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kenthom

Auto throttle, AP problem

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While in flight the speed wants to increase eventually running me into overspeed.  The FMC speed is set and the autothrottle is is armed and on.  The plane reacts by increasing pitch taking me past the assigned altitude while climbing to FL.  

 

Any suggestions?

 

Thanks

 

PS, I am new to the 777 but I have flown other flights and this has not happened.  I believe I followed all the correct procedures in setting up the flight.

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Hi,

 

What is displayed on the FMA when the problem occurs?

I guess you are using a hardware for the thrust levers (throttle quadrant)? How is it configured?

The fact the FD commands a pitch up to reduce the speed make me think that something may be wrong with your throttle quadrant and the autothrottle is not able to reduce the thrust because a signal from your hardware overrride it, hence the pitch up.

 

You may try disconnecting your throttle quadrant when the problem occurs to see if it fix the issue.

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I have flown two more flights since the last posting.  I am using the Saitek yoke and throttles.  P3Dv3.   I set up a flight with everything programed into the FMC.  The one thing that I noticed happening is that I planned the flight for 35000 and after doing all of the flight management entries and after takeoff I noticed that the FMC had the cruise altitude at 37000.  So I did the flight level change by putting /350 into the FMC at each of the waypoints.  It computed them all.   About 2/3 way through the flight the auto pilot/auto throttle starts increasing the speed way into the red zone.  I have to disconnect the auto throttle and then use manual flight to get the plane to reduce speed and drop back down to the correct altitude.  I re-initiated auto throttle and all was fine.  However, it did not begin decent to the next waypoint as programmed.  When I tried to make a correction the sim crashed to desktop.

 

The flight prior to the one describe essentially did the same thing.  The FMC changed my cruise altitude.  I corrected.  Later in the flight the auto throttle went crazy. 

 

I believe the FMC is set up correctly.  I have never experienced this behavior in the NGX or any other plane.  Is it possible the 777 is so sensitive to the throttle that it would spike like that?

 

I have looked at the manual and watched videos.  I will do both again to be certain I am doing everything as directed.  Just thought I should post this in case there is something else I can to fix it that you are familiar with.

 

Thanks.

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Regarding the change of flight level:

_ Do you mean that in the init page you set FL350 but then the cruise flight is changed but the FMC to FL370? That's strange. Where do you check the flight level?

_ To change the flight level, you don't need to set the new flight level for each waypoint and it is not the correct way. Just go to VNAV cruise page and enter 350 on the top left. 

 

Regarding the hardware config, do you use fsuipc?

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I do not assign my hardware through fsuipc.   I am on a flight now and I unpluged the throttle as you suggested.  I monitored it for awhile and it seemed to be very smooth with no oscillation.  I plugged the throttle back in and so far so good. 

 

Yes, in the init page I set the FL350.  And yes, at some point during the early part of the flight I noticed the FL370 on the FMC.  I am going to assume it was something I was doing wrong.  I took it all very slowly while programming this flight.  There was no change on the FL.  

 

A question.  When in flight and prior to the FL changing in the decent, do you have to change the FL manually on the altitude panel just before you get to that waypoint.  In other words, while at FL350 can you manually set the altitude at FL250 and then expect VNAV to fly at FL350 until it gets to that waypoint.  Or, do you have to wait until the last waypoint just prior to the decent waypoint.  I hope you can understand my rambling. 

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I'm not sure I have understood exactly what you want to know but here are the different cases I can figure out from your question:

 

1) You are at cruise level, FL350. So FL350 is on the VNAV cruise page (and at all the waypoints until the calculated TOD) and on the altitude box of the MCP. 

 

_ If you only decrease the altitude in the box of the MCP without pushing the altitude button, then the AP will maintain FL350 until the calculated TOD where the FD will command the descent automatically down to the altitude set in the MCP.

 

_ If you push the altitude button after having reduced the altitude set in the MCP, then it will command an immediate descent to the altitude set in the MCP.

