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Gregg_Seipp

Weirdness during descent

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Hi,

 

I'm new to the NGX so I'm still in the process of making mistakes and learning by trial and error.  Here's a situation I ran into.  Not sure what's related to what but it was in the situation.

 

I'm flying fix to fix...no airways.  I have a perfect flight as I reach TC.  There are a couple of fixes that are out of the way on up ahead and I'm my own ATC so I go into LEGS and remove them by line selecting a later fix and replacing the first of the two fixes with that one.  Looks good.  I execute it.  About 30 seconds later I get an error message on the FMC that says "No descent path after X" which was the fix before the modification.  (What does that error mean?)  Ok, so I keep flying.  I'm getting close to TD so I dial in 13000 feet, select DES and hit DESCEND NOW.  I need to be at 13,000 when I cross a fix up ahead.  The restriction is in the FMC.  I start descent and the airplane crosses the fix right at 13,000.  Perfect.  Then it gets weird.  My next restriction is a simple at or above 3400 at the IAF.  It's a ways off but I'm thinking to get started down early.  The fix that had the error is between where I am and the IAF.  I turn the dial to 3400 and click ALT INV.  No effect.  I'm hanging at 13,000.  I click it again.  Nothing.  I click ALT HOLD.  The airplane starts climbing...climbing fast.  My airspeed is falling (I'm at idle)...220 down, down to 150...falling.  (I hate computers.)  I put my thumb on my yoke and push it to nose it over.  I press V/S and dial -2000 into  the window.  I take my thumb off but I'm still hanging between 17,000-18,000.  I'm about to shut off the autopilot and just fly it but I reach over and press Speed and dial in 200.  Finally!...it starts down.  After I'm sure things are okay I look back down at my LEGS and recheck...no altitude restrictions except the IAF, IF, and FAF...all perfect.  I sit back, looking at the screen.  There's no way I'm going to make it down so I just shut the sim down.  Time to ask some questions.  I'm hoping someone can shed some light on what happened here.

 

One other question:  If you depart flying by hand with the autothrottle engaged, does the autothrottle respect the 250 KIAS restriction below 10,000 or do you have to manage the throttle yourself? 

 

Thanks in advance.


Gregg Seipp

"A good landing is when you can walk away from the airplane.  A great landing is when you can reuse it."
i7-8700 32GB Ram, GTX-1070 8 Gig RAM

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Hi,

 

I'm new to the NGX so I'm still in the process of making mistakes and learning by trial and error.  Here's a situation I ran into.  Not sure what's related to what but it was in the situation.

 

I'm flying fix to fix...no airways.  I have a perfect flight as I reach TC.  There are a couple of fixes that are out of the way on up ahead and I'm my own ATC so I go into LEGS and remove them by line selecting a later fix and replacing the first of the two fixes with that one.  Looks good.  I execute it.  About 30 seconds later I get an error message on the FMC that says "No descent path after X" which was the fix before the modification.  (What does that error mean?)  Ok, so I keep flying.  I'm getting close to TD so I dial in 13000 feet, select DES and hit DESCEND NOW.  I need to be at 13,000 when I cross a fix up ahead.  The restriction is in the FMC.  I start descent and the airplane crosses the fix right at 13,000.  Perfect.  Then it gets weird.  My next restriction is a simple at or above 3400 at the IAF.  It's a ways off but I'm thinking to get started down early.  The fix that had the error is between where I am and the IAF.  I turn the dial to 3400 and click ALT INV.  No effect.  I'm hanging at 13,000.  I click it again.  Nothing.  I click ALT HOLD.  The airplane starts climbing...climbing fast.  My airspeed is falling (I'm at idle)...220 down, down to 150...falling.  (I hate computers.)  I put my thumb on my yoke and push it to nose it over.  I press V/S and dial -2000 into  the window.  I take my thumb off but I'm still hanging between 17,000-18,000.  I'm about to shut off the autopilot and just fly it but I reach over and press Speed and dial in 200.  Finally!...it starts down.  After I'm sure things are okay I look back down at my LEGS and recheck...no altitude restrictions except the IAF, IF, and FAF...all perfect.  I sit back, looking at the screen.  There's no way I'm going to make it down so I just shut the sim down.  Time to ask some questions.  I'm hoping someone can shed some light on what happened here.

 

I often edit my routes, removing, adding waypoints etc as I go and have never experienced anything like this, sorry without pictures or video I am at a loss as to what was going on.

