B737 NG Descent profiles
Thread Starter
Join Date: Feb 2002
Location: east coast
Posts: 33
Likes: 0
Received 0 Likes
on
0 Posts
B737 NG Descent profiles
Howdy all.... Just started polling around 737 Ng series [700/800] and having never flown jets before am having a little trouble on descents. Tell me folks what do you think is the best profile or method to descend these things???
I've heard and been shown a few methods..ie starting descent early so that it will hold the profile in VNAV Path, using Speed in leiu of path etc, etc. It appears with a bit of a tailwind the Aircraft wont hold VNAV Path and tends to go into ' overspeed disconnect' fairly early on descent. Any thoughts, profiles from experience, FMC manipulations, methods or calculations that may help the process would be appreciated.
Thanks.
I've heard and been shown a few methods..ie starting descent early so that it will hold the profile in VNAV Path, using Speed in leiu of path etc, etc. It appears with a bit of a tailwind the Aircraft wont hold VNAV Path and tends to go into ' overspeed disconnect' fairly early on descent. Any thoughts, profiles from experience, FMC manipulations, methods or calculations that may help the process would be appreciated.
Thanks.
Join Date: Nov 2002
Location: Brisvegas
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
I certainly do not know all the answers, however,
Nothing you put in the FMC will make any difference. On the descent page there is provision for anti-ice on and off levels but as the VOL 2 states it does not affect the a/c performance.
Most jet jockeys that have been around for a while will tell you that having "gates" on your descent is the way to go. I.E. an ALT at a given distance. It is simple mental gymnastics that provide a simple back up to what the FMC is doing. NEVER EVER have total blind faith in the FMC!
The FMC always flies a 3 degree profile in path and will generally hit profile limits pretty close to the mark. You have probably found that the heavier the a/c the more difficult it is to hold speed when descending in path. Some pilots swear by speed at all times and forget about the path (as long as they are above or below any limiting steps). I always put 500 feet below a AT or BELOW step on a STAR. This will account for any speed or path excursions that you have to fix at short notice, ATC restrictions etc.
Dont be affraid to use the airbrake at the higher levels if the speed starts to get away from you. Disregard the "he man" B.S. some testosterone heads carry on with about using the airbrake. It is there for a reason. I have found that overspeed disconnect is generally only a problem at higher levels. I never descend early for no other reason than economy, although that can be argued against as the saving is minimal. I rarely have disconnect problems any more due to a bit of experimentation, and as I said they all started at higher levels (above 30,000 feet).
Good luck.
Nothing you put in the FMC will make any difference. On the descent page there is provision for anti-ice on and off levels but as the VOL 2 states it does not affect the a/c performance.
Most jet jockeys that have been around for a while will tell you that having "gates" on your descent is the way to go. I.E. an ALT at a given distance. It is simple mental gymnastics that provide a simple back up to what the FMC is doing. NEVER EVER have total blind faith in the FMC!
The FMC always flies a 3 degree profile in path and will generally hit profile limits pretty close to the mark. You have probably found that the heavier the a/c the more difficult it is to hold speed when descending in path. Some pilots swear by speed at all times and forget about the path (as long as they are above or below any limiting steps). I always put 500 feet below a AT or BELOW step on a STAR. This will account for any speed or path excursions that you have to fix at short notice, ATC restrictions etc.
Dont be affraid to use the airbrake at the higher levels if the speed starts to get away from you. Disregard the "he man" B.S. some testosterone heads carry on with about using the airbrake. It is there for a reason. I have found that overspeed disconnect is generally only a problem at higher levels. I never descend early for no other reason than economy, although that can be argued against as the saving is minimal. I rarely have disconnect problems any more due to a bit of experimentation, and as I said they all started at higher levels (above 30,000 feet).
Good luck.
Join Date: Dec 1999
Location: UK
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
50 Cal
Most guys here use the DES NOW prompt and start down a bit earlier than normal to keep the speed from running away, especially at the higher levels (>35K). I only put in descent tailwinds above say 30 kts but normally anticipate going to FLCH around ten grand.
Program in a sensible descent speed(I don't know what CI you use, we're around twenty), we leave the Mach No and change the IAS to 280 kts. SOP is 250kts below 10k also so we leave that in at the planning stage.If you program say 315 kts it limits the margin to increase speed to gain the path if you end up getting high.
I'm not that keen on getting above 300 kts now as we can't use the speedbrake there, so unexpected turbulence at >300kts is more of a consideration now.
The speed brake isn't ideal but often it's the only sensible option, it's there so use it if you need to.
Keep the track miles updated and use 3 x height plus tailwinds/ Anti ice and a few miles in proportion to speed.
