The two out of five channeling...? HLD + HLD FRQ

[J van E]

I like to know EVERYTHING there is to know about the awesome A2A Comanche, which is my one and only plane at the moment, and all the available manuals have helped me a lot. The various devices (radio's, ADF, etc.) can do a lot more than most people think they can do! I just love to actually use all those options too (specially since I navigate without GPS). However, there are TWO options I don't get.

On the upper radio you can set the DME function switch to a few very nice functions but I just can't understand what the manual is trying to say about the HLD and HLD FRQ functions.

I do understand something is being held somehow but what exactly? The line that is totally confusing me 'The two out of five channeling to the DME'. 'The two' what? Out of 'five' what? And what does 'channeling' and 're-channeling' mean?

These are the only two things I don't understand about the Comanche right now. I have the feeling it is something quite simple but I just don't understand it. If someone could enlighten me I can call myself 'Master of the Comanche' and I'd like that.

[Nick - A2A]

Hi Jeroen,

I understand the "two out of five channeling to the DME" is a reference to an ARINC data transmission protocol. The technicalities of this are probably a matter best explained by someone with real-world avionics/electronics experience (such as Esa) but this link gives a bit of info.

As regards operation of the HLD/HLD FREQ modes, here's a brief description I posted in the 'Avionics' thread a while ago...

The DME is normally tuned by selecting a VHF frequency on the NAV1 radio. This instructs the DME transceiver (which is actually a separate bit of kit) to transmit to the ground station using a 'paired' UHF frequency. However, the DME hold function temporarily prevents the DME transceiver from having its frequency altered by the NAV1 radio, until the DME function knob is placed back in the 'FRQ' or 'TTS' position.

What this means in practice is that if you tune the NAV1 radio to show a DME readout for navaid A, then switch to the 'HLD' position and tune it to navaid B instead, the distance to navaid A will still be shown. (If you switch to 'HLD FRQ' instead, the frequency of navaid A will be displayed instead of its distance from you.)

Hope it helps a bit.

Nick

[J van E]

Thanks, Nick. That post in that other topic makes sense to me. I will give it a try during my coming flight (which will start in a few minutes) and I will read that pdf during cruise!

EDIT
Yes, got it! These options enable you to keep track of the distance to a specific VOR while actually having another VOR active for navigation. The HDG and HDG FRQ options work together: you can either set it to HLD or HLD FRQ and then you can switch to another frequency. After doing that HLD, being the main function, shows the distance to the previous active VOR, while HLD FRQ, as a secondary function, shows the frequency of that VOR, as a reminder. Setting the knob to anything else removes this functions from memory, so to speak. Nice! Happy to know all there is to know now! As I said, I like to use all possible functions during my flights: keeps me busy and entertained!

EDIT 2
Also read the pdf. I kinda get what it all means but it is of no real importance to me as a pilot, I think. All I need to know is how these functions work in practice and that I do now. Thanks again!

[AKar]

I understand the "two out of five channeling to the DME" is a reference to an ARINC data transmission protocol. The technicalities of this are probably a matter best explained by someone with real-world avionics/electronics experience (such as Esa) but this link gives a bit of info.

I've never met this at all in avionics as in, in practical, hands-on experience, but it is of some interesting technicalities. When you've got some to spare, just follow the apparent rabbit holes you linked, rewind your mindset back to the world of being hardware-limited, and read & study. Actually, the technicalities would be best explained by not starting from the avionics standpoint. Note that the ARINC is not directly related. I don't think any of this is defined much at all in directly related pubs.

This makes an extremely good study topic for hardcorers. What comes to two-out-of-five, you soon google up the bar codes and stuff - then think about of DME frequencies.

If this won't quite guide you, think about TACAN as well.

-Esa

[Nick - A2A]

All I need to know is how these functions work in practice and that I do now. Thanks again!

You're welcome. In practice, I suspect there are fairly few excuses to use the HLD function but on one or two occasions I've found it useful if NAV1 is tuned to an ILS frequency which doesn't have DME, but there's a separate VOR/DME on the field.

[...] Actually, the technicalities would be best explained by not starting from the avionics standpoint. Note that the ARINC is not directly related. I don't think any of this is defined much at all in directly related pubs.