 

 

One other question: If you depart flying by hand with the autothrottle engaged, does the autothrottle respect the 250 KIAS restriction below 10,000 or do you have to manage the throttle yourself?

 

Thanks in advance.

 

 

On climb the autothrottle will hold CLB power (or whatever N1 power setting you have selected) unless you reach a restriction or MCP altitude. It is your job to adjust your pitch to hold the speed at 250.

 

So, if flying by hand with autothrottle it is up to you to maintain the speed you wish with pitch. If you're going faster than 250/10000 then pitch up to bring the speed down etc. If you set the modes in the MCP you can follow the flight director to make it easier. 


Jay Vorkapic

 

pmdg_trijet.jpg

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On climb the autothrottle will hold CLB power (or whatever N1 power setting you have selected) unless you reach a restriction or MCP altitude. It is your job to adjust your pitch to hold the speed at 250.

 

Ah...so, ATC gave me a hold down on departure that wasn't in my FMC...it was in my MCP.  So...I'd have to throttle back then.  Yes?

 

I only mentioned the editing of the route because of the errror (don't know what it means) and, perhaps, it was related.


Gregg Seipp

"A good landing is when you can walk away from the airplane.  A great landing is when you can reuse it."
i7-8700 32GB Ram, GTX-1070 8 Gig RAM

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Hi.

 

"No descent path after FIX" means the FMC is unable to calculate a path descent given the current constraints. And example would be... You give it a constraint at point X to cross at 13,000ft, then give it cross at 3,000ft the next waypoint and that waypoint is only 20nm from the first. The aircraft wants to descend at roughly a 3/1 rule... That is 3nm per 1000ft (or 30nm per 10,000ft).

 

I don't know your situation but if your 3,400ft constraint was less than say 27nm fom your 13,000ft constraint, you would get this error. I see now that your next constraint was 3400A so more likely the constraint that as triggering your error was the runway.

 

FWIW, there's actually no need to use Desc Now in normal circumstances. Of you correctly program your descent and arrival and select a lower alt in the MCP before TOD, the aircraft will begin descent automatically.

 

For your second question, the normal procedure would be:

 

- Climb at v2+10-20kts until accel height (usually 1000-1500ft AFE)

-When CLB thrust is indicated in the EICAS(you might even hear the engines throttle back), set flaps up speed in the MCP and retract flaps on schedule.

-When flaps are up you have some option but the easiest would be select VNAV and this will allow the FMC to control the auto throttle which will also respect your 250/10000 speed restriction.


Greg Barber
VeeOz Virtual
VATPAC

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"No descent path after FIX" means the FMC is unable to calculate a path descent given the current constraints. And example would be... You give it a constraint at point X to cross at 13,000ft, then give it cross at 3,000ft the next waypoint and that waypoint is only 20nm from the first. The aircraft wants to descend at roughly a 3/1 rule... That is 3nm per 1000ft (or 30nm per 10,000ft).

 

I don't know your situation but if your 3,400ft constraint was less than say 27nm fom your 13,000ft constraint, you would get this error. I see now that your next constraint was 3400A so more likely the constraint that as triggering your error was the runway.

 

That makes sense.

 

 

FWIW, there's actually no need to use Desc Now in normal circumstances. Of you correctly program your descent and arrival and select a lower alt in the MCP before TOD, the aircraft will begin descent automatically.

 

I was just practicing the descent method in Tutorial 2...no particular reason other than that.  I'm going to load up the flight again today.  I was scratching my head and asking myself, why was the airplane overriding my desire to descend...I mean, no matter what?  The answer to that has to be in the FMS...something I had in there.  The next thought is, apart from shutting off the autopilot (a guaranteed solution), what would I do if I *had* to override what the FMS was doing?  For example, I can always press Heading Sel and override the FMS heading.  What if ATC had assigned me 3400 or 5000?  There has to be a way to override and specifically tell the airplane that that is exactly what you want.

 

It's probably shows my ignorance but, where do you find the flaps up speed?  Is that what's indicated by the 'up' tag on the speed tape?  I'll admit that, on departure, I just stuck 240 in.


Gregg Seipp

"A good landing is when you can walk away from the airplane.  A great landing is when you can reuse it."
i7-8700 32GB Ram, GTX-1070 8 Gig RAM

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There are a couple of fixes that are out of the way on up ahead and I'm my own ATC so I go into LEGS and remove them by line selecting a later fix and replacing the first of the two fixes with that one.

