PPRuNe Forums - 737NG VNAV PATH QUESTION (new S/N)

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737NG VNAV PATH QUESTION (new S/N)

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... :ok:

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

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

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.

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.

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

I guess it's just another Boeing option.

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.

737NG VNAV PATH QUESTION (new S/N)

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

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.

It's "lose" FFS not "loose"

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

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???

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.

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.

Maybe energy conservation is what we are on about here?

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.

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.

Quote:

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.

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

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.

Okay, I've read this thread again.

I don't see the problem. PATH reverts to SPD.

If it was an automatic reversion, it'll rejoin PATH automatically, so you can't get a PATH prompt.

latetonite:

Quote:

please explain how VNAV can maintain a PATH, if limited in speed?

As VNAV PATH initially increases speed in descent from ECON SPD, up to but not exceeding KIAS 330 and/or M.081

If you want to prevent VNAV reaching that speed when (very) high on profile, speed brake can be used to allow VNAV to dive but without the speed increasing too rapidly (good airmanship...).

Either way, it would make sense that when VNAV increases the speed to adjusted a VNAV PATH profile in the FMC with a higher speed so not to allow VNAV to dive too much but to reflect the remaining energy contained within the aircraft.

OK465, Try instead of using SPD INTV to increase the descent speed in the FMC.

VNAV PATH will recreate a profile for idle descent at that speed, but as you increased the speed will throttle up (THRUST REQUIRED message) and maintain a 1000'/min rate until establishing the VNAV PATH (from below if increasing speed). The further speed restrictions are maintained should they be required.

Quote:

Not until the path deviation limit indicators on the PFD turn amber will thrust then increase, and then....mode switches back to VNAV PATH and aircraft will attempt to rejoin the path with A/T thrust at the SPD INTV speed (300).

That's not how the system works. If you enter SPD via the SPD INT, it doesn't attempt to recapture the path. It'll honor altitude restrictions, but not path.

Like Skyjob said, that's where the FMC DESC SPD is a better option.

(Assuming IDLE DESC option.)

PHX STARS are just sucky, regardless. Maybe it works in a FULL GEO PATH plane, but not in our IDLE DESC planes.

In descent phases, clean config, VNAV PATH changes to SPEED, if you select Speed Intervention . With flaps down, in a VNAV approach, it will stay on VNAV path.
But that still has nothing to do with my experience, that high on a descent leg, the PATH would lead me to an overspeed, if I did not intervene. Is it because the AP does a bad job? Or was it just looking like that? I obviously prevented it from happening.

Skyjob - on one occasion VNAV has put me into the overspeed regime, when I wasn't paying quite as much attention as I should have been. It's tried to do it to me on several other occasions but I had learnt my lesson...

Have noticed this as well on our newer −800Ws and −900ERW 's as well .I actually like this feature , there's no drastic pitch change as the aircraft dives to regain the path and the changeover to VNAV SPD is quite straightforward.All I do then is use DES SPD to modify the path once again and it's back on PATH again.

I've noticed though some of our −700's used to have some issues when on VNAV and the aircraft not behaving quite as expected in VNAV. I suspect Boeing have modified their software and removed these chinks in the system .

Regardless, I think that over-reliance and modifying the FMC too much to get the aircraft behave the way want it to in VNAV/LNAV is a common problem and my first reaction to a VNAV problem is take over on the MCP , then sort out the issue before re-engaging VNAV..which is what I suspect most 737 drivers do....

On our brand new 738 the vnav remains in path mode and and dives to capture the path, rate of descent might reach 4000+ fpm. The speed increases to the red tape and we even get the clacker sound. Anyone know if this is normal or an error by boeing perhaps.

Hi guys...

To tell the truth, I didn't expect this thread to go so long !

I'm really enjoying some of the info being posted here. This exchange of experiences, IMHO, is really worthy.

As some might have noticed, english isn't my first language, so, please, be patient with my grammar mistakes.

Boeing says that VNAV has overspeed protection, so, as far as I understand, the pitch mode should revert to a more conservative mode before the activation of the clacker.

Anyway, I feel myself more comfortable (and the passengers probably feel the same) using V/S or LVL CHG to dive and recapture the calculated path. Sometimes I even go a little bit under the path in order to trade the excess of energy for altitude. But all that depends on the guy sitting on the left seat !

Some of the "VNAV maniacs" don't feel like this being a "good practice", but it often works and avoids the excessive usage of speedbrakes.

Cheers, :)

rsbessa,

Agreed. Never trust VNAV to avoid the clacker. I also use LVL CHG to regain the path, dip under and then wash off the excess speed before regaining the VNAV path.

Re-setting the new higher speed on the descent page and then re-engaging VNAV is not a good solution in my mind. Firstly, you are no longer descending at the same cost index as before. You may or may not care about that. Secondly, the next time you are kept high you won't have much of a speed margin with which to regain the VNAV path. You have reduced your options.

