Aircraft too high on a VNAV descent

[skysurfer 76]

Ray.

I jumpseat on NG very often and I asked pilots same or similar questions. NG is a bit slippery airplane, I witnessed it myself many times. If you descend in a quite environment with no destruction from ATC and well planned descent profile in CDU  you most of the time will be OK. However in RW things go sideway very often. ATC may speed you up to 300 kts close to the airport and may change STAR and Rwy several times then your constant and smooth descent will be out of the window. In such cases you just have to improvise. NG offers you many options for that like VNAV, FL CHG, Vertical speed, speedbrake etc. With the practice you will find your way to manage off path descent and it also a lot of fun!     

[richjb2 195]

2 hours ago, Ray Proudfoot said:

I don't call hundreds of feet a bit high. The descent rate is a paltry 800fpm when it should be around 1400. Why would the aircraft not descend more quickly especially when the small display on the ND shows it's higher than it should be? I can understand if you put a mandatory altitude against a waypoint and got "path unachievable" - that's understandable.

I'm not putting winds in on the descent page. Does that make a significant difference to how the autopilot functions? And what winds am I putting in? For a particular waypoint or the destination? I have Active Sky.

Was it necessary to use the speedbrake n every descent? Seems odd. Maybe when conditions are unusual. It suggests you were probably too high to start with.

Ray,

VNAV is both an autopilot and FMC function.  The Boeing FMC works a bit different in that it uses a performance based descent as opposed to a geometric path descent (there are exceptions, like the altitude constraints between two waypoints, and even that is optional - PMDG modeled the geometric option between two waypoint constraints). 

The FMC will compute a TOD based on an idle thrust descent profile (fuel efficiency).  For it to compute that idle descent point, the FMC needs to know the wind profile it will encounter during the descent, ISA deviations (which affect TAS), and the planned use of A/I (affect the amount of thrust output at the idle stop).  With those parameters entered into the VNAV DES page, the FMC can compute a theoretical idle descent point.  I say theoretical, because things may not be as forecast during the descent.  In those cases, the descent path may not be followed, and the autoflight system & FMC -- quietly -- transitions from VNAV PATH to VNAV SPD.  This catches real pilots as well, and one reason why it is considered "good practice" to verbalize FMA mode changes when they occur.  In VNAV SPD, the descent is a "speed on elevator" control, and path will not be followed.  

Also, when the descent starts the thrust node in the FMA indicates RETARD, and then when idle is obtained the FMA thrust mode changes to HOLD.  Depending on the PMDG option you have set, if the physical throttle is not at the IDLE stop, then the actual thrust will not be at idle.  I have B737 non-motorized throttle quad, and occasionally, I'll forget to pull the physical thrust levels back to idle.  she'll come off path rather quickly in that case.  

Just some things to consider.

Thanks,

Rich Boll

Wichita, KS 

 

Edited January 31, 2019 by richjb2
forgot a word...

[Ray Proudfoot 8,921]

Edward,

Real world experience counts for a lot when flying sims. Although I’m a relative beginner with the 737 I have a lot more experience flying Concorde. Sadly, only in P3D.

Descents were far more dramatic. At Mach 2 at the decel point and close to FL580 you spool the engines back to 94% N2 (from 105% where they stayed for most of the flight) and when IAS drops to 350kts you engage Speed Hold and the descent starts.

Gradual to start with. Maybe 2000fpm but at Mach 1.5 a further power reduction is made to 77% but still with a speed hold of 350kts means the rate steepens dramatically, up to 4500fpm.

Then passing through Mach 1 throttles go to idle and with the speed hold still engaged descent rate can be 6000fpm. At 10,000ft 250kts is engaged and it becomes just another subsonic aircraft. FL580 down to 10,000ft in around 15 mins. Even on a simulator it is exciting.

Compared to that the Boeing is a baby and should be relatively easy to handle. But as you say, it’s slippery so appearances can be deceptive.

All I need is more flights to gather experience but sadly, the 737 does lack the thrill of a supersonic icon. 😢

[skysurfer 76]

Ray. 

To confuse you even more 🙂 you may find this video interesting: 

Looks like RW pilot demonstrates how to deal with high energy approaches. I know you said you were new to NG but this video may be useful to understand how it all works. 

 

[Ray Proudfoot 8,921]

Thanks Edward. Will watch it later on my big telly. 👍

[vc10man 732]

Hats off to you, Edward. A great tutorial video. Despite having flown the PMDG 737NGX since its inception, I too learned a lot form that tutorial vid.

[Ray Proudfoot 8,921]

13 hours ago, skysurfer said:

Ray. 

To confuse you even more 🙂 you may find this video interesting: 

Looks like RW pilot demonstrates how to deal with high energy approaches. I know you said you were new to NG but this video may be useful to understand how it all works. 

 

Just watched it. Very helpful. FL CH is your friend and increasing speed on the CDU to get a greater rate of descent is also a good tip.

[Adrian123 1,015]

21 hours ago, Ray Proudfoot said:

Seems odd VNAV is ‘broken’ for such a good product. 

So, is it still Broken or can it be said it required more training? 😉

[Ray Proudfoot 8,921]

3 hours ago, Adrian123 said:

So, is it still Broken or can it be said it required more training? 😉

Had a flight from EGNT to EGKK today and VNAV behaved itself after I reduced throttles to idle. Makes all the difference. 😁

[sloppysmusic 169]

I too felt the same way until I finally got around to reading all the manuals including the massive fcoms. Then it all makes sense for the big picture. All of these systems work in unison to provide info for the pilot to get the passengers safely to their destination. The pilot of course is in control. Would you actually want an autopilot that if off descent threw on the speedbrakes and plunged into a 5000 fpm dive just to get back on track? 

