B777 VNAV question
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B777 VNAV question
A slightly esoteric question for those of you well versed in the VNAV for the B777:
During descent, when on the geometric descent path between two altitude constraints, that put us below the ideal econ descent speed/idle thrust profile, once clear of the first constraint, how can we "redraw" the idle path to the next constraint, so as to obtain the most efficient path?
Can we merely go 'direct to' the next constraining point, or is there a easier trick?
After nearly two decades on the Airbus I'm finding the VNAV on this old beast a little clumsy! But don't get me wrong, apart from the seats and the VNAV.... I love it!
Thanks to the Boeing aficionados in advance.
During descent, when on the geometric descent path between two altitude constraints, that put us below the ideal econ descent speed/idle thrust profile, once clear of the first constraint, how can we "redraw" the idle path to the next constraint, so as to obtain the most efficient path?
Can we merely go 'direct to' the next constraining point, or is there a easier trick?
After nearly two decades on the Airbus I'm finding the VNAV on this old beast a little clumsy! But don't get me wrong, apart from the seats and the VNAV.... I love it!
Thanks to the Boeing aficionados in advance.
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B777 VNAV question
You can go to descend now I believe. However I think that jus goes into thrust/FLCH SPD. Or, a lot of guys are going into the cruise page ad entering the new cruise level mirror your current alt. No, not that intuitive. Kind of lacking a lot.
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Hi Honk,
Matchspeed is pretty wide of the mark regarding the Des Now function.. 'Des Now' is an option to start an early descent. In all instances when you're in Descent Mode but under the glide path, VNAV will command a 1250ft/min descent in the hope of capturing the idle path. It'll do this in IDLE/HOLD or THR, along with VNAV SPD. When within 150ft of the idle path, it'll transition to IDLE/VNAV PTH.
To answer your question - you obviously know that VNAV goes into point-to-point descent path construction, whether that is from the first to subsequent constraints, or from your present position to the next constraint (ie. If you recycle the next constraint waypoint in the legs page).
So, how to create an idle descent path in VNAV after the. First constraint, rather than the point-to-point profiles it will draw, there are 2 options.
The first, and the one I prefer to use; is...
As you approach the first waypoint constraint on descent, let's say it's MUSEL FL130; once you're assured you'll make the constraint, but before it Has captured the ALT mode (IDLE/HOLD will change to SPD) - delete the restriction in the legs page. Or, if the MCP alt is set to a lower level than FL130, just push the button.
VNAV will then re-calculate an idle descent based on the next constraint, you've removed the first one before you've 'obeyed' it.
The second, is simply to re-cruise the FMC at FL130 once you're there, as Machspeed said. That is, set 130 in the VNAV CRZ page, as the cruise altitude. Again, it'll plan an idle descent based on the next constraint.
Hope this is of some use.
With all that being said, in the environs we operate in these days, idle descents are basically just a wish, so I generally don't bother doing either; just stick to the 3 times tables, and when made to descend below the path, do it in V/S500 or VNAV SPD, and nudge the thrust levers up a bit when in HOLD, to achieve a shallow descent.
Cheers.
Matchspeed is pretty wide of the mark regarding the Des Now function.. 'Des Now' is an option to start an early descent. In all instances when you're in Descent Mode but under the glide path, VNAV will command a 1250ft/min descent in the hope of capturing the idle path. It'll do this in IDLE/HOLD or THR, along with VNAV SPD. When within 150ft of the idle path, it'll transition to IDLE/VNAV PTH.
To answer your question - you obviously know that VNAV goes into point-to-point descent path construction, whether that is from the first to subsequent constraints, or from your present position to the next constraint (ie. If you recycle the next constraint waypoint in the legs page).
So, how to create an idle descent path in VNAV after the. First constraint, rather than the point-to-point profiles it will draw, there are 2 options.
The first, and the one I prefer to use; is...
As you approach the first waypoint constraint on descent, let's say it's MUSEL FL130; once you're assured you'll make the constraint, but before it Has captured the ALT mode (IDLE/HOLD will change to SPD) - delete the restriction in the legs page. Or, if the MCP alt is set to a lower level than FL130, just push the button.
VNAV will then re-calculate an idle descent based on the next constraint, you've removed the first one before you've 'obeyed' it.
