744 speed logic
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744 speed logic
The 744 FCTM states that speed should be manually controlled when on approach and this requires use of speed intervene.
The question I have is if you are positioning on base leg in VNAV PTH and select Flap 1 and do not open the MCP speed window then what speed does the speed bug now go to? (assuming no limiting waypoint or ALT speed constraint):
A. F1 reference mark on the speed tape
B. Top of the amber band on the speed tape
C. 5 kts (or other margin) above the top of the amber band
D: Other. If so could you please explain.
Many thanks
The question I have is if you are positioning on base leg in VNAV PTH and select Flap 1 and do not open the MCP speed window then what speed does the speed bug now go to? (assuming no limiting waypoint or ALT speed constraint):
A. F1 reference mark on the speed tape
B. Top of the amber band on the speed tape
C. 5 kts (or other margin) above the top of the amber band
D: Other. If so could you please explain.
Many thanks
Last edited by cowpatz; 23rd Sep 2015 at 03:42.
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Why not? Might be positioning on an RNAV arrival for an approach.
It is the behaviour of the speed bug in VNAV PTH, when selecting flap, that I'm interested in.
It is the behaviour of the speed bug in VNAV PTH, when selecting flap, that I'm interested in.
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My guess is the commanded speed would remain unchanged, flaps do not change speeds on the 744 ( they do on the 748).
On an RNAV approach you would have the MCP speed window open, since after passing the approach point ( which is the point on position R6 on the dep/arr page when you select the approach) system does allow speed intervene with the aircraft remaining in VNAV path.
I might be wrong, I didn't look it up, just from memory.
On an RNAV approach you would have the MCP speed window open, since after passing the approach point ( which is the point on position R6 on the dep/arr page when you select the approach) system does allow speed intervene with the aircraft remaining in VNAV path.
I might be wrong, I didn't look it up, just from memory.
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What speed do you have in the FMC to start with? In most cases it is the default 240kts below FL100.
Speed will (most likely) stay the same. Airspeed should be commanded by what is programmed/set or is the default in the FMC.
IF your speed was above/close to the max F1 speed, then the FMC will limit the speed to 5 kts below max F1 (280-5=275).
Also, assuming you were above the amber min maneuvering speed to start with? Otherwise A/T would have been actively protecting you and keeping you at/above that. So adding flaps to 1 should cause that speed to decrease. IF so, then speed would drop closer to whatever FMC speed is, but will still keep you above amber min maneuvering speed, and 5 kts below max flap placard speed.
Speed will (most likely) stay the same. Airspeed should be commanded by what is programmed/set or is the default in the FMC.
IF your speed was above/close to the max F1 speed, then the FMC will limit the speed to 5 kts below max F1 (280-5=275).
Also, assuming you were above the amber min maneuvering speed to start with? Otherwise A/T would have been actively protecting you and keeping you at/above that. So adding flaps to 1 should cause that speed to decrease. IF so, then speed would drop closer to whatever FMC speed is, but will still keep you above amber min maneuvering speed, and 5 kts below max flap placard speed.
Last edited by bigduke6; 23rd Sep 2015 at 04:56.
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Assume that it is below 10,000ft so the 240/10000 speed restriction would be active on the descent page. I know that the autothrottle will protect the speed going below min manoeuvring but I'm not sure where the command speed bug would be or if in fact it is even influenced by the min manoeuvre speed for that flap position. It is possible that the speed bug just looks at what the FMC is commanding as in 240 below 10,000 and then the next altitude or waypoint speed constraint ie 180/3000 at XXX. In other words in VNAV PTH the bug would be 240 kts until a deceleration to 180 was required and then it would go to 180. This would explain why the speed should be manually controlled. I spent 14 yrs on the old girl and still can't be 100% certain as to what would happen!
Last edited by cowpatz; 23rd Sep 2015 at 05:14. Reason: spelling error
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I think you are right, speed bug would move to whatever speed is commanded by the next FMC waypoint ( e.g. 180). It would however not command speed below placard, which means it would stop there and only move to 180 after flaps are set accordingly.
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You've basically got it now. The answer to your original question was D. Other - programmed VNAV speed.
The programmed speed is displayed on VNAV DESCENT page, as well as LEGS page. Typically it is the 240/10000 restraint, followed by any restraint programmed on the legs page or from the STAR/Approach. A simple approach might be the 240/10000 restriction, 220/3000 at the IAF and 180/2000 FAF. If you don't speed intervene, VNAV will fly you at 240K until approaching the IAF, then slow to 220K and slow again approaching the FAF to achieve the 180K constraint.
The programmed speed is displayed on VNAV DESCENT page, as well as LEGS page. Typically it is the 240/10000 restraint, followed by any restraint programmed on the legs page or from the STAR/Approach. A simple approach might be the 240/10000 restriction, 220/3000 at the IAF and 180/2000 FAF. If you don't speed intervene, VNAV will fly you at 240K until approaching the IAF, then slow to 220K and slow again approaching the FAF to achieve the 180K constraint.