 

Remember that your T/D is being calculated by the systems to include those "out of the way" legs.  Since you mentioned they were "out of the way," they probably increased the flown distance between your current position and the destination.

 

If you're close to your T/D and then start deleting legs that decrease flown distance, you're going to get that kind of message.  Essentially, the plane was expecting to be able to descend over 100nm (example value only), and by cutting out those legs, you brought it down to 75nm.  If it needed all of that 100nm, and you allowed it to descend earlier, there would've been no problem (e.g. you deleted those legs well prior to the T/D so that it could recalculate the T/D and likely move it back).

 

Always remember: trash in, trash out.

Your automation is only as good as the data you give it.  If you feed it data and let it assume you're doing that and then surprise it, it's going to surprise you back.

 

ATC Note:

You normally don't get too many directs on a STAR, mainly for the reasons you experienced.  There are some STARS that have flatter sections where you can get away with it, but there aren't many cases where you're going to get a significant direct to cut too much distance.


Kyle Rodgers

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Remember that your T/D is being calculated by the systems to include those "out of the way" legs.  Since you mentioned they were "out of the way," they probably increased the flown distance between your current position and the destination.

 

If you're close to your T/D and then start deleting legs that decrease flown distance, you're going to get that kind of message.  Essentially, the plane was expecting to be able to descend over 100nm (example value only), and by cutting out those legs, you brought it down to 75nm.  If it needed all of that 100nm, and you allowed it to descend earlier, there would've been no problem (e.g. you deleted those legs well prior to the T/D so that it could recalculate the T/D and likely move it back).

 

Always remember: trash in, trash out.

Your automation is only as good as the data you give it.  If you feed it data and let it assume you're doing that and then surprise it, it's going to surprise you back.

 

ATC Note:

You normally don't get too many directs on a STAR, mainly for the reasons you experienced.  There are some STARS that have flatter sections where you can get away with it, but there aren't many cases where you're going to get a significant direct to cut too much distance.

 

Yeah, when you're a noob, you tend to create a lot more 'garbage' and unexpected situations for the systems to handle.  I'm in that boat.  (My instructor is snoozing in the co-pilot's seat.)   The trick, of course, is to learn and figure out what information that's in front of you is important and prioritize.

 

This morning, wide a wake, I took a second look at my flight plan and did, in fact, find that I had an altitude restriction at that fix I was getting the error from (actually, in the STAR it was just an expect to cross that I entered as a restriction since "I'm ATC" so it wasn't an estimate like I thought.).  Doesn't explain the climb at all because it was a lower restriction...8000.  I did some tweaking and got rid of the error and flew the flight...same livery...same everything else.  This time it flew through 13000 like water.  Pretty much everything went well until about 3 mile final where there is an updraft on final (I know this airport well).  The airplane was trying to stay on final but the A/P light started flickering and it let go of the approach and I hand-flew it the rest of the way in.  So, as far as I can tell right now, the 'garbage' was that error and the effect was weird (and add to that the fact that I couldn't figure out how to override to make it descend).  Still have a lot to learn about this autopilot.  If anyone wants to share some best practices, I'm all ears.

 

BTW, in terms of the direct-to, this particular star (KAYOH4) serves several airports, KSNA (my destination) being one of them.  Not sure if they funnel everyone to SLI and fan them out or not but it goes right by SAGER, the IF for the approach, on the way.  I just mimicked what I usually get out of software.


Gregg Seipp

"A good landing is when you can walk away from the airplane.  A great landing is when you can reuse it."
i7-8700 32GB Ram, GTX-1070 8 Gig RAM

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BTW, in terms of the direct-to, this particular star (KAYOH4) serves several airports, KSNA (my destination) being one of them.  Not sure if they funnel everyone to SLI and fan them out or not but it goes right by SAGER, the IF for the approach, on the way.

 

Being your own ATC in the SoCal area is pretty rough.  The NY Area is even worse.  There are a lot of quirky things done in that area to get traffic around, over, or under other aircraft that don't show up on the charts (as they're controller instructions/vectors/altitudes).

 

Looking at the KAYOH4, it looks like you'd be getting a vector between JOGIT and SLI.  As to when that occurs, I couldn't tell you.  For what it's worth, I've found it very helpful to look to see if the STAR has LOST COMMS procedures and follow those.  The KAYOH4 is an "old nav" STAR, too.  I'd suggest using RNAV STARS where possible, as they're generally more explicit in what you're supposed to do, and the NG is quite capable of them.  Take a look at the KEFFR1 and its description page.  It's a lot more explicit about what you should be doing.