The second, is simply to re-cruise the FMC at FL130 once you're there, as Machspeed said. That is, set 130 in the VNAV CRZ page, as the cruise altitude. Again, it'll plan an idle descent based on the next constraint.
Hope this is of some use.
With all that being said, in the environs we operate in these days, idle descents are basically just a wish, so I generally don't bother doing either; just stick to the 3 times tables, and when made to descend below the path, do it in V/S500 or VNAV SPD, and nudge the thrust levers up a bit when in HOLD, to achieve a shallow descent.
Cheers.
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Hi Honk...
@honkozzie,
Sorry that I cannot help you, but I did enjoy your thoughts about enjoying the B77, '...except for the seats and and the VNAV...' My guess is that you will adapt to the 77's VNAV quirks quickly, but I'm most curious about the seats. What part of the seat component is uncomfortable for you? You can bet that some Boeing designers and engineers are reading this space. Obviously, this is about Pilot Comfort, not some fluffy "F" Class seat. If someone does not like the B-Driver's Chair, the Boeing folks need to know about it. Blind yourself and your airline as necessary, but please... tell them about your issues. IME, they are very responsive folks.
Sorry that I cannot help you, but I did enjoy your thoughts about enjoying the B77, '...except for the seats and and the VNAV...' My guess is that you will adapt to the 77's VNAV quirks quickly, but I'm most curious about the seats. What part of the seat component is uncomfortable for you? You can bet that some Boeing designers and engineers are reading this space. Obviously, this is about Pilot Comfort, not some fluffy "F" Class seat. If someone does not like the B-Driver's Chair, the Boeing folks need to know about it. Blind yourself and your airline as necessary, but please... tell them about your issues. IME, they are very responsive folks.
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Seats on the "Tupolev7/77" are way too hard; vnav is antique and the lights in the Flightdeck just rubbish!! oh..and the yolk needs a table!! Other than that, its not a bad platform!!
VNAV on the 777 is somewhat flawed, IMHO. That doesn’t mean you can’t get it to do what you want but you have to be aware of quite a few issues. Our FCOM has many pages devoted to all kinds of VNAV anomalies.
One of the peeves I have with VNAV is that it is sometimes not very good at telling you in advance what it’s going to do - whether it will fly level, follow a particular path or start an idle descent is not susceptible to prediction in some cases. There are un-annunciated modes which have a big effect on system behaviour. This means it is sometimes a bit difficult to say whether it is behaving oddly because of finger trouble or because it has chosen a different way of doing things *today*.
The seats are crap but it doesn’t help that no-one except me seems to snag worn out cushions in the AML. A decent car seat would be much more comfortable on a long flight but I suppose it forces me to get up and walk around fairly often, which is a good thing. My glider, which has a thin layer of crash foam over bare carbon, is peachy for an 11hr+ flight and there is very little room for manoeuvre in there!
One of the peeves I have with VNAV is that it is sometimes not very good at telling you in advance what it’s going to do - whether it will fly level, follow a particular path or start an idle descent is not susceptible to prediction in some cases. There are un-annunciated modes which have a big effect on system behaviour. This means it is sometimes a bit difficult to say whether it is behaving oddly because of finger trouble or because it has chosen a different way of doing things *today*.
The seats are crap but it doesn’t help that no-one except me seems to snag worn out cushions in the AML. A decent car seat would be much more comfortable on a long flight but I suppose it forces me to get up and walk around fairly often, which is a good thing. My glider, which has a thin layer of crash foam over bare carbon, is peachy for an 11hr+ flight and there is very little room for manoeuvre in there!
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777 VNAV
Hi HONKOZZIE:
Welcome to the Boeing side.
The VNAV system in the 200 leaves much to be desired, I've seen the
DRAG REQ message many times, and of course the "what's it doing now,"
message.
Where the system is so crazy, may I suggest that on a decent especially
when a slow down is required, to cross an intersection! EX Cross XYZ at
10,000 250KTS. That you build a crossing point about 10 miles prior to the
fix, at the required altitude. EX XYZ/-10, insert it into the legs page, then
place the alt restriction at that point. The aircraft will level, and then start
slowing. Where the FMC programing is to meet altitude restrictions, then
airspeed restrictions, doing this will ease the worry about the crossing.
I retired off the 777, with 29 years with my company! Miss the flying, and
the laughs but don't miss the associated crap of the job. Have fun on the
Boeing side!
Welcome to the Boeing side.
