Geometric Path confusion during VNAV PATH Descent.
 

Having gone through FCOM Chapter11 on MCNP SPD INTV, VNAV PATH, SPD descent modes, I still am not able to clear some of the materials.

This happened once during descent in actual flight and we were doing nominal descent from CRZ ALT of FL240. Here is the information for each waypoint.

CRZ: - FL240

Waypoint A (Start of STAR) : FL180 by ATC
Waypoint B : FL160A
Waypoint C : FL120A

During descent from FL240, having passed numerous waypoints on airways, waypoint A was the first restriction to cross AT FL180 assigned as such by ATC.
Then, ATC re-cleared us to delete FL180 and continue descend to FL150. I instantaneously pressed ALT INTV to delete FL180 from FMC then FMA changes from VNAV PATH to VNAV SPD.

Here is the confusion. Did the FMA change to VNAV SPD because the PATH no longer exists between present position to Waypoint A or we there were insufficient information to remain in VNAV PATH by TOD (as mentioned in FCOM chapter 11) ? FCOM states that the mode will change to VNAV SPD when insufficient info exists.

Next, the definition of phrase 'point-point(geometric path)' from FCOM, and its meaning in terms of altitude restriction being shown on FMC LEG page? There can Above, At or below, or hard restriction to be crossed at certain waypoint however FCOM doesn't specify the parameters.

Many Thanks,

John

For the avoidance of doubt, what aircraft type are you asking about?

? Boeing..

in my experience if you are descending in VNAV Path towards a altitude constraint and you then delete that constraint the “system” will initially almost always transition to SPD (“speed on the elevators”) whilst the machine crunches out a new path, so you are probably right it is a case of at least temporarily insufficient information... ... it’s then up to you get the thing back into path using power/speedbrake as appropriate.

Generally, the VNAV path extends upwards from the runway, through various waypoints, speed and altitude constraints, deceleration points etc. When you program it, it draws a line based on prevailing conditions.

When you're on the path (in a Boeing at least) you'll see SPD / PATH (AT / pitch).
Any time you aren't on the path, you'll either be SPD / VNAV ALT in level flight (possibly ALT HOLD) or EPR(THR HLD) / VNAV SPD in a descent.

There are two ways to leave the path in descent:

1: move the aircraft from the path by slowing down or speeding up (Speed intervene)
2: move the path away from the aircraft by reprogramming it.

For example, if you delete a speed/alt constraint, VNAV will recalculate for an idle descent to the next constraint instead, which will immediately put you high or low on the new path. Since you're now not on the path anymore you'll default to VNAV SPD in a descent.

It behaved correctly, as you deleted the constraint, and you were in clean config. It would have returned to PATH once it met the next alt constraint and had a geometric path from B to C.

cheers

Boeing 737.

Thanks rudestuff. What I do not understand is that point to point is geometric path as mentioned in FCOM, and does not specify if it only applied to altitude constraints including at or at or below altitudes. In the example I wrote above, I was descending from FL240 to first waypoint A, was the FMC actually following the geometric path?
In other words, can point to point be applied from CRZ level to waypoint A in this case?

What about the ABOVE altitude restrictions? They are not taken into account for building up Geometric PATH?

From cruise to waypoint A the FMC will generate a path based on idle descent at whatever descent speed you've put in the box. (75 & 76 this is a default 250 it's so we generally overwrite with .78/290 or similar, and VNAV always gives 240 below FL100)

This will give a top of descent point (green circle). If you increase the descent speed (a steeper descent) then the TOD will move closer to the destination.

If you wind down the MCP window and descend, you'll leave the path and descend in VNAV SPD with a bit of power applied and eventually regain the path the throttles will come back and it will descend in IDLE PATH. If you reset the MCP window and do nothing else, it will get to the TOD and descend by itself in IDLE / PATH

That is all assuming you've programmed it with the correct winds: if you have a headwind you will need thrust, if you have a tailwind you will need drag.
If ATC ask you to increase your descent rate you'll need to leave the path (if you stay on the path you'd need to increase your speed!) - basically you need to put it into a pitch speed mode and either dive or use the speedbrakes. FLCH works, V/S works, or force it into VNAV SPD with SPD intervene. Once you get "own rate" again you can set -500fpm and drift back to the path.

Another point to note is deceleration: let's say you're at FL240, flying to ALPHA (220/FL120) with a descent speed of 290 in the box. The VNAV path will allow for an idle descent at 290 to a point several miles before ALPHA, at which point it will slow the rate of descent to allow almost level deceleration to 220. It basically does a dive and drive only slowing down at the last minute. If you've got an unexpected tail wind, you'll definitely need speedbrakes and there will definitely be anxiety on the flight deck. Those who understand VNAV will program a descent restriction of 220/FL130 and trigger the deceleration approximately 3 miles early: those who don't will have selected FLCH already!

