Central Flying Academy Follow The VASI
Should You Follow The VASI On Final Approach?
It's a question debated by flight instructors and pilots around the country... after flying a traffic pattern, should you use the VASI as a descent aid for final approach, or should you land close to the numbers to avoid wasting valuable runway?
The short answer? Use the VASI when you can, and here's why...
PAPI vs. VASI
Before diving into how you should plan a descent using visual guidance systems, it's important to know a little bit about how they work. For the purpose of this article, we'll use "VASI" as a synonymous term for any vertical guidance lighting system.
According to the AIM, the Visual Approach Slope Indicator (VASI) is a system of lights arranged to provide visual descent guidance information during the approach to a runway. These lights are visible from 3-5 miles during the day and up to 20 miles or more at night. The visual glide path of the VASI provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 NM from the runway threshold.
If you see two red lights over two white lights, you're on glide path. Although normal glide path angles are 3 degrees, VASI lights at some airports may be as high as 4.5 degrees to give proper obstacle clearance.
The Precision Approach Path Indicator (PAPI) uses light units similar to the VASI, but are installed in a single row of either two or four light units. These lights are visible from about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 3.4 NM from the runway threshold.
Two white lights and two red lights mean you're on the established glide path.
Choosing Your Aiming Point
In almost every case, following the VASI will give a single engine piston aircraft more than enough room to land and stop well before the end of the runway. It's much more likely for a single engine piston to land short when aiming for the threshold than to overrun the runway after touching down near the aiming points.
The runway aiming points (commonly called the 1000 foot markers) are a perfect target to descend towards, and you should plan to touchdown within 200 feet of them. If landing performance allows, having some of runway prior to your point of landing will ensure that you don't land short. There's rarely a time when landing on the numbers is safer than landing near the aiming point.
Configuration changes, tailwinds, stop and go landings, and tailwinds are a few reasons why you might plan to land before the aim point markers, to ensure you have enough usable runway left. Because of that, it's not a firm rule to follow every time you land. But in the majority of cases, using vertical guidance to land slightly further down the runway is a safer option.
How You Should Fly Referencing The VASI
Whether flying during the day or night, vertical guidance lighting is one great reference for you to use on descent. On the base leg of your pattern, if the lights show that you're on glide path, you'll probably end up slightly low. The greater rate of descent typical of turns means you'll lose more altitude quicker than a straight-in descent. Because of that, it's OK to fly slightly higher on base than what the VASI indications suggest.
On final approach, use VASI indications to ensure you're flying a stable approach to the runway and aren't chasing the glide path. While it's entirely possible to fly a stabilized approach slightly high or low, the 3 degree glide path has been established as a consistent way to approach the runway without landing short or long.
Landing From An Approach
One of the most common mistakes students make during instrument checkrides is to chop the power and dive for the runway after reaching minimums on an instrument approach. This maneuver inherently de-stabilzies the approach. Unless conditions require the extra few hundred feet of available landing distance you might gain, there's little reason to abandon a glide path in a single engine piston airplane.
There are a tiny number of airports in the country that have extremely short runways coupled with vertical guidance lights. And for these exceptions, you should brief the approach as such.
Entering The Flare From The Pattern Or An Approach
As you approach your flare, stop using references from vertical guidance. Transition your attention to references like the end of the runway or runway edges to interpret your height. The closer you get, the more sensitive the VASI or PAPI becomes and it can be dangerous to attempt following the lights at this point.
Simply put, don't chase the VASI at low altitude over the runway.
Exceptions Are Few And Far Between
At nearly every airport equipped with vertical guidance to the runway, you'll fly a standard glide path of about 3 degrees. But this isn't the case everywhere.
Pilots are prohibited from referencing the 4 light PAPI at Molokai's Airport (PHMK) beyond 1.8 NM from the landing threshold due to rapidly rising terrain. At Molokai, the PAPI is situated at a steep 4 degree glide path and is installed as a reference for straight-in approach traffic. If you fly the standard traffic pattern at Molokai, there's more than enough room to maneuver and land without terrain conflicts by flying a standard 3 degree glide path. In cases like this, there's nothing wrong with ignoring the visual glide path indications.
At Steamboat Springs, Colorado, the 2 light PAPI for Runway 32 is situated at 4 degrees due to rising terrain. The RNAV(GPS)-E approach to this runway requires an extremely steep 7.75 degree descent path from MDA to landing if an aircraft breaks out at minimums. Because of this, instrument traffic usually flies well above the vertical guidance indications provided. The PAPI is really there if you visually acquire the runway well before MDA, or as an aid for visual traffic.
Exceptions are few and far between. Unless you see notes in the chart supplement's airport description, there's no reason to plan on ignoring a VASI or PAPI.
It's An Easy Way To Ensure A Safe, Smooth Touchdown
The VASI/PAPI should be used in almost all cases. You don't want to solely rely on GPS, but you still use it. You don't want to solely rely on an autopilot, but you still use it. Vertical guidance from a PAPI or VASI is the same. It's a great way to ensure you fly a standard, stable approach with plenty of runway on either side of your touchdown. While you shouldn't solely rely on it, there's rarely a good reason to ignore vertical guidance indications when they're available.
This is not an original article. It first appeared on boldmethod.com on 17 August 2017. To read the original article click here.
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