Newbie Question about FMC error "Unable ??? KTS at "waypoint

[wheels1911 1]

A quick question.

I am wondering why I always seem to get this error message? It seems like every fight I do I have at least 1 waypoint that I get "Unable 280 KTS AT (Waypoint)"

Im curious if this is my mistake or is the FMC just isn't correctly inputting speed restrictions for my flight.

Ill put a picture of the Legs page if for further information

I dont understand how it goes from "Crowy" doing speed 253 KTS and next waypoint is 280 KTS.....Thats a 27 KTS difference... Why would that automatically be inputted? 

 

https://postimg.cc/0rFM8CMJ

https://postimg.cc/JHkhy9DH

Thank You

Edited November 16, 2018 by wheels1911

[JRBarrett 3,049]

It appears that you are in a descent on the IRNMN2 arrival into KLAX, which has a required speed of 280 KIAS at MUPTT.

But - looking at the legs page, you appear to be descending on a Mach 0.73/253 knot profile, and that is where the problem lies. The waypoint CROWY, which comes before MUPTT has you crossing at 253 knots, as does the waypoint MDOTS which comes after MUPTT.

VNAV is calculating your descent path based on maintaining a constant airspeed. Because your programmed descent speed is only 253 knots, the FMS won’t allow you to speed up to 280 to cross MUPTT only to slow right back down to 253 again at MDOTS

One way to fix this would be to go into the DES page and manually enter that you want to descend at 280 knots (once below below the  Mach transition altitude). Then the calculations in the FMS descent path will all reference 280 knots at each waypoint, and there will be no error message.

Perhaps you are using an extremely low cost index to have ended up with a calculated 253 knot descent?

In general, if a STAR contains constraints requiring you to cross specific waypoints at a specific IAS, you need to insure that your programmed descent airspeed is at least as high as the waypoint constraint airspeed. 

[wheels1911 1]

3 minutes ago, JRBarrett said:

Perhaps you are using an extremely low cost index to have ended up with a calculated 253 knot descent?

yeah i believe sim brief gave me a Cost Index of 13 on this flight.   Anyway thanks for the replay Ill test it out. See if I can figure it all out. Thanks much!

[BlueStar 323]

You can always do SPD INTV. Another option is to go to the DEScent page and put 290 in the Target Speed.  

One of my real world practices is to make sure my descent speed is at least as fast as my first crossing restriction speed.  In the PMDG B737 I use .78/320.  I always set the bottom of the altitude restriction in the ALT window and the crossing speed using SPD INTV. In your example I would have had 280 in SPD INTV and 23000 in the Altitude window for MUPTT.  Once I am below 10,000 msl I use FLT CHG exclusively.  

I'm not very trusting of the magic and besides if I bust a speed or altitude restriction, it will not be the "magic" standing in front of the CPs desk or answering a "Love" letter from the FAA. 🤣

Grace and Peace, 

[wheels1911 1]

13 minutes ago, Bluestar said:

Once I am below 10,000 msl I use FLT CHG exclusively.  

So you never follow the FMC really in decent? just kind of use it as a guide line I guess? You still use LNAV though right?

Once ALT INTV is pressed VNAV disengages correct? or I may be wrong.. Actually I believe it just changes your current FMC data right? VNAV will still be active.

[FalconAF 213]

16 hours ago, wheels1911 said:

yeah i believe sim brief gave me a Cost Index of 13 on this flight.

SimBrief will do that depending on some different factors you input about the flight.  Like most times if I use a SWA flight designator (for Southwest Airlines) I get a really low Cost Index used for the flight plan.  It's not unusual to get a single-digit one that will make my descent speeds much lower than many, many STAR crossing speed restrictions.  You can see them in the FMC waypoints list when you verify your flight plan using the "Step" sequence after entering the flightplan in the FMC.  That is when you can initially make the adjustments recommended above in previous posts to make sure you have a descent profile that fits the crossing restrictions for STAR arrivals in the plan.

