INS System Depth Questions

[Joshua Green]

1. Are the INS systems triple mixing the position information?  If not, is it coming? If so, is the mode configurable?

2. Is remote mode planned so that waypoints and position data can be set simultaneously across all three?

3. Is there an inflight position update process either available or planned? DME/DME or even just a manual update at a known position?

Edited April 3 by Joshua Green
addition

[MD82]

I read somewhere that triple mix as well as remote will come at a later date. Regarding the LTN-72 (Not the RH models) I believe only manual updating was possible and described in the manual. Search RNAV 5 or RNAV 10, I believe it's mentioned somewhere there.

[richboy2307]

1. The LTN-72 is not capable of triple-mixing IRL, so not simulated by design. The AP on this equipment configuration can be driven by INS-1 or INS-3 only. At the moment, only INS-1 drives the autopilot in the simulator. Team is investigating feasibility of coupling INS-3 in a future update.

2. Remote functionality is planned for a future update.

3. INS-Drift is simulated, and is a user-configurable option via the PFB Aircraft Settings (Note: Further refinement of drift-rate is in progress).
As MD82 mentioned above, there is no automatic position updating on the LTN-72 (manual excerpt attached). The manual position-updating procedure is explained starting Page 105 in the manual which can be obtained here.

[Joshua Green]

Thank you for that information. However, there are some aspects of it that I find confusing.

A few follow-up questions about the nuances of the update procedure:

1. All the info about the DME/DME fix and intersection fix accuracy are relevant for systems like the CIVA, which can compute position corrections based on it, but it sounds like the LTN-72 can't do that.  Also, the pilot can't manually derive lat/long coordinates from a DME/DME fix either, without a lot of math. 

So it seems like directly overflying a radio navaid with known coordinates is the only way to do this update procedure. Am I right?

If so, that leads to the next question:

2. The aircraft is moving at multiple miles per minute, and tends to sequence to the next waypoint slightly before arriving at any particular one. How do you indicate to the INS that it should overfly the fix rather than cutting the corner, so that you fly precisely over the coordinates you intend to update? (Manual mode? or is their an OVFLY function?)

3. Does pressing the "hold" key, as indicated in the procedure, cause any newly entered position apply a correction to a) where the INS thought it WAS at the moment you pressed the "hold" key or b) does that entered position become where the INS thinks it is RIGHT NOW?  I'm hoping it's the first option. Otherwise, any delay in typing the corrected position in would cause miles of error given the aircraft's speed. 

 

Edited April 3 by Joshua Green

[richboy2307]

1 hour ago, Joshua Green said:

1. All the info about the DME/DME fix and intersection fix accuracy are relevant for systems like the CIVA, which can compute position corrections based on it, but it sounds like the LTN-72 can't do that.  Also, the pilot can't manually derive lat/long coordinates from a DME/DME fix either, without a lot of math. 

So it seems like directly overflying a radio navaid with known coordinates is the only way to do this update procedure. Am I right?

The information provided applies to navigation in general. The key difference is that the Delco Carousel IV-A (CIVA) can perform these calculations automatically, while the LTN-72 requires the crew to calculate positions manually using navigational charts.

These updates are usually planned in advance during flight planning. In practice, they’re often done during a VOR-to-VOR segment, where radio navigation is used temporarily while updating the INS. Full flight navigation via random fixes only was very uncommon at the time, so there would typically be a suitable cruise segment to carry this out.

Overflying a radio navaid with known coordinates would be the most straight-forward way. Alternatively it would be the procedures detailed in the manual with the help of navigational charts. These days you could use something like SkyVector or LittleNavMap for guidance.

1 hour ago, Joshua Green said:

2. The aircraft is moving at multiple miles per minute, and tends to sequence to the next waypoint slightly before arriving at any particular one. How do you indicate to the INS that it should overfly the fix rather than cutting the corner, so that you fly precisely over the coordinates you intend to update? (Manual mode? or is their an OVFLY function?)

There is no OVFLY function as far as I'm aware but I'll ask the developer regarding that too. The way to go about it is manual flight, or reverting to radio navigation / heading hold temporarily during a VORDME-to-VORDME segment ideally.

1 hour ago, Joshua Green said:

3. Does pressing the "hold" key, as indicated in the procedure, cause any newly entered position apply a correction to a) where the INS thought it WAS at the moment you pressed the "hold" key or b) does that entered position become where the INS thinks it is RIGHT NOW?  I'm hoping it's the first option. Otherwise, any delay in typing the corrected position in would cause miles of error given the aircraft's speed. 

I believe it is the former. You are telling the INS what the *actual* coordinates were for the position which you told it to hold, thereby correcting the drift/error that was present in the system. However, I will confirm with the developer as well. 

