FMC says Descent rate not Achievable

[DBCorkran75 0]

Hello, was just wondering if others are receiving this message?  So the plane does not follow the proper descent and I have to react quickly to get back on course.  Any one else having this problem... and does this really happen in real life and what is anyone doing about it?  I mean sometimes the plane just needs to drop a thousand feet in 8 nautical miles and then the FMC displays the message and stops descending and then I am having to descend very step to get back on course.  Understand the problem? 

[maple-syrup 40]

It means exactly what it says, it must not be able to descend at a safe rate to make it to the right altitude at the next waypoint. The fix for this is change the next waypoints altitude lock. Or simply add a way point between the two.

[Mickel 42]

Or you can chuck out the brakes, which would lower the nose to maintain airspeed, assuming idle throttles. That will increase your rate of descent. Possibly too late at this point.

 

Or you could hit level change and increase your airspeed (and throw out some drag). That increases the rate of descent.

 

If the last waypoint had a between X & Y altitude and the aeroplane hit the top end of the range, that may have left you too high for the subsequent one.

 

If worst comes to worst, two clicks with the virtual left thumb, two clicks with the virtual right thumb and the plane is yours. Don't rely on the FMC and AP to be faultless.

 

Mike

[WhiteKnuckle 11]

I often get this warning.  Engage speed brakes and engage VS (disengaging VNAV).  Basically you have to slow the aircraft down and the situation (glide path) will stabilise and you can engage VNAV again at a lower altitude.

[Guest]

The remedies seem to be fair but which are the causes of this message considering the warning comes straight from the same device which generate the plan itself ? 

 

If fixes are easy to apply why the FMC doesn't apply them by itself ? I think only a sudden weather ( wind ) condition change can justify that.

 

 I think no dangerous corrections have to be made.

[Spin737 102]

Hello, was just wondering if others are receiving this message?  So the plane does not follow the proper descent and I have to react quickly to get back on course.  Any one else having this problem... and does this really happen in real life and what is anyone doing about it?  I mean sometimes the plane just needs to drop a thousand feet in 8 nautical miles and then the FMC displays the message and stops descending and then I am having to descend very step to get back on course.  Understand the problem? 

What does the FMC actually say? I don't think DESCENT RATE NO ACHIEVABLE is an FMC message.

 

Is it UNABLE NEXT ALT?

 

Where are you in your descent? Are you just starting down? Are you on approach? What is your descent speed (Cost Index)?

 

Some STARS/APP require you to configure early. There's one in to Osaka Kansai that you need to configure a long way out to make a few alt restrictions.

 

Or maybe you need to go faster. If your plane is planning on gliding down at 256kts, it has a fairly low rate of descent. 280 or 290 will go down steeper.

 

And, all this can be done in VNAV. You can simply use SPD INT and spin the speed up (tactical) or change the descent speed in the FMC (strategic).

[scandinavian13 5,192]

 

 

Any one else having this problem... and does this really happen in real life and what is anyone doing about it?

 

First, it's not a "problem" in the sense that it's a problem with the software.  The plane's automatics are just objecting to the current plan of action.

 

Remember that the FMC only has the information that you give it, and that information may actually conflict with what it's taught to do.  As an example, depending on your CI, your VNAV ECON DESCent speed may actually be rather low.  This requires a shallower descent that a higher speed descent would.  If the STAR you're flying requires a higher descent rate (as many often do, notably in the Southern California area), then you will need to override that DESCent speed on the VNAV DESCent page, or by using speed intervention.

 

In some other cases, as with the LENDY STAR into JFK, the FMC is assuming too much.  If you're not familiar with the STAR, it has a restriction of FL190 only 23 nm away from the airport (in a jet, it's assumed that you'd need about 60 nm to descend from FL190 to approach).  Since the last fix does not have some logical flow to get you down to the airport that's so close, the FMC isn't able to figure out a logical descent path (it's too steep, and, unless you've selected an approach, there may not be other lower altitude information).  In the case of the LENDY STAR, you're more likely to see a NO DES PATH AFTER [FIX] message, but the concept is similar: the automation can't find a logical solution to the picture it's seeing.

