The minimums listed on an instrument approach describe the lowest altitude and visibility allowed during the final approach segment of an instrument approach.
The purpose of approach minimums is to ensure that final approach obstacle clearance is provided from the start of the final approach segment to the runway or missed approach point. Whichever occurs last.
At some point, the pilot at the controls must allow the aircraft to proceed below the prescribed minimums listed on an approach.
Under Part 91.175, the FAA requires pilots to meet three requirements before descending below the published Minimum Descent Altitude or continue an approach below the authorized Decision Altitude or Decision Height.
For a pilot to descend below minimums, one of the following 12 visual references must be distinctly visible and identifiable:
The first visual reference that allows descent below minimums is the approach light system.
With the approach light system is distinctly visible and identifiable a pilot may descend no lower than 100ft above the touchdown zone elevation. This makes the approach light system unique, as it is the only visual reference that does not allow a pilot to descend from an instrument approach's minimums to the runway.
However, approach light systems that include red terminating bars, or red sidd row bars, that are also distinctly visible and identifiable in addition to the approach light system grants a pilot the ability to descend below minimums to the runway to land.
The FAA individually lists the runway threshold, threshold markings, and threshold lights as visual references that can allow a pilot to descend below approach minimums.
It may seem redundant to list 3 different visual references solely for the runway threshold. However, depending on environmental conditions and even the runway itself, the capability of the pilot is increased with no decrease in safety by listing each type of threshold visual reference.
For instance, in low visibility conditions and low light conditions it may be impossible for a pilot to identify the threshold of a runway without the threshold lights. Without the threshold lights, absent any other visual reference, a pilot may have to perform a missed approach.
Contaminants on the runway surface such as snow may also make it challenging for a pilot to identify the beginning of the runway. In this case, using the threshold markers as a visual reference may prove to be too difficult. Even worse, some runways do not have threshold markings painted on them. In this case, the threshold markings physically can not be used as a visual reference.
Runway Edge Identifier Lights (REIL) are a common type of runway lighting installed at approximately 800 airports.
REILs provide rapid and positive identification of the approach end of a particular runway.
The REIL system consists of a pair of synchronized white flashing lights located on each corner of the runway landing threshold. The individual lights may be either unidirectional or omnidirectional and face the approach area.
REILs are most effective at night and can be visually seen from about 20 miles away at night, and 3 miles during the day.
REILs are also extremely effective for identifying a runway surrounded by a large number of other lights, such as a city or suburb, identifying runways that lack contrast with the surrounding terrain, and during periods of reduced visibility.
Due to these abilities, it becomes pretty clear why a REIL is listed as a visual reference that allows descent below minimums to land.
Visual glideslope indicators provide a pilot a visual indication of the quality of their current glide path.
The visual glide slope system will provide adequate obstacle clearance within 10 degrees of the extended centerline up to four nautical miles from the runway threshold. Additionally, pilots that follow the visual indications will be lead to touch down within the touchdown portion of the runway.
These traits, therefore, make the visual glideslope indicator a perfect candidate for continued flight below minimums to land.
The FAA lists both the runway and touchdown zone, and their respective lights and markings, as visual references that allow descent below approach minimums to land.
Similar to the explanation presented with the runway threshold, the FAA lists respective visual references along with its markings and lights to account for certain environmental conditions that may have one aspect of the visual reference not be suitable. As previously explained, runway contaminants, fog, and low lighting conditions are just a few examples of potential conditions that may have one aspect, such as the lights, of a visual reference, be more preferred than another.