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With the speed window closed, I think your FMC target speed in descent mode (speed window closed) is normally going to be whatever is on VNAV 3/3, or your max flap limit speed, whichever is lower.
If you select a flap setting whose max speed limit is less than 240 knots (more than flaps 10), then the FMC target speed should try to prevent you from exceeding that flap limit speed. In your scenario, the speed window is closed and you are selecting flaps. If you were already at 240 knots (that's what you would probably have in VNAV 3/3), when you select flaps 20, the FMC target speed should go to 230 knots or perhaps five knots less than that.
Notwithstanding that, I would prefer to have the speed window open well before I got to flaps 20 on a VNAV approach and not rely on the FMC to prevent a flap overspeed.
[EDIT] It looks like AtoBsafely got it before me. You can also manually enter a speed at the course fix or final fix on the LEGS page and I think the aircraft will slow to that speed when within a certain distance of it. As mentioned above, some programs even have lower speeds coded in on the LEGS page which is handy to prevent the power surge when you select VNAV and the speed window closes just prior to the FF during a VNAV approach.
If you select a flap setting whose max speed limit is less than 240 knots (more than flaps 10), then the FMC target speed should try to prevent you from exceeding that flap limit speed. In your scenario, the speed window is closed and you are selecting flaps. If you were already at 240 knots (that's what you would probably have in VNAV 3/3), when you select flaps 20, the FMC target speed should go to 230 knots or perhaps five knots less than that.
Notwithstanding that, I would prefer to have the speed window open well before I got to flaps 20 on a VNAV approach and not rely on the FMC to prevent a flap overspeed.
[EDIT] It looks like AtoBsafely got it before me. You can also manually enter a speed at the course fix or final fix on the LEGS page and I think the aircraft will slow to that speed when within a certain distance of it. As mentioned above, some programs even have lower speeds coded in on the LEGS page which is handy to prevent the power surge when you select VNAV and the speed window closes just prior to the FF during a VNAV approach.
Last edited by cf6-80c2b5f; 23rd Sep 2015 at 07:53. Reason: Previous post
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As mentioned above, some programs even have lower speeds coded in on the LEGS page which is handy to prevent the power surge when you select VNAV and the speed window closes just prior to the FF during a VNAV approach.
Not sure what you mean by this as on the 744 the speed window remains open when in the "on approach" mode.
Not sure what you mean by this as on the 744 the speed window remains open when in the "on approach" mode.
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I don't think so. I think the speed window can be closed while you are in on-approach logic, although it is not something that is normally done because at that point you would want to be in speed intervention. The speed window, however, cannot be closed while doing an ILS.
If you are outside the course fix in ALT on a VNAV approach, you are usually not in on-approach logic until crossing the CF. So when you select VNAV 2 nm prior to the CF (sometimes we will start VNAV from the CF instead of FF), the window will close and the aircraft will usually accelerate. If there is a lower speed entered or coded at the CF, it will prevent this sudden thrust surge when the window is closed.
Also, if you extend the centerline from the FF, I don't believe on-approach logic would start until the FF. So you would also get the thrust surge between the CF and FF if you extended from the FF.
If you are outside the course fix in ALT on a VNAV approach, you are usually not in on-approach logic until crossing the CF. So when you select VNAV 2 nm prior to the CF (sometimes we will start VNAV from the CF instead of FF), the window will close and the aircraft will usually accelerate. If there is a lower speed entered or coded at the CF, it will prevent this sudden thrust surge when the window is closed.
Also, if you extend the centerline from the FF, I don't believe on-approach logic would start until the FF. So you would also get the thrust surge between the CF and FF if you extended from the FF.
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Try it in the sim.
Manually enter the speeds you want to be at by certain waypoints such as the next lowest flap maneuver speed as calculated by Vref plus 20,40,60, and 80. You might have a default of 240 below 10K but that can be modified to flaps up maneuver speed.On the VNAV descent page can enter a lower speed as appropriate for weight to be commanded below a certain altitude such as 210/3000. Perhaps enter flaps 5 speed at a stepdown fix and flaps 20 speed at the next fix(or flaps 10 speed as well if you have enough fixes. Then enter approach speed at the FAF.
Just prior to reaching the next waypoint with the next lower flap maneuver speed select the next greater flap setting.
Manually enter the speeds you want to be at by certain waypoints such as the next lowest flap maneuver speed as calculated by Vref plus 20,40,60, and 80. You might have a default of 240 below 10K but that can be modified to flaps up maneuver speed.On the VNAV descent page can enter a lower speed as appropriate for weight to be commanded below a certain altitude such as 210/3000. Perhaps enter flaps 5 speed at a stepdown fix and flaps 20 speed at the next fix(or flaps 10 speed as well if you have enough fixes. Then enter approach speed at the FAF.
Just prior to reaching the next waypoint with the next lower flap maneuver speed select the next greater flap setting.
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Window closed the BUG can go anywhere between MERS and -5 (VMO/MMO/VFE) the flap bugs mean nothing.