As for above and below restrictions: let's say you're routing ALPHA, BRAVO, CHARL then RW14.
With no restrictions, the VNAV path will project backwards from RW14 to the cruise level ignoring all the waypoints. Each waypoint will have an altitude next to it on the LEGS page, in small numbers, showing where the path crosses the waypoint, but it will be an idle descent all the way down.

If you put in hard altitudes, they will show as large numbers, and between each hard alt the will be a different path, some steep, some shallow.

If you put in a conditional alt such as /FL120B then the path might change, but it might not: imagine a rubber band stretched between ALPHA (FL250) and CHARL (FL150) representing an idle descent. If BRAVO is exactly in the middle, it's predicted alt will obviously be FL200.
A /FL220B restriction will make no difference (because it was already going to do that) but an /FL190B will bend the elastic band down, forcing a steeper path ALPHA-BRAVO and a shallower path BRAVO-CHARL.

In a real world scenario, I might be in a descent and told "descend to FL160 level by DELTA" - I'll look at the LEGS page, find DELTA and look at the predicted altitude next to it in small numbers. If it says FL142 I know the path will meet that constraint anyway. If it says FL174 then it won't, so I'll overwrite /FL160B and it will calculate a new path from my position to DELTA, and of course I'll now be 1400' high on that new path - so out come the brakes or up goes the speed!

Hi John,

Now that we know the aircraft type it is a bit easier to give you some answers.

First of all you will find that VNAV, and specifically VNAV descent, is the least understood part of the automation in the 737. You'll come across people who say things like 'I do not trust VNAV', when they really mean 'I do not understand VNAV'. To find out how it works you have to play with it, but as a lot of captains don't really understand it (your profile suggests you're a first officer) you have to find the ones who do and will allow you to experiment.

How it behaves exactly in the situation you describe depends on a few things. One of them, over which you have no control, is how the FMC is set up. It can be set to full geometric path or approach only. I think most operators use approach only, but the other option does exist and complicates matters greatly. There are two ways to find out. You can look it up (but not change it) in the maintenance pages of the FMC or you can tell from the ND. When you're in the cruize the ND will show a T/D. Once you initiate your descent to waypoint A as you describe and provided this forces the aircraft into an early descent, have a look at the ND and see if it shows 'T/D FL180' on the ND somewhere after waypoint A. If it does, your FMC is set up for approach only geo path descent. Worse case scenario is that you fly a mixed fleet of pre-loved airframes and they are not all set the same depending on what the original operator specified.

If the FMC is set to full geometric path descent the behaviour of the aircraft will differ depending on whether you press ALT INT before or after reaching FL180

The ABOVE restrictions are taken into account when calculating the descent, but only influence things if they put you high on profile after the relevant waypoint. It might show a 'Steep descent after XXX' message in the scratch pad.

If at any time you want to know if the aircraft is in an idle descent or a geometric path descent, the easiest way to find out is to press the SPD INT button (provided you have that option). If the speed window displays a speed and the FMA shows 'VNAV PTH' you're in a geometric path descent.

It is difficult to answer your question without being there as it happened as we are dealing with too many variables and we also depend on your exact recollection of what you did in what order. You have to find the captains who understand VNAV and ask them to explain to you what the aircraft is doing as it is happening.

Although different Boeing models broadly follow the same philosophy there are subtle differences in the implementation. When asking these type of questions make sure you include the exact type so the answers you get are relevant to the type you fly. Rudestuff's profile shows the 757 as the type, and I am sure the information provided is correct for that type, but the 737 behaves in a slightly different manner.

Thanks both of you.
 

Thanks for the explanation on VNAV path.

My next question after having read your comment was, yes, VNAV does build up idle path starting from all the way down to TOD but when when I descend at TOD, sometimes FMA still remains in FMC SPEED instead of ARM.

Also, in the example I provided above, I was already in a descent, and FL180 was cancelled by ATC and pressed ALT INTV. Then FMA changed to VNAV SPD. Now we do not know whether FMA will remain inPTH or SPD by pressing SPD INTV and confirm that geometric path is active from TOD as you mentioned, then is not FMC going to build another PATH starting from point B to present position to allow idle or non idle descent path?

In another words, did it change to VNAV PATH because FMC did generate new PATH starting from point B and we were still high above the path so the plane dived in SPD mode?

You're overanalysing this single event. Unless you have access to the OFDM data you can never be 100% sure what exactly happened in what order. Take what you have picked up here and see what happens next time you find yourself in a similar situation.

As I pointed out before, rudestuff's explanation refers to the 757. The 737 is subtly different. For instance rudestuff wrote:

The 737 will do the same for the A-B path, but the B-C path will depend on the setting in the FMC I talked about before. If the FMC is set to geometric path on approach only it will calculate a level segment after B before intercepting an idle descent to C. If it is set to FULL geometric path descent it will behave as rudestuff describes. You really have to find out what your aircraft is set to before you delve too deep into these problems.