Early Descent to promote engine life.
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Early Descent to promote engine life.
I'm curious to know how many boeing drivers use the "DES NOW" function in the FMC when told to "descend at your discreation" before TOD. Is it common practice to use this facility as a means of "smoothing" the transition to an idle profile with the aim of reducing the impact of cooling on the engines? Or is it considered more economical to maintain cruise altitude until crossing the TOD?
Finally, do Airbus have a similar function in the FMGS that allows an early descent?
Finally, do Airbus have a similar function in the FMGS that allows an early descent?
I tend to avoid using any procedures that cause "discreation", however, on the 767 "Desc Now" is very useful, especially to reduce the chances of an overspeed when initiating descent from a level with a strong tailwind.
On the 744, the autothrottle and VNAV algorithms seem to do a much better job.
I do not know about the Airbus.
On the 744, the autothrottle and VNAV algorithms seem to do a much better job.
I do not know about the Airbus.
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I do not believe there are any cooling issues on a jet engine.
On the'bus you can start descent either by selecting open descent (like flch in a boeing) v/s mode or what i think you are getting at which is managed descent, selecting this before the TOD will give you 1000'/min until you merge with the calculated profile.
On the'bus you can start descent either by selecting open descent (like flch in a boeing) v/s mode or what i think you are getting at which is managed descent, selecting this before the TOD will give you 1000'/min until you merge with the calculated profile.
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Originally Posted by esr
to reduce the chances of an overspeed when initiating descent from a level with a strong tailwind
a) makes a slightly 'smoother' transitition for pax
b) increases fuel burn
c) satisfies an ATC request to 'descend now' earlier than planned TOD although v/s does the same at a potential 'legal' 500fpm rather than the computer's 1000fpm, this marginally improving fuel burn. I cannot see any effect on engine 'life'.
You will finish up on EXACTLY the same VNAV profile you would have done if you had not intervened.
The 737 models descent planning based on ACTUAL wind at the point of TOD (plus pilot input of descent winds lower down) - I believe it uses 2/3 of the 'cruise' figure for planning. Using 'Descend now' on a 737 merely delays the point at which a potential overspeed may occur, especially if there is a stronger tailwind than planned lower down in the descent. I have consistently found it a rarity that the 737 FMC calculates a 'professional' descent plan and occasionally have had it try to 'overspeed' when when the planned tailwind becomes a headwind. Personally I find it a badly 'written' computer function, not helped by a dubious pitch/speed performance in the 737 that I do not quite understand.
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[QUOTE][This is a misunderstanding I have seen time and time again (737). The function does nothing to prevent that./QUOTE]
BOAC
I agree with esr. It certainly used to help on the 737-700 from high levels because it only goes to IDLE at a lower level after intercepting the path from below at 1000 fpm so the initial descent at high level is not commenced so aggressively. I don't find it such a problem on the -800.
Also I remember Britannia using Descend Now on the 767 for a while (long time ago admittedly)and I seem to remember it was something to do with engine cooling.
BOAC
I agree with esr. It certainly used to help on the 737-700 from high levels because it only goes to IDLE at a lower level after intercepting the path from below at 1000 fpm so the initial descent at high level is not commenced so aggressively. I don't find it such a problem on the -800.
Also I remember Britannia using Descend Now on the 767 for a while (long time ago admittedly)and I seem to remember it was something to do with engine cooling.
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Stan - it all depends on when you select it! A lot of the 'mistaken' (IMO) selections are made about 10-15 nm before TOD (ie 1-2 mins) due to anxieties about VNAV performance 'to avoid overspeeding' and this does NOT work for the reasons I said. Obviously if you select it, say 60nm before TOD you have a better chance of avoiding it due to less time in pure VNAV 'descent' - but then you are essentially simply in a v/s mode, but with a built in 'alarm clock' to ensure you revert to planned descent - and look at the increased fuel burn!
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BOAC
I must have been 'mistaken' then.