Ie, you programme 180kts the BUG will go straight there if it's above MERS and below -5 (etc) if not it will rest on the MERS stick or sit at -5 and go to 180 when it's allowed.
A simple way to show it is to pull the speed brake while descending at highish altitude with the BUG near MERS (ATC slow down and go down) as MERS increases with speed brake extension it will carry the BUG with it = safe!
Window open gives you "airspeed low" if fitted as MERS goes above the BUG.
You see the -5 VFE example every departure as the aircraft tries to accelerate to 250kts or +100
Window open can go outside MERS and VMO but the autopilot will try not to obviously.
Ie, you programme 180kts the BUG will go straight there if it's above MERS and below -5 (etc) if not it will rest on the MERS stick or sit at -5 and go to 180 when it's allowed.
A simple way to show it is to pull the speed brake while descending at highish altitude with the BUG near MERS (ATC slow down and go down) as MERS increases with speed brake extension it will carry the BUG with it = safe!
Window open gives you "airspeed low" if fitted as MERS goes above the BUG.
You see the -5 VFE example every departure as the aircraft tries to accelerate to 250kts or +100
Window open can go outside MERS and VMO but the autopilot will try not to obviously.
Last edited by Avinthenews; 23rd Sep 2015 at 17:29.
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Perhaps enter flaps 5 speed at a stepdown fix and flaps 20 speed at the next fix(or flaps 10 speed as well if you have enough fixes. Then enter approach speed at the FAF.
I always seem to learn something from your posts. If on the LEGS page you enter 190 at the CF and 170 at the FF, with the speed window still closed (non-standard) and flaps at 25, at what point will the aircraft start slowing to 170? Does it slow so as to cross the FF at 170 or does it start slowing as soon as it cycles the CF? Or is there a specific distance prior to the FF that it starts slowing?
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Try it in the sim.
Manually enter the speeds you want to be at by certain waypoints such as the next lowest flap maneuver speed as calculated by Vref plus 20,40,60, and 80. You might have a default of 240 below 10K but that can be modified to flaps up maneuver speed.On the VNAV descent page can enter a lower speed as appropriate for weight to be commanded below a certain altitude such as 210/3000. Perhaps enter flaps 5 speed at a stepdown fix and flaps 20 speed at the next fix(or flaps 10 speed as well if you have enough fixes. Then enter approach speed at the FAF.
Just prior to reaching the next waypoint with the next lower flap maneuver speed select the next greater flap setting.
Manually enter the speeds you want to be at by certain waypoints such as the next lowest flap maneuver speed as calculated by Vref plus 20,40,60, and 80. You might have a default of 240 below 10K but that can be modified to flaps up maneuver speed.On the VNAV descent page can enter a lower speed as appropriate for weight to be commanded below a certain altitude such as 210/3000. Perhaps enter flaps 5 speed at a stepdown fix and flaps 20 speed at the next fix(or flaps 10 speed as well if you have enough fixes. Then enter approach speed at the FAF.
Just prior to reaching the next waypoint with the next lower flap maneuver speed select the next greater flap setting.
JamedStab,
I always seem to learn something from your posts. If on the LEGS page you enter 190 at the CF and 170 at the FF, with the speed window still closed (non-standard) and flaps at 25, at what point will the aircraft start slowing to 170? Does it slow so as to cross the FF at 170 or does it start slowing as soon as it cycles the CF? Or is there a specific distance prior to the FF that it starts slowing?
I always seem to learn something from your posts. If on the LEGS page you enter 190 at the CF and 170 at the FF, with the speed window still closed (non-standard) and flaps at 25, at what point will the aircraft start slowing to 170? Does it slow so as to cross the FF at 170 or does it start slowing as soon as it cycles the CF? Or is there a specific distance prior to the FF that it starts slowing?
In the end, speed intervention is just easier and probably safer than having to get it just right on approach. With speed intervention, the pilot is slowing down only when he feels it is OK to do so while the closed window method risks a command speed decrease below the flap maneuvering speed if one delays the next flap selection for whatever reason.
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JammedStab,
I think I misunderstood your post. I thought you had already tried this in the sim. It would be nice to have access to a Level-D sim with the time to actually experiment with this kind of stuff.
I think I misunderstood your post. I thought you had already tried this in the sim. It would be nice to have access to a Level-D sim with the time to actually experiment with this kind of stuff.
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Everyone has got it wrong
The answer is A.
Everybody is saying that the speed bug will go down to the pre-assigned speed entered into the LEGS page on the FM(G)C. However, the question states otherwise... "assuming no limiting waypoint or ALT speed constraint".
Everybody is saying that the speed bug will go down to the pre-assigned speed entered into the LEGS page on the FM(G)C. However, the question states otherwise... "assuming no limiting waypoint or ALT speed constraint".
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It's actually D, the BUG does nothing and stays right where it was (most likely 240kts) continue to configure and it will be pushed down by Vfe -5 eventually.
(On a -8 the answer would be A.)
(On a -8 the answer would be A.)