and types of lights you can see.
It's a better plan to know the rules and do your best to fly within them--even if someday you're arguing, "The flight visibility
honestly looked like half a mile ..."
If it's not, we still can shoot the approach and land there but we'll need to increase the procedure's specified minimum flight visibility
. How much?
Expect the required flight visibility
, cloud and terrain/ obstruction clearance all the way to the airport
Airspace and Flight Visibility
Distance From Clouds Altitude Class E, less than 3 Statute Miles 500 feet below 1000 feet 10,000 feet msl above 2000 feet horizontal Class E, at/above 5 statute miles 1000 feet below 1000 10,000 feet msl feet above 1 statute mile horizontal Class G (1200 feet or less agl) Day (see FAR 1 statute mile Clear of clouds 91.155(b) Night (see FAR 3 statute miles 500 feet below 1000 feet 91.155(b) above 2000 feet horizontal
local showed a 900-foot overcast with 10 miles visibility, plus a 2-degree temperature-dewpoint spread (13/11) that could have made flight visibility
It states that the "RVR to Visibility Conversion" table may be used for converting RVR to flight visibility
. The AIM also says: For converting RVR values that fall between listed values, use the next higher RVR value; do not interpolate.
The flight visibility
starts to fade away in haze, rain or fog.
However, a layer of smoke generated by California forest fires over the preceding days had drifted over Utah and may have impaired horizontal flight visibility
. According to the NTSB, a Civil Air Patrol pilot who searched for the missing aircraft opined the flight visibility
in the accident site area would have been very bad at night.
The regulations require the requisite flight visibility
in order to land, and that might be better than reported visibility.
The requirements to land following an approach are the same for both Part 91 and Part 135: All you need is one of the standard visual references and the required flight visibility
specified on the approach plate, meaning the visibility from the cockpit in flight.