Thanks!

[phoenixegmh]

On 4/3/2026 at 1:27 PM, Joshua Green said:

1. All the info about the DME/DME fix and intersection fix accuracy are relevant for systems like the CIVA, which can compute position corrections based on it, but it sounds like the LTN-72 can't do that.  Also, the pilot can't manually derive lat/long coordinates from a DME/DME fix either, without a lot of math. 

The way I have done it during my testing is to plot my route on LittleNavMap, find two DME stations near to the route, then at a relevant point, hold the INS, not the two DME distances and draw range rings around each DME on LNM. Where the two rings intersect, Add a User Waypoint and bingo, you have a lat/long to enter into the INS. It's theoretically exactly how crew did it, only we don't have to use rulers to identify the lat/long of the ring cross as they would have done.

[Wothan1967]

Before flight You could plan a segment that goes VOR1-VOR1 with DME info.

Then select a DME distance from VOR2, f.ex. 20 nm before reaching VOR2 and write down the position.

During flight, once passing that point (VOR2 / DME÷20nm), press HOLD and enter the position You wrote down.

Using the VOR position and then update when overflying it, will be less accurate, since there will be a vertical cone where the signal disappears.

Otherwise the more accurate  way would be to have 2 DME stations tuned, preferable to either side of Your route.

Then when pressing HOLD, write down the DME distances from both, use a map, like Litte Navmap to draw a circle around the DME's with the measured distances as radius and then get the position where they intersect. Just be aware that You need to know where You are relative to these DME's since there will be tow possible positions.

 

[MD82]

All these points are moot at the moment because I believe the INS Drift still has not been implemented, correct?

[Squatchpilot]

I thought I remembered something about tuning to an omega station to update position..am I just imagining this? I used to have a list of the stations 

[MD82]

Wouldn't you need to have a separate Omega Unit though?

[Mantock]

I have to be honest, in all the flying I've done so far in the Tristar, I've never come across ANY INS drift. With it turned on in the efb, the aircraft is still crossing all its navpoints with laser-like accuracy.

Also, when turning any of the INS units to DSR TK/STS, I would expect to see the accuracy index gradually go from 01 back up to (eventually) 90. But it sits at 01 for hours and hours with no gradual inaccuracies being introduced as one would expect.

 

Also, (just currently going an EGLL-CYVR longhaul) about a half to two thirds of the way through my longhaul, my ADI has become stuck and the needle now does not move to the fresh track heading when going through the waypoints, and the orange digital 'time' and 'miles' readouts above the ADI needle have also frozen. I got to a waypoint 9 and expected the aircraft to auto select and input the next 9 waypoints as it should, but this didn't happen, so that the aircraft began circling at cruise level as seen in the image below.

[richboy2307]

On 4/10/2026 at 7:58 AM, MD82 said:

All these points are moot at the moment because I believe the INS Drift still has not been implemented, correct?

It is already implemented, toggled via the PFB INS Drift setting. But the rate at which it was compounding in certain cases needed further tuning which has been done for v1.0.2 update (expected release sometime this week). Please try again after that update and let us know how you fair.

22 hours ago, Mantock said:

Also, when turning any of the INS units to DSR TK/STS, I would expect to see the accuracy index gradually go from 01 back up to (eventually) 90. But it sits at 01 for hours and hours with no gradual inaccuracies being introduced as one would expect.

That’s true for a Delco Carousel IV-A (CIVA) INS, but not for the simulated LTN-72. The LTN-72 doesn’t display navigational performance data this way. Its DSR TK/STS page’s right numeric field shows only alignment status, changing in flight only if an alignment error occurs. Otherwise, it counts from 90 to 02, then switches to 01 when MSU is set to NAV.

Both systems track gyro bias and reset it during alignment or position updates, but only the CIVA displays it (as PI values); the LTN-72 does not.

 

 

23 hours ago, Mantock said:

Also, (just currently going an EGLL-CYVR longhaul) about a half to two thirds of the way through my longhaul, my ADI has become stuck and the needle now does not move to the fresh track heading when going through the waypoints, and the orange digital 'time' and 'miles' readouts above the ADI needle have also frozen. I got to a waypoint 9 and expected the aircraft to auto select and input the next 9 waypoints as it should, but this didn't happen, so that the aircraft began circling at cruise level as seen in the image below.

We’re currently investigating similar “hang” issues and working to reproduce them internally. Could you please provide the following:

• Your full flight plan (or a SimBrief OFP PDF, if available)
• Whether you imported SID/STAR waypoints as well (this is an option in the PFB)

Thanks!