The prime directive when it comes to drift down has changed over the years.
A decade or more ago, the idea was you've just lost an engine, you have to descend, fuel will be critical, so maximize your forward distance. Set the maximum continuous thrust available on the operating engines, allow the speed to decrease to a magic value that helps you descend as slowly as possible, and then descend as slowly as possible while heading to an alternate. You are an emergency aircraft, everyone else can get out of your way.
Current thinking realizes that it does you no good, or anybody else for that matter, to descend slowly into another airplane. This is especially (and morally) true for us flying business aircraft. I often cross the North Atlantic with a crew of two and one passenger. Is it right to endanger the lives on an airliner with hundreds of passengers for the sake of us three? So now the idea codified into international regulations is to offset from any known tracks, descend below those tracks, and then turn to an alternate. This is going to cost you fuel and perhaps the ability to make it to your alternate if you didn't plan for this. More about this: The Great Escape.
So you may be wondering why we need drift down procedures at all. Well, you do need them. We often consider these possible emergencies in a worst case scenario. But let's say you are at 30 West when that critical engine stops and you need to head downstairs. If CPDLC is working, or if you have good connectivity on the HF, why not explain the situation to Gander or Shanwick and see what they can do for you? Chances are you will still have to dive below the tracks, but it doesn't hurt to ask. Or let's say you are stretching your airplane's legs from London to Tokyo over the most barren parts of Canada. You aren't on or above any known tracks but are rightfully worried about other aicraft doing the same thing you are. If air traffic control says you have that part of Canada to yourself, perhaps a proper drift down will get you to your alternate with a little extra gas in case you need to make more than one instrument approach.
You need to know how to do a proper drift down. What follows are drift down notes for the Gulfstream G450, G500, G550, G600, and G650. For those of you flying other aircraft, the generic scenario and procedures will apply to you. The aircraft-specific considerations that follow will provide ideas that should help you add your own aircraft-specific notes.
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