Driving down mountains is one of the most dangerous things you can do in a large commercial vehicle. Before I go any further, click on the link below to see what happens when someone goes down a mountain too fast.
Donner Pass on I-80 at the California/Nevada border
You can go down mountains too slow for the rest of your life, but you can go down too fast only once. I’ve heard the rule where you use the same gear that you came up the mountain with to go down it. Disregard that because especially in the western U.S at higher elevations, you’ll be at 6000-8000′ elevation for hundreds of miles and may start to descend abruptly.
Sometimes there are posted speed limits for trucks going down mountains. In my experience, if I stay below that limit, I’ll be fine as long as I’m under the 80,000# gross weight limit. Oversize loads must use even more caution and slower speeds. Remember, all information that I’m sharing is under the assumption that your truck and trailer are in fine working order with no brake problems.
If they’re no speed limits posted, I usually get in the proper gear for 35 mph before I descend, operate my four-way flashers and use my brakes lightly if needed to stay in that gear. If you’re empty, you may be able to go a little faster if permitted. I keep alert for safe speed signs for the upcoming curves and look for the runaway ramps in case I need them. The important thing is to keep you speed low because once you gain momentum, your truck will require even more braking to slow down, much less stop that rolling physics experiment. If that happens, your brakes will heat up and start fading away until the scene from the video takes place.
Keep you eye on your trailer wheels as you drop down the mountain. If you start seeing smoke coming off of them and you have plenty of mountain left, you need to try and stop your vehicle immediately to let them cool or you’ll lose your brakes completely. This will take at least 30 minutes and it wouldn’t hurt to pour water on the brake shoes. I had a friend lose his brakes on I-64 in the Blue Ridge mountains of Virginia and he crashed into a truck in front of him which got him stopped and no one was hurt. My friend walked away from trucking that day and never plans to return. Stay safe out there.