> However there is two cases:

_ If you push the alt button while you are more than 100nm (if I recall well) of the TOD, the FMS will enter in ECON cruise descent mode which means that the new altitude you set in the MCP becomes your new cruise altitude and the aircraft will descent down to that altitude and level off for a new cruise. The TOD will be recalculated accordingly.

_ If you push the alt button within that distance of the TOD (100nm TBC), then the FMS enters in descent mode and will start a shallow descent until it either reach your new altitude or until it reaches the calculated descent path and then adjust the descent rate to follow the path until it reaches your altitude set in the MCP.

 

2) You are at cruise level, FL350. So FL350 is on the VNAV cruise page on the altitude box of the MCP. But you have enter a lower altitude at or several waypoints in a raw in the leg page of the CDU, then the FMS will consider these altitude as constraints to be taken in account for the descent and calculate a TOD accordingly. Therefore if you that altitude in the MCP, the FMS will go to descent mode and command the descent automatically when you reach the calculated TOD.

 

I hope it is clear.

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Wow, thanks so much for your explanation.  This plane is going to take awhile to learn all the nuances.  It is like a cross between a Boeing and an Airbus. I have flown the A320 and NGX 737 for so long and thought I knew a little bit about them.  This is a whole different beast.  I have skimmed through the manual but have not looked at it in depth.  I only have the manual on my sim computer so I am going to download it onto my tablet so I can spend some time in it when I am not flying.

 

Thank you for your patience.  By the way, the flight went smoothly after I plugged the throttle back in.

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You are welcome.

 

If you haven't yet, I would strongly recommend you to fly both tutorials. Tutorial 1 is provided with the addon and the 1.5 is available for download.

 

Then the FCTM along with the FCOM v1 will teach you how to fly the aircraft and the FCOM v2 will teach you the aircraft systems.

Last, the QRH along with electronic checklist system will help you to handle the failures and emergency situation once you are comfortable with the normal operations.

This addon has a lot to give if you take the time to explore it.

 

Enjoy!

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Yes, in the init page I set the FL350. And yes, at some point during the early part of the flight I noticed the FL370 on the FMC. I am going to assume it was something I was doing wrong. I took it all very slowly while programming this flight. There was no change on the FL.
The change from FL350 to FL370 on the legs page is probably the result of step climb to FL370 planned by the FMC at some point in the future. If you want the airplane to remain at FL350, you need to replace RVSM with 0 on the Init Page. Leaving RVSM on the Init Page will result in step climbs of 2000 feet. If the flight is long enough you would probably also see FL390 on the Legs Pages after the pages that had FL370.

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Silly me, I completely forgot the step climbs calculations... but I understood that the cruise level changed itself in the VNAV cruise page which cannot be unless the option auto step climb is active.

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That is exactly what it was.  I began a flight and then sat down to read tutorial1. I had observed FL370 on the LEGS page and later in the flight it changed to FL390.   I did not find anything about the step changes in the tutorial and decided to ask the question on the forum.  To my surprise, the answer was already here!    Bud, I probably mislead you about the cruise level changing in the VNAV page.   I thought it had because I saw the higher altitudes on the LEGs page. 

 

So to be clear.  IF the option auto step climb is active, and, I have a cruise FL350, the plane will climb to FL370 during the flight all by itself?  I have to take no action? 

Secondly, IF I deactivate the auto step climb option the plane will reflect cruise FL350 on the LEGs page?

Thirdly, is the step climb real world procedure for full efficiency or some other reason?

 

As always, thanks.  I am really enjoying the plane.

 

Ken

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So to be clear.  IF the option auto step climb is active, and, I have a cruise FL350, the plane will climb to FL370 during the flight all by itself?  I have to take no action? 

 

If you have auto step climb on, yes.

 

 

 


Secondly, IF I deactivate the auto step climb option the plane will reflect cruise FL350 on the LEGs page?

 

No. It will still show FL370, because the aircraft is planning the step climbs in its calculations (see the PERF INIT page: STEP CLIMB value).

 

 

 


Thirdly, is the step climb real world procedure for full efficiency or some other reason?

 

Fuel efficiency/.

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Hi Ken,

 

Just to make it clear, the auto stemp climb option doesn't exist in the real aircraft.

