Descent after TOD : some waypoints heights impossible to achieve

[GSalden]

With about 3 out of 5 flights I have a descent issue.
Eg :
TOD  40000 ft -> first waypoint --> After 80 nm / 20900 ft -- no issue
Waypoint 20900 ft --> After 8 nm / 16900 ft -- impossible
Waypoint 16900 ft --> After 7 nm / 13900 ft -- imposdible
Waypoint 10900 ft --> After 9 nm /  7500 ft -- impossible

This kind of impossible descent values I see often.
Fuel has been calculated. All weights set. Wind en temp set.
Derate set. All speeds set.

Sometimes the aircraft tries to reach the new height At the cost of the speed restriction.
Then the aircraft is way to fast or also to high for the Star.

All advice is welcome.

[KyleH]

Questions:

1) Aircraft type. (I'm assuming 737)
2) Are you using VNAV or a different mode?
3) Using speed brakes to increase descent rate?

It does seem like a steep descent. Are all the waypoints restrictions? What STAR is it?

[GSalden]

Questions:

1) Aircraft type. (I'm assuming 737)
B737-800 --> PM 737-800 flight model

2) Are you using VNAV or a different mode?
VNAV and LNAV

3) Using speed brakes to increase descent rate ?
At a speed of 310 kias an aircraft does not use speed brakes as far as I know.
I tried it under 18000 ft but it only slows down the aircraft with max 10 kias

4) It does seem like a steep descent. Are all the waypoints restrictions? What STAR is it?
From the TOD the distance till the Star is almost 140 nm.
The max v/s is 2000 ft as with any higher the speed keeps getting up ( + 310 kias ).
In reality the aircraft descents at 2200-2700 ft/min. Then it can be done.
In the flightplan the descent speed is 0.74.
Star : GENRO.

Do I need someting to set different at the DES page ?

[jackpilot]

140 nm is way enough (40000'X3 =120nm)
Maybe excessive speed during initial descent.
Try it "manual" (no VNAV)
Idle at TOD, glide config
Stabilize speed with pitch
Trim
Check what VS you get
Having no thrust, pitch control should increase drag and control speed

[NedH]

Those descents aren't impossible in a 737 with speed brake.

Try speed brake or start TOD earlier or lower cruise altitude.

[KyleH]

You can use the speed brakes at any speed in the flight envelope for the 737.
I should have specified, what STAR and what airport is this approach for?

-2700 ft/min is achievable. If the VNAV mode has gone into VNAV SPD mode, the aircraft is trying to hold an airspeed in descent and not attempting to get on the descent path.

There was a discussion on descents here not too long ago, where Jack and I explained it in more detail. Try searching for that.

Edit: Here it is http://www.cockpitbuilders.com/community/index.php?topic=5134.0

[jackpilot]

 

[GSalden]

Here page 4 and 5 from the Legs Page :
- NVS   .75/FL400
140 NM
- VBZ  .74/FL400
109 NM
- GENRO  250/6850
6.6 NM
- FR607  250/5750
6.0 NM
- GEBTI  220/4000
7.2 NM
- C128  180/2000
2.0 NM
- F127  180/2000
4.0 NM
- RW28  180/10

ECON PATH DES
Crz Alt
FL400

TGT SPD
.735/270


PERF INIT. --> at start
GW LB
160.7 (154.0)
FUEL LB
39.1
ZFW LB
121.6
RESERVES LB
3.0
COST INDEX
30


In VNAV the aircraft does not want to descent quicker than 2140 ft/min trying not to exceed 270 kias.

But for some waypoints at the end I need -2400-2700 ft VS.
However , even with the speedbrakes on the aircraft enters the Star at 280 instead of 250.
And slowing down from 250 to 220 with speedbrakes up seems also difficult.

With throttles at idle the approach is at 160-170 kias instead of 140-150 kias.

I am really wondering if I need to do something about the drag in the aicraft.cfg file....

What would you guys advise ?

