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scottb613

GTN and the dreaded Holding Pattern ?

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Hi Folks,

 

RE: Learjet 35A

 

Just documented the turns for the holding pattern issue - even 2.8 can't seem to make the turn...

 

Reported this on Flight1 as well - maybe they can offer some insight ?

 

Is the plane just not turning fast enough or is the holding pattern too small ???

 

 

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Flight1 Link: http://simforums.com/forums/in-progress-adjust-holding-pattern-size_topic57547.html

 

Hi Tiberiu,
 
Sure - here's a hold at PVD - speed is held at around 200 kias throughout the maneuver - no auto-throttle... I noticed the pattern size does change with speed but it doesn't even appear close to what is needed... 200 kias is pretty slow for a Lear and I'm turning with a 25 degree bank which seems reasonable... If I slow down to like 160 kias - for a procedure turn as part of an approach - it does seem to make it around... Happens every time regardless of winds... It happens to everyone with this plane - not just me... 
 
 
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And I am slung from orbit into oblivion...
 
 
Regards,
Scott

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Scott,

Myself and a fellow I fly with often have found you need to keep the IAS around 150Kts or so to have a change of holding the turns in a holding pattern --- and even that doesn't always work.  So I keep an eye on the bank angle in the turn and just maintain that if the Nav goes to Nav Arm, and sometimes the AP will recapture the course.

Al

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Hi Al,

 

So that autopilot adjustment that (you ?) posted a while back doesn't really work and 2.8 hasn't fixed this issue ? Flight1 says the Lear 45 works fine with the GTN hold and has a 25 degree bank - so I'll have to try a comparison as I know the Lear 35 also uses a 25 degree bank angle... I'll post a comparison of both planes at 200 knots - I'm curious if the actual size of the holding patterns the GTN presents are different based on the aircraft ? I know the size does change with speed... It seems like the Lear 35 is making a normal turn at the same bank but just can't turn tight enough ? Perhaps the 35 is starting to turn later ? For some reason the Lear 35 turn radius is substantially larger than the Lear 45 ?

 

I have most of my stuff back together now so maybe I could look at some issues next weekend - like that other test you wanted me to try...

 

Regards,

Scott

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So that autopilot adjustment that (you ?) posted a while back doesn't really work and 2.8 hasn't fixed this issue ?

 

Hi Scott,

 

The only AP adjustment I recall was changing the nav_proportional_control value in the aircraft.cfg, which I see is now incorporated into ver 2.8. That change was aimed at improving the intercept of radials or localizers. I seem to recall that sometimes the L35 uses less than standard rate turn bank angles when in a holding pattern, or it may be that happens when it drops out of Nav back to Nav Arm. I'll have to pay more attention to see what happens.

 

Al

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Hi Folks,

 

Well - I think I figured out what's happening - compared the Default Lear ro the FSW Lear - doesn't appear to have anything to do with the turn radius like I first suspected... When a plane flies the hold - the CDI always references the leg the fix is based on - so the CDI is centered when flying the leg crossing the fix - then the CDI is offset for a given distance when flying the opposite leg... So I did some testing at 250 KIAS on both planes with no wind - the GTN creates a holding pattern around 5.5 miles wide - the Default Lear autopilot stays connected throughout the entire holding pattern even when the CDI is deflected 5.5 miles... The FSW Lear autopilot disconnects instantly any time the CDI reaches a 2 mile deflection - which also explains why we thought the speed was causing issues... If you slow down enough to where your turn radius is less than 2 miles the FSW Lear will fly the pattern but it is totally based on CDI deflection distance not speed itself... All said and done - I don't know how to fix it but at least it's identified...

 

Regards,
​Scott 

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Scott,

 

Interesting info. A 2NM CDI deflection implies a 1NM turn radius in the holding pattern. At 180Kts TAS a 1NM turn radius requires a 25 deg bank angle, and at 240Kts TAS a 1NM turn radius requires a 40 deg bank angle. This assumes no wind, of course.

 

Al

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Guys, I see similar behaviour in my GTN 750. Regarding hold radius and procedure turns to FAF.

 

I managed to get my GTN 750 working, using the Flight 1 "beta" dll fix from their forum. I still have no charts in the aircraft GTN 750, while they show up in the Trainer?....GTN 750 also crashes every now and then...another story.

