When To Start Descent After Getting Cleared For The Visual

I recently went to recurrent simulator training and the instructor had a different scenario planned for one of the practice sessions. Apparently, there have been some recent GPWS "saves" of aircraft going into KLAS (Las Vegas, NV - USA) after being cleared for a visual approach to runway 26L. The aircraft are about 20 miles east/southeast of the SHAND fix and are descending to the minimum crossing altitude of 4900' for that fix. If you look at the IAP for 26L, you see a mountain peak at 5365'. There have now been several reports of aircraft descending early before the mountain peak to 4900' and getting GPWS "Terrain, Terrain, Pull Up!" alerts.

Remember that when you accept the visual approach, YOU accept responsibility for terrain separation, so this is not an ATC issue. The instructor gave me this scenario to see how I would react. Luckily, I have some friends in the training department and knew it was coming so I already had it figured out. The instructor had us level at 10,000 feet, proceed direct to SHAND, and we were "cleared for the visual." That's an altitude difference of approximately 5000'. I used the 3:1 rule of thumb (300 feet/nm) and decided I wouldn't start down until 15 miles from SHAND (5000 feet to descend x 3 = 15 NM). It's a technique I use all the time when on final on a visual approach (5 mile final x 3 = 1500' AGL). Use the GPWS/TAWS display, especially when in mountainous terrain, and use the Peaks Display option to determine if the altitude you are descending to clears all the obstacles and terrain in your flight path.

Follow The Agonic Line

The Agonic Line blog focuses on aviation training. Advanced Aircrew Academy brings you articles written by subject matter experts in their field on topics of interest for business aviation flight department managers and pilots. Through insightful content it is our goal to reduce declination and show the course direct to true north on aviation training issues.

Agonic Line - An imaginary line on the Earth's surface connecting points where the magnetic declination is zero. The agonic line is a line of longitude on which a compass will show true north.