The ILS glideslope bar (on ADI) doesn't concide with HUD glideslope - ED Forums
 


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Old 02-23-2018, 08:20 AM   #1
Maverick Su-35S
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Default The ILS glideslope bar (on ADI) doesn't concide with HUD glideslope

Hi,

For some reason, the ADI's ILS horizontal bar (the bank needle is otherwise correctly matching the real localizer) drifts away from the real glideslope as distance decreases to touchdown.

Although the VOR/ILS instrument bars on the ADI work as a flight director (thus it tells you to climb or bank in advance to the movement of the HUD's ball indication) similar to a modern airliner, the error / difference in displacement between the ILS glideslope bar (up-down pitching needle) and HUD ball becomes very great as the distance to the ideal touchdown point decreases. At higher distances the indications are quite close although the ADI needles predict when you should climb or turn. There may be nothing wrong with the error between the ADI's horizontal bar indication and the HUD's ball (in pitch) at higher distances, but as you get closer to the touchdown point, the ADI ILS glideslope bar starts to drift more and more away from the real glidelsope path that is correctly measured by the HUD's indication only.

In reality, the flight director bars (which our ADI ILS indicator mimics) may be having a great error from the glideslope and localizer indications if the airplane isn't already aligned with them, but as the distance to touchdown decreases, no matter if the plane is more or less aligned with the glideslope and localizer (as directed from the VOR/ILS facility), the difference or error between the flight director bars and the glideslope & localizer indications will also decrease.

Here's a shot proving the high drift between ILS and HUD indication and track of a landing trial:

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Name:	Su-33 ILS instrument versus HUD glideslope error.jpg
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VOR-ILS instrument and HUD glideslope indications error.trk

If our ADI's ILS bars don't work like flight director's bars, let us know. Otherwise there's no reason for the ADI's glideslope bar to start deviating from the real path during the most crucial landing phase.

Regards!
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Old 02-26-2018, 03:41 AM   #2
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Second. I've been having a hell of a time trying to carrier land IFR. Seems pretty broken TBH.
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Old 02-26-2018, 10:52 PM   #3
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Quote:
Originally Posted by Maverick Su-35S View Post
Hi,

For some reason, the ADI's ILS horizontal bar (the bank needle is otherwise correctly matching the real localizer) drifts away from the real glideslope as distance decreases to touchdown...
That is probably because the ADI in the Su-33 is just an ADI, not an ADI/ILS combination. If you look at the gauge, there are no glideslope/localizer flags to indicate when you have captured the ILS signals.
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Old 02-27-2018, 06:55 AM   #4
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Quote:
Originally Posted by Wrench View Post
Second. I've been having a hell of a time trying to carrier land IFR. Seems pretty broken TBH.
Therefore, I use the indications on the HUD and the monitor (also for night landings).
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Old 02-28-2018, 11:21 PM   #5
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The ADI and HSI ILS bars work just fine for me in LDG mode
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Old 03-01-2018, 02:18 AM   #6
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Quote:
Originally Posted by IvanK View Post
The ADI and HSI ILS bars work just fine for me in LDG mode
They do. The OP was saying that the ADI's ILS bars weren't functioning properly.
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Old 03-01-2018, 09:39 PM   #7
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Quote:
Originally Posted by wavn View Post
Therefore, I use the indications on the HUD and the monitor (also for night landings).
How do you use the HUD? The 2-Circle Glide-path indication system makes no sense to me.
The way it seems is counter-indicated by forums posts, videos, and even the training missions, but I swear all of that is wrong.

The closest I've been able to figure is that the one should place the larger circle on the center of the Roll and Back Indicator, using the throttle to control descent rate. I can fly right down the glideslope until the ILS on the HSI goes live, and then the rate at which the aircraft moves relative to glide-path seems to increase exponentially, and I can never stay "locked" the glide-path like I can in literally every other airframe I've ever flown.

Also, What indications do you use on the monitor?
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Old 03-02-2018, 12:27 AM   #8
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Think of it as a three dimensional picture of the glideslope at two distances; the larger circle being your current position and the smaller circle being farther away.
Put the thing on the thing, and use your AoA indexer on the left side of the HUD and you're golden.
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Old 03-02-2018, 03:08 AM   #9
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This is what happens if I "put the thing on the thing"
I know this was far from a perfect demonstration, given my airspeed was high, and AOA low, but what I'm really worried about is how the HUD works, for for the scope of this video it makes no difference.
I couldn't keep the circles perfectly concentric, given as my flight path continued to devolve, it was increasingly difficult to fight my natural urge to do literally anything other than what I was doing. However, it does illustrate the point just fine.

Last edited by Wrench; 03-02-2018 at 03:35 AM.
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Old 03-02-2018, 09:49 AM   #10
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Quote:
Originally Posted by Wrench View Post
This is what happens if I "put the thing on the thing"
I know this was far from a perfect demonstration, given my airspeed was high, and AOA low, but what I'm really worried about is how the HUD works, for for the scope of this video it makes no difference.
I couldn't keep the circles perfectly concentric, given as my flight path continued to devolve, it was increasingly difficult to fight my natural urge to do literally anything other than what I was doing. However, it does illustrate the point just fine.
Wrench, it's very simple. Ignore the small circle. Just follow the flight path indicator (the big circle on the HUD). Ideally, as you fly either from point-to-point or follow the glideslope and localizer beams down to touchdown, you simply keep it centered in the aircraft datum. In reality, when there are large changes in either course or altitude, you pull it into the aircraft datum center more gradually. Otherwise you end up chasing it all over the sky as you blow through transitions. But your goal is to always have it centered on the datum as you fly.

Keep it centered and it'll bring the small ILS deviation index circle there as well and, then, you'll be where you need to be. As the ILS funnel narrows, if you haven't settled your aircraft on the glidepath, things will start to get squirrelly because slight changes have bigger consequences. So be settled in by the time you reach the 2 km mark and you should have far fewer problems.


EDIT: Here's an example of what I'm saying. I purposely set it up so that there are a few some significant changes in altitude and direction to reach the AIF. So I'm not in any hurry to center the director circle on the aircraft datum prior to the IAF. But take note how, after passing through the IAF, flying to keep the director circle centered on the aircraft datum maneuvers your aircraft into the correct position on the localizer and glideslope and, then, keeps you there. If it start shifting to the left, you shift to the left to recenter it. It is starts moving up, you move up, etc. The one thing you are not doing, is chasing the ILS deviation circle. The system is designed to have the flight director bring it to you. Your only task is to keep the director circle centered on the datum...except, of course, for minor technicalities like managing airspeed, etc.


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Last edited by Ironhand; 03-02-2018 at 01:25 PM.
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