SPD RESTR
250/10000

[Bob Reed]

What aircraft model are you using?

[GSalden]

I use the PM FSX 737-800 from the Magenta site ( at the bottom of the dl page )

I just recently went back to PM because the Ifly CBE will not be upgraded to P3Dv3.
With the Ifly I was able to make all Stars and approaches.

However I have to say that the fluidness of the PM suite is better than the Ifly CBE ...

[KyleH]

...
In VNAV the aircraft does not want to descent quicker than 2140 ft/min trying not to exceed 270 kias.
...

No it won't. If it's treating 270kts as the restriction it won't descend any faster. Clear that limit out. See the thread I linked.

I would like to be able to lookup the actual STAR chart, which airport is this for?

[blueskydriver]

Hi,

Looking at your PERF INIT page, it shows the GW of 160.7 (154.0). That says you're inputting the 160.7 weight into the CDU yourself or through an import, and then the 154.0 is what your aircraft model airfile in MSFS thinks the weight really is. Now, this could be the cause of your problems, because your're actually using a heavier 160.7 weight to calculate the parameters, and if you intended the lighter 154.0 weight too be the value for the PM CDU to calculate with, it's not using that value n MSFS.

In other words, those two numbers should be the same, so where are you getting your GW value from? How are you loading the aircraft? Are you using the FSX Fuel/Payload page or another addon program to load those items? Try this first: setup your flight as before and whatever the number is in the brackets, type that value and place it into the GW LB line of the CDU PERF INIT page; the numbers should now match.

The next thing too check is your CG%. You did not list it in your previous post; what is the value of your CG on the CDU page? You could be too nose heavy and you're descending to fast as a result of it. Also, where is your fuel loads being located...Center Tanks, Wing Tanks or both? The proper method is too load the wing tanks only, if you do not have enough fuel for a full load. In the full or beyond the capacity of the wing tanks loads alone, you would have the wing tanks full and the center tank full or whatever the amount is beyond full wing tanks, then during the flight you burn the center tank fuel first and the wing tanks fuel last (if no center tank load, you just burn the wing tank loads).

Why is this important? When you're burning the fuel the CG% is going to change slightly, but maybe not enough too affect your descent value/speed; however, if you entered the wrong GW LB versus the MSFS airfile, it could affect things now. Additionally, if the CG% is wrong that is going too cause more issues with the descent. When you input the values for payload, how are you doing that? By the way, if your model is not set too match the payload locations of how PM works, you will find that even though you enter the payload the way you expect it too be which would give you a correct CG%, it could still be loaded wrong throwing of the CG anyway.

For example, if you use TopCat for data input and the model in that program is not the same as the MSFS FSX and PM model, it'll calculate the values incorrectly, then when you transfer those values into FSX, you can see that they're wrong, but if you didn't know this, your flights would be off and yet, you wouldn't know why. A good way or program to really check the values and do your loading is by using Luis Gordo's Instructor Station program, it has some really good features too assist with all of this.

Therefore, please recheck your values for Fuel, Payload, GW LB, CG% and try flying the flight again. Plus, always repeat the same flight for testing, as well as turn all weather off. If you install the PM model, make sure you are actually using the PM CDU model that matches it. Here is a link to the manual; check out pages 120-121:

http://www.projectmagenta.com/wp-content/uploads/downloads/2011/05/MagentaDoc15.pdf

John

[Trevor Hale]

WOW.. Great answer John.    That's awesome.  Thanks for taking the time to post that.  I bet you hit the nail on the head.  I hope he reports back that this solves his issue.

Thanks,

Trev

[GSalden]

Quote from Chock at Avsim.

