WAAS Questions: VDP Missed Point Adherence & CAT Nomenclature

September 2012

The question has been asked if a pilot must adhere to the VDP missed point when a VDP icon displays on the profile view of a WAAS approach plate, and the pilot is using the lowest LPV minima.

We have also been asked if CAT terms are used in reference to WAAS, or if they are ILS nomenclature only.

Let's dive in.

VDP Missed Point Adherence

Visual Decent Point or VDP is sometimes referenced on non-precision straight in approaches for WAAS-(Wide Area Augmentation System) equipped aircraft. VDP is the point in the approach where if the pilot has the runway in view, they may continue their descent using the standard 3-degree descent angle to the runway threshold. If you do not have the runway in sight at this point, you must initiate a missed approach.

VDPs are seen on approaches farther out than normal Minimum Descent Altitude (MDA) and this is normally due to terrain or obstacle considerations.

CAT Nomenclature

We have also been asked if the terms Category (CAT) 3, CAT 2 etc. are used when speaking of WAAS, or if they are only Instrument Landing System (ILS) nomenclature.

CAT terms are ILS approach nomenclature, and don't apply to WAAS. CAT 1 is the approach that we see with the ILS approaches that takes the aircraft down 200 feet Above Ground Level (AGL) and requires one-half mile visibility. This is used by the vast majority of pilots in the U.S. A CAT 2 approach allows descent below 200 feet (but not less than 100 feet) with only 1,200 feet of visibility. This approach involves greater expenses, as it requires additional training for the crew and stringent testing and calibration of the navigation equipment including the radar altimeter and other navigation systems. CAT 3 is the most precise and this will take an aircraft below 100 feet with visibility of about 650 feet. Of course, this is the most stringent approach and requires further approvals and training.

If you have any questions about WAAS, or any other avionics installation-related question, please feel free to contact me. I will be happy to help!