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LecLightning56

Maintaining runway centreline in manual approach

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I have been enjoying the use of the FSiPanel tool to practice approaches with the PMDG 777.

 

Naturally, I have been disengaging autopilot to fly the approach manually to gain the most benefit from such an experience, having established on the localizer and glideslope.

 

As an experiment I have been comparing the relative merits of an Airbus A320 with the 777 in such approaches under conditions of zero wind to assess the handling qualities of both aircraft.

 

I have been flying manually from about 1000-800 feet above ground level and I have found that the A320 requires very little aileron control under these conditions to perform a perfect landing, as one might expect under zero wind.

 

With the PMDG 777 however, I have been finding that as soon as one gets to 400 feet and below, that the aircraft starts to veer off the extended runway centreline (typically to the right at EGLL RW27R), requiring some aileron input to correct. I am assuming that there should be no wind to correct for under my test conditions. I am using a CH Eclipse yoke which has null zones in pitch and roll so I am assuming that the result reported is unaffected by yoke calibration.

 

Is this phenomenon the result of an attempt to model small atmospheric fluctuations in close proximity to the ground within the PMDG 777 simulation? I have disabled turbulence effects in FSX and have also disabled the winds aloft option within FSX. It is also possible that FSiPanel itself is 'throwing in' a wind component that the A320 is largely unaffected by and the 777, with a much larger surface area, is responding to, although I have not selected weather options for the approach itself for the aforesaid tests in FSiPanel itself.

 

Any thoughts about your own personal experiences/ knowledge of 777 approaches in terms of what the PMDG 777 actually injects into the FSX weather model would be very welcome to enhance my understanding of flying this wonderful simulation.

 

 

Paul Hermon

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Aircraft differences aside, enable thermal visualisation to see if there are any on your approach.

 

I was dogged by severe thermals on finals to EGPN RW09 for years regardless of weather settings until a fellow simmer helped me out.

 

Simply delete the ThermalDescriptions.xml file in your root FS directory, backing up if required.

 

It contains all the default thermal data, which I've never missed as I use AS2012.

 

I hope this helps.

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With the PMDG 777 however, I have been finding that as soon as one gets to 400 feet and below, that the aircraft starts to veer off the extended runway centreline (typically to the right at EGLL RW27R), requiring some aileron input to correct. I am assuming that there should be no wind to correct for under my test conditions. I am using a CH Eclipse yoke which has null zones in pitch and roll so I am assuming that the result reported is unaffected by yoke calibration.

 

Hi, Paul,

 

I have not experienced a veering off course to one side on short final and I haven't seen any other reports of this.  So I would guess it's some problem in your setup.  I suggest that you try a couple of things.  First, be very sure your yoke is properly calibrated -- if possible try a different yoke or joystick.  You can also look at the synoptic flight controls display and see if there is some uncommanded aileron or rudder input or imbalance on short final.   Second, try a circuit without using FSiPanel (not familiar with it, but it might be the source of the problem).  Third, try a different airport (However I often practice landings at EGLL and never saw this problem). 

 

Hope this helps!

 

Mike


 

                    bUmq4nJ.jpg?2

 

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With the PMDG 777 however, I have been finding that as soon as one gets to 400 feet and below, that the aircraft starts to veer off the extended runway centreline

 

I'm going to go with user procedure on this one.  As with any troubleshooting, in order to isolate the cause, it's best to turn everything off.  You started to do this by turning weather off, but if there are other issues, it's best to try turning everything off (like FSiPanel, since it does include weather processing).  If anything, the only thing I'm really doing on final is adjusting pitch to maintain the glidepath - wind or no wind.


Kyle Rodgers

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Thank you all for your useful advice. After extensive testing I finally concluded that I was not fully able to eliminate the problem with the 777.

 

Let me explain a bit further. What I have encountered is the result of disengaging the autopilot having established on the localizer and glideslope, then hand-flying the aircraft for the remainder of the approach. My observations are based on minimal aileron and elevator input in conditions of zero wind. I am satisfied that the controllers, particularly my yoke have sufficient null zones to take up any failure to properly centralize and hence provide no input to the flight controls. If little or no controller input is used under these conditions, then it is observed that the aircraft suffers an element of drift away from the extended runway centreline (as visualised by monitoring the localizer marker on the PFD). The phenomenon starts to occur very close to the ground at approximately 300 feet above runway level and below. The nature of the drift itself is that the aircraft maintains the same magnetic heading appropriate to the ILS localizer (it being understood that localizer capture by the autopilot has been disengaged at this point), but is seen to drift sideways very slightly by some invisible influence. The sideways drift ultimately requires some correction but I am not convinced that it can be attributed to a small but imperceptible crosswind component under conditions of nil wind.