This may be a conservative approach to descent planning but it serves me well and keeps the stress levels down.
Cheers
Most guys here use the DES NOW prompt and start down a bit earlier than normal to keep the speed from running away, especially at the higher levels (>35K). I only put in descent tailwinds above say 30 kts but normally anticipate going to FLCH around ten grand.
Program in a sensible descent speed(I don't know what CI you use, we're around twenty), we leave the Mach No and change the IAS to 280 kts. SOP is 250kts below 10k also so we leave that in at the planning stage.If you program say 315 kts it limits the margin to increase speed to gain the path if you end up getting high.
I'm not that keen on getting above 300 kts now as we can't use the speedbrake there, so unexpected turbulence at >300kts is more of a consideration now.
The speed brake isn't ideal but often it's the only sensible option, it's there so use it if you need to.
Keep the track miles updated and use 3 x height plus tailwinds/ Anti ice and a few miles in proportion to speed.
This may be a conservative approach to descent planning but it serves me well and keeps the stress levels down.
Cheers
Join Date: Jun 2001
Location: UK
Posts: 1,178
Likes: 0
Received 0 Likes
on
0 Posts
Monitor the FMC using the following:
Press the Metres button - use the readout to give the track miles required to touchdown. It gives you a little over 3 x FL at height (which allows for the greater speed) and closer to 3 x Ht as you get closer to mother earth.
Less than 250kts requires exactly 3 x Ht for a constant descent approach. Works every time (sensible correction for wind).
Fat Dog
If you get a VNAV overspeed disconnect, going to Lvl Chg and increasing speed is not always a good option.
Speedbrakes are fitted in this aircraft to be used if required. All speedbrakes are uncomfortable for God's sake.
Getting high in the latter stages of an approach in this aircraft is most definately the wrong place to be.
Practice the fast and high approach procedures any day at BCN when Rwy 07 in use!!!!!!!!!!
Press the Metres button - use the readout to give the track miles required to touchdown. It gives you a little over 3 x FL at height (which allows for the greater speed) and closer to 3 x Ht as you get closer to mother earth.
Less than 250kts requires exactly 3 x Ht for a constant descent approach. Works every time (sensible correction for wind).
Fat Dog
If you get a VNAV overspeed disconnect, going to Lvl Chg and increasing speed is not always a good option.
Speedbrakes are fitted in this aircraft to be used if required. All speedbrakes are uncomfortable for God's sake.
Getting high in the latter stages of an approach in this aircraft is most definately the wrong place to be.
Practice the fast and high approach procedures any day at BCN when Rwy 07 in use!!!!!!!!!!
Join Date: Dec 2000
Location: on the golf course (Covid permitting)
Posts: 2,131
Likes: 0
Received 0 Likes
on
0 Posts
Sorry guys - bit of thread creep here ....
Noticed the first time yesterday on a 738's tail a second exhaust? outlet above the apu exhaust. Whats that all about?
Cheers
TB
Noticed the first time yesterday on a 738's tail a second exhaust? outlet above the apu exhaust. Whats that all about?
Cheers
TB
Join Date: Feb 2001
Location: Pub
Posts: 122
Likes: 0
Received 0 Likes
on
0 Posts
It's the cooling air inlet. Air enters, circulates, passes through the oil cooler and vents through the normal outlet.
Also, as CS noted, a good way to distinguish between NG's (female) and others (male).
Merry Xmas
Also, as CS noted, a good way to distinguish between NG's (female) and others (male).
Merry Xmas
Join Date: Nov 2002
Location: Brisvegas
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
Fatdog, thanks for the intellectual (?) slant on things!
I foolishly assumed the descent winds would have been applied on ALL sectors. So i guess I should aim at the lowest common denominator to make it easier. So point taken, putting some winds in the FMC will help your profile. Nothing else in the FMC will alter the a/c performance that I am aware of!!
Perhaps I should not have said the FMC ALWAYS calculates a 3 degree path, as it can vary from 2.5 to 3.5 depending on the speed, temp, a/c weight etc. However GENERALLY a 3 degree profile is pretty close for a .78/300 kt descent which is what we use down here most of the time. To back that up the 3 times distance -/+ wind corrections is usually pretty close.
Cranking up the speed on descent is fine if you don't have speed restrictions. The airbrake is by far the best option for corrections to flight path in all other cases. If it wasn't the thing would be permanently stowed on the wing excepting RTO and after landing lift dumping! "Horrible device, fairly ineffective and uncomfortable". Now that my learned friend is wrong in my opinion! Teaching that kind of neanderthal thinking (or lack of) to new pilots on type is incomprehensible.
