American Airlines B767 V/S mode behaviour
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"One of the caveats when using v/s in the 75/76 is that you lose 'alpha protection'"
In VNAV and FLCH, the autopilot and autothrottle won't let the speed get too low or too high for the current aircraft configuration. The aircraft pitch and thrust will change to prevent this. With V/S, however, there is no speed protection at the limits. E.g. If you dial in a 6000 feet a minute climb, it's not likely that a 75/76 will be able to maintain this climb rate for long periods (even at low weights and max thrust). However, the autopilot/autothrottle will keep aiming for that climb value... until the aircraft stalls. Similarly, if you dial in a large descent value, the aircraft will exceed airframe/gear/flap/mach limits.
Regards.
NSEU.
In VNAV and FLCH, the autopilot and autothrottle won't let the speed get too low or too high for the current aircraft configuration. The aircraft pitch and thrust will change to prevent this. With V/S, however, there is no speed protection at the limits. E.g. If you dial in a 6000 feet a minute climb, it's not likely that a 75/76 will be able to maintain this climb rate for long periods (even at low weights and max thrust). However, the autopilot/autothrottle will keep aiming for that climb value... until the aircraft stalls. Similarly, if you dial in a large descent value, the aircraft will exceed airframe/gear/flap/mach limits.
Regards.
NSEU.
aka Capt PPRuNe
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Seems the same across Boeing fleet
On the B737 which I fly at the moment, there is full speed protection in VS mode but you can still fly away from the selected ALT, again used for non precision approaches.
As far as I remember, on the B757/767, you had to preselect another ALT before you could use VS if you were already in ALT HLD mode. On the B737, selecting VS whilst in ALT ACQ only arms VS as denoted by a white V/S annunciation. A different ALT has to be selected (more than 1000ft difference) before you can actually engage VS.
I stand to be corrected though.
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On the 737 you cannot fly away.
757/767 technically you can fly away.
In practise you should move the ALT window away from your actual altitude in all these types because on the 76/57 if the actual altitude is even a few feet off the one in the ALT window the a/c will go 'ALT CAP' and screw up your plans!
757/767 technically you can fly away.
In practise you should move the ALT window away from your actual altitude in all these types because on the 76/57 if the actual altitude is even a few feet off the one in the ALT window the a/c will go 'ALT CAP' and screw up your plans!
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Sorry was away on a trip!
You guys are right it DOES work amazingly enough.
My humble apologies for being wrong on this occasion, still don't understand why it's done like that but it is.
But i get confused by Danny\'s post aswell.
In the 737 it is absolutely impossible to fly away from a MCP selected altitude. The non precision approaches are done by for instance if the platform is 3000\' then select 4000\' on the MCP when level at platform to ARM V/S. Then dial the knob and the aircraft will follow the V/S command. However if you leave 3000\' in the MCP IT WILL NOT follow the command.
You guys are right it DOES work amazingly enough.
My humble apologies for being wrong on this occasion, still don't understand why it's done like that but it is.
But i get confused by Danny\'s post aswell.
In the 737 it is absolutely impossible to fly away from a MCP selected altitude. The non precision approaches are done by for instance if the platform is 3000\' then select 4000\' on the MCP when level at platform to ARM V/S. Then dial the knob and the aircraft will follow the V/S command. However if you leave 3000\' in the MCP IT WILL NOT follow the command.
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Not correct, Stan or Shaka. You can fly away from a selected altitude in the 737.
If you are in ALT HOLD mode at the preselected altitude, then select a lower altitude, which arms V/S, you can then climb at +6000 fpm, and the aircraft will attempt to hold that - NOT SAFE!!! Similarly, at 2,000 AGL you can select 3,500 in the Alt window and -1500 fpm in the V/S window and only the ground will prevent you going lower than 0 AGL.
If you are in ALT HOLD mode at the preselected altitude, then select a lower altitude, which arms V/S, you can then climb at +6000 fpm, and the aircraft will attempt to hold that - NOT SAFE!!! Similarly, at 2,000 AGL you can select 3,500 in the Alt window and -1500 fpm in the V/S window and only the ground will prevent you going lower than 0 AGL.
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Captain Stable
Sorry but I am correct.
The point and the difference between the types is that on the 75/76 you don't need to 'select a lower altitude' before V/S will arm.
Sorry but I am correct.
If you are in ALT HOLD mode at the preselected altitude, then select a lower altitude, which arms V/S, you can then climb at +6000 fpm
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Exactly! You're missing the point Capt Stable.
You CANNOT fly away from an MCP selected altitude when it's in the window and VNAV PATH or ALT HOLD is annunciated just by whizzing the V/S wheel!
