737NG VNAV PATH QUESTION (new S/N)

[rsbessa]

Hi everyone...

I'm flying for an operator with a "large" 737NG fleet and I've noticed something different on newest acfts.

During descent (VNAV PATH), when updating an active waypoint, sometimes the path goes all the way down (nothing new here), right ?

Older models used to do the energy compensation in order to loose altitude and chase the path. The newest ones are automatically changing the pitch mode to VNAV SPD, reducing the speed for the planned descent speed (269kts in our case) and removing the PATH option from the CDU's DES page.

I've searched the FCOM over and over but couldn't find any information about this "new behavior".

Does anyone knows why this happens with those new acfts (and where is this information ?)

Thanks in advance...

[despegue]

Forget about Vnav and select Level Change or V/S. Problem solved.

[rsbessa]

That's pretty much what I'm doing when that happens... Switching to LVL CHG, V/S or using the speed intervention!

[Skyjob]

What version of the FMC software is being used?

Apart from certain settings, VNAV is pretty much defined in FCOM and FMC Supplementary Manuals, available to Flight Operations.

[pilotho]

I find it only really happens if the forecast winds in the DESC page aren't accurate.

I try to give it as much information as I could and then leave it unless I get a DIRECT of course.

As far as I'm aware, when you reselect the waypoint, then it will use the current wind it can sense, which might be of a stronger tailwind than before.

[ImbracableCrunk]

You have a VNAV SPD / PATH prompt? I thought Common VNAV did away with that.

I guess it's just another Boeing option.

[Skyjob]

Ok, without knowing the VNAV setup, just a quick comment on the latter.
I think a review of what VNAV does internally to determine a descent path might be a good starting point.

Forecast winds entered, if correct, gives VNAV a good chance to determine the groundspeed based on the fact it knows what constant IAS and Mach to fly during descent as planned in the Descent pages of the FMC, or VNAV page 3, subject to FMC setup.

Essentially:

• VNAV draws a line from runway, working its way up past FAF and IAF with known Speed and Height restrictions as coded.
• VNAV attempts to create a idle thrust profile (most fuel efficient) within the constraints given, and as long as they are adhered to will fly it.
• VNAV corrects for pressure altitude correction if forecast QHN is entered in Descent Forecast pages.
• VNAV takes into account higher engine idle settings for Anti-Ice usage for the altitude region entered into Descent Forecast pages.
• VNAV assumes 0 wind on touchdown, tapers it for the wind in the Descent Forecast pages to the lowest (if entered) value/FL, above which tapers it to the middle (if entered) value/FL, then upper (if entered) value/FL, then to last entered cruise wind (which is why during descent you are unable to enter winds in Leg RTE Data).
• The resultant of the above is a T/D point which, in theory, should allow for all variants during descent until touchdown and thus should enable a CDA idle thrust from T/D.

However, forecast models are just that, never accurate to the minutest details.

Thus, during descent, when a direct waypoint is executed, VNAV recalculates the actual profile from present position, using actual winds as new T/D wind then implements the forecast winds again as explained above.

This may result in being either remaining on profile or a vertical displacement from it, becoming high or low on the recalculated VNAV idle descent path, based on the (reduced/increased) track miles remaining and actual height.

The two way the aircraft can correct for this are by either:

• Reducing speed, thus creating a greater descent angle which can be used for longer [VNAV calculates this as most favourable energy management solution to avoid becoming hot & high later on in flight]
• Increasing speed, diving off the excess height and dissipate the excess enegery using other means later [Autopilot, Flight Director commands this]

On the speedtape this anomaly can be observed by the IAS bug being driven lower by VNAV in accordance with energy management, but the aircraft increasing speed by diving it off to regain the recalculated VNAV PTH profile for an IDLE descent then to follow it (advising you when flying at VNAV IAS +10kts of DRAG REQUIRED)...

pilotho, I hope this explains why and how the VNAV recalculates the remaining path and recreates the new idle descent profile and why VNAV acts the way it does.

[JeroenC]

But it shouldn't go into VNAV SPD though, as the TS mentioned...