(This is the EDIT part btw: The overspeed I'm on about is right at the top of descent - not later on the path, that may or may not happen anyway. The distance before you start down using 'DESC NOW' is really nor relevant to the overpeed problem at TOD with the proviso that it will obviously gain the path at a much lower level from 60 miles. Also gaining the path at a lower level is helpful )
It's only a problem at high levels (around 370 and above) so yes, effectively putting it in VS -1000 is less aggressive than stuffing the nose down to 3000fpm or more at IDLE.
Then when it captures the Path it has two further advantages:
1) It's already descending so the change is less abrupt.
2) We are somewhat lower than the initial high level.
I haven't flown the -700 for over five years so maybe the software has improved but I know what worked for me then.
I must have been 'mistaken' then.
(This is the EDIT part btw: The overspeed I'm on about is right at the top of descent - not later on the path, that may or may not happen anyway. The distance before you start down using 'DESC NOW' is really nor relevant to the overpeed problem at TOD with the proviso that it will obviously gain the path at a much lower level from 60 miles. Also gaining the path at a lower level is helpful )
It's only a problem at high levels (around 370 and above) so yes, effectively putting it in VS -1000 is less aggressive than stuffing the nose down to 3000fpm or more at IDLE.
Then when it captures the Path it has two further advantages:
1) It's already descending so the change is less abrupt.
2) We are somewhat lower than the initial high level.
I haven't flown the -700 for over five years so maybe the software has improved but I know what worked for me then.
Last edited by Stan Woolley; 28th Jan 2009 at 10:51.
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I do not believe there are any cooling issues on a jet engine.
although v/s does the same at a potential 'legal' 500fpm rather than the computer's 1000fpm,
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BOAC,
My understanding of "descend now" or "managed speed" functions is that the command a fixed rate of descent (1000 fpm on the bus and 1200 fpm on the boeing).
If descending out of a jetstream in flch or open descent under certain conditions the aircraft may enter a higher than desired RoD or an overspeed or both. Therefore by selecting what is essentially a v/s mode early you can avoid a potentially dangerous/uncomfortable porpoise. Therefore you're using the engines to help dampen out the high level windshear. At high level I've never seen a situation that requires the engines to run up and down erratically as a result of a v/s descent.
My understanding of "descend now" or "managed speed" functions is that the command a fixed rate of descent (1000 fpm on the bus and 1200 fpm on the boeing).
If descending out of a jetstream in flch or open descent under certain conditions the aircraft may enter a higher than desired RoD or an overspeed or both. Therefore by selecting what is essentially a v/s mode early you can avoid a potentially dangerous/uncomfortable porpoise. Therefore you're using the engines to help dampen out the high level windshear. At high level I've never seen a situation that requires the engines to run up and down erratically as a result of a v/s descent.
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The fuel nazis have had a recent purge on using this function in our mob.Smoother transition/engine cooling etc are based on pure urban legend and "Heard it from a friend" sources so they've got a small point I suppose (although the boeing can be a bit "pitchy" in vnav .)
The best reason I often find for using it is "watched kettle" syndrome. The "reset mcp alt" prompt appears at tod/-15 and in a busy atc environment the request for descent is usually made upon this prompt. Assuming you managed to get the call in quickly, the aircraft will then chunter along for about 2 minutes( ..one mississippi, , two mississippi,,count to 120..)) before it commences its descent on the vnav path. I often imagine the controler in a busy sector looking at the block and wondering why the hell we are still sitting at cruise level when we just barged in on a busy rt stream with a request for descent.
When does "at pilot's discretion" just become "when the box decides to get on with it"?
The best reason I often find for using it is "watched kettle" syndrome. The "reset mcp alt" prompt appears at tod/-15 and in a busy atc environment the request for descent is usually made upon this prompt. Assuming you managed to get the call in quickly, the aircraft will then chunter along for about 2 minutes( ..one mississippi, , two mississippi,,count to 120..)) before it commences its descent on the vnav path. I often imagine the controler in a busy sector looking at the block and wondering why the hell we are still sitting at cruise level when we just barged in on a busy rt stream with a request for descent.
When does "at pilot's discretion" just become "when the box decides to get on with it"?
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Stan - I can see that, except all the occasions when my little b****d has tried to run away to the barbers have been further down the descent - 300-350'ish - I have not seen the 370+ 'runaway'.
