FSX ADI Heading Steering Bar

[n4gix]

The ADI on the T-38A has a bank steering bar* that provides a visual cue on which direction to steer the a/c for an intercept.

When in TACAN Mode and the Normal/Manual switch is in Manual position, the bank steering bar is visible and will provide a cue to steer based on the heading bug's position.

The bank steering range limits are +/- 35º

*Note: the bank steering bar looks like a yellow, vertical needle like one that's used for a flight director.

The 3d "bank steering bar" is keyframe animated from frame 0 to frame 254, so the center point is frame 127.

Here is the animation script for the bank steering bar for this specific mode:

    (L:T38_HSI_NormMan_Switch,bool)
    (L:T38_HSI_SourceSwitch,enum) 0 == and
    if{
    25 (>L:T38_FD_VerticalBar,enum) <!-[COLOR="Red"]- Unhides bank steering bar -->[/COLOR]
    (A:PLANE HEADING DEGREES GYRO,degrees) (A:AUTOPILOT HEADING LOCK DIR, degrees) - 180 + (>L:HdgError,enum)
      180 (L:HdgError,enum) - 35 &lt;=
      180 (L:HdgError,enum) - -35 &gt;= and
          if{ 180 (L:HdgError,enum) - 3.6285714 * 127 + (>L:Attitude_Bank_Steering,enum) }
    }

It works quite well until the heading bug is set within the range of +/- 20º due North, when suddenly the computed (L:HdgError,enum) value goes "wonky" as a result of the wierd way that FSX "does the math"...

Has anyone any suggestions on how to modify the formula to "unwonky" the computed heading error value?

 

[Roy Holmes]

Bill,
The F-4 had a similar bank steering bar that was intended to give an asymtotic approach to a TACAN or INS Target radial and I guess it was like the T-38 one. The bank limits were 30 degrees and the error limits were 60 degrees. Never met anyone who actually used it and the Manual basically said ignore it.

Seems to me the wonky results would occur if the returned value was greater than 254.

I put the Autopilot Heading as 270 and the aircraft heading as 300 in Excel and got an answer of 888. Pretty sure I followed the code correctly.

For my edification, why use enum and multiply by 3.6285714? Also if the problem occurs round North would you not use dnor?

Roy

 

[n4gix]

Oh, I had quite forgotten that I'd posted the question, so of course I forgot to post the solution as well...

Here is the "unwonkied" version:

  (L:T38_HSI_NormMan_Switch,bool)
  if{
      25 (>L:T38_FD_VerticalBar,enum) <!-- Unhides bank steering bar -->
      (A:AUTOPILOT HEADING LOCK DIR, degrees) (A:PLANE HEADING DEGREES GYRO,degrees) - dnor
      s1 180 &gt; l1 360 &lt; and if{ l1 360 - } els{ l1 } -35 max 35 min 35 + 3.6285714 *
      (>L:Attitude_Bank_Steering,enum)
  }

Notes: The "s1" stores the calculated results in register 1. The "l1" (lowercase L) loads the value of register 1 for subsequent evaluations. What we're doing is dividing the task into two 180º halves, then subtracting 360º for the "left half of the circle." The end result is that the bank/turn steering bar will always indicate the shortest direction to turn to achive an intercept with the heading bug's current position.

The scalar of 3.6285714 is necessary to "scale the calculated value" to the model's keyframe animation of 254 frames:

-35 * 3.6285714 ~= -127
 0  * 3.6285714  =    0
 34 * 3.6285714 ~=  127

 

[Roy Holmes]

Bill,
Thanks for the explanation.

Do you know how the flight director is programed in FS. Supposedly it can be active when the autopilot is not. I have tried using it and I get odd results. I have had readouts of AUTOPILOT DIRECTOR PITCH & BANK and studied what they did but have been unable to make any sense of it. Basically I'd like to be able to use it on manual ILS approaches.

I'm still curious about the function of the bank steering bar. I can see how it gives you a left/right indication of the quickest way to turn to the heading set on the bug. But after the turn you would be on the correct heading but offset from the track/radial to the TACAN unless something else turns the heading bug. The F-4 bank steering bar caused you to overshoot the turn initially then indicated a slow turn in the opposite direction to get you on the radial. Like an s-turn to intercept the radial. Of course, I'm not familiar with the T-38 systems.

Roy

 

[n4gix]

The unfortunate answer about the "FD system" is that it is throughly useless except when the AP is active, and some lateral navigation mode is engaged (localizer or GPS steering). Then the steering (lateral and pitch) will anticipate and indicate proper lateral and pitch commands.

With the AP on but no modes active, the lateral/pitch bars simply follow along and provide no anticipatory cues.

Which is why, after spending several weeks on this part of the project, we ultimately decided to offer the user the option of using the mostly non-functional "FD system" or a "mirror image" of the CDI and GS indicators when in ILS mode, and a primitive solution which will provide steering guidance TO the localizer station, which is of course not much help at all with regards to a proper intercept guidance TO a specific radial.

Specific to the T-38A/C, there is a SPDT switch that will toggle the lateral steering bar on the ADI between "Normal" and "Manual" modes. In "Normal" mode, it is coupled to the FD system. In "Manual" mode it is coupled to the HDG Bug tracking system.

Typical usage would be to set the OBS to the desired localizer radial, then use "Manual" (HDG Bug) tracking to plot your intercept. As the CDI begins to close towards the selected radial, one would flip to the "Normal" mode to then provide fine steering cues to track the selected radial.

 

[Roy Holmes]

Bill,
Thanks. You confirmed my conclusion about the FD.
Since the FD system only works with AP engaged it is truly useless. Pity.

I remember a gauge from many years back that had a great FD and you could fly ILS manually down to touchdown with it. Seen nothing like it since and would love to recreate one, big task though, needs a lot of thought.

Thanks also for the explanation of the T-38 procedure.

Roy

 

[n4gix]

One other note about the FD system. None of the FD token variables will provide output signal when the sim is paused...

...which, while it makes sense also makes it very difficult to calibrate if one is using 3d XML driven bars or wings...