The AP is no fortune teller or weather forecaster. If you give it all the information you have entered in the correct places it will do a great job but never perfect. VNAV is just one system at the command of the pilot to get to ED. The tutorial might make it look like that's the way you are supposed to do it but they are keeping it simple for your first flights. 

As soon as you take control of all your systems you will feel the power and control at your fingertips. This plane is mind blowing and I'm not easily impressed. Just finished 4 weeks flying nothing but this bird and working through every page of all the manuals and practicing every procedure. I don't even use VNAV any more as I have much more fun with ALT INTV, SPD INTV, FLCH, and VS. I love planning my own descents too so I can arrive at IAF flaps 5 and ready to intercept!

If you left it too late for descent you've already wasted the fuel so might as well be creetive in your descent. Enter a HOLD for your next way point. Or divert to a nearby NAV aid by entering into your legs. Or fly a 5 mile parallel offset between the next few legs. Or slow down manually and use VNAV SPD for descent. Or choose another star or approach that takes you circling or dme holding just to add distance. Anything but that speed brake lever. Just so lazy that is! 

The depth IS in this plane and it's all detailed in the manuals. Prepare to become one with your NGX

😎

Edited February 1, 2019 by sloppysmusic

[sloppysmusic 169]

2 hours ago, Ray Proudfoot said:

Had a flight from EGNT to EGKK today and VNAV behaved itself after I reduced throttles to idle. Makes all the difference. 😁

LOL you disconnected the A/T? Been there done that! The 'proper' way to achieve the same thing without turning off A/T is to use VNAV SPD, set the speed low enough (280 is default for descent between machj levels and 10000ft) until you reach idle throttle or preferably just the right amount is set for your planned descent. SPD INTV or LVL CHG will change mode for you but you need to read up on them first if I quote from memory I might make a mistake.

If you can keep throttles at idle all the way from T/D to IAF while STILL using AP/AT then you deserve a pat on the back. That's perfection and definitely doable!

😄

[Ray Proudfoot 8,921]

37 minutes ago, sloppysmusic said:

LOL you disconnected the A/T? Been there done that! The 'proper' way to achieve the same thing without turning off A/T is to use VNAV SPD, set the speed low enough (280 is default for descent between machj levels and 10000ft) until you reach idle throttle or preferably just the right amount is set for your planned descent. SPD INTV or LVL CHG will change mode for you but you need to read up on them first if I quote from memory I might make a mistake.

If you can keep throttles at idle all the way from T/D to IAF while STILL using AP/AT then you deserve a pat on the back. That's perfection and definitely doable!

😄

Where did I say I disconnected the throttles? I said I reduced them to idle at ToD. The PFD does say Retard. That presumably means draw them back to idle. I’m leaving things on LNAV and VNAV as long as possible until I learn the systems fully.

[sloppysmusic 169]

4 minutes ago, Ray Proudfoot said:

Where did I say I disconnected the throttles? I said I reduced them to idle at ToD. The PFD does say Retard. That presumably means draw them back to idle. I’m leaving things on LNAV and VNAV as long as possible until I learn the systems fully.

Well you didn't say that but you said you reduced the throttles to idle. So either you already disconnected the AT, or never had it activated in the first place! If you use manual override in your throttle setting sin PMDG CDU set up then moving your throttle levers would disconnect the A/T at once. Will stop here as I do not know anything about your set up, but I do know that you can not just magically reduce your throttles to idle while also keeping A/T activated! Would be interested for sure in knowing HOW you reduced them to idle when PFD said RETARD, as that would normally be done by the A/T and not the pilot. 

[Ray Proudfoot 8,921]

26 minutes ago, sloppysmusic said:

Well you didn't say that but you said you reduced the throttles to idle. So either you already disconnected the AT, or never had it activated in the first place! If you use manual override in your throttle setting sin PMDG CDU set up then moving your throttle levers would disconnect the A/T at once. Will stop here as I do not know anything about your set up, but I do know that you can not just magically reduce your throttles to idle while also keeping A/T activated! Would be interested for sure in knowing HOW you reduced them to idle when PFD said RETARD, as that would normally be done by the A/T and not the pilot. 

You seem to be offering your advice when I have already said today’s flight went fine. I have Saitek throttles. I physically moved the two levers back to idle. I could see N2 reducing anyway but moved them to match the sim.

A/T remained engaged. I trust that is clear. I do know when the A/T is engaged and don’t like your suggestion I didn’t know if it was engaged or not.

Does that answer your question? I’m no beginner with the 737. I used Project Magenta for many years but since switching to Concorde around 5 years ago I’ve become a little rusty. Things are starting to fall back into place.

[sloppysmusic 169]

50 minutes ago, Ray Proudfoot said:

A/T remained engaged. I trust that is clear. I do know when the A/T is engaged and don’t like your suggestion I didn’t know if it was engaged or not.

Sorry if you didn't like it but before your latest post you didn't clarify you were just matching the cyan indicator in sim to match the throttles when the AT moved them to idle, you actually said you moved the throttles to idle and inferred that was why your flight went so well. As moving the Saitek throttles as you did had zero effect on the 737 during your flight. That was what confused me and may do others who read these posts. This is a place you will more often find help than insults, but always we have to be precise to diagnose the exact fault and solution. I personally HAVE many times disconnected the A/T and moved the throttles to idle manually before I knew what I did now but would still do so again if I want to take control of the plane. That's not ignorance it's what a pilot is supposed to do. I was crediting you for it whilst confirming it was what you actually did, which we now know was not.

We good now?

😄

 

Edited February 1, 2019 by sloppysmusic
darn autocorrect