RKL-41 C AUT position question

[virgo47]

In the L-39 manual, p154, one can read this about the mode switch selector of RKL-41:

“C AUT” (Compass automatic) and “C MAN” (Compass Manual) are RKL primary operating modes, NDBs’ direction determines automatically. The only difference is that in “C MAN” mode there is no automatic switching from outer NDBs to the near one.

My question is: How exactly does that C AUT switching from outer to inner NDB show? I flew over the outer NDB of Mozdok aerodrome, the radio compass turned around to 18 and nothing else happened. I thought that highlighted O next to the beacon switch would change to highlighted I or something and also that my radio compass would start tracking the other beacon (which clearly points to 15 when I switch to it manually).

I can't find more specific info about this automatic working of NDB. Please, L-39 pilots, if you know more, let me know.

[portman]

This only works if you fly over the outer marker within the limits of +-30° to the runway axis and with the landing gear extended. Your radio compass should then switch to tracking the inner NDB.

[virgo47]

1 hour ago, portman said:

This only works if you fly over the outer marker within the limits of +-30° to the runway axis and with the landing gear extended. Your radio compass should then switch to tracking the inner NDB.

Thank you, great stuff! I wanted to understand it because I was creating an element for it on my Android panel and was wondering when/how the other letter (I/O) can be highlighted when the switch is the opposite way. Now I tested it and it works as you mentioned, the landing gear is the key, and if the approach is not right, then the I/O changes back and forth.

Do you know how does it know that I'm ±30° to the runway? Does it use some other system for it? Because this kind of NDB should ignore the actual azimuth or am I wrong?

[portman]

vor 18 Stunden schrieb virgo47:

Do you know how does it know that I'm ±30° to the runway? Does it use some other system for it? Because this kind of NDB should ignore the actual azimuth or am I wrong?

I guess it has to do with the course selector, which should be aligned with the runway axis on approach, but I don't know exactly, sorry.

[virgo47]

On 3/21/2024 at 10:42 AM, portman said:

I guess it has to do with the course selector, which should be aligned with the runway axis on approach, but I don't know exactly, sorry.

I've tested the behavior around Mozdok which has two NDBs, and I turned off RSBN/SDU just in case it is somehow involved as well (although it definitely is not necessary as this works with aerodromes without RSBN as well).

After a few tests, these are the findings for C AUT behavior:

• As mentioned above, landing gear must be extended (down).
• This works only on NDB with a marker. If you overfly non-marker NDB, nothing special happens.
• If on course to the RWY:
  • If there are two markers and you overfly the outer one it switches from O to I, as advertised.
  • If there is a single marker (e.g. Kutaisi), both O/I blink very rapidly. (If you have I selected, no switching occurs, of course.)
• If you are not on course to the RWY - that is you overfly the marker e.g. in the perpendicular direction, or fly straight away from the RWY:
  • For two markers: It switches to I if you overfly the inner marker. Around the outer marker, it keeps switching between O and I, eventually settling on O. This switching is slower than rapid blinking over the single marker.
  • For a single marker, the behavior is the same as when you approach the RWY properly - it blinks rapidly.
  • The course to the RWY is somehow detected without RMI course set and it is not RSBN-related (there are a few two-marker aerodromes without RSBN in the Caucasus anyway).

I've found a manual T.O. 1T-L39C-1 where this is described on p1-80. It doesn't mention the direction of flight necessarily - instead, it says: "If the aircraft drifts from the approach entry sector, i.e. ± 30° from the runway axis, the system is automatically re-tuned back to outer beacon frequency to enable to initiate the missed approach procedure."

So, somehow it works and it works well for 2 markers and the proper approach. The details are still a mystery to me and I couldn't find more, but I can leave it at that.

[portman]

I've had a look at the document "The radio and radio measuring equipment of the L-39 aircraft" of the East German Air Force again. According to this document, switching from O to I only takes place if the course angle to the beacon (i.e. the RKL needle) is within a range of +-30°. If the course angle to the radio beacon is outside +-30°, the aircraft switches back to O. This means that the direction of the runway is not taken into account at all, but only the position of the RKL needle.

[virgo47]

5 hours ago, portman said:

I've had a look at the document "The radio and radio measuring equipment of the L-39 aircraft" of the East German Air Force again. According to this document, switching from O to I only takes place if the course angle to the beacon (i.e. the RKL needle) is within a range of +-30°. If the course angle to the radio beacon is outside +-30°, the aircraft switches back to O. This means that the direction of the runway is not taken into account at all, but only the position of the RKL needle.

Ah, that would make sense. I guess, the RKL direction to the next (I) beacon is meant by that, at least that makes the most sense to me.

Thank you very much for all this information, @portman!