Strange SID path in my B738 FMC

[Marenostrum 4]

Taking off from EBBR 25R SID DENUT 6C the route requires a 3.8% up to 700, then 7.0% up to 3200 to minimize noise disturbance. There are two altitude restrictions: at 700 and above 1700 following the sid to DENUT

The chart reports a simple take off at 245° then a right turn 297°. Nothing strange, it seems, from an LNAV and VNAV perspective.

If it is so, why does my FMC show a 360° turn left (it  seems to gain  altitude to pass at 700 (first altitude restriction) and a second 360 turn to cross the second altitude restriction above 1700?

I avoided to follow the FMC and flew the SID  manually following the SID path on the charts and had no problems.

Is it a real FMC behaviour or a PMDG bug?

I know it has been discussed at least once, but I finally did not understand the reason.

Thank you

Stefano

Edited September 3, 2018 by Marenostrum

[Spin737 102]

Looks like the SID was coded wrong to me. Probably not something you did.

I think someone will be along in a bit to tell you how to check the SID and make sure it says R-turn and not L-turn in the airport data files, or some such.

[Budbud 227]

Hi,

Have you already entered the takeoff speeds and reduction and acceleration heights?

Sometimes, it resolves such things.

[downscc 1,327]

Wow a double loop... I've often seen the ND render such nonsense only to have it correct itself after takeoff.  Of course I'm ready to take control if LNAV is going to wander off clearance but it usually works out.  There are rare occasions where Navigraph gets something coded wrong but it is pretty rare.

Just one more reason we need PMDG to build a new  FMC for the entire product line that uses ARINC navdata instead of the obsolete macro language that's been around since the 90s.

[Marenostrum 4]

19 minutes ago, downscc said:

Wow a double loop... I've often seen the ND render such nonsense only to have it correct itself after takeoff.  Of course I'm ready to take control if LNAV is going to wander off clearance but it usually works out.  There are rare occasions where Navigraph gets something coded wrong but it is pretty rare.

Just one more reason we need PMDG to build a new  FMC for the entire product line that uses ARINC navdata instead of the obsolete macro language that's been around since the 90s.

That's means that in the real world such a thing never happens..right?!

by the way, my acceleration altitude setting, vspeeds, trim, winds, ...were already set as usual. I have tried also to change the initial climb setting...but I always got the picture shown.

It is like the FMC was calculating "its" way to pass at 700ft and above 1700ft  to respect the altitude restrictions within the SID: in this case the runway is quite long and I took off pretty soon and I was able to manually flying passing at the two restrictions without problems...but if the FMC calculates the distance between the rwy and the first wpt from the end of the rwy, in that case, to be at 700ft would have required a steep angle and, for this reason, the FMC has calculated the 360 turn.

is this  a possible reason?

 

[Johannvr 3]

I think the FMC SID navdata might be suspect. I checked Navigraph's PMDG SIDSTAR Navdata (AIRAC 1809) for EBBR SID DENUT6C off rwy25R and this line: "...TRK 244 UNTIL 700 TRK 297 INTERCEPT RADIAL 278 TO FIX D278J..." does not make sense. The "Intercept Radial 278" part does not relate to any fix, there should be a " from fix BUB" I think and the "to fix D278J" could further confuse the FMC. The part to intercept R278 BUB VOR, as part of the SID, can be substituted by the line "to fix D278J" in the FMC Navdata and no need for the "intercept radial" part. I suggest contact Navigraph and ask them to look into this.

I just notified Navigraph on their Support Forum and wait their response.

Edited September 3, 2018 by Johannvr

[Marenostrum 4]

19 minutes ago, Johannvr said:

I think the FMC SID navdata might be suspect. I checked Navigraph's PMDG SIDSTAR Navdata (AIRAC 1809) for EBBR SID DENUT6C off rwy25R and this line: "...TRK 244 UNTIL 700 TRK 297 INTERCEPT RADIAL 278 TO FIX D278J..." does not make sense. The "Intercept Radial 278" part does not relate to any fix, there should be a " from fix BUB" I think and the "to fix D278J" could further confuse the FMC. The part to intercept R278 BUB VOR, as part of the SID, can be substituted by the line "to fix D278J" in the FMC Navdata and no need for the "intercept radial" part. I suggest contact Navigraph and ask them to look into this.

I just notified Navigraph on their Support Forum and wait their response.

Interesting consideration. Let us know their answer.

thank you in advance.

[downscc 1,327]

1 hour ago, Johannvr said:

TRK 244 UNTIL 700 TRK 297 INTERCEPT RADIAL 278 TO FIX D278J

This is perfectly alright in the syntax used by PMDG.  The 278 radial is to fix D278J, as a pilot you expect all radials to be from but in this language it can be either from or to.  This is the same as "intercept radial 098 from fix D278J,"  both work the same.

Yes, this is not a real world thing.  I expected the OP to advise if the ND rendering corrected itself once the airplane was airborne.

[andreh 125]

I would fly the SID as is. Seems like a fun ride! Combine with Max rate in the climb page! 🙂

 

[MarkJHarris 128]

Can you post the climb page word for word from the FMC screen please.

 I don't have access to my simulator at the moment but as I fly the real ones means I can take a look and see if I can spot an error. It should stand out quite easily if there is one. I'll look at my SID info on our LIDO Chart App on the company iPad and compare that to the Navigraph chart when I get a chance.

 

[MarkJHarris 128]

Ok. I’ve now taken a quick look. Basically the SID restrictions are all about noise abatement, so typically, operating on this SID, crews would run their performance for the runway, then set an acceleration Altitude of Aerodrome plus 3,000ft, or 3,200 as here, with a climb thrust reduction at 1,700ft. 

Remember that noise is irrelevant to an engine out case, so the 737 has no climb restriction issues as you said.

often, the FMC will put a spurious turn or circle pattern in if it cannot decide if it can make a restriction. Your FMC coding probably has an initial fix with the right side as A700. This enables the LNAV to track from 400ft as it is within 5’ without issue. 

 

I’ve often found Missed Approach paths to show erroneous tracking if it is a restricted turn after an altitude when you are on approach. For example, if the misuse£ approach is ahead to 2,000ft the turn right, you may see the right turn at the threshold as you start the approach, but as you descend further it will move away progressively from you. It,s recalculating based on your speed and ability to climb.

 

clearly, parked on the ramp with zero speed, it can’t climb that well.

Never credit the aircraft with more intelligence than PDMG coders... it’s not that smart. You’d think it would see the FMC climb profile and know the speed, but even after weight and thrust inputs it mostly doesn’t, at least for a while. Some will correct once the FMC position updates on TOGA selection. Some do it sooner. 

In real life, a picture like you’ve shown wouldn’t surprise me. I’d fly it and see. Usually by the time you line up it changes. Hopefully. Welcome to the 737!