 

You're correct in setting the "EXPECT" as a restriction.  It's on the chart as the controller will give that to you 90% of the time, so it's prudent to ensure the FMC has the restriction set.

 

 

 

As far as the autopilot issues, it seems like your controller (yoke/stick) might be sending conflicting trim values.  Do you have a trim axis assigned?  Other than that, Tutorial 1 and 2 explain the different autopilot modes pretty well.


Kyle Rodgers

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<p>KEFER1 is a lot more explicit.  I flew KAYOH4 because that's what most flight plans RW seem to be following.  But I'll try KEFER. </p>
<p> </p>
<p>

<br />
Being your own ATC in the SoCal area is pretty rough. The NY Area is even worse. There are a lot of quirky things done in that area to get traffic around, over, or under other aircraft that don't show up on the charts (as they're controller instructions/vectors/altitudes).<br />

</p>
<p> </p>
<p>Yeah, that's what I've been thinking about in the back of my head and trying to sort out.  I'm trying to understand how to do some basic things with the about the autopilot.  There's a lot of stuff in the manual but it's all rote knowledge and not procedural.  So example:</p>
<p> </p>
<ul>
<li>ATC assigns a heading and altitude after departure.  You set V2+20 into the speeed dial, tune the altitude and heading, press VNAV but stay in ROL mode.   Once airborne you click HDG SEL and CMD1.  Seems to work.</li>
<li>Then they give you a turn and tell you to proceed on course.  You make the turn and wait until your course is fairly well aligned with your programmed course.  Press LNAV.  Seems good.</li>
<li>Now, if they ask you to reduce speed in your climb....what *seems* to make sense is that you press Speed.  Nope...nothing happens and the speed window remains blank.  Maybe, turn off VNAV and press speed and roll the speed down to the restriction.  Bigger nope.  I was doing about 230 and I dialed in 210.  This resulted in a 35 degree pitch up and speed going way, way down to 140.  No idea what to do in this situation.  Need some help on this one.</li>
</ul>
<p>I might go through tutorial 2 one more time (though I am *NOT* looking forward to going into that valley again).  In terms of my yoke, it doesn't have a problem with any other aircraft and right now I'd assume it's me.</p>
<p> </p>

 

No idea what the editor did to this post.


Gregg Seipp

"A good landing is when you can walk away from the airplane.  A great landing is when you can reuse it."
i7-8700 32GB Ram, GTX-1070 8 Gig RAM

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I flew KAYOH4 because that's what most flight plans RW seem to be following.

 

Yeah - I just checked a SWA flight into SNA and they seem to be using the KAYOH as well.  I couldn't tell you why, but from experience, sometimes airlines have some odd reasons for not using procedures.

 

Then again, facilities (ATC facilities) occasionally reject the procedure as well.  I know Potomac wouldn't accept/use the PRYME SID, so much to the point that the FAA finally dropped it.

 

Still, in the sim, I'd use RNAV STARs where available.


Kyle Rodgers

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Well, I redid the flight a few different ways and did not have another occurrence.  I spent some time studying the MCP and learning how it works (with a flight plan and performance data in the FMC) with an eye toward the kinds of things I'd expect ATC to do in a terminal and enroute environment.  I read some of the manual on the MCP...tough to absorb.  Here's what I found:

 

Assuming I have a flight plan and performance data in the FMC

  • If ATC issues an altitude - dial it into the MCP on the ground.  On departure press VNAV and either use the autopilot or the FD.  If you have a lower restriction programmed in be sure to press ALT INV to keep going up to the altitude assigned by ATC.  (The airplane will not climb above the altitude dialed into the altitude in the readout.  If you're in ALT HOLD it won't climb until you press ALT INV.)
  • If ATC issues a heading and altitude - follow the instructions for altitude above.  For heading, put the heading in the MCP and press HDG SEL on departure and use the autopilot or FD.
  • If ATC gives you a speed restriction when you're climbing hit the LVL CHG button and dial in the restricting speed.  If you're level hit ALT HLD and dial in the restricting speed.  (Not completely sure about the last sentence.)  If they raise the restriction then just dial in the new speed.  If they remove the restriction just hit VNAV.

Not sure if this is what everyone does or if there are better ways or rules of thumb but that's the best I figured out.  .


Gregg Seipp

"A good landing is when you can walk away from the airplane.  A great landing is when you can reuse it."
i7-8700 32GB Ram, GTX-1070 8 Gig RAM

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