question about when to descend

[Nick]

3 to 1 rule for when you need to descend. Need to come down 5,000'? Start 15 miles out. Gotta come down 10,000'? Start 30 miles out.

How fast? GS/2*10. Moving at 400 knots? Come down at 2,000 FPM. Moving at 500 knots? Come down at 2,500 FPM. Need to meet a speed restriction? Add 10 miles.

Works in my plane at least

If you wanna save gas, I learned this about a month ago that works great for a flight idle, no speed restriction descent as long as there is no big tailwind.

Each 10,000 feet to lose is 20 miles.
Each additional 1,000 feet to lose is 2 miles.
Slowing from 320 knots to 250 is 3 miles.

FL370 to cross SOMTO at 11,000 and 250 knots:
20+20+12+3= 55 miles at idle.

 

[mikecweb]

If you wanna save gas, I learned this about a month ago that works great for a flight idle, no speed restriction descent as long as there is no big tailwind.

Each 10,000 feet to lose is 20 miles.
Each additional 1,000 feet to lose is 2 miles.
Slowing from 320 knots to 250 is 3 miles.

FL370 to cross SOMTO at 11,000 and 250 knots:
20+20+12+3= 55 miles at idle.

That's a 2:1 decent. Saves a ton of fuel.
Some things to consider especially since we are going into winter. I don't know what's it's like on those RJs but we can't run the anti-ice with the power at idle without losing the cabin. We'll still do a 2:1 decent but we'll pop the spoilers and bring the power up to keep the bleeds going.

 

[kellwolf]

3 to 1 rule for when you need to descend. Need to come down 5,000'? Start 15 miles out. Gotta come down 10,000'? Start 30 miles out.

How fast? GS/2*10. Moving at 400 knots? Come down at 2,000 FPM. Moving at 500 knots? Come down at 2,500 FPM. Need to meet a speed restriction? Add 10 miles.

Works in my plane at least

Amazingly, it works in the CRJ, too. I'm willing to bet it works in a 737, 767, 747 and even those darn Airbuses (Airbi?) Thing is, they're all just airplanes, folks. John hit that on the head, and I think a lot of guys get wrapped around the whole jet things and forget that.

I use the same formula when I'm bored at cruise and to make sure the FMS isn't programmed wrong. Some people jack with the VNAV stuff in the default page, and that can throw you off if you don't know it. The easiest thing to do in the CRJ is program in the crossing altitude, wait for the FMS to tell you when you hit a 3.0 degree descent, and start down at whatever rate it tells you. Keep in mind that if you change your airspeed in the descent, the rate will change, too. Now, if the FMS fails or is deferred, this won't work. Which is why it's ALWAYS a good idea to keep the mental math skills sharp.

 

[taseal]

I have managed to hit crossing restrictions within 100 feet of assigned altitude as we passed of the 0.1 DME distance from our point to cross using the 3 to 1 rule. It works... and it works well! I use it all the time, rather than any of the VNAV glideslope guidance we can program into the FMS.

Altitude to lose x 3 = DME to start down

For instance, you are at 11,000 feet and need to cross XYZ VOR at 3,000 feet. That is 8,000 feet to lose. Multiplied by three, 8 x 3 = 24. You start down at 24.0 DME to cross the VOR at 3000 feet.

If you were flying 120 knots, divide your groundspeed by three, and that is your descent rate. If you had a 40-knot tailwind, your groundspeed is 180 knots. Divided by two, add a zero, and your descent rate should be 800 feet per minute. Like someone said, in an unpressurized airplane, that is a comfortable descent rate. Anything over 1,000 FPM is pushing it with passengers.

so how do you work that out in a 145? descending at 120 IAS is pretty slow?

I would assume you guys descend at about 250-270

lets say 250 and thats 125, so youd be descending at 1250 with that 3 to 1 rule and descent rate formula. isn't that starting to get steep?

 

[derg]

Use the 3 to 1 about 250, plan to hit the marker at 180 in lieu of any ATC requests.

Descending at 120 KIAS? Nope! Nevar!

 

[taseal]

Amazingly, it works in the CRJ, too. I'm willing to bet it works in a 737, 767, 747 and even those darn Airbuses (Airbi?) Thing is, they're all just airplanes, folks. John hit that on the head, and I think a lot of guys get wrapped around the whole jet things and forget that.

I use the same formula when I'm bored at cruise and to make sure the FMS isn't programmed wrong. Some people jack with the VNAV stuff in the default page, and that can throw you off if you don't know it. The easiest thing to do in the CRJ is program in the crossing altitude, wait for the FMS to tell you when you hit a 3.0 degree descent, and start down at whatever rate it tells you. Keep in mind that if you change your airspeed in the descent, the rate will change, too. Now, if the FMS fails or is deferred, this won't work. Which is why it's ALWAYS a good idea to keep the mental math skills sharp.

how do you put that into work with faster planes though?

I assume you gotta slow down before the descent? doing 300 knots, or 250 would be way over 1,000 fpm. most you'd be able to do is 200, which is actually putting out the slats in some planes

 

[derg]

I assume you gotta slow down before the descent? doing 300 knots, or 250 would be way over 1,000 fpm. most you'd be able to do is 200, which is actually putting out the slats in some planes

Well, it's about 10 knots per mile to slow. So maybe in the last three or four miles before the 30 mile point, you start shallowing out your descent in order to slow, hit 250 around 10-ish and continue.