 

However, I believe the inability of our Lear to follow the GTN 750 hold, is due to the GTN not "talking" to the Lear. If you examine the holds in aircraft with an integrated FMC hold, e.g. NGX, Mjc Dash8, Airbus, the hold parameters are set via the FMC (time/distance). Then the hold computation gets fed to the Lnav in the simulated aircraft.

 

My layman's way of saying, the GTN, as a third party gauge, is not able to communicate with our Lear, which is not computing the turn via an FMC. Another way of looking at it is that we are missing the components that the FMC requires to compute and fly the hold.

 

You can fly it manually of course, or in alt hold and Hdg mode, thus using the hold course drawn as an approximate guide.....;)

 

 

The GTN 750 is a marvellous add on (when it works!) but can not replace a fully integrated FMC.

 

Perhaps in a future FSW product, they will model an FMC with the necessary code?

 

Just my thoughts.

 

Regards

 

David.

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Hi Folks,

 

Thanks for the responses and your insights...

 

Hah - Al - that's where I started - I started jacking up the allowed bank angle for the autopilot until I had the plane following the displayed hold exactly - and still the autopilot disconnected...

 

Yep - David - I would think this would happen anytime the CDI deflected over two miles... I also read about your tribulations with the GTN - so I've frozen any related updates until I've seen this issue is sorted... Yeah - FMC's would be nice - even a basic one - I used Ernie Alston's ISGsim FMC products for years in FS9...

 

So - churning this over this morning - it has to be retaled to the FSW autopilot's "ARMING" function - it's probably coded that any time the CDI is deflected 2 miles - it's in "ARM" mode...

 

Still - I don't know if it can be fixed without losing the real world arm function...

 

Regards,
​Scott

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Scott,
Myself and another fellow have noticed that often the AP goes to Nav Arm during the turn to the inbound leg after having flown outbound OK'.  So if the AP didn't lose lock on the outbound leg, why then during the inbound turn?

The other thing I've noticed is, if 'hand flying' (AP ENG off) an ILS using the FD, the GS goes to GS Arm if you get about 1 dot off the center of the GS. That doesn't seem right to me. Shouldn't you get valid FD guidance as long as you don't 'pin the needle' at one extreme or the other?

Thx,
Al

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it's probably coded that any time the CDI is deflected 2 miles - it's in "ARM" mode

Scott,

 

You are correct. It is coded to go to Nav Arm if the GPS Cross Track exceeds 2NM in magnitude. As an experiment, I increased this to 6NM and also increased the Glide Slope capture range so capture occurs, and is maintained, starting at about a half dot below the top of the GS range. A friend and I have been testing these changes and they seems to work fine so far, but there certainly could be unintended consequences.

 

In the US 200Kts IAS is the max holding pattern speed at or below 6000ft MSL. So it does not seem unreasonable to expect the L35 AP/GPS to be able to maintain 'capture' at this speed, especially since above 6000ft the max holding pattern speeds go up (230Kts IAS 6000-14000ft, and 265Kt IAS above 14000ft).

 

As for the GS, it seems to me that the system should maintain capture even if, for example, turbulence knocks you a dot low on the GS. Seems to me this is not a situation where you want the GS to fall back to arm as it does in the unmodified ver 2.8.  However, when I was flying years ago we didn't have GPS, so those with GPS experience please feel free to chime in.

 

Al

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Al,

 

I would confirm your GS comment, in normal conditions I never experienced an issue with the FD losing capture during an ILS approach.With a UNS-1C FMS (GPS) the system flew very precise holding patterns through out the performance range at both low and high altitude.If the installed GPS nav system is certified for enroute, terminal and approach modes it will fly the procedures as published.

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Gary,

 

Thanks for the real world input -- very valuable!

 

I like to fly approaches either without the FD, or with Nav and GS selected but with the AP off so I still get FD guidance. In the latter case I certainly still want FD guidance if I get a dot low on the GS for some reason.  Since you are suppose to capture the GS from below, I could see that maybe initial capture might not happen until you are within a dot and a half or so of being centered, but even then, once you have capture it seems dangerous if you can lose it by being 'too' low on GS. I noticed that with the unmodified L35 you don't lose FD capture for being 'too' high on the GS, the indicator just gets pinned at the bottom of the display. So once you have initial capture, if necessary why not just have the indicator pin at the top of the display in a similar manner (and maybe get an alarm   :wink: )?

