F-14B TACAN Radials are effected by MAD compass error

I'm not certain this is a bug as I don't wholly understand how the MAD is modelled but it seems likely.

I've noted that the F-14B's HSI compass and HUD are effect by compass errors due to effects from Magnetic Azimuth Detector.

This is excellent and to be commended.

Unfortunately the compass error also appears to effect the TACAN radial, which AFAIK it shouldn't as the radial information is encoded in the signal not calculated/measured by the aircraft.

F-14B HUD tape looks correct (if -2° error is due to MAD or similar)

However the TACAN thinks we're on the 117°M radial, despite being on the 119°M radial

Although tested in DCS 2.5.6.43931, AFAIK the bug has been present for a while.

Are you on the TACAN or the ILS? Because the tacan station isn’t along the runway centerline.

Are you on the TACAN or the ILS? Because the tacan station isn’t along the runway centerline.

At 10.8 Nm the 390 ft difference between the TACAN and runway center line isn't significant at 0.34°.

A larger problem is that DCS tends to round numbers down, so 119°M may actually be 119.1°M or 119.9°M i.e. there may be upto 1° of error, rather than +/- 0.5°

However, I am comparing the F-14B's TACAN behaviour to other modules such as the A-10C, F-5, AV-8B, etc. which may or may not be appropriate.

I wouldn't normally be worried by a 1-2° error but I've seen regular heading errors of near 10° flying TACAN approaches to intercept the carrier FB / ICLS. As this is nearly 2x the stated 4° ships magnetic effect, I wondered if this was the cause.

Unfortunately a airspawn off the carrier's FB @ 10 Nm doesn't have the same effect, so it likely requires some manoeuvring to build up the compass/heading error I usually see flying east to west.

Hopefully HB will have the tools to see if MAD error, etc. is effecting the displayed TACAN radial (i.e. by injecting a large MAD error) or if I'm incorrect.

Btw: TACAN radial is given as from station. In this case 117 is bearing.

This sounds like this problem.

https://forums.eagle.ru/showthread.php?t=238390

This part:

If your directional gyro/synchro is shifted, it may result in erroneous interpretation of the TACAN direction on all instruments. You might have the carrier straight in front of your nose, but the BDHI will show the TACAN a few degrees to your left or right. However, the radial (bearing) reading would be correct; it would be just your magnetic heading wrong.

This part:

I am aware of this ...

If your directional gyro/synchro is shifted, it may result in erroneous interpretation of the TACAN direction on all instruments. You might have the carrier straight in front of your nose, but the BDHI will show the TACAN a few degrees to your left or right. However, the radial (bearing) reading would be correct; it would be just your magnetic heading wrong.

the error I suspect is in this ...

However, the radial (bearing) reading would be correct;

With compass error, when on the correct radial the course deviation bar should be centered, even when the course select pointer isn't pointing towards the TACAN station i.e. due to compass error.

I suspect I've seen the deviation bar centered while not on the correct radial - but I need to test further (i.e. induce a significant compass error) before I can confirm or refute my suspicion.

He's correct. The TACAN should not use instrumentation other than the TACAN radio itself for radial determination. This is quite clear if you actually know how a VOR or TACAN radio determines the radial.

Hey Ramsay!

Actually, you weren't precisely on the radial 117° (297°) but probably around 118-119. The deflection needle isn't accurately aligned, and the direction to the TACAN station (that little arrowhead) is slightly shifted to the right from your heading.

You are right that the sensed magnetic north direction and the declination shouldn't affect TACAN radial readings. However, we the TACAN itself has some limited accuracy, and according to the specification for the AN/ARN-84(V) the error can be as large as between 0.5° and 2.0° for the digital bearing and between 1.0° and 2.5° for the analogue bearing. The error depends on the strength of the signal received. We measure the signal reception, and we model the errors, so it is possible that in your case that's the reason for the difference you observe.

BTW these TACAN bearing measurement errors are the main reason why TACAN fixes for the INS may introduce INS errors even more significant than before the fix.

Actually, you weren't precisely on the radial 117° (297°) but probably around 118-119. The deflection needle isn't accurately aligned, and the direction to the TACAN station (that little arrowhead) is slightly shifted to the right from your heading.

As I understand it, the little arrowhead points to the currently detected radial (118-119°) and was shifted right (with the compass scale) due to the ~2° compass error.

... the TACAN itself has some limited accuracy, and according to the specification for the AN/ARN-84(V) the error can be as large as between 0.5° and 2.0° for the digital bearing and between 1.0° and 2.5° for the analogue bearing. The error depends on the strength of the signal received. We measure the signal reception, and we model the errors, so it is possible that in your case that's the reason for the difference you observe.

6° compass + 2° TACAN error = 8° which IIRC is close to the error I've seen flying inbound on the FB radial @ ~15 Nm.

Thanks for checking.

Learn here (fr)

Nope. the Arrow head has nothing to do with compass error. It just is conveniently put on the compass for ease of use. You want the 117 Inbound BEARING TO you dial the course to 117.

The Magnetic HEADING you see when you keep the CDI centered won't necessarily be 117 for a TON of reasons most of which you won't notice because the big reason is of course WIND, but your COURSE indicator has to be on the actual course you want.

You fly to keep the needle centered, not to a magnetic heading when flying a Course.