I think SimBrief has been designed to determine Cost Index values based on the airline designator used.  I might be wrong about that though.  In any case, Southwest flight plans consistently get lower Cost Index values when I use SimBrief, because in real life, Southwest does use lower Cost Index values.  That works for the climb and cruise portions of the flights, but the pilot(s) would still have to adjust the descent profiles to comply with STAR crossing restriction speeds if needed.

Edited November 16, 2018 by FalconAF

[DaveCT2003 1,820]

By the way, there are quite a few such "unable" messages that you might see.  FCOMv2 has a list of all them, and the one for the message you received can be found on page 1128.  Anyway, it's a great reference!  I found them by searching the manual for "Unable", and one can search all the PMDG documentation in Adobe Reader by selecting Shift-Control-F.

THIS IS NOT A RTFM reply, just passing along some helpful info.

 

Best wishes for happy flights!

 

 

 

[wheels1911 1]

4 hours ago, FalconAF said:

You can see them in the FMC waypoints list when you verify your flight plan using the "Step" sequence after entering the flightplan in the FMC.  That is when you can initially make the adjustments recommended above in previous posts to make sure you have a descent profile that fits the crossing restrictions for STAR arrivals in the plan.

Thank you again. Yes so I understand how to do this in steps. That part is pretty easy. However Lets say that I don't put in my landing information until im about 50 miles for T/D right. While in flight ATC gives me my clearance information for what runway to expect. So I go to input my STAR. Once I press EXCT how do I edit them considering there are already executed.

 

20 hours ago, JRBarrett said:

One way to fix this would be to go into the DES page and manually enter that you want to descend at 280 knots (once below below the  Mach transition altitude).

I tried this by pressing DES page in FMC and I got very confused. how would you input the information? "Like this

280/23000"?     How does it know what way point I want to adjust?

[DaveCT2003 1,820]

18 minutes ago, wheels1911 said:

Thank you again. Yes so I understand how to do this in steps. That part is pretty easy. However Lets say that I don't put in my landing information until im about 50 miles for T/D right. While in flight ATC gives me my clearance information for what runway to expect. So I go to input my STAR. Once I press EXCT how do I edit them considering there are already executed.

 

I tried this by pressing DES page in FMC and I got very confused. how would you input the information? "Like this

280/23000"?     How does it know what way point I want to adjust? 

 

You insert that next to the waypoint using the Right Line Select Key (LSK).

 

Edited November 16, 2018 by DaveCT2003

[wheels1911 1]

1 minute ago, DaveCT2003 said:

You insert that next to the waypoint using the Right Line Select Key (LSK).

In the FMC "DES" page?

[DaveCT2003 1,820]

9 minutes ago, wheels1911 said:

In the FMC "DES" page?

No sir, the Legs page, which is the only place you can add those.  Changing values in the Climb or Descent pages is specific to those stages of flight.

Remember, the Flight Management Computer (FMC) is a black box that sits in the aviation bay in most airliners... no way to operate it even if it was on the flight deck.  The CDU is the pilots interface to input, select and output information from the FMC. Why is this important?  Terminology is everything in our hobby, especially when it comes to searching the Internet or even PDF files so that you come up with the relevant data.  Flight sim has been around for well over 35 years so there is a TON of excellent information on the web.  Also, the aircraft we use mimic the real ones, so info for the real aircraft or the flight sim aircraft usually answers all questions and has the advantage of providing immediate, accurate answers rather than having to wait for them.  Also, we see a lot of incorrect info provided in the forums, so going to the documentation is usually the best thing we can do!

Best wishes my friend!

EDIT:  There are also a ton of EXCELLENT tutorial videos on Youtube for flight sim products.

 

Edited November 16, 2018 by DaveCT2003

[wheels1911 1]

4 minutes ago, DaveCT2003 said:

No sir, the Legs page, which is the only place you can add those.  Changing values in the Climb or Descent pages is specific to those stages of flight.

Sorry I apologize I'm learning, new to all this. So anyway if I select the CDU "LEGS" Page in fight already with flight plan Executed at least for me it wont let me edit anything even if I Line select.

