I was reading the IF guide, specifically the descent section and got confused about the rate of descent so I have a couple questions.
The calculation of TOD is Altitude / 1000 x 3
So if we’re for example at FL380, the TOD would be 114nm.
ROD for a 3 degree descent is calculated; Groundspeed x 5 so for example if we’re cruising at 500kts, the ROD would be 2500fpm.
If we get to our TOD and start descending at 2500fpm, we would need to keep changing the rate depending on the groundspeed changes until I reach the desired altitude to catch the glideslope, correct?
But let’s say for example you don’t know your groundspeed, how would you calculate the ROD?
What I’ve always done is set a specific time for my TOD, say 25 mins ETE and I want to be on approach at 3000ft (assuming airport elevation is 0ft). Id do 38000 - 3000 is 35000 then 35000 / 25 which is 1400 fpm and start descending. Is there a way to do the same thing but instead of the time you use the TOD?
Yeah, you do have a valid point. As the GS changes, you will need to recalculate both to TOD and the ROD with the formulas
The easiest way to manage this is to split the climb into different stages so that you can make adjustments before any massive changes in GS. (Eg. split your descent to FL350 to FL200/FL200 to FL120/FL120 to 3000ft).
Of course in theory you could try to keep your GS the same but won’t work realistically considering procedures, restrictions etc.
Yes but it depends on the waypoint restrictions and how close they are, if you have 2 waypoints that are far away from eachother, the vnav would still wait to get closer to that second waypoint and descend at -1800 rather than descend at say -1100. That’s what I’ve noticed
Not really. VNAV in IF isn’t set to a 3° descent as I believe it should be. It’s a bit shallow, hence why it depends from so far out.
The truth is that decent calculations are actually really complicated, and there’s no way that you’re going to be able to come anywhere near to a flight management system’s accuracy by running a simple multiplication.
You’re really going for an idle continuous decent. Tricky thing is that “idle” doesn’t mean 0% throttle in IF. “Flight idle” is higher than the full (“ground”) idle that you get when you pull the IF throttle to 0%. It’s usually about 40%-50% N1 I think. It’s a bit like this tutorial, just in reverse. You’re trying to maintain speed and thrust, which will dictate a V/S. That’s obviously not quite how it should work, but I think it’s the best way of doing it.
What I like to do is use Simbrief. It provides altitudes for every waypoint in the flight log, including descent. I don’t know what and how much data it uses, but I find if you set your VNAV to follow the altitudes given in the flight log from one of it’s OFPs, it works really well, giving a nice, continuous idle descent. It doesn’t incorporate any STAR altitude constraints, so you have to deal with those separately if you want to.
I don’t think I’ve explained that very well 😂
I might make a full tutorial to explain that better because I think I’ve pretty much nailed it. I could be completely wrong but it works and looks good to me.
Only way this makes sense to me is if the plane starts descending before the TOD to get a less steeper V speed (-1800 rather than -2500) as indicated in my example.
If anyone else has any calculations or suggestions I’d love to hear it :)
