VNAV Path construction and Flap Speed

[Flat Cap]

Hello Technical Types,

I am trying to sort out a question regarding the above. Particularly the 737 with 10.8a.

When constructing the planned VNAV path is the FMC expecting you to reduce to minimum maneouver speed for the current config. before selecting flap ie. within about 10kts of UP speed before selecting F1 and so on; or is it expecting you to select flap just under flap limiting speeds, which would seem like using flaps as drag to me, but there you go.

I've consulted all the usual sources and cannot find a reference either way, the implication in Vol.2 and Billy Bluffer seems to hint to the former. However many of my colleagues are prepared to stake their mortgage on the latter "...its why you always end up hign using VNAV!"

Any thoughts or better yet references you may have are gratefully recieved.

[ImbracableCrunk]

"Always end up high"? I don't have that problem.

I bet that's the same person who exits VNAV because LVL CHG will magically give them a better descent rate than VNAV SPD at the same IAS.

I'll do some digging, but I bet your sources are as good as mine.

[Skyjob]

VNAV expects a continuous deceleration to the proposed flap setting when coming back to clean configuration.
A seasoned aviator will roughly know how long it takes for flaps to extend, thus ensure you select the next flap setting required for phase of flight with that time in hand, e.g. approximately clean speed +20kts works reasonably well to get flap 1 and immediately afterwards 5 out in time for approach segment, depending on angle and weight flap 10 may be required to maintain speed on the subsequent final descent angle coded.
Think of it that thrust levers stay idle as long as possible until throttling up to maintain speed on intermediate approach segment before gear goes down and further deceleration takes place...

Unfortunately I have yet to find references to the question you're asking...

[Intruder]

From the 747 FCTM 4.21:

Maintain cruise altitude until the proper distance or time out for the planned descent and then hold the selected airspeed schedule during descent.

Since the flap speed for FLAPS 1 is 280 KIAS, and the descent profile has a speed reduction to 240 at 10,000', extending flaps at or near flap maneuvering speed should be assumed.

Further (4.22):

Plan the descent to arrive at traffic pattern altitude at flaps up maneuver speed approximately 12 miles from the runway when proceeding straight-in or about 8 miles from the runway when making an abeam approach.

...and on the next page:

Normal descents are made in the clean configuration to pattern or instrument approach altitude.
. . .
Extend the flaps when in the terminal area and conditions require a reduction in airspeed below flaps up maneuver speed. Normally select flaps 5 prior to the approach fix going outbound, or just before entering downwind on a visual approach.

Going forward to 5.7:

Flap Extension
During flap extension, selection of the flaps to the next flap position should be made when approaching, and before decelerating below, the maneuver speed for the existing flap position.

HOWEVER, the FCTM doesn't seem to address a constant descent approach. So, in order to be able to decelerate while descending, our FCOM used to say to start extending flaps at 17 miles instead of 12 (it is now silent on the subject...).

[Capn Bloggs]

constant descent angle approach

"Angle"? A CDFA is a Constant Descent Final Approach and a CDA is a Constant Descent Approach. I have not heard of a Constant Descent Angle Approach and it would be illogical if there was one, as you would, as you say, have to add drag at various points to stay on the constant angle to reduce speed, thereby wasting fuel.

[Avenger]

VNAV is a path both vertically and with a speed consideration. The most common mistake is for guys to chase the magenta path indication using VS and then arrive at the decel point way above the VNAV path calculated speed. Re engaging VNAV at this point will cause the aircraft to fly level until with the FMC speed parameters, thus giving the impression that VNAV leaves you high. The ND clearly shows the green decel dots, the point at which the vertical rate will erode along with the speed to the speed schedule shown in the legs pages. Obviously if one fails to extend the lift devices, or take account of tail winds or anti ice then the issue is basic airmanship and lack of understanding of the systems. Equally, the messages " unable next altitude" or " bypass" should be an indication that somehow you have managed to overcook the approach, a quick check of the Legs Data will then show that according to your advanced planning you have asked the machine to arrive in two places at the same time , or loose 2000 feet and 40 Knots in 1 mile for example. The Systems manual navigation section has comprehensive descriptions of the process: for instance:
Normally, the target speed is economy speed above the airspeed restriction altitude and 240 knots below that altitude, until deceleration for approach. VNAV will not permit descent below the airspeed restriction altitude until the airspeed is at or below the restricted value plus ten knots. The start and end of the airport speed restriction deceleration segment is shown on the map as a green open circles with no labels.
The descent path assumes deceleration to reach the final approach fix (FAF), or the glideslope intercept point at VREF 40+20 knots.
Target speeds are changed by entries on the DESCENT page. Entries made on the LEGS page are “at or below” and may limit the target speed. Wind and thrust assumptions are changed on the DES FORECASTS page.
Deceleration points show on the map as green open circles with the label DECEL. Deceleration points show prior to:
• airspeed constrained waypoints
• holding patterns
• approach flap extension.
If more than one deceleration segment exists in the flight plan, only the next deceleration point shows. Deceleration points can also show prior to cruise holding patterns or other speed reductions.