That is my 2 bobs worth. May not be worth that much but I would like to think I am open minded when it comes to learning new techniques etc. Refering to someones opinion as garbage underlines a reprehensible arrogance.
50 Cal , I hope you are enjoying yourself and experimenting with various techniques.
I foolishly assumed the descent winds would have been applied on ALL sectors. So i guess I should aim at the lowest common denominator to make it easier. So point taken, putting some winds in the FMC will help your profile. Nothing else in the FMC will alter the a/c performance that I am aware of!!
Perhaps I should not have said the FMC ALWAYS calculates a 3 degree path, as it can vary from 2.5 to 3.5 depending on the speed, temp, a/c weight etc. However GENERALLY a 3 degree profile is pretty close for a .78/300 kt descent which is what we use down here most of the time. To back that up the 3 times distance -/+ wind corrections is usually pretty close.
Cranking up the speed on descent is fine if you don't have speed restrictions. The airbrake is by far the best option for corrections to flight path in all other cases. If it wasn't the thing would be permanently stowed on the wing excepting RTO and after landing lift dumping! "Horrible device, fairly ineffective and uncomfortable". Now that my learned friend is wrong in my opinion! Teaching that kind of neanderthal thinking (or lack of) to new pilots on type is incomprehensible.
That is my 2 bobs worth. May not be worth that much but I would like to think I am open minded when it comes to learning new techniques etc. Refering to someones opinion as garbage underlines a reprehensible arrogance.
50 Cal , I hope you are enjoying yourself and experimenting with various techniques.
50 CAL -
I use descend now at about 20 nm to run to the descent point.
I also put a QNH of about 8 hPa higher than the ATIS. This has the speed sitting at about 7-8 knots below bug (in path). This gives you the option of going to speed or lvl chg to increase the ROD (it will accelerate to pickup that 7-8 knots thus increasing ROD).
I have found that this works really well and I only very rarely have an overspeed disconnect.
many guys use speed rather than path and forget to monitor the vertical restrictions on STARS, and get embarrased at the last minute....
PS I like the idea about using the metres button for a quick calculation of track miles to get down..good idea Flaps one!
Spermbank - I am not a fan of amending the vertical restrictions by 500' as you suggest. It works if you are paying attention, but I have seen a couple of people put in 200 ft below the restriction at BEROW when they are going straight in 16 in SYD - DOH!
I am also not a fan of deleting altitude restrictions (without executing) just to see where the aerroplane will be vertically - I prefer to use the fix page and the ABM LSK, which gives you a height crossing without risking being distracted and executing the delete...
And while I am at it - we get a lot of RTA's with a 250 on descent beyond the RTA waypoint. I stick in the RTA and then have a look at the crossing height at that waypoint. I then 250 below that altitude in the descent page. That will allow the RTA to be accomplished whilst still accounting for the 250 speed beyond the RTA waypoint...
I use descend now at about 20 nm to run to the descent point.
I also put a QNH of about 8 hPa higher than the ATIS. This has the speed sitting at about 7-8 knots below bug (in path). This gives you the option of going to speed or lvl chg to increase the ROD (it will accelerate to pickup that 7-8 knots thus increasing ROD).
I have found that this works really well and I only very rarely have an overspeed disconnect.
many guys use speed rather than path and forget to monitor the vertical restrictions on STARS, and get embarrased at the last minute....
PS I like the idea about using the metres button for a quick calculation of track miles to get down..good idea Flaps one!
Spermbank - I am not a fan of amending the vertical restrictions by 500' as you suggest. It works if you are paying attention, but I have seen a couple of people put in 200 ft below the restriction at BEROW when they are going straight in 16 in SYD - DOH!
I am also not a fan of deleting altitude restrictions (without executing) just to see where the aerroplane will be vertically - I prefer to use the fix page and the ABM LSK, which gives you a height crossing without risking being distracted and executing the delete...
And while I am at it - we get a lot of RTA's with a 250 on descent beyond the RTA waypoint. I stick in the RTA and then have a look at the crossing height at that waypoint. I then 250 below that altitude in the descent page. That will allow the RTA to be accomplished whilst still accounting for the 250 speed beyond the RTA waypoint...
Join Date: Apr 2002
Location: Lands End
Posts: 55
Likes: 0
Received 0 Likes
on
0 Posts
I program the VNAV descent page for .79/300kts. I display the VNAV descent page and keep an eye on 4R, which displays V/S required to make what's displayed in 3R.
