Skypoint

Desent Path Unachievable

55 posts in this topic

Hey Gang,

 

I've flown this route several times with the PMDG 737 slippery 800 and less slippery 900 and have had the same problem of "descent path unachievable" message. The route is KSAN-KSFO FL320 PDRZ1 IKAYE RIZIN SPLNT SERFR2 RWY 28R. I've had this problem more often than not with other flight plans with the PMDG. The flight is good until about 10,000 in the STAR and I get the message we're too high. Fuel and payload is within range. By the time I get the message there's no way to descend manually that won't make a passenger puke. This is a reasonable flight plan with an established STAR. I understand the 737 FMC is delicate, can I expect this with most flights?

 

Thanks in advance for your help!

Jim

 

 

 

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It's likely not a problem of you're being too high, but rather too high and too fast under your current configuration OR the speeds and distances between two points.  The message is based on basic math, so all you have to do is change one or more parameters (speed and/or altitude).  It's pretty basic from that point forward.  Remember that IRL altitude and crossing restrictions are based on a number of different factors (other airways, flight paths, ground issues, etc.), and have controllers who will or may directively get us down or slower sooner than published.  You'll simply have to work this out if you're flying without human ATC.

I'm sure you can take it from there.  Best wishes,

 

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What is your descent speed? If it's 256, try making it faster, say 280.

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Not enough info...., landing 28R you should cross EPICK at 10000 280 KIAS... the VNAV might be getting you across between the altitude constraints of 10000A15000B so you might have to intervene.  The NGX simulation has occasional glitches in the VNAV path and it's not unusual to be right on vertical path then suddenly after crossing a fix you are a 1000 too high.  It's buggy and PMDG finally got it working well in the B744.  If you are suddenly too high the pitch mode reverts to VNAV SPD which means the pitch is controlled by speed so first thing to do is get all the speedbrakes out there and set up a stable steep decent.  Don't worry about pax, they are only aware of changes in vertical speed... a steady 4000 fpm decent feels the same as a1000 fpm one to them. Don't delay getting the drag out our you won't make 4000 at MENLO. To prevent this from happening you need to be ahead of the airplane and if it looks like it is going to cross EPICK too high then get rid of VNAV and switch to FLCH and use your thrust and drag to control the arrival.

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16 hours ago, Skypoint said:

I understand the 737 FMC is delicate, can I expect this with most flights?

Welcome to the forum, Jim. Sounds like you're new to the NGX and still getting to know the aircraft and how it handles. Most important to remember is that you, not the flight computer, are the pilot and that the A/P is not "set it and forget it" technology. Safe operation of the aircraft often requires human intervention and to intelligently intervene the pilot has to know the route, SID, STAR and Approach requirements and always anticipate the A/P's capacity to handle on its own the kinds of descent restrictions you flew into. More often than not it can, but you should never find yourself in a situation of expecting more from the aircraft's automation than it can deliver and be caught off-guard. There are more than enough things that can go wrong on their own during a flight, but a "Descent Path Unachievable" warning should not one of them. It's almost entirely avoidable.

In my early days flying the NGX I encountered many situations exactly like you describe and these were teachable moments. When I learned about the necessity of intimate familiarity with the entire route and advanced planning it substantially deepened my satisfaction in flying this beautiful bird. Other folks here have offered specific techniques for managing descent anomalies but my only point is a more general one that flying the aircraft may not be as automated as you expected. It's your job to be prepared for those points in the flight when it will be necessary to get your hands on the yoke or otherwise manually make happen whatever is needed to get your aircraft and the folks onboard safely to their destination. So, study, learn, plan, anticipate and have fun.

 

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Have you tried to enter ISA temp and winds in the desc. forecast page?

 

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12 hours ago, Spin737 said:

What is your descent speed? If it's 256, try making it faster, say 280.

I think it was about 260 so that probably had an effect. Thanks!