[FlightDetent]

Bloggs,

CDFA = as you say
CDA = continious descent arrival

Hm?

[Lantirn]

Off topic,

Pans ops 8168

CDFA is the constant descent final approach and there is no level off segment.

CDA is constant descent angle type of approach and there is a level off segment!

[Capn Bloggs]

CDA is constant descent angle type of approach and there is a level off segment!

Ah, bureaucracy at it's best...

[cf6-80c2b5f]

On a related note, regarding VNAV path construction, aside from speed brakes, flaps or gear, the only other way to reduce airspeed during the deceleration phase is to reduce the descent rate. I think the original poster's question -- whether you are expected to add flaps near the maximum flap extend speed to decelerate and stay on the path, or reduce descent rate until close to minimum flap maneuver speed -- can be asked in a different way:

How does the FMC deal with the deceleration "knees," which must be calculated in advance and inserted into the predicted overall idle descent path? I noticed the vertical deviation indicator stays centered while the vertical speed decreases to about 500 fpm during the deceleration segment. Obviously, the path itself is not a constant gradient if you can change vertical speed from 2400 fpm to 500 fpm while decelerating (aproaching 10,000' or some other speed restriction point) and continue to stay on the path. I suppose you can keep the 2400 fpm descent rate going and decelerate if you add flaps just as you get below each max flap placard speed, but this is usually not what is preferred.

There must be some target speed that the FMC is anticipating for these deceleration segments. Is it the minimum flap maneuver speeds, maximum flap placard speed, or some speed in between?

[Lantirn]

Originally Posted by Capn Bloggs

Ah, bureaucracy at it's best...

Indeed. But that's how they had to separate the 3 types of NPA approaches back then to establish design criteria for each procedure. But it's there written.

CDA today means for me constant descent arrival, like LHR. Which doesn't really have to be constant in terms of angle, but rather a constant descent achieved.

[RAT 5]

I suppose you can keep the 2400 fpm descent rate going and decelerate if you add flaps just as you get below each max flap placard speed, but this is usually not what is preferred.

Indeed, frowned upon and I doubt very much you could achieve the performance you speculate on. There is not enough drag in the flaps to succeed in that.

Flat Cap: I can only tell you what I've observed. First some basics, which Avenger has touched upon. VNAN PTH is an energy profile not a strict descent profile like a glide slope. If you are descending towards a WPT with an ALT/SPD constraint the ALT has priority at the sacrifice of speed. When descending to 250/100 then 250kts has priority.
The forecast winds entered on DESC page help with the calculation of TOD but are then not updated during descent, automatically. If the real world & winds are different the a/c will try to stay on the calculated path profile and the speed will be sacrificed. You can update the FMC to actual wind by reselecting DGT TO the active WPT and this actual wind is now used as if it was at TOD from np]ow to landing.
When decelerating approaching FL100 it generally goes 500fpm if on an accurate path and accurate descent speed. If the speed is higher than FMC speed it could even level off at FL100. This can cause you to go high. It could then dive for the path for the next WPT constraint and accelerate above 250kts in doing so. You'll have seen 2 green rings on the map showing the calculated start & end points for decel to 250kts and then continued descent.
The next green ring you'll see is DECEL. This is the calculated point where F1 is expected. Assuming this whole profile is flown in VNAV PTH with A/T as you approach this DECEL marker you'll see the ASI speed bug start to reduce towards UP. The a/c goes 500fpm to decelerate. If its got it correct you should hit DECEL at UP - UP +10kts. Taking F1 s now your decision. The FMC has calculated the reduction in energy from this point in space to achieve the next constraint which maybe IAF or OM. You'll see the SPD/ALT in LEGS. There are no more DECEL rings, but you'll see the ASI speed bug reducing and you are supposed to match the flap setting as the speed bug reduces below flap man speed.
It is a real gotcha if you forget the flaps and end up slow, but there is a speed buffer between Flap Man bug and absolute Min Man speed; that is why it is very common, IMHExp, for many operators to either not use VNAVPTH below FL100, or t go SPD INV. You then control speed and not FMC. A variety of reasons: it improves your direct control of the a/c; the ATC route may not be the same as FMC route and so the energy profile will be different.

