question about when to descend

[taseal]

Like guys that I know that fly the -800. Headsup displays, flat panel LCD's, beautiful avionics... WILL EAT YOUR LUNCH and STEAL YOUR WOMAN if you try to do a VNAV descent! Heck, even the 757/767 requires a lot of babsitting during VNAV descents and it's hit or miss.

I thought the VNAV was like a fire and forget missile I guess not eh?

 

[NJA_Capt]

Not a damn thing! We can't even draw holds or curved lines for that matter. Honeywell Primus!! Nothing but the best for a Third World airplane.

The Primus 2000 does both, but it doesn't have a banana bar/bottom of descent mark, only TOC/TOD . I guess the airlines didn't spring for the upgrade.

I thought the VNAV was like a fire and forget missile I guess not eh?

Never turn your back on VNAV.

 

[Dispatch1]

I 'had to put that formula to the test yet. I am only on my third flight and my instructor keeps me under 4000 ft. After my fifth flight he will start increasing the altitude. I don't know how high we will actually go yet. I am training in a C172. But sooner or later I guess I'll have to try that formula.




Any landing you can walk away from is a good landing...LOL

 

[woodreau]

Is there a quick little formula that you can use that will help with figuring out when to descend?

lets say you are on airway V3, and you need to cross some intersection at 3,000. you are currently at 11,000 and you are doing about 120 knots.

at what DME from that intersection will I need to start my descent, and what should my rate be?

It isn't a jet - it's only a turboprop - we descend at Vmo - so I use 5 miles a minute for groundspeed even though the ground speed may be higher (anywhere from 240-360kts) in cruise at FL250.

Going from 11,000 to 3,000 = 8000ft - it will take 4 minutes to descend @ 2000fpm x 5 miles/minute = 20 miles. Start the descent at 20 miles from the point you want to level at 3000ft. One common descent instruction I usually get is to cross Topeka VOR at 7,000 from FL230 - we start the descent 40 miles from Topeka.

Since our Vmo (248kts) is less than 250 we don't worry about having to slow or anything at 10000ft - other than to turn on the lights. The formula has worked in all the time I've been flying the 1900.

Once we're in the terminal area below 1000ft, we try to stay in a discretionary descent of about 700-1000ft/min so that we can keep the speed up at Vmo in order to stay in sequence and/or keep up with the jets. If we're forced to level anywhere, all we can do is 235kts in level flight and the jets will start catching up.

Anyways it works real well in straight-wing 1900 flying - I'm discovering it's not working so well when you fly something that goes faster than 250.

 

[derg]

I thought the VNAV was like a fire and forget missile I guess not eh?

Oh HELL no!

 

[launchpad]

I 'had to put that formula to the test yet. I am only on my third flight and my instructor keeps me under 4000 ft. After my fifth flight he will start increasing the altitude. I don't know how high we will actually go yet. I am training in a C172. But sooner or later I guess I'll have to try that formula.

Any landing you can walk away from is a good landing...LOL

You CAN use the formula at any altitude. If you're coming back from the practice area at 3000' and want to arrive 2 miles from the airport at pattern altitude, that's 2000' feet to lose....you'll need 6 miles for that plus the 2 out you want to get there at.......start your descent at 8 miles and it'll work out.

in pattern work it still applies. Abeam the threshold on downwind, you're approximately 3 miles from landing.....1000'. as you approch the middle of your base leg, you're approximately 2 miles from the runway, and should be at around 600 feet or so. As you turn final, you should be 1 mile from the runway and at 300 feet.

it's a rule of thumb, but even with approximations, it can help you plan descents from any altitude. The added bonus of starting to think about it now is that you'll have a big advantage when you start your instrument training and need to plan descents on non-precision approaches, or need to calculate your required descent rate on an ILS........

Use the formula....see if it works. If it doesn't, go over it in your head or work it out with your instructor...it's good stuff....

 

[jtrain609]

I've had situations where we had a sick passenger in back, and we would start our descent early with a rate between 1000 and 1500 FPM down, so it only descends the cabin at a rate of -300 FPM or so. Helps keep people's ears from popping and what not.

Yeah about that. I had to do that for myself yesterday (I was actually flying with Brad from NC, sounds like you went through initial and upgrade with him). Felt like an ice pick was being run through one of my eye balls. Just got home from taking myself off the trip for being sick.

 

[CaptBill]

I'll use VNAV descents for a bit and watch it like a hawk, but usually in the high teens, I'll transition to a combination of "level change" and "vertical speed".

Nothing beats the old fashioned, fleet-common method of doing what you have to do in order to get the aircraft to where it has to be without the crutch of automation.

Amen. My thoughts exactly !!!