How Go Down?

totally off topic but having to do with pax comfort. so on tuesday i took my friend to atlantic city. there was an airmet for moderate turbulence along the route of flight. winds were gusting to about 25 and 30 degrees off the runway so i had a pretty good x wind. he was fine the entire flight even though we were getting tossed around. we got a visual into acy and all of the sudden he gets really quiet. i look at him and know he is going to puke. he ends up chucking all over his shirt and on the seat right as i was on short final! so i had to deal with him puking, a good x wind, a wake turbulence from a heavy dc 10 who decides to do a touch and go. looking back, i could handle the winds but for my passenger it was not too pleasant.
 
flyingcheesehead said:
Couldn't there be a good reason to stay at the higher altitude for a while? :dunno: I got discretion from 10,000 to 4,000 125nm from my destination a couple weeks ago. Whaaa? Uh, OK. Read it back, stayed at 10.
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Sure, but I'm talking about setting the ALT window on the autopilot not actually descending. When you have 15-25 altitude exchanges with center. Not setting "the bottom" in the window can be a trap...Last week into YYZ (Toronto) we were in mod and some severe turbulence...during the decent (cleared descend via)I told YYX center I would not be complying with the restrictions on the arrival due to turbulence and waiting until the absolute last second to descend.
 
taters said:
Sure, but I'm talking about setting the ALT window on the autopilot not actually descending. When you have 15-25 altitude exchanges with center. Not setting "the bottom" in the window can be a trap...
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Right, but in the given scenario - Descend and maintain 320 right away, discretion to 240 - Would you be putting 240 in the preselect and then trying to punch the altitude hold when you got to 320? Or would you put 320 in the window until you got there, and after you were holding 320 put 240 into the preselect? Personally, I'd go with the latter - But that's just due to the particular capabilities of the planes I fly. Do you have a mechanism for telling it to stop the descent at 320 when you have 240 in the preselect?
 
Here's a question for the OP:

Why did you level off first and then start reprogramming the box?

If you were initially cleared to a specified altitude and then given a new clearance for a lower one, can you not reprogram the box while continuing the descent?
 
Fearless Tower said:
Here's a question for the OP:

Why did you level off first and then start reprogramming the box?

If you were initially cleared to a specified altitude and then given a new clearance for a lower one, can you not reprogram the box while continuing the descent?
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That's the point. I could, but I want to be on that first profile of high and steep. By leveling off I get back on that profile. If I just keep going I'm on the 2nd or worse, the 3rd.
 
Baron2PG said:
I've received these clearances many times (albeit at a much lower altitude) and it is a manual mental exercise to comply usually leading to a 1500 fpm decent rate over the last two miles.
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Found some good advice for you:
J3 Driver said:
I can't believe how many of you depend on you gps. It's very simple. 3 time your altitude loss is how far out you start down. 5 times your groundspeed is how fast you come down. And guess what, it keeps you on a perfect 3 degree glide slope.
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Keep a running check every thousand feet or so, so you don't get behind. It helps to pad a few miles for the level off too.

Captain said:
I'd much rather fly a 4.0 degree path for fuel savings by delaying the descent as long as possible.
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Is that 4° slope based on a headwind, tailwind or zero wind? I'd be interested in your calculations that show exactly how much your particular airplane will save vs other descent angles.

dtuuri
 
dtuuri said:
Is that 4° slope based on a headwind, tailwind or zero wind? I'd be interested in your calculations that show exactly how much your particular airplane will save vs other descent angles.

dtuuri
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No calculations. It works like this; the steeper you descend the more you need to pull back power to keep from red lining the airplane. At some point it's too steep for pax comfort.

So, you find a degree that works with the above. 3.5 works well with pax onboard and 4.0 works well on empty legs. Purely from experience. No charts or graphs.

I will say this though;
I used to fly B-727's. Since we often flew to small airports in South America and could descend as we wished we would make a game out of trying to chop the power at cruise altitude (FL370'ish) and land from that. The rules were that you had to cut the power completely, flight idle all the way...fly at the barber pole during the descent...slow at 10K to 250 kts....configure on speed (gear and flaps)...and only push the power up at 1,000 feet AGL on final with a normal descent rate.

You were allowed to adjust your ground track (obviously) and you got pretty good at the game. I could do it consistently after a few weeks. Headwind / tailwind weren't as big a factor as weight. If heavy you had to delay the descent. If light start early. The wind mattered but weight and landing runway mattered more. (straight in or have to fly a downwind?)

To answer your question, flight idle burns WAY less gas than with power. PLUS you fly at higher altitudes longer. That is win/win for fuel savings. No graph needed to see the logic.
 
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Captain said:
No calculations. It works like this; the steeper you descend the more you need to pull back power to keep from red lining the airplane. At some point it's too steep for pax comfort.

So, you find a degree that works with the above. 3.5 works well with pax onboard and 4.0 works well on empty legs. Purely from experience. No charts or graphs...

To answer your question, flight idle burns WAY less gas than with power. PLUS you fly at higher altitudes longer. That is win/win for fuel savings. No graph needed to see the logic.
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The steeper descent angle is not necessarily the most fuel-efficient. Consider this study's conclusion:
"The constant flight path angle descent at constant Mach/calibrated-airspeed, a procedure used by regional and business jets but not by large jets, was found to be more fuel efficient compared to the standard descent procedure at idle-thrust and at constant Mach/calibrated-airspeed. The difference in fuel consumption with these two procedures ranged between -8% and +10%."​
It seems to me that the descent profile one uses or wishes to use ought to be matched with the cruising power setting chosen, i.e., long-range, normal or high speed. What's the sense of ignoring fuel burn for three hours at max cruise, then suddenly getting anal about making a fuel-efficient descent profile?