Real NG 737s have six fuel switches located on the left side of the overhead, plus a crossfeed rotating valve switch. On the 737-800, that's all you get unless it is a customer special variant with more than the standard three fuel tanks (unless we get into talking about the real thing in ,much more detail, in which case there are some other switches too, but not really of relevance to cockpit operation). Special versions with auxiliary tanks have four additional feed valve switches located above the electrical bus panel for the auxiliary tanks, but that is of course not there on an 800 since it only has three tanks as standard.The 737 should use fuel from the centre tank first and then the wing tanks, since the centre tank maintains a pressure level approximately twice that of the wing tanks. If it has auxiliary tanks fitted, these feed into the centre tank, fuel is forced in there by pressure bled off from the cabin pressurisation system and the engine bleed air, since the auxiliary tanks do not have dedicated pumps, unlike the three main tanks, which have two pumps each (hence six switches).

This says that first the center tank will be used and than the wingtanks.
That is also as PM shows the fuel usage on the Eicas.
At startup I had 98% in the center tank and 65% in both wing tanks.
The fuel values where calculated with Flight Sim Commander 9.6.
I use the default P3Dv3 Fuel/Payload page.

Why there was 160.7 GW I do not understand as I put 154.0 on the scratchput.
The CDU put 160.7 ...

The CDU also uses the 737-800 file ( came with the PM 737-800 dl ).

Tomorrow I will try again and report here.

Great help. Thanks !

CoG was 30.

[KyleH]

Yes, fuel feeds from the center take first. Loading fuel, you fill the wing tanks to 100% first before adding the remainder required to the center tank.

Your zero fuel weight plus your fuel load add up to 160.7. Did you enter both ZFW and TOW in the cdu?

[GSalden]

Just did some digging in the airfiles from the PM 737-800, the default 737-800 and the iFly 737-800.
The iFly was tested by real airline pilots. I used the iFly CBE a lot and never had the Descent issues.

Here the entries


Spoilers section :

Lift iFly  0.44
Lift Default   0.48
Lift  PM 0.0489

Pitch iFly  0.01
Pitch Default  0.01
Pitch PM  0.005

Drag iFly  47
Drag default  261
Drag PM  15

Roll iFly  0.11
Roll Default  0.11
Roll PM  0.0552 

The spoiler_limit value in all aircraft cfg files are the same : 60.
Looking at the above I can imagine that the Airbrakes from the PM 737-800 are not that effective. The airbrakes from the iFly give a 3x more drag.
The Default 737-800 has a "6 year old child can slow down the aircraft" value...


I also looked at the Lift coefficients :

Cl.adot iFly   6497                   ( Cl.adot =  Lift AOA rate - coefficient of lift due to rate of AOA change )
Cl.adot Default  6497
Cl.adot PM  4494

Cl.q iFly  17398
Cl.q Default  17398
Cl.q PM  21393

CL.de Elevator iFly  590
CL.de Elevator Default  590
CL.de Elevator  PM  734

Cl.iht Horiz Stab  iFly  2088
Cl.iht Horiz Stab Default 2088
Cl.iht Horiz Stab PM  3876

Cl.de Flaps iFly  1.47
Cl. de Flaps Default  0.9
Cl. de Flaps PM 1.6588

Cl.ds Spoiers iFly  0.44
Cl.ds Spoilers Default  0.48
Cl.ds Spoilers PM 0.0489     


You can make your own judgement about the PM airfile  ...

[blueskydriver]

By looking at the values you listed, Ifly is not much different then the default airfile. Almost all are the same, except a few like the spoilers and flaps... What happens if you fly the default FSX B737?

Did you try flying with the suggestions I gave you? Can you post your values from the Payload and Fuel loads of FSX, the CG% value, the GW in PM CDU (both numbers), your Descent Restriction Not To Exceed Speed from PM CDU, and how you're setting up loads (manual or through a program like TopCat or IS-instructor station).

By the way, you are using PM Systems right? If so, are you using Wing and Eng Anti Ice during descents, as well as making sure PITOT Heat is on?

John

[GSalden]

@John,

Tomorrow I will get back with all the info.

In the mean time a question about a complete Autoland .
With my Ifly Cockpit Builders Edition I could autoland where my throttles went to idle at 27 ft flare.