It just takes experience and knowledge of the systems you're working with.

VNAV is a great tool, but will burn you on a variety of advanced airplanes.

 

[FlyChicaga]

so how do you work that out in a 145? descending at 120 IAS is pretty slow?

I would assume you guys descend at about 250-270

lets say 250 and thats 125, so youd be descending at 1250 with that 3 to 1 rule and descent rate formula. isn't that starting to get steep?

We descend from cruise altitude with a groundspeed of anywhere between 400 to 600 knots. This means 2000-3000 FPM descent rate. Even at 3000 FPM, it is not uncomfortable. In our pressurized cabin, that is just about -600 FPM cabin rate. I've done flight idle descents at 4000 FPM, and that's about my limit. Of course, I've been kept high and fast before on approach, and descended a lot faster than 4000 FPM to make the runway (still being stabilized and configured by 1000 feet AGL; just a disclaimer).

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.

 

[taseal]

Well, it's about 10 knots per mile to slow. So maybe in the last three or four miles before the 30 mile point, you start shallowing out your descent in order to slow, hit 250 around 10-ish and continue.

It just takes experience and knowledge of the systems you're working with.

VNAV is a great tool, but will burn you on a variety of advanced airplanes.

yeah easy enough on the FMS

legs = put speed/alt

reset MCP alt

vnav = page 3, DES NOW, and thats it

or wait til you hit T/D, reset MCP alt, at T/D it will do it for ya to meet the alt target you put in LEGS page

 

[taseal]

We descend from cruise altitude with a groundspeed of anywhere between 400 to 600 knots. This means 2000-3000 FPM descent rate. Even at 3000 FPM, it is not uncomfortable. In our pressurized cabin, that is just about -600 FPM cabin rate. I've done flight idle descents at 4000 FPM, and that's about my limit. Of course, I've been kept high and fast before on approach, and descended a lot faster than 4000 FPM to make the runway (still being stabilized and configured by 1000 feet AGL; just a disclaimer).

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.

oh right right... I'm sorry... I misread. anything above 1,000 is too much for unpressurized cabin... it makes no diff in pressurized cabins. duhhhhh :crazy:

 

[derg]

yeah easy enough on the FMS

legs = put speed/alt

reset MCP alt

vnav = page 3, DES NOW, and thats it

or wait til you hit T/D, reset MCP alt, at T/D it will do it for ya to meet the alt target you put in LEGS page

"Theoretically".

But in actual practice, in my experience if you depend on the FMS to do the brainwork for you during the transition from cruise descent to terminal area descent, you're going to carry a high risk of busting speeds, altitudes and crossing restrictions.

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".

Often times, by the moment you determine that VNAV isn't giving you the desired performance, it's worked you into a corner where it might not be recoverable.

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.

 

[HercDriver130]

Yea...that last sentence says it all. Use a bit of airmanship and with time you will know what you can and cant do in whatever plane you are flying. Its called being the PILOT!!! And you flying the plane vs. the plane flying you.

 

[derg]

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.

 

[ERfly]

That's a 2:1 decent. Saves a ton of fuel.
Some things to consider especially since we are going into winter. I don't know what's it's like on those RJs but we can't run the anti-ice with the power at idle without losing the cabin. We'll still do a 2:1 decent but we'll pop the spoilers and bring the power up to keep the bleeds going.

The ERJ bumps the idle up when the ice comes on (aren't FADECs great?), but you have to be on top of it. It's a pretty substantial bump. If you're right on track at idle, that little bump can throw you way off. If I know we'll be getting into ice, I shallow out the descent and give some wiggle room.

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.

But you guys have a great tool in your airplane...the Banana Bar (don't know the real name of it). What I wouldn't give for that...stupid crappy Honeywell avionics.

 

[derg]

Banana bar?

 

[ERfly]

Banana bar?

Yeah. The little curved line that shows where you'll end up on your EHSI/ND. I thought it was a Boeing feature.

 

[derg]

The green semi-circle? Heck, even a mad dog has that! You don't? What indicates 'bottom of descent' on your ND?

 

[ERfly]

The green semi-circle? Heck, even a mad dog has that! You don't? What indicates 'bottom of descent' on your ND?

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.

 

[Nick]

After just 500 hours in the plane I am convinced that the cockpit in the EMB-145 was simply deemed "good enough" by the engineers and test pilots.

The B-757/767 rolled out in the early 80s with autobrakes.

The ERJ rolled out in the late 90s (after the 777) with a bicycle handle, no autobrakes, a squiggly line on the MFD course, half a dozen master cautions per flight under normal operations, and -- oh forget it, that's all for another thread.

 

[kellwolf]

how do you put that into work with faster planes though?

I assume you gotta slow down before the descent? doing 300 knots, or 250 would be way over 1,000 fpm. most you'd be able to do is 200, which is actually putting out the slats in some planes

1000 fpm in the CRJ is when you're on an approach or a SLOOOOW descent. If you want a 3.0 descent at 290 kts (our profile descent IAS), you're looking at about 2400-3000 fpm. You won't need spoilers either. 1800 fpm at idle = 250 kts, give or take.

VNAV can really burn you if your TOD is close to your crossing restriction. It'll go from 1.0 degrees to a 3.0 degree descent in a few seconds. So, it's better to do the math in your head at that point.