Skypoint

Desent Path Unachievable

55 posts in this topic

1 hour ago, downscc said:

It is indeed normal to deploy speedbrakes for minutes at a time.  Set the option ON to always show your throttle position, and in your descent give your throttle and little nudge up and down watching the N1 gauges.  You'll be able to tell if the aircraft throttle is close by comparing it to the blue indication of your controller position.  The only NGX I've noticed not always keeping to idle is the B736 but haven't really figured out why that happens.  279 is okay but usually there is a 280 constraint somewhere so I select that to avoid the annoying "unable 280 at" messages.  I usually use 280, although sometimes I will hear Houston using 310.  Just depends on location and controller.

I do have the ON option currently set.  I thought the PMDG 737 NGX had issues with the throttle at idle? Hence why some people recommended pressing F1 to ensure it is at complete idle?

So, is this the process I should be following?

Upon descent, throttle back joystick controller to idle.

Under 30k, hit F1.

Monitor speed at 280 / flight path, using speedbrake as needed.

Under 10K, should I switch to FLCH with a speed of 250? Or should I just follow the path still? I was having trouble slowing down quick enough for the flap increments when coming up on the final leg. Once again, riding the speedbrakes were my friend.

Oh, and thanks for your patience.

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Robert:  NGX throttles work just as expected, any discussion of going to idle problems is usually associated with the landing flare and throttle vs reverse thrust.  Flight idle during descent should not be a problem. No need to hit F1, just bump your throttle controller (open slightly then close) to make sure the closed throttle lines up with the displayed N1.

I advise against a blanket policy of switching pitch modes at 10000.  It depends.  The NGX FMS is very powerful, and will control speed for you all the way to touchdown if you set up the speeds for it.  For example, after setting approach Vref the LEGs segment for the landing will show Vref+5 and I copy that to the FAF waypoint.  The FMS will automatically adjust speeds such that you cross FAF at that speed if you set flaps on a schedule that allow the FMS to retain speed control.  Even if it drops out of FMS and into MCP speed control it's just a matter of you being the pilot and setting the appropriate speeds.  Normally there is a transition between the 240 kts you get below 10000 and F5 speeds so plan on slowing down before you need flaps for example a few miles before intercepting final course get the speed down to 210 then 180 or whatever.  Don't stay at 240 until final.

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23 minutes ago, downscc said:

Robert:  NGX throttles work just as expected, any discussion of going to idle problems is usually associated with the landing flare and throttle vs reverse thrust.  Flight idle during descent should not be a problem. No need to hit F1, just bump your throttle controller (open slightly then close) to make sure the closed throttle lines up with the displayed N1.

I advise against a blanket policy of switching pitch modes at 10000.  It depends.  The NGX FMS is very powerful, and will control speed for you all the way to touchdown if you set up the speeds for it.  For example, after setting approach Vref the LEGs segment for the landing will show Vref+5 and I copy that to the FAF waypoint.  The FMS will automatically adjust speeds such that you cross FAF at that speed if you set flaps on a schedule that allow the FMS to retain speed control.  Even if it drops out of FMS and into MCP speed control it's just a matter of you being the pilot and setting the appropriate speeds.  Normally there is a transition between the 240 kts you get below 10000 and F5 speeds so plan on slowing down before you need flaps for example a few miles before intercepting final course get the speed down to 210 then 180 or whatever.  Don't stay at 240 until final.

I will try copying the Vref +5 speed over to the FAF waypoint. I haven't been doing that.

Do you have trouble reducing the speed to get to the next flap level? Or do you use the speed brakes for that as well?

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1 hour ago, 787WannabePilot said:

Do you have trouble reducing the speed to get to the next flap level? Or do you use the speed brakes for that as well?

Not normally, and it would probably call for gear down rather than speedbrakes.  I'm not sure what you are using for a flap schedule, and it varies but I normally have F5 when within 3 nm of final course, gear dn when GS is half scale, F15 at GS intercept, and final F30 about 1800 AGL. There are ways to keep the speed up or to slow it down as required but it helps when you are learning to keep it simple.

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15 minutes ago, downscc said:

Not normally, and it would probably call for gear down rather than speedbrakes.  I'm not sure what you are using for a flap schedule, and it varies but I normally have F5 when within 3 nm of final course, gear dn when GS is half scale, F15 at GS intercept, and final F30 about 1800 AGL. There are ways to keep the speed up or to slow it down as required but it helps when you are learning to keep it simple.

Yeah, I am coming in way to quick. I hope to have F5 by the time I turn into the final course, but sometimes I am turning as I reach it.


Sounds like I need to be a bit more hard on the speedbrakes.

