Kicks out of VNAV PTH during NPA

[tommchowat 1]

Hi Chaps,

 

Was doing an EGLL - LLBG run this afternoon and was happy to have online ATC, and happier that they told me to expect the RNAV GNSS approach for runway 30! Have yet to do an NPA in this thing so was looking forward to it.

 

Anyway they cleared me for the approach just before the downwind and I was at clean speed, LNAV and VNAV PTH. I cross checked all the FMC altitude and speed constraints with the charts and all were in there. I put the MDA in the MCP on the downwind, which I know I would get into trouble for on a line check but hey, no one was watching!

 

Anyway as it was quite a short base turn (the approach is pretty much like a circle to land with prescribed tracks and RNAV points) I was flaps 15 with speed around 175 at the start of the turn, VNAV PTH, LNAV and now speed intervent with the bug at F15 manoeuvre speed of around 165. Around the turn it started only descending at around 100fpm, getting gradually above path. I let it do it just to see what would happen. At around 300 feet above path (VNAV PTH still annunciated) it kicked out of PTH and went to VNAV SPD. I intervened here as I had the bug at 165 and it would have pitched up to maintain it, getting even more above path. Quick flick into vertical speed then back to VNAV PTH fixed it and rest of the approach completed normally.

 

My question then, why did it allow itself to get above path then change to VNAV SPD?

[Driverab330 74]

It changed to VNAV SPD because the FM couldn't slow down the airplane to the target speed which was set at 165kts. My guess is you were doing 175kts passing waypoint PIDET which has a max of 170kts on the chart. With the descend profile from PIDET is pretty much 3 deg it will take time for the airplane to slow down 10kts without speed brakes or the gear. In you case the FM chose to slow airplane down first by letting the airplane to pitch up until it's 150ft above the profile then it will revert to VNAV SPD. However you should get "drag required" msg first. Anyways I just guessing.

 

Never been to LLBG ( Tel Aviv ) before. By looking at the charts the base leg is only 3.1nm from PIDET to MODKA. Which to me seems to be quite short. Normally base leg is usually about 4nm for most of the airports I go to.

 

The other possible cause is the due to the coding of the Approach procedure in the FM, because I don't think you have done anything wrong apart from just being slightly fast. Next time try to put a speed of 160kts or flap 15 speed at PIDET for the turn so that the airplane have a smaller turning radius and will be able to follow the magenta line more closely and see how it works.

 

Usually for type of Rnav appr which involves a tight turns on final I would go for gear down then flaps 20 passing PIDET (in your case) then landing flap just before MODKA in the turn. And try to be stabilized a bit earlier.

 

The problem with this airplane's FM is that it would not tell you if the airplane can slow down in a descending leg, until you fly the that leg and you will see "drag required" Fm msg. Unlike the airbus which will give you a "too steep path" msg on the legs page. Therefore we will need to be more aware and be active to put in our own speed control on the STAR or approach as long as those speed does not violate the procedural speed or ATC restrictions.

 

Hope I can answer your question. Happy flying

[tommchowat 1]

Thanks for the reply. I'm lucky enough to fly the 737-800 for my day job, and assuming the logic of VNAV is the same on the 777, when you have the flaps down you can speed intervent and the VNAV logic will keep it in PTH. So on the 737, if I've got flaps five and I speed intervent to 165, VNAV will still be in PTH, and it'll follow the path. Just because I've bugged 165 doesn't mean it will try to decelerate to that speed, it's still following the path and may not be able (most of the time actually) to get to 165. A very slippery plane.

 

I don't know if the 777 is the same, but judging by the FMA, VNAV PTH was still annunciated and I was expecting it to follow the path regardless of what speed I had bugged. Annunciating VNAV PTH and just sort of floating above it then changing to VNAV SPD doesn't really make sense to me!

 

The approach was fine other than that, speeds were fine, config sequence was fine, and I use the same stable approach criteria as the company I fly for, which were of course met

[777simmer 117]

Just because I've bugged 165 doesn't mean it will try to decelerate to that speed, it's still following the path and may not be able (most of the time actually) to get to 165. A very slippery plane.

 

 

The 777 (I fly that now) works the same in that regards as the 737 (I flew that before).

 

It think what you saw was just a little screw up by the simulation.

I have seen it happen a couple of times as well (on the PMDG, not real life).

Something then seems to screw up the pitch mode untill you go to another mode (Vnav SPD).

But like you said, once you re-capture the Path, everything continues just fine.

 

could be either Navigraph data that has an error coded in it, or it could be the PMDG FMC that is not calculating right.

 

Just try the same approach again and see if the same thing happens.

Or try another Vnav approach and see how that goes.

 

 

If it happens again,;

Changing to vertical speed mode is not necessary though (but works as well ofcourse).

In Vnav speed the throttle would have been in HOLD, so all you needed to do is pull the TL to idle.

