RSBN box issues

[Bestandskraft]

Pages 99-100 of the "MiG-21 Pilot's Operating Instructions", a .pdf of which can be found linked at http://forums.eagle.ru/showthread.php?t=138880, describe the RSBN box ("Wide Rectangular Approach") that is detailed on pages 120-122 of the game manual in a somewhat different way.

 

I have been unsuccessfully trying to recreate the supposedly "realistic" RSBN box, a to-scale drawing of which is attached.

 

By looking at the in-game HSI marks (2, 3, 4) I have visually derived the depicted bearings of approximately 35° from the approach course for the downwind and base turns, as well as 15° for the turn to final. Crosswind and downwind turn radii have been drawn to fit the ranges/bearings given in the description and are more or less consistent with actual turn radii at the mandated speeds and a bank angle of 30°. The given distances for the crosswind and downwind initiation as well as the distance abeam the station are approximately what can be observed in-game and calculated mathematically.

 

However, it seems to be impossible to get the aircraft to a point 17-21 km away from and at a 15° bearing off the station while simultaneously adhering to the previous (and congruous) box parameters. Also, even if this were possible, starting the final turn at 500 km/h and 30° AoB at the given position results in a significant undershoot.

 

Please note that the mentioned issues arise not just in the drawing, but in the same way when actually trying to fly the box in the game "by the book".

 

I have four theories why this might be:

 

1) The "4" mark is incorrectly placed in the game. This theory is supported by page 101 of the MiG-21 manual, which states that the final turn commencement index is 10-12° off the final approach course, not 15° as I see it in the game. When adjusting the final turn bearing to 10° in my drawing, the undershoot problem is virtually solved, however, the aircraft will still fly a 23 km instead of a 21 km base (remember that base distance is supposed to be 17-21 km, so the drawing is already very conservative).

 

2) The real-world manual's description of the box is wrong.

 

3) I am not understanding the manual correctly.

 

4) The RSBN box base turn is not designed to be a 90° turn, but an approximate 135° turn. This would be consistent with the drawing on page 104 of the MiG-21 manual, and it would be partially consistent with page 89 of the manual, which states that in a visual pattern, the base turn is a 100-110° turn (the number of degrees still does not match, which might however be easily explained by one being a visual, the other an instrument maneuver). However, it seems unlikely that such an important fact would not be explicitly stated in the manual. Also, even with a 135° turn, the aircraft barely reaches the 21 km point. It should be noted however, that a 135° turn would cause the aircraft to reach the 17 km point if a 10° bearing was assumed (see point 1). Maybe both 1) and 4) are correct. Then again, if the 135° turn theory is valid, there would be no need for a "4" mark at all.

 

Does anyone have any information that might explain these discrepancies? Alternatively, could anyone post a track or an .acmi of an RSBN box flown successfully according to the MiG-21 manual?

Edited June 14, 2015 by Bestandskraft

[effte]

To my mind, that's effectively a lead-in radial, i e the heads up to start an intercept of the final approach course. I don't expect to roll out on the extended centerline, but rather fly a 30 to 45 degree intercept at that point. Otherwise, you'd have overshoots half the time.

[Bestandskraft]

To my mind, that's effectively a lead-in radial, i e the heads up to start an intercept of the final approach course. I don't expect to roll out on the extended centerline, but rather fly a 30 to 45 degree intercept at that point.

 

I'll call that theory 5.

 

While this is certainly a possibility, it does not explain how the aircraft is supposed to reach the 015° @ 17-21 km point ("4") coming from the 035° @ 23-25 km point ("3"). Theory 5 also contradicts page 122 of the game manual, which depicts the RSBN box as a quasi-rectangle, not a trapezoid.

[effte]

From abother thread here:

 

That removes some uncertainty. I also think you need to look at the azimuth you use for the downwind turn.

 

I'm on the road, doing the calcs literally on the back of an envelope so I can't really post them, but I find a theoretical lateral distance from final if rolling out blind of around 1200 to 1800 meters at slightly short of the expected distances (17.4-19.6 km). It must be considered that in practise it takes time to initiate and roll out of the turns as well, extending the pattern.

 

The intention isn't precision either, but to get close enough to a decent pattern even though the speeds dictate you'll rarely be visually oriented to the runway as you'd be at 80 KIAS.

 

Finally, I've found it to work as expected in actual practise. Does your mileage differ? Post a track?

Edited June 14, 2015 by effte

[Art-J]

It might actually add some uncertainty, because it's an outdated picture from post-release times, when the red needle's head and tail were reversed :). At least it gives some insight on how the marks on real instrument are painted.

 

The marks on "our" NPP seem to be painted more or less correctly, but RL manual, DCS manual and box tutorial mission all suggest different bank angles during turns, which might be the source of the problem.

[Bestandskraft]

Please find attached a track and corresponding .acmi showing my unsuccessful attempt to fly a "by-the-book" RSBN box.

 

effte, I understand your box works as advertised. If at all possible please upload a track or .acmi.

Tacview-20150614-171816-DCS.txt.acmi.zip

RSBN box.trk

Edited June 14, 2015 by Bestandskraft

[Bestandskraft]

Just used a ruler on the photo effte provided, and this confirms that the real life instrument, like the virtual one, uses 35° and 15° for the "3" and "4", respectively. The "2" might be 34° instead of 35°, but this discrepancy should be insignificant.

 

This seems to disprove theory 1, but contradicts page 101, which states that the final turn commencement index is 10-12° off the final approach course. Maybe there are/were different versions of the HSI.

 

I'm still confused about how a "proper" RSBN box is supposed to look.

[Bestandskraft]

RL manual, DCS manual and box tutorial mission all suggest different bank angles during turns, which might be the source of the problem.

 

According to Western doctrine, turns in IMC conditions are normally performed at 30° AOB. Since the whole point of an RSBN box is to land the aircraft in IMC conditions, I can hardly believe that it is designed around a bank angle of 45° (I'm not saying that you say it is).

 

Of course Russian doctrine might be/might have been different, but since Russians have the same cochlea and are therefore susceptible to spatial disorientation in the same measure as Westerners I'd only believe that if I saw it in writing in a real-world document.

[Bestandskraft]

As some further food for thought on theory 1, page 44 of the MiG-21 manual states that Landing mode should be selected at no more than ±10° degrees from the final approach azimuth. In addition to what I already stated in the OP, page 101 (as well as page 100 et al) also imply that the HSI (CCI) localizer channel failure warning flag closes its window (presumably meaning the localizer signal is being received) at 10-12° of azimuth deviation, which should occur simultaneously with the relative bearing pointer indicating that azimuth delta AND pointing at the "4".

 

In the game, according to page 116 of the game manual and verified by in-game behaviour, the localizer is received only within 2° of the centerline.

 

Since we have an actual real-world photograph standing against an actual real-world manual, the most likely explanation seems to be different mod states of instruments and approach aids.

Edited June 16, 2015 by Bestandskraft