Now with PM the throttes do not go to idle , but AT goes to off and manually I have to pull them back to idle.
The rest of the landing is fine including autobrake.

Perhaps I have to change a setting in the cdu or mcp ? ...

[blueskydriver]

Hi,

Look forward too your future results...

As for the flare altitude, you set it in the CDU or MCP depending on what you have for PM software. If you have MCP only, with no CDU, you set it in the MCP.ini file. Otherwise, if you do have the CDU and MCP, you set it in the CDU Pages here:

If running the CDU the flare altitude must be set. If you do not change a/c types then this only needs to be set once. The flare altitude will determine at what point on an Auto Land the aircraft will start the Flare. The flare altitude will probably be set at 30 by default; you may want to change this value depending on what a/c type you are using.

From the INDEX page select MAINT. From this page select the LSK for A/T. In the A/T page if you want to assign a new flare value, enter it into the scratch pad, and then enter this value by pressing the LSK next to the FLARE AGL setting. The new value will be added.

Check page 33 of PM manual.

Post your reseults please.

John

[GSalden]

Yesterday I have had contact with the KLM Simulator department.
They told me that at desend with the use of the Speedbrakes the speed needs to go back slowly.
Also with flaps at 40 and a Gross Weight around 140 the aircraft mus be able use a low speed of 135-140, which for me was impossible.   
The aircraft stayed at a 0 degree angle and no 3-7 degree nose up....!

I also went to a friend who uses ProSim. We did the same flight and with the help of the spoilers he was able to follow the descent path perfectly.
As I have modified the CaptainSim 757 flightdynamics in the past for the guys at the forum, I made some small adjustments at the spoilers and flaps drag data and also added 5% to the take off elevator in the aircraft.cfg file.
Also I upped the Fuel comsumption by 10% as for a flight from Amsterdam to Faro the aircraft used about 12000kg of fuel instead of 13.500..

Now I am able to follow the glidescope with V/S and the use of the speedbrakes ( 75% of the time ) coming from 40.000ft and descending to the Star.

The only thing that remains now is the throttles not going back to idle when doing the flare.
They engines remain at around 57% N1.

I have a test Approach at Schiphol / rw 06/ ILS 110.55 the speed is around 160 .

CDU at Approach :
GW 120.3 --> 54.681 kg according to the default Fuel/Load calculator
LGw 110.0
Flare 50
Flaps 30
Vref 135 KT
Speed set at 130
Fuel left  13% both wing tanks/ 15% center tank
Default load = 70%

CDU maintenance :
PM Autopilot  mode=PM / Pitch=2 ( no difference with 0 as far as I can see ) / Bank=75

CDU A/T Factors :
Left side  --> All 100
Thr Sync=On
Autocoord=Off ( does not change when pressing RSK2)
Flare 50
T/O N1 = 100
CLB N1=100
F/D<200 = 100
F/D>200=100

Like I wrote, when at 30 ft I manually disable AT and pull back the throtles I can make a very nice landing.
Ref Speed for landing, Radio 50, CMD ( 2 AP ) on my PFD.   

I only want the Autoland to work too.
With Autoland mode I cannot het the second Ap to engage, below 1500 ft to get CMD.
I tried other runways but no succes.

The aircraft captures the localizer ( Loc ) , then the glidescope ( App ) and follows it.
I have set Speed to 130 ( Fref said 124/30 ). 2x Course at rw heading.
Autobrakes at 2. Speedbrakes are armed.
Below 1500ft I press Ap B and it stays on Single Channel. 
Also the throttles are trying are going up and down to keep the speed around 150 with a pitch down...
With the iFly and ProSim it was no problem to land...

Probably I have some lack of knowledge and am doing something wrong / wrong settings.

-------

Additional :

I tried to make a new Approach at Schiphol rw 06.

Here a vid of what is going on :  https://youtu.be/z9L3nMQQ_FQ