 

It's assuming you're going direct from that last fix to the airport - FL190 to airport elevation in 23 nm - and based on its calculations, it can't do that.  *DING* I CAN'T DO WHAT YOU'RE ASKING ME TO.

 

Again, it's not a "problem," as much as it's a prompt for you to give it more information, change the plan, or change AP modes from LNAV/VNAV to something more conducive to the situation.  As an example, the LENDY issue I mentioned earlier is only an issue to the FMC because of the extensive amount of vectoring.  The FMC doesn't know that the controller actually vectors the aircraft another 60 nm.  In order to accommodate the vectors, you won't be using LNAV or VNAV anyway, so in cases like these, you just ignore the messages and utilize HDG SEL and FL CH or V/S.

 

Always remember:

Your brain understands the situation a lot better than the FMC.  Granted, the FMC is a lot faster than you at crunching numbers, but if it doesn't have the right numbers for the situation, that number crunching isn't too helpful.

 

"Solutions" for the DESC PATH UNACHIEVABLE message:

• If you're flying an RNAV STAR with altitude constraints ahead of you:
  Increase the descent speed through either speed intervention, or the selector on the VNAV DESC page - monitor the vertical path closely to ensure compliance with altitudes.
• If you're flying a STAR with no altitude constraints ahead of you:
  Increase the descent speed through either speed intervention, or the selector on the VNAV DESC page
• If you're approaching the end of a STAR and the message appears (or you get the NO DES PATH AFTER [FIX] message):
  Change to FL CH and adjust the speed in the window to achieve an appropriate rate of descent

If you're curious as to how different the picture is between the LENDY and the actual vectored path, have a look at the chart, and compare that direct line (what the FMC is assuming exists between the LGA VOR and JFK) with this (click on any of the options under "JFK" - the aircraft coming over TEB are on the LENDY).

 

 

 

DESCENT RATE NOT ACHIEVABLE

 

I think he's referring to "DES PATH UNACHIEVABLE."

[DBCorkran75 0]

Thank you to all the responses.... by droping the cost Index from 80 down 30 had me descending at 272 instead of 317 and when the NVAV comp could not descend, 

using LVL change working just fine.  So thanks again for all responses... 

[Andrea1 6]

Hi,

as Kyle as well explained YOU are the Captain and have to find the best strategy for a correct descent path and you MUST say to your autopilot/FMC how to descend according to that strategy...thiking ahead of the plane being the most important thing in all phases of flight but especially during descent when it's necessary to close monitoring the automatics to prevent problems and/or screwing the approach.

Sometimes it's simply question to insert a "soft constraint" (i.e. 210/4000A) insted of an hard one (210/4000) to get that message cancelled some other times it's necessary applying speedbrake, speeding up a little, early lowerint the gear etc. etc.

I remember a session in that B738 fixed based simulator I'm attending: after my flight another guy asked me to spend another hour with him acting as training captain (I was in the right seat acting as PNF and training captain) to help him (unexperienced simpilot) to improve his technique etc..he had planned a LIRF-LIMJ flight but I cannot remember the best route (neither him) and approach charts books weren't in the library where I was used to find them (the proprietor was gone out for a while)..so we planned a route arriving directly to the IAF (not exactly a good idea)..during the descent phase the proprietor arrived and passed me an ipad containing the charts database, while I was eyes down to check the charts looking at ipad screen I gave to my fellow the task to "check the descent vs distance and advising me if he had saw something wrong)...but, while I was looking at the computer screens every now and then I was glancing at ND screen and altimeter and I had the sensation to be "high on approach" so I was mantaining 240 KTS (at 20-15 miles from touchdown) and applying speed brakes, when I returned on the gauges I had received confirmation we were a little high...(so I have thought two strategy: lowering our gear or building an holding) so I called "gear down" and my fellow "argh..but we're at 240 knots!..and I "perfect: 30 knots lower than the max landing gear extension speed..I want our gear down NOW" and I lowered the gear...we arrived perfectly at the "rendez-vous" with the glide path and my colleague executed a good landing on rrwy 29...

Ciao

Andrea Buono