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1 hour ago, 787WannabePilot said:

Sounds like I need to be a bit more hard on the speedbrakes.

You simply need to get ahead of the airplane.  It will come in time.

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2 hours ago, 787WannabePilot said:

Sounds like I need to be a bit more hard on the speedbrakes.

The other option is to start descending sooner!

Descent management is often seen as a dark art, but it's really just physics: it's all about energy management. Rather like a rollercoaster, the objective is to get from flying at high speed and high altitude (where you have oodles of both kinetic and gravitational potential energy) to stopped on the runway (where you have none of either). This means that you need to convert all of that energy you had at top of descent in to other forms.

The engines convert the chemical energy stored in the fuel in to kinetic energy. Thus when you initiate the descent, you close the thrust levers in order to stop pumping more energy in to the system (obviously a jet engine at idle will generally still produce a small amount of thrust, but it's negligible for the purposes of this discussion). You can then lower the nose to maintain airspeed: by doing so you are now converting some of your gravitational potential energy (from your height above the Earth) in to kinetic energy (to 'replace' the energy you were putting in with the engines before).Meanwhile, some of that energy is being transferred to the surrounding air in the form of sound, heat and movement (as the aircraft moves through the air) -- i.e. drag. The more drag you put on the airframe, whether that be through the use of speedbrake, flaps or gear, the more energy you are transferring away to the air and therefore (if we assume we still have negligible thrust from the engines) you will need to increase the rate of descent in order to convert more GPE to KE in order to maintain airspeed; OR you can maintain the same rate of descent and the airspeed will reduce. Hence the old maxim about it being possible to slow down OR go down, but not both at the same time!

An approximate descent path may be approximated by divide your height above the airfield (in thousands of feet) by three (which actually gives a slightly steeper profile than three degrees) or multiply the track distance to go by three (which results in a slightly shallower profile than three degrees). In any event, this tells us that we should be at 3,000 feet above the airfield about 9-10NM out; 10,000 feet above the airfield roughly 35 miles out; 20,000 feet above the airfield roughly 70 miles out and so on.

However, this only tells us half the story: because we are not just talking about reducing height but about reducing the overall energy level of the aeroplane, it should be clear that we need to add speed to the equation to give us the full story.

Thus, 10,000 feet, 35 NM and 250 knots is about on profile. However, if you are at 10,000 feet, 35 NM to run and 320 knots then you are HIGH! Conversely, at 10,000 feet, 35 NM and 210 knots you are actually somewhat low and will almost certainly need to add thrust at some point.

When you extend the speedbrakes to increase drag, you are 'throwing away' energy in order to increase your rate of descent. Now, where did that excess energy come from?

That's right -- it came from the fuel that you burnt up at 35,000 feet before you closed the thrust levers to start your descent!

Thus it follows that if you had started your descent earlier, you wouldn't have put so much energy in to the system and therefore you wouldn't be having to throw it away now (and you wouldn't have burnt that fuel in vain).

Of course, reality is such that often there will be air traffic control restrictions etc that preclude a nice continuous descent without the use of speedbrake; but in the absence of those, it should be perfectly possible to plan and execute the descent such that close the thrust levers at top of descent and only open them again at 1000 ft aal, on the glide path, at Vref + 5 with landing flap and gear extended, without getting the speedbrakes out at all. However, VNAV will probably not do that for you: the computer can do many things but in this regard it is not, generally, smart enough.

Other ways that you can do to manage the descent:

  • Change the airspeed (either in FLCH or VNAV with speed intervene). Remember that drag increases with the square of speed, so going faster very quickly increases the amount of drag and therefore enables you to descend at a steeper angle (even though you are also covering the ground faster). However, remember the speed/height equation: if you increase the airspeed you will need to be lower at each of your "gates" than you would otherwise need to be because you will at some point need to level off (or at least shallow off the descent) in order to reduce the airspeed. So if you are bombing down at 320 knots, you will actually need to reach 10,000 feet at around 42-45 NM out instead of 35 (allowing around 1NM per 10 knots of airspeed). Remember that the VNAV path will have been computed at the speed entered in the VNAV Descent page - so if you change the airspeed without changing it also in the VNAV Descent page your VNAV profile pointer will not be giving you accurate information!
  • Likewise, if you are being held high for some reason then don't keep pumping loads of energy in to the system: slow down (if you can). This will give you the chance when you do start descending to wind the speed up and attain quite impressive rates of descent.
  • Think ahead and keep cross-checking your height against your range. VNAV has a nasty tendency to give you lumps of thrust when not really necessary, with the result that it's bunged a load of unnecessary extra energy in to the system and then later demands MORE DRAG to get rid of it. Anticipate the steepness of each segment of a stepped STAR, for instance, and if you are levelling off approaching a 'steeper' segment then it may be better to go to FLCH and reduce the airspeed to shallow off the descent in anticipation of winding it back up again on the 'steeper' section

