Quote:
Originally Posted by Sky_bacon
I have another question, what is the formula for calculating your descent rate?
I might be misunderstanding your question, but we don't really ever try to calculate a descent rate (i.e. ft/minute), even though we might have a rough idea of what looks about right.
For example, when flying an ILS we'll usually see a descent rate of ~700 fpm, give or take. The actual number is a function of groundspeed which obviously varies depending on the winds aloft. (With a little trigonometry, you could derive the exact fpm descent rate required for a given groundspeed to fly a 3° glideslope.)
The only real "calculation" we do on the fly (didn't mean that as a joke or a pun, but let's keep it in there!) is using the "3-to-1" rule for figuring when we need to begin a descent to make a crossing restriction. So if we're at FL270 and ATC tells us to cross the IRONS intersection at 11,000', we'll figure on starting down when we're about 48 nm from IRONS ((27-11)*3). If there's also a speed restriction, we'll figure in an extra mile for each 10 kts of speed we have to lose. So for the same example I just used, let's say we were indicating 320 kts and the controller said "cross IRONS at 11,000' and 250 kts," then we would start down around 55 nm from IRONS.
These are just "rule of thumb" calculations and we don't have to be OCD about it because we can always adjust our descent rate (within limits) to make up for a late descent, and we've also got spoilers for drag if we need to lose airspeed a little faster.
Did this answer your question, or did I misinterpret what you were asking?