Yes you CAN fly away but there is one action needed, namely selecting a lower or higher altitude to fly to and by that you ARM V/S on the FMA, then you are able to fly both higher and lower "away" from the selected altitude. But THIS wasn't the question.
You CANNOT fly away from an MCP selected altitude when it's in the window and VNAV PATH or ALT HOLD is annunciated just by whizzing the V/S wheel!
Yes you CAN fly away but there is one action needed, namely selecting a lower or higher altitude to fly to and by that you ARM V/S on the FMA, then you are able to fly both higher and lower "away" from the selected altitude. But THIS wasn't the question.
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OK - I see the difference. I'm a bit out of date on the 75 - it was a while ago! The difference between us was based merely on teminology - on the 737 you can fly away from a selected altitude, but not from a selected and held altitude.
My apologies for the confusion.
Either way, V/S is a dangerous mode to use without lots of careful attention.
My apologies for the confusion.
Either way, V/S is a dangerous mode to use without lots of careful attention.
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easy 2 do, english is my second language 
can somebody tell me then how the difference between the 73 and 75/76 has arisen, is it just simply different software?
and how do the other Boeing A/C compare?
can somebody tell me then how the difference between the 73 and 75/76 has arisen, is it just simply different software?
and how do the other Boeing A/C compare?
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To get back on the subject, and to confirm still
-> On the 57/67 you can fly away from the selected altitude EVEN if it was the same altitude held at the moment when engaging V/S ?
Tero
-> On the 57/67 you can fly away from the selected altitude EVEN if it was the same altitude held at the moment when engaging V/S ?
Tero
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Teropa
On the 757/767s that I fly (not for AA which are the aircraft the initial post was about), you CAN fly away from the current selected altitude by using V/S.
Example:- you are flying along at 6000ft, in ALT HOLD with 6000ft selected in the altitude window. By pressing the V/S button you will open the vertical speed window. This will open at the current vertical speed - 0000ft/min as we are in level flight. We can now wind in any vertical speed from 8000ft/min down to 6000ft/min up. The aircraft will now do what we have told it to do until either we stop it, by reducing the selected vertical speed to zero or by pressing the ALT HOLD button, or we select a new altitude in the altitude window - in the direction of the climb/descent - and then the automatics will capture the new altitude, the wings fall off because we overspeed in the descent, we stall in the climb, or we hit the ground. As others have said, this needs to be understood and monitored as things can go wrong in V/S mode, although the speed protections in other modes such as Flight Level Change are not quite as clever as some people may think. For overspeed protection to actually occur, you have to have told the aircraft that you want to go faster than the speed limit - i.e. have wound the speed bug above the barbers pole. The same for Alpha protection - you have to have wound the speed bug down to a figure below the stall speed for the configuration.
As I said, this is not necessarily what AA aircraft do as this is what the original question was about. I too would be interested to hear from any AA pilots as to whether or not this is different to their aircraft.
On the 757/767s that I fly (not for AA which are the aircraft the initial post was about), you CAN fly away from the current selected altitude by using V/S.
Example:- you are flying along at 6000ft, in ALT HOLD with 6000ft selected in the altitude window. By pressing the V/S button you will open the vertical speed window. This will open at the current vertical speed - 0000ft/min as we are in level flight. We can now wind in any vertical speed from 8000ft/min down to 6000ft/min up. The aircraft will now do what we have told it to do until either we stop it, by reducing the selected vertical speed to zero or by pressing the ALT HOLD button, or we select a new altitude in the altitude window - in the direction of the climb/descent - and then the automatics will capture the new altitude, the wings fall off because we overspeed in the descent, we stall in the climb, or we hit the ground. As others have said, this needs to be understood and monitored as things can go wrong in V/S mode, although the speed protections in other modes such as Flight Level Change are not quite as clever as some people may think. For overspeed protection to actually occur, you have to have told the aircraft that you want to go faster than the speed limit - i.e. have wound the speed bug above the barbers pole. The same for Alpha protection - you have to have wound the speed bug down to a figure below the stall speed for the configuration.
As I said, this is not necessarily what AA aircraft do as this is what the original question was about. I too would be interested to hear from any AA pilots as to whether or not this is different to their aircraft.
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Pogop,
Indeed, I would like to hear from AA pilots wrt this issue. The way you describe the functionality is exactly what I have encountered in the big sim and read in the books. I would like to know if this differs somehow in AA aircraft.
Thanks!
Tero
Indeed, I would like to hear from AA pilots wrt this issue. The way you describe the functionality is exactly what I have encountered in the big sim and read in the books. I would like to know if this differs somehow in AA aircraft.
Thanks!
Tero
