Auto throttle hunting

[Bob Reed]

It has been a long time since I saw this but.. When I am in decent, I have selected "Level change" and brought the throttles to idle. Auto throttle is engaged. The plane starts to come down as it should but the throttles are "hunting" They go to idle and then for some reason there is a command for like 90% throttle and then they spool back down and then back up. So my speed is a constant 10 to 15 faster then commanded. Any ideas? I am going to do a flight tonight and use Vnav and see what it does.

[GSalden]

Just to check : try with only Spd and VS and see how the throttles behave.
You can use LNav too.

[Bob Reed]

I will give this a try when I get home. I am using the PM flight model as well.

[GSalden]

Bob, I have tried with FlCh but I do not have your issue.
My motorized TQ is from Cockpitsonic.

[Bob Reed]

Bob, I have tried with FlCh but I do not have your issue.
My moto
rized TQ is from Cockpitsonic.

I am not using a motorized throttle. Shes the old fashoned kind....  When I am descending I pull the throttle back to idle.

[KyleH]

Is the vertical speed constant when its doing this?

What is the throttle mode displayed on the PFD? (top left)

[Bob Reed]

Is the vertical speed constant when its doing this?

What is the throttle mode displayed on the PFD? (top left)

Going to fire up the sim and will have a few of these answers. More to come.

[Bob Reed]

Ok I just flew the milk run. Takeoff to TC fine. Cruse to TD fine. Before TD I dialed in 220knts on the MCP. At TD select VS dialed in the - altitude  to get me to my desired altitude at my waypoint, and down we go an so starts the hunt. Air speed stayed 10 to 20 knots faster then MCP commanded speed all the way to landing. Kyle, upper left "MCP" 

[KyleH]

Ok. V/S mode does not control airspeed.
The plane will try and maintain the airspeed set in the speed window using throttle, but in V/S mode, the selected vertical speed is the driving restriction.
FLCH/LVL CH mode descends via the speed set in the speed window. It sets the throttles to idle, then pitches for the selected airspeed and gets whatever vertical speed that happens to correlate to.

V/S mode is not meant to be used for long climbs or descents. It is typically only to be used for short periods of time.
For descent from cruise use LVL CH or VNAV.

I need to do a comprehensive write up or video series on this as it keeps coming up. If I was arriving early enough, I'd do a training session at Worldflight.

[Bob Reed]

Well it was a very short decent. 5000 to 2600. I am having the hunt in FLCH LVL change as well and speed running 10-25knts faster then commanded. It does this through approach as well.

[KyleH]

There is secret sauce going on with LVL CH as well for short descents.
The formula it uses is: V/S = change in altitude/2.

So: if your descending from 35,000' to 12'000 as would be normal, it would try to give you a 11,500 foot per minute descent. Obviously that would cause an overspeed, so the airspeed is limited to that which is in the MCP speed window.
But, descending from 5000 to 2500', it will give you 1250 foot per minute v/s, and use thrust to maintain the airspeed set in the speed window.
That's how the real thing works, not sure if PM emulates that.

What does it do if you pull the speed brakes? If it throttles up, then it's trying to maintain some speed set somewhere.
The spooling up and down though sound's like a potential software/hardware issue.

[Bob Reed]

There is secret sauce going on with LVL CH as well for short descents.
The formula it uses is: V/S = change in altitude/2.

So: if your descending from 35,000' to 12'000 as would be normal, it would try to give you a 11,500 foot per minute descent. Obviously that would cause an overspeed, so the airspeed is limited to that which is in the MCP speed window.
But, descending from 5000 to 2500', it will give you 1250 foot per minute v/s, and use thrust to maintain the airspeed set in the speed window.
That's how the real thing works, not sure if PM emulates that.

What does it do if you pull the speed brakes? If it throttles up, then it's trying to maintain some speed set somewhere.
The spooling up and down though sound's like a potential software/hardware issue.

Well there is no hardware involved. I do not have a motorized TQ. If I throw the boards the plane still does not slow down as I think it should so someplace there is another speed I guess.. Time to hunt through the CDU pages I guess. Never had this issue before...

[Bob Reed]

My thought is I am missing a setting while setting up the CDU. Any thoughts on this?

[Trevor Hale]

Bob, I am thinking this is an N1 Limit setting in the CDU.  I am guessing here.  But levelCH I believe uses the N1 Limit setting in the CDU. What is the N1 Limit setting set to?

Trev

[KyleH]

If the mode in the PFD is MCP, then it is trying to set the speed from the MCP not the CDU.
The N1 setting in the CDU is used for climb, not descent.

