American Airlines B767 V/S mode behaviour

[teropa]

Hi,

A pretty specific question, but here goes:

Do the American Airlines B767s need a lower or higher MCP ALT (than what the a/c is flying at that moment) to enable the usage of V/S mode, or does the
V/S window open no matter what is selected on MCP ALT if V/S mode is chosen ?

Just so no one wonders, this is related to a certain flightsim development.

cheers
Tero

[Shaka Zulu]

Opens up both ways....

[None]

One of the cautions associated with using vertical speed on the 757/767 is that the jet will disregard the MCP altitude setting in certain scenarios (climb or descend away from MCP altitude). Vertical speed is very useful on almost every flight. However, it is prudent to carefully watch its progress.

[teropa]

Thanks guys,

The reason I asked is because there was an assumption, that AAL 767s had this feature differently than in other 57/67s, ie. that the V/S window will open ONLY if a lower or higher MCP ALT is dialed in.

Tero

[teropa]

Oh btw,

Does anyone here have a differing opinion on this ? Has someone seen AAL 767s where V/S window will NOT open unless you dial in a lower/higher MCP ALT ?

Tero

[Localiser Green]

Check out 757.org.uk (http://www.757.org.uk/sops/sop7.html) for useful info on the 757/767, quote:

VS is a pitch lock and requires caution when used. Although the autothrottle will attempt to control airspeed within the power available, when very high rates of climb or descent are selected this is obviously not possible. Secondly, unlike VNAV and FL CH, when in VS mode it is possible to fly away from the altitude selected in the ALT select window, depriving the crew of ALT CAP protection and in some cirumstances, of the white ALT ALERT lights.

[teropa]

Loc green,

Yes, I'm aware of the standard operation of the V/S mode. I have Boeing 757/767 AOMs (many versions even), and none of them say that V/S could have the option, that it will not operate unless using MCP ALT lower/higher than present.

I heard from the sim programmer that AAL 767s have it setup that way, that they indeed need a different altitude setting for V/S to operate, and I find this hard to believe :).

Tero

[Shaka Zulu]

U need to dial a diff MCP altitude before V/S unlocks and is annunciated on the FMA...
Seems the same across Boeing fleet

[teropa]

Shaka Zulu,

Are you saying that if I don't change the MCP ALT from the level I'm flying at the moment, pressing V/S will have no effect ? That the mode will not engage and the window will not open ?

Tero

[Captain Stable]

That is what he is saying - quite correctly.

[teropa]

Captain Stable,

Understood. Just the confusion. Earlier he posted "opens up both ways", when I asked if it needed a higher/lower ALT or not...

Tero

[NSEU]

"Seems the same across Boeing fleet"

Can anyone confirm that this is true of the B747-400?

I don't know what to make of the following statement/s in our 747-400 engineering training manuals:

"The vertical speed window on the MCP unblanks and synchonizes to the present V/S.

It will synchronize to 000 if the previous mode was:
#ALT (Altitude Hold)
#Takeoff or go-around and the vertical speed is negative"


Thanks in advance.
Rgds.
NSEU

[Captain Stable]

I can't confirm it is true of the 744. However, the following that you quote:-The vertical speed window on the MCP unblanks and synchonizes to the present V/S.

It will synchronize to 000 if the previous mode was:
#ALT (Altitude Hold)
#Takeoff or go-around and the vertical speed is negative"seems clear to me - the V/S window opens at your current vertical speed (and can then be adjusted). If you had been in Alt Hold mode, it will open at zero. If you were in take-off or go-around modes but descending, it will also open at zero - commanding to remain level.

Don't know if that helps or not.

[Shaka Zulu]

Sorry my post was easy to misread!
And indeed it seems to be the same across the Boeing fleet but indeed I can't confirm either if the 744 has got this function. I don't think so since they are you using a similar sort MCP.
And Why would V/S be available if you are in ALT HOLD or VNAV PATH modes?

As Captain Stable says maybe V/S will be annunciated on the FMA but it seems not logic to me that you can actually wind a different V/S in than 0 in ALT HOLD mode.
Definitely not on 73 75 76

[two speed prop 3]

Shaka Zulu,

I'm afraid I disagree with you. If you are in ALT Hold at 20000' with 20000' set in the MCP window and you press VS you will enter VS mode. The window will show 000 which you can then adjust up and down. The same is true with VNAV Path, give it a go.

[None]

As TSP 3 says, it will fly away, unless there is some very unusual modifaction to the AA 757/767 AFS that we are not aware of.....that being the reason for teropa's first post which started this thread. Additionally, if the jet is in a descent and you push V/S, it will enter V/S and maintain the V/S present at the time it was selected. Same for a climb.

[teropa]

Thanks guys,

I was beginning to think everything I had learned from 57/67 afds was wrong :). I think it would be really weird if AAL had this kind of modification in their birds, but IF they do, then I guess it has been added for the sake of additional safety.

Anyone flying AAL here who could make an end to the speculation ?

Tero

[Shaka Zulu]

Ehm it doesn't work....

[teropa]

Shaka Zulu,

You are sure, that the AAL (specifically, NOT generally) 767 needs a lower/higher MCP alt for the V/S mode to engage? By engage I mean that it will show V/S on the FMA and that the V/S window will open (with either a value or 0000, depending on the sit.) and SPD mode engage with current speed.

Because the usual 757/767 config for that mode is that it will fly away from MCP alt, and will engage in ANY altitude, regardless of MCP alt settings.

Tero

[teropa]

Hi again,

So is there a definite answer for this WRT AAL 767s ?

I would appreciate one :). There seem to be conflicting opinions, but...

Anymore input on this, anyone ?

thanks!

Tero

[twonky]

One of the caveats when using v/s in the 75/76 is that you lose 'alpha protection'
Could anyone please explain as I can't find it in the manuals

Thanks
twonky

[NSEU]

"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.

[Danny]

Seems the same across Boeing fleet

Be careful of that kind of quote. I used to fly the B757 and B767 and as has been pointed out above, there is no speed protection when using VS mode and it is possible to fly away from the selected ALT. Normally used for non precison approaches where the go around altitude is selected and the a/c is descended using VS to the MDA.

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.

[Stan Woolley]

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!

[Shaka Zulu]

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.

[Captain Stable]

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.

[Stan Woolley]

Captain Stable

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

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.

[Shaka Zulu]

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.

[Stan Woolley]

Ja! Tune him some grief Shaka. :ok:

[Captain Stable]

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.

[Danny]

Yes, should have said, you can fly away from a selected altitude... as long as it's not the one you're at! :\

[Shaka Zulu]

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?

[teropa]

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

[pogop]

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.

[teropa]

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