 

The question to yourselves in the community is this: should one expect an aircraft of the proportions of the 777 to suffer such drift under nil wind if the controllers are effectively untouched on the approach, once having disengaged the autopilot? It is noticeably the case that a smaller airframe such as the Airbus Extended X A320 from Aerosoft suffers no such drift under the same test conditions, in the absence of any tweaking in FSX or by any other means, although it has to be taken into account that the A320 offers a significantly smaller fuselage surface area than the 777.

 

Your thoughts and experiences of the nature of such drift would be appreciated. I am only trying to fathom whether or not such behaviour truly replicates the real aircraft's characteristics or if I am having issues within the 777 settings themselves, since I am inclined to rule out FSX at this stage.

 

Please note that I have multiple installs of FSX and the PMDG 777 and the phenomenon occurs in all such installations (including a Windows 8.1 installation - yes the 777 does work in Windows 8.1!).

 

I look forward to any further thoughts from personal experiences or perhaps even the PMDG team itself if they can verify that the behaviour is indeed a real-world 777 characteristic.

 

Paul Hermon

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Theoretically, if you don't do anything NIL wind, disconnect the AP at a very low altitude, all you need is to perform the flare. Airplane set on the correct Track.

 

However, it is possible that at some point of the flare, some roll input is made to the yoke during the flare, or a tendency to try to align the horizon with the glare shield which result the airplane drift off from the centerline.

 

In a level D sim, the theory works because it can create a perfect environment if we wish. In real life, in a brand new airplane perfectly calm condition, I would say "yes", I have done a low level disconnect after about 250ft, due to low cloud base for a manual landing it works pretty well. However I am sure subconsciously I did make some small adjustment but definitely not much.

 

In real life however, with some older airplanes, the airframe is "bended" slightly and it will constantly required some input to maintain its path, feels a driving a car without a proper wheel alignment. This is what you won't experience on the airbus, because their FBW masks this problem. But not on the 777.

 

In conclusion, I did not find myself having to make excessive roll input in flight sim to fly on the centerline even if I did a long 10-15 miles visual approach.

 

Hope it helps.


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 What I have encountered is the result of disengaging the autopilot having established on the localizer and glideslope

Paul Hermon

This would suggest that you are hand flying the entire descent. If so you will be all over the place! You cannot ever hand fly an approach as accurately as the AP.

In real life you make small corrections only otherwise as you get closer your corrections increase in amplitude and you will almost certainly have to go aound.


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Hi vololiberista,

 

Thanks for the advice. I only really hand fly the approach from about 400-500 feet above ground level. I am merely concerned with the correctness of the PMDG 777 model in conditions of nil wind when flying such approaches, but with a caveat that, as an experiment, I have chosen to let the aircraft fly itself once having established on the ILS and autopilot subsequently disengaged at about 400 feet above runway level. Alright, not real life then, but it is interesting all the same to observe any drift and to identify the causes of such drift, either within FSX or the PMDG 777 itself. So purely an academic exercise and I can assure you I have no intention of trying it out on the real thing!

 

P.S. I appreciate your comments about the difficulties in correction the closer you get and that is indeed my own experience in FSX.

 

 

Paul Hermon

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Hi vololiberista,

 

Thanks for the advice. I only really hand fly the approach from about 400-500 feet above ground level. I am merely concerned with the correctness of the PMDG 777 model in conditions of nil wind when flying such approaches, but with a caveat that, as an experiment, I have chosen to let the aircraft fly itself once having established on the ILS and autopilot subsequently disengaged at about 400 feet above runway level. Alright, not real life then, but it is interesting all the same to observe any drift and to identify the causes of such drift, either within FSX or the PMDG 777 itself. So purely an academic exercise and I can assure you I have no intention of trying it out on the real thing!

 

P.S. I appreciate your comments about the difficulties in correction the closer you get and that is indeed my own experience in FSX.

 

 

Paul Hermon

 

well, if you disconnect the AP and do nothing then the a/c will maintain the last instructions given to it by the ap. So if the ap was correcting drift when you disconnected then that correction will continue until you do something or reconnect the ap.


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Super VC10 into LOWI with PF3 at a cinema near you

https://www.youtube.com/watch?v=298UDyNmgUA

 

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Thanks for response and comments. What I am trying to isolate is the cause of the drifting in the first place and seek to eliminate it from the simulation. I appreciate that, in the absence of a/p that no further correction of drift will take place. As a footnote, a member of the real-world airline pilot community has contacted me privately and has let it be known that he had experienced the same issue as myself with the PMDG 777 and managed to eliminate it by some means: hopefully he may be able to replicate the situation and check settings, both within FSX and the 777. If I get any further feedback from him I shall report here for everyone's benefit.