Could you shoot a video of this Bob. Otherwise we are just stabbing at things.

[Bob Reed]

Bob, I am thinking this is an N1 Limit setting in the CDU.  I am guessing here.  But levelCH I believe uses the N1 Limit setting in the CDU. What is the N1 Limit setting set to?

Trev

I think you hit the nail on the head! I did a little more looking around inside the CDU last night and found that speed reference. But.. This does not explain the hunting during approach.

[Bob Reed]

If the mode in the PFD is MCP, then it is trying to set the speed from the MCP not the CDU.
The N1 setting in the CDU is used for climb, not descent.

Could you shoot a video of this Bob. Otherwise we are just stabbing at things.

As a matter of fact I think I can. I am still pretty sure it is the way I am programming the CDU. And I also have to wonder if it has something to do with the Decent speed issues that Gsaldan is having in the other thread. I have used the PM model in the past with out this issue so I do not think it is the model. Maybe I should try it with the default 73 model?

[KyleH]

Unless your using VNAV the CDU does not affect descent.

[Bob Reed]

Unless your using VNAV the CDU does not affect descent.

Ok this is what I thought. But I may still have a setting wrong in the software. I will probably not get another chance to fly it until I get back from WF at this point, but we can talk more about it while we are all there!

[GSalden]

Bob,
I am using the FSX default 737 in P3Dv3.
Even without PM the "hunting" like you described it appears a little and the speed stays 15 knots above the dialed mcp speed.

I did modify the flaps and spoilers drag a little to match more like what I was used to with the PMDG 737 NGX and Ifly CBE 737-800.

We can further dicuss it here when you getback.

[GSalden]

If one of you guys is interested in trying an improved PM 737-800 flight model ( airfile + aircraft.cfg file ) sent me a PM.

Several years ago I did the same for the Captain Sim757 and it was published on their website.

This model "feels" more real , follows the descent path better and has improved drag, thrust and lift values.

[GSalden]

http://www.mycockpit.org/forums/showthread.php?t=10876&highlight=cycle+time

[Bob Reed]

Keep in mind my problem is not a hardware issue as I do not have throttles that move. This is a software issue of some kind. It may not be PM at fault. When I get home I will do more digging.

[GSalden]

Quote from blueskydriver about this issue at MyCockpit :

"It's the aircraft model from PM... I switched to the Prosim737 aircraft model along time ago and PM works great with that model. I still use PM that way now. "

I have asked him what exact model and aircraft.cfg he is using.
If he helps I will share them with you to try.

[GSalden]

Update :

Three flightmodels I tested with :
- default FSX 737
- Project Magenta 737 NGX
- custom made flightmodel based on the Ifly

In LNAV and VNAV no issues.
With MCP speed there is throttle hunting which causes the speed to stay +- 15 knots faster than the dialed in speed.
Altitude Hold is also not stable. The aircraft either descents a bit and then climbs back, or climbs a bit and then descents back.

This is with all flight models.
Disabling the PM mcp and using the default mcp solves these issues.

I tried playing around with the cycle time in the MCP menu. Low till 25 ms and all the way up to 100 ms.I uses 30 now.
I tried with Descent/Speed control on/off.
Nothing fixes this.

In the CDU Maintenance / AT page 1: all sensivities set to 120 for spd / spd / thr / lvl/ mach . Page 2 P/D both 120.

At altitudes higher than 11.000ft the speed is accurate with mcp speed. Alkso no issues with pitching up/down.
250 knots = 250 knots
220 knots = 220 knots
210 knots = 210 knots. 

In the CDU Maint AT pages there are settings for <11000ft and >11000 ft.
My tests showed that higher than 11000 ft there was no issues...

The approach issues where the aircraft starts to zig-zag or pitch up/down I was able to fix with my custom flight model.
My workaround : 

1 Manually : When capturing the localizer I turn off AT and apply thrust manually. In this way I am able to land with VorLoc and App and make a nice landing.

2 Autolanding : When capturing the localizer I set the mcp speed to 15 knots below the speed I need to fly. Then the aircraft flies at the speed it has to.
At 500 ft I set the MCP speed to Vref -5 instead of Vref + 5...
This causes a very nice autolanding with a beautifull flare....


I can conclude that the PM mcp alone or icw the CDU is causing the speed/altitude/throttle hunting issues...
In several replies years ago Jonathan wrote that they never found the cause but it was something in FSX...
With Fs2004 they never had the issue.

Therefore we have to find a better workaround ourselves...

Unfortunately I have sent my flight models to 3 people and no one has given feedback up to now.
Imho we can make things better if we do this together.

So come on guys, lets solve this...