Approach Light Geometry and ILS Visibility Requirements

If an airport’s ceiling is below an IFR approach’s minimum descent altitude (MDA) or decision height/altitude (DH/DA), or the approach’s published visibility minimums, a pilot can’t complete the approach. A missed approach procedure is required.

Unless, that is, a pilot can make out the airport’s approach light system which allows them to descend to 100 feet above touchdown zone elevation. Once a pilot has the threshold markings, runway lights, VASI, runway end identifier lights, or other markings, they can continue the descent below 100 feet above the touchdown zone elevation. At that point, visibility permitting, a pilot can either decide to land. Bottom line—a pilot’s flight visibility can trump those of any official airport visibility reports.

When flying an ILS to minimums to complete a mission, it is very satisfying to hear the pilot monitoring state “approach lights only, descend to XXX’,” followed by, “I am visual, I have the controls.” This statement means the helicopter is in a continuous position for a safe landing land and that the helicopter crew has the required visibility to land (typically a 1/4 sm for helicopter conducting an ILS). In our community, we land the helicopter safely but we could use some work articulating how we met the visibility requirement.

So how do you determine the visibility? The only time this question may arise is if there’s a disagreement that bears on the outcome of an accident investigation or other enforcement action. Nonetheless, a disciplined professional pilot should be able to answer this question. 

Fortunately, ILS and precision approach lighting design allows helicopter pilots to easily determine the standard 1/4 sm visibility at the bottom of an ILS. Let’s take a closer look: 

A standard ILS has a three-degree glideslope and a DA of 200’ AGL. 

A standard instrument approach lighting system for a precision approach is 2400-3000’ long or about a half mile. 

Coincidentally, on a three-degree glideslope at about 3800’ (a little over half mile) from the touchdown zone, the aircraft should be approximately 200’ HAT. Continuing to descend on the three-degree glideslope about a quarter mile from the touchdown zone, the aircraft should be approximately 100’ HAT. Therefore, if you see the runway environment at 100’ HAT, you have roughly the ¼ sm visibility required to land.