The VNAV system in the 200 leaves much to be desired, I've seen the
DRAG REQ message many times, and of course the "what's it doing now,"
message.
Where the system is so crazy, may I suggest that on a decent especially
when a slow down is required, to cross an intersection! EX Cross XYZ at
10,000 250KTS. That you build a crossing point about 10 miles prior to the
fix, at the required altitude. EX XYZ/-10, insert it into the legs page, then
place the alt restriction at that point. The aircraft will level, and then start
slowing. Where the FMC programing is to meet altitude restrictions, then
airspeed restrictions, doing this will ease the worry about the crossing.
I retired off the 777, with 29 years with my company! Miss the flying, and
the laughs but don't miss the associated crap of the job. Have fun on the
Boeing side!
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In all instances when you're in Descent Mode but under the glide path, VNAV will command a 1250ft/min descent in the hope of capturing the idle path. It'll do this in IDLE/HOLD or THR, along with VNAV SPD. When within 150ft of the idle path, it'll transition to IDLE/VNAV PTH.
If you were on path (IDLE||VNAV PTH), descending at 300 knots (FMC-commanded) and then you speed intervened and set 340 knots in the window, the airplane would, of course, pitch for speed and go below the path. Presuming the above quote is true, at what point would thrust be added to attempt a 1250'/min descent? Anyone have any reference that the logic is set up to do this? Thanks.
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To answer your question you would have to close the window once beyond 150' from the path, it would then go THR - VNAV SPD, the 1250FPM wouldn't apply immediately as it would be speed on elevator and be reducing towards the 300kt FMC speed, however once it got back to 300kts with the window closed assuming you never got within 150' of the path that initial THR application would now have the aircraft descending at approximately 1250FPM at 300kts in an effort to regain the path.
It appears the 1250 had been removed from the FCTM as it now just says "VNAV commands a reduced decent"
It appears the 1250 had been removed from the FCTM as it now just says "VNAV commands a reduced decent"
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Thanks for the answer. So, the 1250'/min will not be maintained with thrust unless the speed window is closed?
If, after I reached 330 knots, I inserted 330 for a descent speed into VNAV 3/3 and closed the window, would it attempt to shallow the descent with thrust to 1250'/min to intercept the path?
ADDENDUM: For what it's worth, I just tried this scenario above on my computer with some old Aerosim 747-400 training software as well as the PMDG 747-400 software and found that thrust will not increase until the speed window is closed. Close the speed window with 330 knots in the CDU and the thrust comes in right away to reduce the descent rate to 1250 FPM. So, if those are modeling the airplane correctly (presuming the -400 operates like the 777 in this regard), then I have a partial answer.
It would be nice to see something on this in writing though. I have a 777 FCTM, Rev. 4 (Feb 20, 2014) and I don't see anything that would explain the above action.
If, after I reached 330 knots, I inserted 330 for a descent speed into VNAV 3/3 and closed the window, would it attempt to shallow the descent with thrust to 1250'/min to intercept the path?
ADDENDUM: For what it's worth, I just tried this scenario above on my computer with some old Aerosim 747-400 training software as well as the PMDG 747-400 software and found that thrust will not increase until the speed window is closed. Close the speed window with 330 knots in the CDU and the thrust comes in right away to reduce the descent rate to 1250 FPM. So, if those are modeling the airplane correctly (presuming the -400 operates like the 777 in this regard), then I have a partial answer.
It would be nice to see something on this in writing though. I have a 777 FCTM, Rev. 4 (Feb 20, 2014) and I don't see anything that would explain the above action.
Last edited by cf6-80c2b5f; 10th Mar 2015 at 07:07. Reason: Additional Information
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Simply put closing the window allows the FMC to get back to VNAV PTH, above it, it will go IDLE/HOLD - VNAV SPD below it will go to THR/HOLD - VNAV SPD the 1250' may only be in the ground school computer based training.
Once in HOLD I often add thrust to reduce the descent rate even further and allow the FMC to catch the path as early as possible.
The 1250 is only a initial attempt I believe based on the FMC performance figures for current weight etc. once in HOLD it may end up more or less however it'll always be better than an idle descent.
Once in HOLD I often add thrust to reduce the descent rate even further and allow the FMC to catch the path as early as possible.
The 1250 is only a initial attempt I believe based on the FMC performance figures for current weight etc. once in HOLD it may end up more or less however it'll always be better than an idle descent.