Simple technique. Comfortable, elegant and in control. When the V/S required reads 2000'/min....I ask ATC for descent clearance. This is generally 20-40 miles from top of descent point T/D, depending on winds. No wind results in a V/B vertical bearing of approximately 2.7 degrees. Result is a part-throttle descent. No brainer.
It can be tough to slow down and get down in an NG. I find the speedbrakes to be ineffective in all phases of flight and generally ARM them before descent and rarely use them until landing. The best way to slow down, get down is to level off. Much more effective than speedbrakes. Just go V/S = 0, then when lyour within 10 kts of your speed, hit level change. Elegant and effective.
I'll only use VNAV for climbs, approaches or complex SIDS. Keeping an eye on 4R gives you performance that's as good as VNAV without the extra degree of separtion caused by automation. You actually feel like your in charge and not the computer.
Simple technique. Comfortable, elegant and in control. When the V/S required reads 2000'/min....I ask ATC for descent clearance. This is generally 20-40 miles from top of descent point T/D, depending on winds. No wind results in a V/B vertical bearing of approximately 2.7 degrees. Result is a part-throttle descent. No brainer.
It can be tough to slow down and get down in an NG. I find the speedbrakes to be ineffective in all phases of flight and generally ARM them before descent and rarely use them until landing. The best way to slow down, get down is to level off. Much more effective than speedbrakes. Just go V/S = 0, then when lyour within 10 kts of your speed, hit level change. Elegant and effective.
I'll only use VNAV for climbs, approaches or complex SIDS. Keeping an eye on 4R gives you performance that's as good as VNAV without the extra degree of separtion caused by automation. You actually feel like your in charge and not the computer.
Thread Starter
Join Date: Feb 2002
Location: east coast
Posts: 33
Likes: 0
Received 0 Likes
on
0 Posts
Thanks groovers. Tried a few of those techniques during my last four day trip and I might just be getting the hang of it.
Thanks for your help.....hope I can repay the favour on day
Thanks for your help.....hope I can repay the favour on day
Join Date: Sep 2002
Location: Middle East, Geneva, Middle East, Geneva...
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
Fat Dog - it would appear that you take all of this very seriously, quotes/quoshes and all that good stuff. I think that's commendable.
But maybe (just an opinion, not a fact) maybe you need to get out a little more....
Nuff said. Thank you all contributers. So much good stuff for all descenders. After all; aviation - as someone told me just the other day - has a 100 percent record. We've never left anyone up there...
But maybe (just an opinion, not a fact) maybe you need to get out a little more....
Nuff said. Thank you all contributers. So much good stuff for all descenders. After all; aviation - as someone told me just the other day - has a 100 percent record. We've never left anyone up there...
Join Date: May 2000
Location: SV Marie Celeste
Posts: 655
Likes: 0
Received 0 Likes
on
0 Posts
This thread just goes to show how many different ways there are to skin this particular cat. Your challenge will be to learn them all and be able to deliver them to the right captain at the right time.
A couple of points from my (limited) experience.
To descent always on V/S as suggested seems to me quite inefficient and a bit lazy.
Sorting out a high speed disconect with lvl-chg and speed to the barbers pole will ensure the guy to your left gets the jitters.
The x3 table is really usefu as a check but does not take into account: high winds, unusual descent speeds (after being deiced for example), light weights or altitude constraints. I think it gets more useful as you get closer in.
The most important thing is getting accurate winds in the descent forecast page.
A couple of points from my (limited) experience.
To descent always on V/S as suggested seems to me quite inefficient and a bit lazy.
Sorting out a high speed disconect with lvl-chg and speed to the barbers pole will ensure the guy to your left gets the jitters.
The x3 table is really usefu as a check but does not take into account: high winds, unusual descent speeds (after being deiced for example), light weights or altitude constraints. I think it gets more useful as you get closer in.
The most important thing is getting accurate winds in the descent forecast page.
Join Date: Apr 1999
Location: UK
Posts: 101
Likes: 0
Received 0 Likes
on
0 Posts
I beg to differ Oceanic. The NG is definitely the male version of the 737. From a distance it looks like a perfectly normal aircraft but when you get up close you realise you have absolutely no idea what is going on in its tiny brain.
Join Date: Apr 2002
Location: UK
Posts: 386
Likes: 0
Received 0 Likes
on
0 Posts
What's so difficult about VNAV PTH descend anyway, just fill in the bits and bobs on the forecast page and you won't be far off!
The Descend option is nice but really is no solution, it only goes into a 1000FPM RoD to capture the Idle Thrust Descent path after crossing your normal T/D point..
The only reason I can think of is by lowering stress on the engines after a long cruise by going down a 1000 FPM before Idling it out.....