But it ain't calculated on using flaps at max speed.

[flite idol]

Ahhhh......VNAV.....where art meets science. It's all about energy management old boy.

[RAT 5]

Indeed. And sometimes it threw up some real gremlins, which highlighted serious lack of appreciation of energy, and therefore training. I didn't know the answer at the time, but that was not important: I watched PF RHS sitting idly by in VNAV-PTH, on the pipper, on speed. I suggested they look at PROG 1 and distance to TDZ and then the altimeter and consider if all was well. "But we are in VNAV PTH." so what can be wrong. I pointed out the discrepancy and we decided what was the best course of action. That in itself was a small adventure. On the return sector we analysed the previous 'curiosity' and came up with a reason hidden in the FMC. The PF was astounded how this could happen. It all looked so good. But why were they not applying their own 'on-board computer'? Another lesson ticked off that should have been done in LT and continually since.

[Flat Cap]

Thanks for all the input so far, much appreciated.

I had never heard the "VNAV leaves you high because we take flap at min maneuver rather than flap limit" until I arrived at my latest company.

Anecdotally my on the line observation of what VNAV does seem to agree with most peoples. Such that the VNAV path constructs its "energy profile" (I like that, going to steal it), that if you fly a standard Boeing decent and approach profile as in the FCTM you generally end up in the same place as the VNAV. (Subject to correct FMC programming and the forecast info you've enter being similar to the actual met you encounter.)

I struggle to accept that Boeing would want the FMC to construct a profile that requires you to use flaps as drag on every approach, when they are at such pains to ensure you don't do that when you are working the MCP or manually flying. But I would like to be able to point to it on a page.

Not sure Boeing helped those new to FMC VNAV by labelling the numbers on the path as "Feet" in Vol. II. when as you say it actually combines both speed and height ,foot/knots, units of path, any other name except straight "feet" might have helped.

[cf6-80c2b5f]

So the consensus so far seems to be that "[t]he a/c goes 500fpm to decelerate. If its got it correct you should hit DECEL at UP - UP +10kts."

Flat Cap,
If you do find something in writing that addresses this, I would be interested too. So much of Boeing's VNAV is a mystery to those who operate the airplanes.

[FlightDetent]

CDA today means for me constant descent arrival, like LHR. Which doesn't really have to be constant in terms of angle, but rather a constant descent achieved.

that's why they call it "continious descent arrival, i.e. without recourse to level segment." There's nothing constant about that descent. [OFF]

[RAT 5]

So much of Boeing's VNAV is a mystery to those who operate the airplanes.
That's one good reason, in the early days, to let the automatics do their thing and watch. It's the best way to learn: far easier than reading manuals; and as you say there's not a lot there.

[flite idol]

My most fav VNAV observation that tells me instantly that it is not understood goes like this. "I'm starting down early to get a head of the path." Ok says I and nobby promptly selects DES NOW or mashes that there altitude selector. We now start a shallow descent and THR HOLD presents itself. We continue on that 1200 ish fpm descent until re intercepting our original flight idle descent path that was 40 miles in front of us when we started down on this oddisy. The TLs go to IDLE and "What did we just accomplish" says I to a blank stare.

[cf6-80c2b5f]

This may be airplane specific, and totally a matter of technique, but some guys on the B744 will bring the first constraint on an arrival down to the scratchpad and add a /-4 after it, then insert that waypoint over the first constraint waypoint and insert the same airspeed on the right as the respective constraint waypoint. This fools the FMC into planning on being where it should -- but often isn't -- about 4 miles prior. It seems to work well for them and it beats having to pull out the boards after it becomes apparent that the airplane is not going to make it down.