[latetonite]

Ours go into VNAV speed, if the airplane cannot follow the path, within certain speed limits. Applying speedbrake usually helps getting back into VNAV Path.

[spelling_nazi]

It's "lose" FFS not "loose"

[seven3heaven]

FCOM 2

VNAV reverts to VNAV SPD if
a limit speed will be exceeded, with COMMON VNAV and U10.6 or later.

[Gas Bags]

spelling_nazi,


For a bunch of guys who generally compare themselves to surgeons....It is incomprehensible that so many of them can't spell the word "lose".


Perhaps you could start a new thread on the spelling of one of the simplest words in our language???

[RVF750]

lose and loose are both words in the language so a spelling checker will not correct. However, This is a tech log thread populated by native English speaker and also non Native English speakers so really it's quite pedantic and unnecessary to correct something so minor.


Interesting topic though. Thanks.

[ImbracableCrunk]

I re-read the OP and now I'm confused.

Energy compensation while in descent? That's not possible, is it? Energy compensation is for level flight in VNAV after TOD, right?

It sounds like some of your planes differ in the Common VNAV option is all.

[VH-ABC]

Maybe energy conservation is what we are on about here?

[B737NG_Pilot]

Rsbessa's observation is correct. This is common on the new B737NG's. After checking with senior pilots, the only reason for is to prevent the VNAV from going into a clacker.

Hence as seven3heaven mentioned VNAV reverts to VNAV SPD if
a limit speed will be exceeded, with COMMON VNAV and U10.6 or later

Although lot of pilots, are of the opinion, you can never touch the clacker with VNAV, irrespective of the FMC version.

[latetonite]

There must be different options around. My previous operator NG's would fly straight into the red zipper with an unforeseen or not programmed tailwind.
The sin I am in now does not hesitate to revert to VNAV speed. Have to admit: now I am lost.

[Skyjob]

B737NG_Pilot - the only reason for is to prevent the VNAV from going into a clacker.

Unfortunately incorrect.
VNAV will prevent the aircraft flying into the clacker as it is restricted to KIAS 330, Mach .81

In VNAV PATH, it will dive to try regain the path and increase the speed doing so (crew could assist VNAV by reducing the speed increase using the speed brake of course).
When reaching KIAS 330 the aircraft will forego on its effort to regain the path by not further increasing the speed but by maintaining the maximum of KIAS 330 or M.81, until the calculated PATH is regained and VNAV PATH resumes.
Should at ANY stage a VNAV SPD/LIM be triggered such as e.g. KIAS240/FL100 then it will level off at that level, wait until the speed is within the restricted limits (in this case 240 +10) then drop the nose again and obey the new restricted speed +10 knots.
The message UNABLE NEXT ALTITUDE will be displayed when VNAV is then unable to cross subsequent altitude restrictions coded into the FMC.

VNAV PATH is very clever, and does what it is supposed to do: PROTECT THE AIRCRAFT AND ITS CREW from an overspeed (M.81 and KIAS 330, or coded speed restriction +10 its) or an under speed (-15 kts or lower band limits).

In VNAV SPD however, any descent speed restrictions are ignored if you are controlling the speed using SPD INTV with an open speed window on the MCP, a practise regularly observed on the line. VNAV SPD will obey limitations if selected through the FMC on DESC page or VNAV page 3.

It is a very common misunderstanding and incorrectly taught by unknowledgeable trainers (to mostly new cadets) of how VNAV (PATH) works when saying it will go into the clacker.
Hence my original posting (22/12/13) asking for the FMC version used, to determine the small ambiguities and changes as time has progressed in VNAV-land.

[ImbracableCrunk]

You've never had VNAV attempt to overspeed at altitude in descent?

[latetonite]

Skyjob: please explain how VNAV can maintain a PATH, if limited in speed?
With A/T in RETARD, speed is the only variable it can play with, to maintain the path.
That is why it changes over to VNAV SPEED, as the AP cannot maintain the path in the first place, without your personal input with braking devices.

And to B737NG-pilot: your statement seems pretty bold for an inexperienced pilot you claim to be in other posts in this forum. There it looks like you never did the type rating.