Descent Profiles / Crossing

[#1 FLAMED OUT]

What is the best way to calculate the following to teach your students?

Your at 40,000 feet and need to cross a VOR at 30,000 feet. Descend at 30 miles away from the VOR. 10,000 feet to loose times 3 is 30 miles? Now what descent rate do you descend at? Speed is 200 knots? Any ideas to help students?

 

[flyguy75000]

Well, to finish it up you need to find out how many miles a minute you are moving. At 200 kts, your covering roughly 3.5 miles a minute. You have 30 miles to cross, which means you'll be at the VOR in roughly 8-8.5 minutes. So you have 8 minutes (being conservative) to lose 10,000 ft. Descending at 1000 ft a minute is too little (8,000 ft), 1500 ft a minute for 8 minutes is 12,000 ft, so 1250 ft a minute will give you 10,000 ft. However, its better to descend just slightly quicker (say 1300-1400 ft/min) and be sure you make the restriction. You wont get busted for being there early, only late.

I hope that makes some sense.

 

[#1 FLAMED OUT]

Yeah, I understand that way with the miles per minute. Is there a quicker way to figure it out using your speed?? Like 200 knots meaning 2,000 fmp??

 

[midlifeflyer]

You can use a base formula of

Distance to descend = Altitude to lose/Descent Rate X GS/60

for all of these calculations, turning it around depending on what you have and what you're looking for. Plug in the numbers that you have (should be everything except one) and find for the one you don't.

Plugging in your numbers gives me a descent rate of 1111 FPM.

You can also plug in and pre-calculate certain parts of the equation. For example, if you are flying an airplane that typically flies 200, you can preclaculate the GS/60 as flyguy suggests (always keeping in mind that the variable is =ground= speed, not airspeed, so we're talking about an estimate). If you do that the formula becomes

Distance to descend = Altitude to lose/Descent Rate X 3.333

 

[Lead Sled]

What is the best way to calculate the following to teach your students?

Your at 40,000 feet and need to cross a VOR at 30,000 feet. Descend at 30 miles away from the VOR. 10,000 feet to loose times 3 is 30 miles? Now what descent rate do you descend at? Speed is 200 knots? Any ideas to help students?

Keep it simple. Back in my airline days we simply used 3 times the altitude loss at 6 times the groundspeed. When I left the airlines and went back to corporate flying it was the only technique that I used for the 3,000 hours that I had in the front row seats in Lears. Nowadays we've got FMSes with coupled VNAV, but the old method still works just fine. In your example you'd start your descent at 3 X 10 or 30 miles out.

As for the descent rate...

You're at FL400 and doing 200 kts? I'll assume that you're talking indicated so that will translate to +/- 360 knots true. The descent rate would be 6 times the groundspeed, so in a no wind situation you're looking at an initial descent rate of roughly 2200 fpm. If your GS is higher then your descent rate would be higher and vice-versa. Remember, your ground speed will change pretty radically as you slow down to meet the 10,000 speed restriction, so out of 10,000 feet you're going to be looking at 250 KIAS. At that point 1500 fpm works well and keeps things simple. It's not rocket science and it's easy to hit those descent restrictions square on.

'Sled