The effect of wind skews the optimal descent angle, too, so without some high-brow mathematical analysis you won't really know what the best descent angle is on a given day. Even if you did, wind components during the descent aren't linearly stratified, especially if the top of descent is above the trop, so descending earlier or later may better mitigate or make use of the jet stream.

Then you have the inherent fuel-efficiency of the engines themselves. Are they fans or straight turbojets? A turboprop probably wouldn't save more fuel on an optimized descent than the line crew spilled during refueling, imo.

dtuuri
 
Captain said:
If heavy you had to delay the descent. If light start early.
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I think you've got that just backwards, but I'll let someone else explain why. Aerodynamics are too theoretical for me.

dtuuri
 
Captain said:
If heavy you had to delay the descent. If light start early.
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dtuuri said:
I think you've got that just backwards, but I'll let someone else explain why. Aerodynamics are too theoretical for me.
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In theory, both are actually incorrect. Your glide ratio does not change with weight.
 
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Baron2PG said:
Curious - What "box" do you program to have the AP fly a smooth decent to hit a specific altitude at a specific point in space?

I've received these clearances many times (albeit at a much lower altitude) and it is a manual mental exercise to comply usually leading to a 1500 fpm decent rate over the last two miles.
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Honeywell or Thales FMS will do it just fine.
 
dtuuri said:
The steeper descent angle is not necessarily the most fuel-efficient. Consider this study's conclusion:
"The constant flight path angle descent at constant Mach/calibrated-airspeed, a procedure used by regional and business jets but not by large jets, was found to be more fuel efficient compared to the standard descent procedure at idle-thrust and at constant Mach/calibrated-airspeed. The difference in fuel consumption with these two procedures ranged between -8% and +10%."​
It seems to me that the descent profile one uses or wishes to use ought to be matched with the cruising power setting chosen, i.e., long-range, normal or high speed. What's the sense of ignoring fuel burn for three hours at max cruise, then suddenly getting anal about making a fuel-efficient descent profile?

The effect of wind skews the optimal descent angle, too, so without some high-brow mathematical analysis you won't really know what the best descent angle is on a given day. Even if you did, wind components during the descent aren't linearly stratified, especially if the top of descent is above the trop, so descending earlier or later may better mitigate or make use of the jet stream.

Then you have the inherent fuel-efficiency of the engines themselves. Are they fans or straight turbojets? A turboprop probably wouldn't save more fuel on an optimized descent than the line crew spilled during refueling, imo.

dtuuri
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Have to admit, this is an excellent read. Problem is it is about 23 grade levels above my head.I'm sure it made an excellent point, I just have no idea what it was.

Still, I can't see how a more than flight idle profile results in less burn. Maybe in crazy weird Jet Stream models, but as a whole...wouldn't flight idle be the best? I've always thought so.

If your report is right I still have no idea how to apply that. When do I do what to save gas?
 
duplicate post.

...stupid phone
 
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Captain said:
Still, I can't see how a more than flight idle profile results in less burn. Maybe in crazy weird Jet Stream models, but as a whole...wouldn't flight idle be the best? I've always thought so.
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The way to get less fuel burn is to use less thrust. The study I linked to compares different "less thrust" scenarios driven by a need to meter traffic in a way that maximizes fuel efficiency. That way ATC can say, "Sure, you're going to arrive late, but look at the fuel we saved you!" :) At the reduced speed, apparently, a constant flight path angle (cruise descent) nets better fuel efficiency than an idling/curved descent path. It surprises me too, frankly.

Captain said:
If your report is right I still have no idea how to apply that. When do I do what to save gas?
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As the study states:

"Computation of fuel-optimal trajectory is not the objective of
this study."​
I only referenced it to show that, IMO, it isn't worth stressing out over an early descent given by ATC in the name of fuel consumption. You have in mind a 4° slope which is steeper than typical ATC descent profiles. I'm skeptical that would net much savings, but often put you at odds with ATC. If you spent the last couple hours at high speed cruise, why suddenly become fuel-conscious for the 20 minute descent?

This is part of the reason I would adjust my descent to cross DIGGY at FL190 rather than level off temporarily. If I did want to level off I'd tell ATC even though, technically, it isn't required.

dtuuri
 
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Well, good points. I'd only add that I'm fuel conscious at cruise too. I don't just turn it on at TOD.

Still, guess ill worry less.
 
Captain said:
Many guys just start down and burn the extra gas. There are three possible profiles;

- stay high long and descend steep,
- descend early and keep a marginal descent rate,
- descend early and steep so as to fly level at the lower altitude.
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Not to sound too stupid...too late.

So that I'm clear on this point, is it correct to assume that ATC has cleared a box of airspace between your present position and the distant fix with a ceiling of your current assigned altitude and a floor of the "crossing" altitude, so that it is essentially a "at pilot's discretion" decent clearance as long as you are at the assigned crossing altitude once you leave the box?
 
Baron2PG said:
Not to sound too stupid...too late.

So that I'm clear on this point, is it correct to assume that ATC has cleared a box of airspace between your present position and the distant fix with a ceiling of your current assigned altitude and a floor of the "crossing" altitude, so that it is essentially a "at pilot's discretion" decent clearance as long as you are at the assigned crossing altitude once you leave the box?
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That's correct baron, in effect matter what you do in that box of airspace, you won't trade paint with another IFR aircraft.
 
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