Yes, after a bit more reading I think I've got a slightly better understanding of fundamentals of this two-out-of-five thing: it's basically a binary decimal notation for simple error detection if I understand correctly. The "two" and the "five" are just bits which encode the decimal digits 0 through to 9. (The "two-out-of-five" part comes because two-out-of-five of those bits are 1s: it's which two that determines the digit the code returns, as shown below.)

0 = 00011
1 = 00101
2 = 00110
3 = 01001
4 = 01010
5 = 01100
6 = 10001
7 = 10010
8 = 10100
9 = 11000

In the context of something like DME frequencies, I gather each bit translates to a frequency selection input to the transceiver which would be variously grounded by different control knob(s) positions to select the different digits.

Anyway, I guess that PDF I linked above was somewhat inappropriate for our old Narco as it wouldn't have a serial bus - just a bundle of wires presumably.

Cheers,
Nick

[AKar]

Yeah, the thing with DME channel selection is that there actually are several ways of how's it coded using parallel inputs which utilize five + "common" wires. At least Bendix/Kings support various of these, and also ARINC 429.

In practice, these are more used in a truth table way instead of actually representing any real digit. The DME channel selection actually uses ten wires (plus the common), or two sets of five: with five values you could represent 32 figures, and you'd actually need 33 when it's done this way.



While one can construct some sort of a scheme by re-ordering the table (the DME frequencies do not run in the same order the VOR frequencies do), it is fairly random on its own. I understand the reasons for calling this 2x5 code are in part historical.

-Esa

[J van E]

In practice, I suspect there are fairly few excuses to use the HLD function but on one or two occasions I've found it useful if NAV1 is tuned to an ILS frequency which doesn't have DME, but there's a separate VOR/DME on the field.

Well, funny thing, today I already made good use of my new knowledge where the exact same happened to me! I had to use an ILS that didn't have a DME but I needed the DME to fly an arc but also wanted to see the ILS on the HSI already, so I set NAV1 to the VOR/DME, checked DST, then switched to HLD, checked same distance, and then switched NAV1 to the ILS freq and set up the HSI to show the proper heading for the approach, and all the time I could fly the arc as needed! It worked perfectly fine! Great stuff!

[AKar]

Yep, ain't it fantastic how even these supposedly simple radios are modeled for their good functionality!

-Esa

[Nick - A2A]

Indeed, though I suspect some of the functions modelled go undetected by the majority. The direct NAV entry mode on the KX 155A is one example: I actually find my using this quite a bit - sheer laziness I guess!

Nick

[AKar]

Yes, and the amount of timers available in some of these things should satisfy any needs. I've actually used ones in the radios quite a bit. The OBS mode is also excellent.

-Esa

[J van E]

I am actually using ALL available options on these devices whenever I can. I fly without GPS so I need them too. The Comanche is as complete as it can be and that's why I love it. Well, complete... oddly enough one option hasn't been implemented unfortunately: the volume knobs on the radio's and ADF are simple on and off knobs: you can't rotate them and you also can't pull them. There must be a reason for this... But I use all other available options on the radio's during every flight. Even when I don't actually need them LOL I just love using it all. Like the countdown timer on the ADF: sometimes I just use it to surprise myself with the sudden alarm. Heck, I even use the circuit breakers every now and then! (For instance to sometimes turn off that rather annoying light next to the gear switch.) Yes, the A2A Comanche is the best!

[Nick - A2A]

[...] oddly enough one option hasn't been implemented unfortunately: the volume knobs on the radio's and ADF are simple on and off knobs: you can't rotate them and you also can't pull them. There must be a reason for this...

Just a limitation of the host sim platform I believe. Apparently making the various COMM and NAV volume settings functional would require some considerable work, though I agree it'd be a nice feature to have. I think the only add-on I've used which has this kind of functionality (e.g. adjustable volume for the individual morse idents) is the Majestic Q400, and it uses its own custom navaid data.

Nick

[J van E]

Well, I am not complaining but since everything is working it would have been nice to also have this working. But I am fine without though. Don't even know what 'Pull test' would do. Well, Yes, it would deactivate the squelch circuit but I have no clue what that means LOL Probably nothing important.