Maybe add a little speedbrakes to quicken the re-capture of Vnav Path a little (or throw the gear). As soon as you are back on path Vnav SPD will change again to Vnav Path without the need to push any buttons :-)

[777simmer 117]

It changed to VNAV SPD because the FM couldn't slow down the airplane to the target speed which was set at 165kts. My guess is you were doing 175kts passing waypoint PIDET which has a max of 170kts on the chart. With the descend profile from PIDET is pretty much 3 deg it will take time for the airplane to slow down 10kts without speed brakes or the gear. In you case the FM chose to slow airplane down first by letting the airplane to pitch up until it's 150ft above the profile then it will revert to VNAV SPD. However you should get "drag required" msg first. Anyways I just guessing.

 

 

Nope, the plane should have gotten faster while maintaining its PATH....at which point you use speed brakes or throw the gear.

 

A Vnav approach works a little bit different than a descend from cruise FL:

 

Normal descend:

If you get too fast during a normal descend from cruise FL then the pitch mode will change from Vnav Path to Vnav SPD and then you will end up above path.

 

A normal descend from cruise FL also changes to Vnav SPD when you use speed intervention.

 

Vnav Approach:

Once certain criteria are fullfilled on approach, the FMC switches to "on approach" mode.

You dont see this indicated anywhere, but at that point you can use speed intervention while the pitch mode remains in Vnav PATH :-)

And even if you get faster than your target speed because the AP is trying to maintain PATH, the pitch mode should not change to Vnav SPD in this phase of flight (Vnav "on approach" mode) unless you get close to flap placard speed.

 

So to me it looks like there is sometimes a little screw up where the the airplane does not know if it is in a normal descend or in approach mode.

One of the things that trigger "on approach" is when the first waypoint of the approach is passed. So there could be a coding problem on Navigraphs part.

Another thing that triggers "on approach" is flaps extention.

 

It can also be an AP screw up (not wanting to descend even while commanded by Vnav Path) that results in you ending up high. I had to look it up, but once you are 150ft above PATH then Vnav Path will disengage and revert to Vnav SPD.

[Driverab330 74]

Nope, the plane should have gotten faster while maintaining its PATH....at which point you use speed brakes or throw the gear.

 

A Vnav approach works a little bit different than a descend from cruise FL:

 

Normal descend:

If you get too fast during a normal descend from cruise FL then the pitch mode will change from Vnav Path to Vnav SPD and then you will end up above path.

 

A normal descend from cruise FL also changes to Vnav SPD when you use speed intervention.

 

Vnav Approach:

Once certain criteria are fullfilled on approach, the FMC switches to "on approach" mode.

You dont see this indicated anywhere, but at that point you can use speed intervention while the pitch mode remains in Vnav PATH :-)

And even if you get faster than your target speed because the AP is trying to maintain PATH, the pitch mode should not change to Vnav SPD in this phase of flight (Vnav "on approach" mode) unless you get close to flap placard speed.

 

It can also be an AP screw up (not wanting to descend even while commanded by Vnav Path) that results in you ending up high. I had to look it up, but once you are 150ft above PATH then Vnav Path will disengage and revert to Vnav SPD.

Yes 777simmer that's the 150ft rule I meant, and it also applies in approach mode as well regardless. But you rightly said that only works when the speed within flap placard speed -5kts. That's why I also suspect there might be something wrong with the approach coding as well. Because unless the speed runs away it should not be a problem on the 777LR ( unless we need to do an overweight landing ) which has quite a high flap placard speed compared to 772/773 classic except at flaps 30.

 

I don't know how the approach is coded, however if you force the altitude restriction at PIDET from 170/2500A to something like 160/2500 it should work normally on the real airplane. Having said that this is flight sim and not all approaches in the FM are tested before used. Even in real life occasionally on some of the nor-precision approach, the airplane would do strange things and the approach would need to be re-code.

 

These are my guesses. Hope you will be able to sort it out.

[tommchowat 1]

It did cross my mind that it might be something to do with it not knowing that it should now be in approach mode. It did it a little earlier on in the approach too with the flaps at 1 and no speed intervent, but when it got about 200 feet high it pitched down and recovered the path, whereas later in the approach it decided to go VNAV SPD. Definitely something wrong I think, when I speed intervent I always double check the FMA (as in real life) to make sure it's still in VNAV PTH.

 

If I remember correctly the FAF on that approach is very close to the airport and should be crossed at only 900 feet, and that's where the FMC coded 3 degree glidepath starts, whereas with most NPA's I've flown the FAF is alot further out, therefore the FMC glide path starts earlier. Wonder if this confused it at all, even though all the RNAV points leading up to the FAF were coded with at or above altitudes and didn't seem to me to be too challenging to adhere to!

 

Thanks for the tip on how to get back onto PTH without a mode change. I guess vertical speed or level change for me is a habit as I usually use them at the end of an initial approach phase to refine a glideslope capture to get a nice CDA