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Thanks again for clearing this up.
And to confirm this in writing, these are excerpts from the Big Boeing Book:
And to confirm this in writing, these are excerpts from the Big Boeing Book:
Selection During Descent:
If Speed Intervention is selected during descent,
while on, above, or below path, in speed reversion,
or on a VNAV PTH segment, pitch control immediately
becomes VNAV SPD and the A/T mode becomes
HOLD.
If Speed Intervention is selected during descent,
while on, above, or below path, in speed reversion,
or on a VNAV PTH segment, pitch control immediately
becomes VNAV SPD and the A/T mode becomes
HOLD.
Transition from Speed Intervention:
When the MCP speed knob is pushed again to
deselect speed intervention, the target speed becomes
the VNAV DES page target speed and path
capture is enabled.
When the MCP speed knob is pushed again to
deselect speed intervention, the target speed becomes
the VNAV DES page target speed and path
capture is enabled.
The THR mode is selected if the
airplane is below the path to set the thrust for
closure onto the path. HOLD is selected when the
thrust is set to allow the flight crew to adjust the
closure rate onto the path.
airplane is below the path to set the thrust for
closure onto the path. HOLD is selected when the
thrust is set to allow the flight crew to adjust the
closure rate onto the path.
Resumption of the descent when the airplane is below the
descent path results in descent guidance of VNAV
SPD and an initial thrust setting (THR) to achieve a
flight path angle that is half the angle for the
descent path. A/T HOLD occurs after the initial
thrust target is reached.
descent path results in descent guidance of VNAV
SPD and an initial thrust setting (THR) to achieve a
flight path angle that is half the angle for the
descent path. A/T HOLD occurs after the initial
thrust target is reached.
Last edited by cf6-80c2b5f; 11th Mar 2015 at 03:01.
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Yes. It is interesting that the Big Boeing Book, in one place, says the descent path is cut to half the idle descent path, but also indicates that the same logic is used for a cruise descent.
Cruise descent is used for enroute cruise altitude change, NOT in the descent phase of flight.
- A cruise descent can be started on the MCP by selecting a lower altitude and pushing the altitude
selector. If you do not have altitude intervention, enter the lower cruise altitude in 1L of the CDU pg.
- In a CRZ DES, the cruise descent guidance law is applied.
It initially uses a predicted thrust which will result in approximately 1,200 ft per minute descent and
at the cruise descent airspeed. When VNAV estimates that the thrust target has been achieved for this
descent rate, throttle clutches disengage (dormant) and FMA annunciates THR HOLD just as in a DES NOW function.
- Once the autothrottle is set to the HOLD mode, the pilot can position the throttles to control desired
vertical speed; a very nice design.
- If a CRZ DES intercepts the planned descent profile, the descent path is captured and DES becomes
active. This is NOT what the CRZ DES feature was designed for.
The cruise descent guidance law - IDLE to 1200 fpm then
THR HLD - is used in the following situations:
1 When descending from one cruise altitude to another.
2 When making an early descent before the end of cruise.
3 When engaging VNAV when below the descent path.
4 When the airplane is flying a descent path and, after a
modification, ends up below the reconstructed descent
path
- A cruise descent can be started on the MCP by selecting a lower altitude and pushing the altitude
selector. If you do not have altitude intervention, enter the lower cruise altitude in 1L of the CDU pg.
- In a CRZ DES, the cruise descent guidance law is applied.
It initially uses a predicted thrust which will result in approximately 1,200 ft per minute descent and
at the cruise descent airspeed. When VNAV estimates that the thrust target has been achieved for this
descent rate, throttle clutches disengage (dormant) and FMA annunciates THR HOLD just as in a DES NOW function.
- Once the autothrottle is set to the HOLD mode, the pilot can position the throttles to control desired
vertical speed; a very nice design.
- If a CRZ DES intercepts the planned descent profile, the descent path is captured and DES becomes
active. This is NOT what the CRZ DES feature was designed for.
The cruise descent guidance law - IDLE to 1200 fpm then
THR HLD - is used in the following situations:
1 When descending from one cruise altitude to another.
2 When making an early descent before the end of cruise.
3 When engaging VNAV when below the descent path.
4 When the airplane is flying a descent path and, after a
modification, ends up below the reconstructed descent
path