 

Once the real aircraft has reached the cruise altitude, it will change flight level on herself at only one condition: it reaches the TOD and the altitude set on the MCP is  lower than the actual cruise level.
 

So if the auto step climb is deactivated, then no matter what the calculated step climbs are in the leg page, the autopilot will maintain the initial cruise level even if you set a higher altitude on the MCP. To perform the climb when reaching the S/C pseudo waypoint, then you have to set the altitude on the MCP to the next level and push the altitude button.

If the auto step climb option is selected then it will simulate these pilot actions for each new step climb.

 

Now regarding the step climbs, yes they are used in real world for efficiency (economy) purpose. Generally speaking and out of other parameters such as the wind, the higher an aircraft flies, the lower the consumption.

But the optimum level depends on the weight of the aircraft: the heavier the lower. So during the cruise, the aircraft consumming fuel will get lighter over time which means that her optimum level will get higher. This is what is reflected by the calculated step climbs of the FMS.

 

Of course the optimum path would be a continuous shallow climb but this is not possible due to the cruise levels in the rules of the airspaces.

Only Concorde could perform this continuous cruise climb due ti the fact it flew way higher than any other aircrafts and therefore with no possible traffic conflict.
 

In the CDU, you can set the high of the step climbs. If you set RVSM, the step climbs will be 2000ft, which is valid for most of the airspaces. 

In some particular airspaces you can find have 1000ft step climbs (specialy over Atlantic ocean) and in some others they were 4000ft (non-RVSM airspaces but they tend to disappear).

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Very good.  Thanks again to all of you for the information and explanations.   Very helpful.

 

Ken

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Hi Ken,

 

I find it useful to remember that the VNAV modes differ for each stage of a flight. So climb, cruise and descent modes need separate understanding.

 

Assuming autopilot and VNAV are engaged, the transition between climb and cruise is automatic when the aircraft reaches the cruise altitude. The transition is also automatic when the aircraft reaches TOD (assuming you have set one!).

 

As Romain pointed out, the manuals are very helpful in describing each mode independently.

 

Good luck!

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While in flight the speed wants to increase eventually running me into overspeed.

 

If this is not triggered by auto step climb (and I assume it's not), then it is a common problem with physical throttles. I mean your Saitek THQ is absolutely ok, but at some point the beast forgets to use its autothrottle values. To put it simple - when you advance throttle levers for take-off, you probably leave them at 100% or something close to it and then at some certain situation plane ignores the signal from FMC and instead of using commanded values overrides them and looks for your THQ signal. I have the same problem and I am not sure how to deal with it correctly, but to avoid such situations I set throttles to ~ 40%after take-off. Real 777 does not have this problem ofcourse, because levers will always follow the A/T commanded setting.

I would like to hear some advice regarding this - is there a better way to do this?

 

P.S. What weather add-on are you using? Not sure about P3D, but FSX is infamous for its terrible weather (winds aloft) system, that often created situations where wind could change for 180 degrees in a matter of second. It was a nightmare for the MD-11s Autoflight system, resulting with same behaviour - nose up, to reduce overspeed.   

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Hi,

 

THe issue is often when a spike from the throttle quadrant remains the addons of the position of the levers of the TQ. As you said, as long as the throttle quadrant levers don't move, they the addon forgets about them.
A good way to avoid that is to said the option A/T MANUAL OVERRIDE to <IN HOLD MODE ONLY.

That way, the autothrottle will ignore the signal of the TQ as long as it is not in hold mode.

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Romain, thanks for explanation! Just to mention - i forgot (havent been simming for couple of years :D ) about spikes, you are right!
And I guess throttle spiking is the answer to the OPs problem - why it shows up randomly, and Saitek's THQ is rather ###### in terms of quality. Sad but true. 
Is that A/T manual override setting inside PMDG Setup, located in CDU? 

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Is that A/T manual override setting inside PMDG Setup, located in CDU? 

 

Yes, have look at the introduction manual to find exactly where, I don't have neither the manual nor the aircraft next to me right now.

Search for "A/T MANUAL OVERRIDE".

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