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I believe that I have found a workaround to eliminate the drift on short final under nil wind with the 777. FSX has to set to clear all weather and uncheck download of winds aloft and, in addition, FSUIPC has to be set to eliminate wind and all atmospheric disturbances under the wind tab in FSUIPC itself (registered copy required for this purpose). If the approach is then flown under these conditions then, at the point of disengaging the autopilot, the primary flight computer (PFC) is disconnected instead, the drift is effectively eliminated and only small changes in pitch are required to land.

 

It would appear that the PFC itself must presumably introduce small imperceptible changes in control surface movement under these conditions where one might expect no such correction laterally, which results in the observed drift. Disconnection of the PFC does not switch off the FBW or disconnect the controls, it merely changes the laws governing the relationship between the control inputs and the control surfaces themselves. A search on the internet has revealed that this may be referred to as a secondary mode on the 777, akin to the alternate law in the Airbus. Whereas I appreciate that this is not standard procedure with 777 operations and may only be used under failure conditions, the exercise I have conducted is purely academic but nevertheless has exposed two things:-

 

1. To completely switch off wind and all atmospheric disturbances e.g. turbulence in FSX, a registered copy of FSUIPC is required: it would appear that FSX is not capable of

completely removing the influence of wind and atmospheric disturbances by itself.

 

2. The PFC appears to be responsible for small changes in control surface deflection under these flight conditions which are sufficient to cause the aircraft to drift away from the extended centreline of the runway.

 

Anyway, the above are my findings on the subject, but how realistic the behaviour of the PFC is under these circumstances within the FBW system I cannot comment on. Perhaps somebody at PMDG can enlighten the community about their experiences based on the shared information from the Boeing Company. Any further thoughts from the community at large would be most welcome.

 

Paul Hermon

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Quote from above: 'A search on the internet has revealed that this may be referred to as a secondary mode on the 777, akin to the alternate law in the Airbus'.

 

 

Correction to this quote before I get my knuckles rapped by PMDG. I believe this is referred to as 'direct mode' in the 777, whereby no commands are processed by the PFCs and hence is probably not akin to the alternate law in the Airbus.

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The question to yourselves in the community is this: should one expect an aircraft of the proportions of the 777 to suffer such drift under nil wind if the controllers are effectively untouched

I disagree......the real question is......do other users see this as well!?

 

So far nobody reported that.

 

You say a real world user has contacted you by PM and confirmed the drift.

 

For the record guys,.......that was not me!

 

But I might be able to give it a shot today and see if I get the drift thingy.

I have not noticed it thusfar.......so if it is there at all......then it must be quite small.

 

Ps, does this drift allways happen?

At every airport?


Rob Robson

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Hi Rob,

 

Yes, in the absence of clearing all weather in FSX by the procedure detailed above and disconnecting the PFC the drift always happens irrespective of airport. Please note that the drift occurs in the complete absence of setting a crosswind in the landing by whatever means, either through use of the FSiPanel tool or weather-injection software such as ActiveSky Next.

 

To reiterate my earlier messages, the issue does not occur with Aerosoft's A318/319/320/321.

 

The drift is quite small but sufficient that, unless corrected for with a little aileron, will result in touchdown well to the side of the runway.

 

Thank you anyway if you manage to try it out. I shall be interested in your findings.

 

 

Paul Hermon

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Hi Rob,

 

Yes, in the absence of clearing all weather in FSX by the procedure detailed above and disconnecting the PFC the drift always happens irrespective of airport. Please note that the drift occurs in the complete absence of setting a crosswind in the landing by whatever means, either through use of the FSiPanel tool or weather-injection software such as ActiveSky Next.

 

To reiterate my earlier messages, the issue does not occur with Aerosoft's A318/319/320/321.

 

The drift is quite small but sufficient that, unless corrected for with a little aileron, will result in touchdown well to the side of the runway.

 

Thank you anyway if you manage to try it out. I shall be interested in your findings.

 

 

Paul Hermon

Two things you need to consider.

Firstly as regards drift. The a/c is quite large and can be affected by any breeze. How much is the drift in knots?

Secondly you may be confusing drift with momentum. Again it's a large a/c and therefore subject to high momentum forces even in zero wind conditions.

It seems likely that the a/c is behaving normally especially if PMDG have modelled wind effect and momentum which I wouldn't be surprised.

There are a host of things on the model that users have never come across before in simming and therefore led to believe are bugs.

You have to think about how the real a/c would fly in real conditions. If you're conducting a handling exercise don't expect it to behave like a Cessna!


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Super VC10 into LOWI with PF3 at a cinema near you

https://www.youtube.com/watch?v=298UDyNmgUA

 

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