For Example If I Line Select

LEGS PAGE: MUPTT (Waypoint) and type 280/23000 it says "Invalid Entry". So I don't understand how you edit pre executed Waypoints

[DaveCT2003 1,820]

35 minutes ago, wheels1911 said:

Sorry I apologize I'm learning, new to all this. So anyway if I select the CDU "LEGS" Page in fight already with flight plan Executed at least for me it wont let me edit anything even if I Line select.

For Example If I Line Select

LEGS PAGE: MUPTT (Waypoint) and type 280/23000 it says "Invalid Entry". So I don't understand how you edit pre executed Waypoints

No worries!  That's why I went into detail my friend!  Always happy to help.

Interesting.  Well, you should be able to do that, I'm not sure why you're not able to.  You are definitely in the Legs Page, entering the speed/altitude in the scratch pad and then using the RIGHT line select key?  No reason at all that shouldn't work for you.

 

 

[DaveCT2003 1,820]

Hey, just a thought... have you flown the tutorial flights yet?

That will help you learn VERY quickly my friend.

 

[JRBarrett 3,049]

1 hour ago, wheels1911 said:

Thank you again. Yes so I understand how to do this in steps. That part is pretty easy. However Lets say that I don't put in my landing information until im about 50 miles for T/D right. While in flight ATC gives me my clearance information for what runway to expect. So I go to input my STAR. Once I press EXCT how do I edit them considering there are already executed.

 

I tried this by pressing DES page in FMC and I got very confused. how would you input the information? "Like this

280/23000"?     How does it know what way point I want to adjust?

On the DES page, second entry on the left side will be “TGT SPEED”. Based on the image you provided in your original post, this would probably have read “.733/253”, meaning that the FMS has calculated a descent profile holding Mach 0.733 initially, then switching to hold a speed of 253 knots once below about 29,000 feet. This is where the gray speed entries next to each waypoint on your LEGS page come from.

The FMS would have chosen that Mach/IAS combo for your descent based on your cost index.

If you enter .733/280 on the scratchpad, and overwrite the existing entry next to TGT SPEED, it will change the speed targets next to each waypoint on your LEGS page. Waypoint MUPTT already had a speed constraint of 280 knots. It is shown in magenta because that constraint, at that waypoint, is built into the STAR you have selected to fly.

If you change the TGT SPEED on the DES page, it will replace “253” with “280” next to the waypoints that come before MUPTT, which will prevent the “unable” error associated with MUPTT. You would probably want to make this change while still in cruise before reaching the calculated top-of-descent point.

Or, as Dave said, you can edit the ispeed/altitude constraints for individual waypoints directly on the LEGS page. You will not be able to change the 280 knot constraint at MUPTT in the LEGS page because that speed is encoded into the STAR procedure in the Nav database. Unless ATC clears you for a different speed, you must cross MUPTT at 280 knots.

Or, as Bluestar suggested, you could use Speed INTV on the flight guidance panel once in descent, and dial in 280 knots. This will tell the autopilot to ignore the 253 knot FMS-calculated speeds at the waypoints before MUPTT, and instead fly your VNAV descent at 280 knots. This will probably result in a change in engine power, and rate-of-descent to keep you on a VNAV path based on 280 knots.

In general, before beginning a descent, you should look at the chart for your planned STAR to see if there are any specific speed constraints for upcoming waypoints. If there are, you would want to check the TGT SPEED entry on the DES page insure that the FMS-calculated descent speed is at least as high as the highest speed constraint in the STAR. If all upcoming speed restrictions are at (or lower than) your calculated descent speed that’s fine. In that case the FMS will automatically calculate deceleration points in your descent path to insure you slow down in time to meet the speed constraints.

But, the FMS will not allow a situation where your speed has to increase during descent over the calculated descent speed. That’s why you had the “Unable 280 KTS” error.

As the pilot, you can always use Speed INTV on the autopilot to change your descent speed.