VNAV is fine for general use provided you input all the winds and temperatures accurately and ATC play your game. With the best will in the world this is a rare occurrence. :)

In my opinion VNAV works nicely if it's managed correctly. Descent winds need to be entered, altitude restrictions need to be managed with speed changes in the box. You have to think of the whole profile and not just think you can press a button, sit back and let it all happen.

We plan .78/280 in the descent and I won't let the plane reach TOD in VNAV if above FL350. It's DESC NOW early or it's clackers.

Hi guys...

By the way, which winds do you usually insert in the DES FORECAST page ?

I believe that the most advanced airlines use the ACARS system to upload this kind of data to the fleet, right ?

Where I'm flying nowadays, the ACARS system is pretty basic, not to say useless, and the only winds we have forecasted are those on our navigation plan. So, when available, I feed the DES FORECAST page with High, Medium and Low altitude winds + the QNH setting at the destination arpt.

For descend forecast page, I follow the following:

Last Cruise waypoint before T/D enter last known cruise wind
FL300
FL200
FL100
QHN
TAI ON/OFF altitude: the altitude range in which you expect to use TAI (as it increases the N1 % thus forward thrust vector)

Hi Skyjob...

That's pretty much what I'm doing too.

When the forecasted winds match the real world and the ATCOs don't :mad: with our plans, it works nice.

Cheers.

Skyjob, have you actually flown the 737, or just read the manuals? VNAV is a bag of nails which has a terrible tendency to overspeed, especially as it first enters descent at high levels with a higher cost index, and will still finish its arrival on height but way fast, making the approach unstable unless you intervene earlier.

The best way of avoiding it is to cheat the FMC descent winds with an extra 30kts tailwind or hit "descend now" or "alt intervent" 10 miles before TOD. Even then, VNAV can occasionally be glitchy, refusing to descend at all or levelling off despite the FMC and MCP settings being lower. When it doesn't do what you want, just over-ride it with LVL CHG or V/S. Use VNAV, but don't trust it.

Skyjob, you talk from the APU orifice.

If I had a £10 voucher for every time my -700 and some -800s tried to get me into the clackers, I could have retired earlier.

During the time taken wondering what update I may have, what winds/QNH/TAI settings/ISA deviation is loaded or what the FCOM or Ops Manual "B" or any other BOOK says, I preferred to WATCH THE DAMN AEROPLANE and intervene as necessary.

It kept me alert, gave me something to do and out of the Asiana syndrome of "everything is all right" so I don't need to work hard til brakes on.

That's what we're paid to do, so why don't we earn it?

I can understand the OP's query, but reading these pages re the thought processes of Mr Boeing and M'sieur Airbus, I recommend we all treat our aircraft like a baby's bottom.

You know it will work eventually,

You know how it's supposed to work,

You stay prepared for it to :mad: all over you, any time, any where.

Happy landings, stay out of the smoking hole, stay out of the office and keep a sense of humour?! :)

Barkingmad and Aluminium shuffler, I and fellow readers must thank you for your knowledge and wisdom shared. FYI: I have and still do fly the old lady, yes, in fact for over 13 years gaining well in excess of 10,000 hours and still flying her today.

Now, please let me share some of the wisdom gained by not just reading the FCOM and flying a plane, quotes from the following document are made available to you by your airplane manufacturer, it's called the FMC Supplementary Data Document. You can find this document through your performance department, or should be able to at the very least.
To all, please excuse the extend of contact quoted, as it has to be read in conjunction with each other. Alas the full manual is not available for sharing for obvious reasons.

Quote:

Because ECON speed schedules are not available in either the Performance Engineer's Manual or the Boeing Operations Manual (References 1 through 3), they are provided in this document.
...
In VNAV operation the FMC limits speed and altitude capability in order to ensure reasonable and safe operation. The VNAV operational envelope takes into account the maximum certified altitude, maximum operating speeds (VMO and MMO), buffet margins at an entered or default center of gravity position, and stick shaker margins. These limits are treated as hard constraints by the FMC guidance function.
...
The data in this document are derived from the Boeing- developed data base used for all FMC performance calculations.
...
The FMC defaults to the economy speed modes for VNAV operation. In addition, by default, a transition climb speed limit is observed below the speed transition altitude during climb (250 knots below 10,000 feet in the USA), and a similar limit, but 10 knots less, during descent (240 knots below 10,000 feet in the USA). The 10 knot margin was added to reduce the probability of exceeding the transition speed limit in turbulence. The FMC increases the transition climb speed at heavier gross weights to equal the flaps-up full maneuvering speed of VREF40 + 70 knots (minimum transition climb speed).
...
The speed schedules provided in this document are a function of one or more of the following variables: airplane gross weight, altitude, cost index, wind, and temperature. No other FMC variables (i.e. drag factor, fuel flow factor) affect the speed schedules.
...
The economy descent speed schedule provides the descent speed associated with the lowest total cost over a fixed distance for a given cost index and TOD gross weight. The method used to derive these speeds is similar to the one used for economy climb speeds. The FMC speed schedules were generated by iteratively computing cruise plus descent trajectories from a common cruise point to a common end of descent point. The descent CAS was varied and the trip cost for each resultant trajectory computed. The CAS value which minimized the cruise-plus-descent cost at each cost index and gross weight was selected as the economy descent CAS. This method allowed the cruise-descent interaction effect on trip cost to be included in the economy descent speed determination.
At a cost index of zero (minimum fuel), the economy descent CAS is near maximum L/D. This stretches the glide and allows an earlier TOD, which allows an earlier thrust reduction from cruise thrust (higher fuel flow), to idle thrust (lower fuel flow). This results in significant trip fuel savings despite the fact that the actual descent fuel burn is higher due to the extra time in descent relative to higher cost index cases. As cost index increases it is necessary to increase speed to reduce the time costs. This results in a steeper descent, more time in cruise, a higher overall fuel burn, but a lower relative cost.
The speed schedules are normally fixed CAS/Mach with the economy descent Mach set equal to the final economy cruise Mach. However, the application of more restrictive speed limits for the FMC descent path calculation may, at times, result in an economy descent Mach number somewhat lower than the final economy cruise Mach number.
Economy descent CAS is defined to not exceed the VNAV speed limit, which in descent is the airplane VMO (340 KCAS) reduced by a 10 knot speed. This limits the maximum economy descent speed to 330 KCAS. A minimum economy descent CAS of 250 knots has been defined to improve compatibility with the air traffic at low cost index values (software versions U10.6 and earlier).
...
The FMC provides protection from operating near the region of airplane buffet. All target speeds are checked against a selectable maneuver margin to initial buffet. The FAA FMC will accept values of maneuver margin to initial buffet ranging from 0.2g to 0.6g, but defaults to 0.3g; the JAA FMC lower limit is 0.3g. If necessary, target speeds are adjusted to maintain that margin. Both the FAA and JAA FMCs accept CG values ranging from 5% to 36% MAC; the FAA defaults to 26.2%, while the JAA default is 5% (forward limit). Appendix B describes corrections for other values. FMC- generated speed targets may not be less than VREF40 plus a 70 knot increment.
...
The VNAV operational envelope is defined by margins to initial buffet, stick shaker speed, and VMO/MMO. It provides margins to initial buffet and VMO/MMO speed as detailed below.

VNAV Operation Envelope Limits (VMO = 340 KCAS, MMO = .82 Mach)
• VMO/MMO - 5 KCAS for Climb & Cruise
• VMO/MMO - 10 KCAS for Descent
• 0.Xg margin to initial buffet, where 0.2 < X < 0.6 for FAA operators or 0.3 < X < 0.6 for JAA operators

VNAV operational envelopes for altitudes of 20,000 feet and above are shown in Figures 4.2.1.1 through 4.2.2.3. The low speed segment of the envelope contains data based solely on FMC computed, initial buffet derived data. An additional constraint is added to the buffet-based low speed limit to provide protection equivalent to a 14% speed margin to VSS, the stick shaker activation speed. This is accomplished by limiting the minimum maneuver margin to 0.3 g at Mach numbers less than 0.45. Above 0.45 Mach, this limit gradually washes out, varying linearly from 0.3 g at 0.45 Mach to 0.2 g at 0.78 Mach. This permits a minimum margin of 0.2 g at the peak of the envelope and higher Mach numbers. The effect of this added constraint may be observed by comparing the low speed portion of Figures 4.2.1.1 and 4.2.1.2. The two figures are identical in these areas.
The data in Figures 4.2.1.1 and 4.2.1.2 are based on a middle CG position of 26.2% MAC. This is the default value for FAA operation, but values forward or aft of 26.2% can be selected by the flight crew to adjust the VNAV operational envelope for expected cruise CG positions. The data in Figure 4.2.1.3 are based on a forward limiting CG position of 5%, the default value for JAA operation. Data are provided in Appendix B to determine the effects of CG position and maneuver margin to buffet for the flaps up VNAV operational envelope.
Below 20,000 feet the VNAV operational envelope is based chiefly on stick shaker derived data with the low speed limit based on 1.14 VSS.
For flaps down operation the low speed limit is defined by 1.14 VSS and the high speed limit is the appropriate flap placard less 5 KCAS.
This information must be used in conjunction with thrust- limited altitude data, Figures 4.2.1.1 and 4.2.1.2, to compute maximum altitude.

Flyboy: you probably include the reading of this document in your ALAR checklist before you descent in VNAV, but remember that if you do not feed the FMC correct, for one, rubbish comes out at the other end.

I still prefer selecting Level Change, than having to explain later:" My FMC had a bad day", when the IAS steams towards the red zipper.