Another point worth noting:

Don't use flaps as speedbrakes. In other words, don't extend flap close to the limit speeds in order to slow the aeroplane down: the flaps are not designed for this sort of thing and it is hard on them! If you are fast then utilise other drag sources: i.e. the speedbrake (although you will need to get that stowed no later than 1,000 ft aal to meet the stable approach criteria, and some aircraft have restrictions on using speedbrake with flap extended (I don't know about the NG: the FCOM should say if so. The 747 manuals do not recommend use of speedbrake with flap >10 on the older aircraft and >20 on newer models - IIRC this is to preserve flap beam life). As Dan says, the landing gear is the greatest source of drag on the airframe and you can and should extend it early if you need to (though once it's down, it's down: don't go retracting it again unless you're going around).

Hope that helps.

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downscc,

I took a flight from the NYC area to Miami last night and it worked out great! The weather was calm to mild, so that helped big time.. but putting the Vref +5 speed at the FAF actually helped quite a bit for preparing and slowing down for the final approach.  I did have one item that I hadn't seen before, with the change of the speed at the FAF put in the LEGS, it removed the "DESC" section of the flight path. But, it actually didn't matter and worked out regardless.

About the idle, I didn't touch it as you stated, but I did 'bump it' to see if the idle was truly idle. It wasn't. However, I am not sure if the PMDG starts idle at lets say 41%, then reduces it as the flight continues.  My concern is because my descent speed was around 277-280, that any lower idle would throw me into VNAV and off the flight path as the plane tries to catch up in speed.  Thoughts?

Should I adjust the descent speed to 280 from 279 at the beginning of the flight when setting up the FMC?

Other than that, it was a good flight. I do need to look into the taxi light however, I don't remember the light being so dim where I can't even see it.

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1 hour ago, 787WannabePilot said:

downscc,

I took a flight from the NYC area to Miami last night and it worked out great! The weather was calm to mild, so that helped big time.. but putting the Vref +5 speed at the FAF actually helped quite a bit for preparing and slowing down for the final approach.  I did have one item that I hadn't seen before, with the change of the speed at the FAF put in the LEGS, it removed the "DESC" section of the flight path. But, it actually didn't matter and worked out regardless.

About the idle, I didn't touch it as you stated, but I did 'bump it' to see if the idle was truly idle. It wasn't. However, I am not sure if the PMDG starts idle at lets say 41%, then reduces it as the flight continues.  My concern is because my descent speed was around 277-280, that any lower idle would throw me into VNAV and off the flight path as the plane tries to catch up in speed.  Thoughts?

Should I adjust the descent speed to 280 from 279 at the beginning of the flight when setting up the FMC?

Other than that, it was a good flight. I do need to look into the taxi light however, I don't remember the light being so dim where I can't even see it.

Arriving Miami when they are landing East can be a challenge on the descent, congratulations.  I love it when a flight comes together.

Idle will change with altitude and speed, and the definition of idle might best be considered minimum fuel flow and N1.  At 400 KTAS and FL300 the N1 might be 41% then decrease to (off top of head) 38% at 300 KTAS FL180.   Good idea to watch these things every flight and get used to what is normal because sometimes a problem announces itself with something not quite right long before the alarms sound.

The change from 279 to 280 is not relevant.  I only do it to satisfy my OCD behavior if there is a 280 constraint.  In general, the speed constraints are "at or below" and instrument flying speed tolerance is +/- 5 kts.  I arrived last night at KMEM and noted their arrival charts say fly mach until 290 then hold 290.  Gotta read the everything on the charts.  On this topic, given a 280 speed target, I might let it increase to whatever if there is not a speed constraint on the arrival until the slow down at 10000 and there I will use speedbrakes just that one time to slow to 240.  The only reason you wouldn't do this is if ATC gave you a speed in the clearance in which case of course you fly your clearance.

The taxi and ground lighting changes vastly depending on scenery.  Be sure you have dynamic lighting enabled in P3D.

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2 minutes ago, downscc said:

The taxi and ground lighting changes vastly depending on scenery.  Be sure you have dynamic lighting enabled in P3D.

I haven't pulled the trigger to upgrade to P3D just yet.  The cost for everything would be just around $400.

I know some kinks have to be worked out in a few of the programs I use, so I am gonna sit idle till things are worked out first.

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