3 x Track Miles to go plus some miles to get the speed off works very well, and speedbrakes can be very effective but you have to have high speed...........
The Descend option is nice but really is no solution, it only goes into a 1000FPM RoD to capture the Idle Thrust Descent path after crossing your normal T/D point..
The only reason I can think of is by lowering stress on the engines after a long cruise by going down a 1000 FPM before Idling it out.....
3 x Track Miles to go plus some miles to get the speed off works very well, and speedbrakes can be very effective but you have to have high speed...........
Join Date: Sep 2003
Location: here and there but mostly lgw
Posts: 166
Likes: 0
Received 0 Likes
on
0 Posts
fat dog , all good apart from the dive dont use speed brake advice.context tailwind, cu, atc level offs.va et al
surfing a wave gets you there faster doesnt peneterate the wave.
tailwind and increase of speed equals the above.
The vnav physics/logic to slow up when high is sound.
the wing is sh1t (or too good depending on your life philosophy),cant achieve it
speed brake makes up the difference, diving just gets u to the scene of excess altitude sooner.
head wind context then i'll dive , dive ,dive with the saltiest 3x hghtplus ten dinosaurs./
just follow the pink one and level at the green one and you'll be cool
surfing a wave gets you there faster doesnt peneterate the wave.
tailwind and increase of speed equals the above.
The vnav physics/logic to slow up when high is sound.
the wing is sh1t (or too good depending on your life philosophy),cant achieve it
speed brake makes up the difference, diving just gets u to the scene of excess altitude sooner.
head wind context then i'll dive , dive ,dive with the saltiest 3x hghtplus ten dinosaurs./
just follow the pink one and level at the green one and you'll be cool
Join Date: Mar 2002
Location: bumf*ck, idaho
Posts: 447
Likes: 0
Received 0 Likes
on
0 Posts
I love guys with elaborate theories on making the computer look right for what they are trying to achieve.
Don't get bogged down with the FMC, its there to help not to rule.
Sure, fill in the relevant gaps like the descent forecasts page, give it the best info you can, but remember to look out the window and look at the raw data. In the NG's you still can tune DME's etc for extra info that you couldn't in the classic.
Don't forget the basics that you may have used in the past, like 3Xheight etc.
Its just a plane and needs to get from crz to finals like any other.
After a while you tend to look past the FMC because of its limitations and use the old mark one eyeball.
Finally, if it looks wrong out the window, it is. Cause your officially out of your comfort zone.
Don't get bogged down with the FMC, its there to help not to rule.
Sure, fill in the relevant gaps like the descent forecasts page, give it the best info you can, but remember to look out the window and look at the raw data. In the NG's you still can tune DME's etc for extra info that you couldn't in the classic.
Don't forget the basics that you may have used in the past, like 3Xheight etc.
Its just a plane and needs to get from crz to finals like any other.
After a while you tend to look past the FMC because of its limitations and use the old mark one eyeball.
Finally, if it looks wrong out the window, it is. Cause your officially out of your comfort zone.
Last edited by Sonny Hammond; 30th May 2004 at 08:38.
Join Date: Apr 2000
Location: uk
Posts: 77
Likes: 0
Received 0 Likes
on
0 Posts
I tend to use VNAV most times. Enter data in the forecast winds page and the QNH. Start the descend around 5-10 miles before your TOD point by selecting DESCEND NOW.
Have your destination in the FIX page to get a direct distance to go and back this up using the good old 3xFL method(remember for this to work your ROD must be 1/2 your GS ie GS 440kts then ROD must be 2200fpm.
Also its good to have a GATE where you must aim to be at a certain speed and ALT.Give yourself 10-15 miles for desceleration.
Another trick which also works a treat when excecuting a NON PRECISION approach is the following. DESCEND PAGE line select 3R. Here enter ie RW27/00250(must be 4or 5 digits otherwise gives a FL) where the rwy27 elevation is 200ft + 50ft gives 250. Underneath it will give you the ROD required to be over the threshold at 50ft.
Have your destination in the FIX page to get a direct distance to go and back this up using the good old 3xFL method(remember for this to work your ROD must be 1/2 your GS ie GS 440kts then ROD must be 2200fpm.
Also its good to have a GATE where you must aim to be at a certain speed and ALT.Give yourself 10-15 miles for desceleration.
Another trick which also works a treat when excecuting a NON PRECISION approach is the following. DESCEND PAGE line select 3R. Here enter ie RW27/00250(must be 4or 5 digits otherwise gives a FL) where the rwy27 elevation is 200ft + 50ft gives 250. Underneath it will give you the ROD required to be over the threshold at 50ft.