 

Al

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The other thing I've noticed with the FD is that in an AP controlled turn, the FD and the orange triangular part of the attitude indicator do not line up particularity well. I would think that once an AP turn is established, the FD and orange triangular indicator ought to line up (fit together) fairly well. I can't recall how well this worked in earlier versions of the L35A. Perhaps this is a FSX limitation -- don't know.

Al

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I should have added above that where I find the accurate alignment of the FD most important is when flying an approach procedure with the FD on but the AP off, since now I'm interpreting what the FD is commanding instead of the AP. I guess this highlights the importance of keeping an eye on the raw data even when using the FD.

Al

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Al,

 

I would confirm your GS comment, in normal conditions I never experienced an issue with the FD losing capture during an ILS approach.With a UNS-1C FMS (GPS) the system flew very precise holding patterns through out the performance range at both low and high altitude.If the installed GPS nav system is certified for enroute, terminal and approach modes it will fly the procedures as published.

Gary,

I would appreciate your thoughts on the following:

 

One approach to fix the GPS capture and hold issue with the FSW L35A is to simply increase the GPS capture and hold range from the default 2NMs to something greater, for example 6 or 10 NMs. With this change, the the AP NAV would go to NAV CAP anytime you were within the capture range, let's say 10 NMs for sake of discussion (we can use any reasonable distance here). If you went outside this range, you would lose NAV CAP until back just within the range, within the 10NM boundary in this case.

 

Another approach is to leave the initial  GPS NAV CAP requirement at 2NMs, but once NAV CAP is established it would stay captured out to the 10 NM point. However, once you lost capture by exceeding the 10 NMs (or for any other reason), then you would have to get back to within the initial 2NM range to reestablish capture.

 

My question is, based on your RW experience which of these options seems the most realistic? I've tried both of these options with the L35A, but don't know which one makes the most 'sense'.  I wonder, for example,  if having an initial GPS NAV CAP point at 6 or 10NMs is unrealistic or an operational problem for some reason since once implemented this would be the case for all published GPS procedures, not just holds?

 

Thx very much,

Al

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Al,

 

I do not recall any published reference as to what distance or angular offset is required for a NAV capture so I can not comment on what it would be.With a VOR radial or an ILS the autopilot would typically switch from arm to capture once the CDI was alive and had movement.With GPS when a leg or procedure was activated NAV would capture when selected, the only exception I can think of would be using a GPS OBS mode.You could specify intercepting a specific radial to or from a fix, use HDG to establish the intercept angle, select NAV and it would capture when reaching the desired track.

 

There was a preflight test with the earlier FC-200 autopilot where you test the NAV arm and capture function using a valid VOR signal on the ground by simulating an intercept and capture.Set the CDI full scale off center, select the HDG and NAV arm as though you are on an established intercept angle to the radial, then slowly rotating the course knob to bring the CDI towards the center and verifying the NAV capture occurs.You can do the same thing with the the FSW L35 530 autopilot and the default GNS 530 will fly a full procedure turn and hold although not very pretty to watch how it does it.I don't have the GTN, maybe load up the stock version and see what you get.

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Gary,

 

I think L35A Nav capture, as far as VORs and Localizers are concerned, works fine and as you have described -- capture happens when the CDI is alive.

 

It has been with the NAV GPS mode that I have been experimenting. I now have 3 different cab file versions:

 

- the first one just expands the original FSW 2NM GPS capture range to either 6 or 10NMs (the 10NM value allows for higher speed holding pattern turns). Get anywhere in that range and GPS capture happens.

- the second one has an initial 2NM capture range requirement (the original FSW value), but then holds GPS capture out to some extended range (either 6 or 10 NMs). If you exceed the extended range then GPS recapture requires you to get back within the initial 2NM range.

- the third version is like the second one with an initial 2NM capture requirement, but if you exceed the extended 6 or 10 NM GPS range, GPS recapture only requires that you get back within the extended range, not the initial 2NM range.

 

I probably like the third option best, although I can't clearly articulate why (except maybe like with the VOR/Localizer, you need to be 'reasonably close' to your course for at least initial capture, and recapture is easy). I'll use it for awhile and see if there might be unintended consequences. In any case, it is kind of fun to experiment a bit. I really like the L35A.

 

Thanks very much for you input,

 

Al

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