TheG - those conditions are normally catered for by planning the descent winds and can easily be handled by using v/s if need be. Let's face it, if you are chuntering into a 100kt jet and plan your descent on that descent wind........................
Homer - that was item 3 on my post#4. If you 'request descent' - and get it - before planned TOD you need to action it - the 'allowable' time has been kicked around here before. Better, IMO, to use v/s at 500?
TheG - those conditions are normally catered for by planning the descent winds and can easily be handled by using v/s if need be. Let's face it, if you are chuntering into a 100kt jet and plan your descent on that descent wind........................
Homer - that was item 3 on my post#4. If you 'request descent' - and get it - before planned TOD you need to action it - the 'allowable' time has been kicked around here before. Better, IMO, to use v/s at 500?
In my experience in the 767, the most potential for overspeed is when descending out of a jetstream tailwind. If this is initiated in "Desc Now" (or V/S -1000) several minutes before VNAV calculated ToD, then the path will be intercepted several thousand feet below the jetstream core with a slower transition )shear if you like) from 160kt or so T/W to much less.
Seems to make bugger all difference to fuel over destination, and surely pulling the speedbrake would be detrimental to ones potential energy situation!
It works - trust me!!
It seems far less important in the 744 - probably due to both a better VNAV programme and the availability of uplinked forecast winds.
Seems to make bugger all difference to fuel over destination, and surely pulling the speedbrake would be detrimental to ones potential energy situation!
It works - trust me!!
It seems far less important in the 744 - probably due to both a better VNAV programme and the availability of uplinked forecast winds.
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Originally Posted by esr
is when descending out of a jetstream tailwind.
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Hope the head is not too bad and the 'tailwinds' are gentle
Same comment to you as to TheG in para 2 of post#13..... How does it go?
"Garbage in = garbage out". The a/c and the FMC rely on the people who press the buttons.
Same comment to you as to TheG in para 2 of post#13..... How does it go?"Garbage in = garbage out". The a/c and the FMC rely on the people who press the buttons.
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I personally have never had any issues with overspeed on either aircraft in managed/VNAV descent (i could maybe foresee an issue if you have programmed the FMC to descend at M.82/320kts).
I have suffered an overspeed once on the A320, this occurred after ATC kept us above our descent profile, I then went into THR IDLE OP DES, and wound up the speed (very new on the bus at the time). This resulted in the aircraft passing through the speed bug into the barbers pole.
I have suffered an overspeed once on the A320, this occurred after ATC kept us above our descent profile, I then went into THR IDLE OP DES, and wound up the speed (very new on the bus at the time). This resulted in the aircraft passing through the speed bug into the barbers pole.
BOAC - no, I think I had it right the first time!
Having thought about it (and consumed a few more beers) - the descent out of a jetstream t/w gives you an decreasing t/w shear (which is equivalent to an increasing h/w scenario). This leads to the possibility of an overspeed in the initial part of the descent as one gets out of the jetstream.
V/S or FLCH, selected a couple of minutes before the VNAV calculated ToD, are probably preferable modes to "Desc Now". Both seem to hold the selected speed better as they are not subject to the VNAV path allowable speed variation.
To correct the thread drift - I can't see that the use of "Desc Now" would have any measurable or significant effect on engine life.
Having thought about it (and consumed a few more beers) - the descent out of a jetstream t/w gives you an decreasing t/w shear (which is equivalent to an increasing h/w scenario). This leads to the possibility of an overspeed in the initial part of the descent as one gets out of the jetstream.
V/S or FLCH, selected a couple of minutes before the VNAV calculated ToD, are probably preferable modes to "Desc Now". Both seem to hold the selected speed better as they are not subject to the VNAV path allowable speed variation.
To correct the thread drift - I can't see that the use of "Desc Now" would have any measurable or significant effect on engine life.
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Originally Posted by esr
This leads to the possibility of an overspeed in the initial part of the descent as one gets out of the jetstream.
) or by a bit of 'mental' to work out when to descend - in a different mode. There is no reason for an 'overspeed' during descent IN the jet since the FMC is flying the profile it calculated FOR the wind conditions IN those conditions - unless you are descending into stronger tailwinds. When you drop below the jet, it will find itself with too too high a RoD.