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3 hours ago, coa1117 said:

Welcome to the forum, Jim. Sounds like you're new to the NGX and still getting to know the aircraft and how it handles. Most important to remember is that you, not the flight computer, are the pilot and that the A/P is not "set it and forget it" technology. Safe operation of the aircraft often requires human intervention and to intelligently intervene the pilot has to know the route, SID, STAR and Approach requirements and always anticipate the A/P's capacity to handle on its own the kinds of descent restrictions you flew into. More often than not it can, but you should never find yourself in a situation of expecting more from the aircraft's automation than it can deliver and be caught off-guard. There are more than enough things that can go wrong on their own during a flight, but a "Descent Path Unachievable" warning should not one of them. It's almost entirely avoidable.

In my early days flying the NGX I encountered many situations exactly like you describe and these were teachable moments. When I learned about the necessity of intimate familiarity with the entire route and advanced planning it substantially deepened my satisfaction in flying this beautiful bird. Other folks here have offered specific techniques for managing descent anomalies but my only point is a more general one that flying the aircraft may not be as automated as you expected. It's your job to be prepared for those points in the flight when it will be necessary to get your hands on the yoke or otherwise manually make happen whatever is needed to get your aircraft and the folks onboard safely to their destination. So, study, learn, plan, anticipate and have fun.

 

You are correct, I'm just getting familiar with the PMDG 737 in FSX. I had the FS9 version but that was sometime ago. Learning the FSX software is very different than FS9 from what I remember. I appreciate your and everyone's tips and comments. I like the flight model will have to put in the time to learn the AC. Thanks again for taking the time to reply!

Jim

 

 

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17 hours ago, Skypoint said:

I think it was about 260 so that probably had an effect. Thanks!

A low CI will set a low descent speed, but you should check the speed constraints on the arrival you select.  Note this arrival has a constraint of 280.  It is true that the constraints are at or below speeds but generally on arrival you can expect ATC to control the flow by getting everyone on the same speed, which in this case will be 280.  I always put the highest speed constraint for an arrival in as my programmed descent speed, regardless of CI.

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I've seen this too when using STAR's in the PMDG 737.  I'm not trained as a commercial pilot (I consider myself to be an "intermediate simmer", so the actual details of how to get around it is more of a trial and error thing for me.  However, I've noticed that if I don't use STARs, and just set the arrival runway/transition in the CDU, I don't get those errors.  It sends you in at a pretty good path with no steep turns or anything, proper waypoints to line you up, etc.  Plus, I don't think STAR's are even used much IRL (don't quote me on that, just what I've heard in the past)

I gave up on STAR's.  Not saying you should, but still works and is a far less stressful decent and less technical

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This guy's vids are awesome. Here he talks about how to deal with some of the issues you might be seeing. 
 

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2 minutes ago, Nick Dobda said:

This guy's vids are awesome. Here he talks about how to deal with some of the issues you might be seeing. 

+1.  I was thinking about that particular video when I was reading this thread.

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On ‎8‎/‎29‎/‎2017 at 0:29 PM, blaird22 said:

I gave up on STAR's.  Not saying you should, but still works and is a far less stressful decent and less technical

Do you think the FAA created SIDS and STARS just for laughs? SIDS and STARS are part of almost all flight plans that appear on FlightAware. They should be used unless your are vectored by ATC off of the SID or STAR. Learn to use them!

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4 hours ago, pcubine said:

Do you think the FAA created SIDS and STARS just for laughs? SIDS and STARS are part of almost all flight plans that appear on FlightAware. They should be used unless your are vectored by ATC off of the SID or STAR. Learn to use them!

where  does  it  say  you have  to use   stars  and  sid  in the  sim, if  he  chooses  not  to use  them  than its  his  choice:biggrin:

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11 hours ago, pcubine said:

Do you think the FAA created SIDS and STARS just for laughs? SIDS and STARS are part of almost all flight plans that appear on FlightAware. They should be used unless your are vectored by ATC off of the SID or STAR. Learn to use them!

I was never implying that SIDS/STARS were created for comical value, I'm not sure where you got that from.  I have used and (somewhat) learned them.  I'm simply stating that they are not necessary and can cause confusion for casual simmers on certain routes (the reason for this post in the first place).  Many real-world approaches are vectored by that point so they're moot.  Also, if you select a runway and transition in the FMC, you get waypoints that line you up and have VNAV calculated properly.  

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