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CEJM
22nd Jan 2008, 17:41
During my last couple of flights i cam across two different views on how to use the HF. I would like to post this over here to get some other opinions.

Flying across the Arabian Sea we are normally in contact with Mumbai on HF. (No CPLDC :{) Often when asking for the SelCal check we don't get the buzzer in the flightdeck. When this happens several of our pilots tune the second box to the same frequency as box number one and ask again for a SelCal check. In 99% the SelCal check is positive and we resume normal ops with both HF boxes tuned to the same frequency.

However, in during my time as an engineer i have been told several times (by avionics engineers) that you never tune both HF sets to the same frequency and than transmit. As this might blow up the other HF receiver.

During a recent discussion about this subject one of the other pilots asked me if i could show him where this was written in the Airbus FCOM. Which ofcourse i could not find.

What are your ideas about tuning both HF boxes to the same frequency?

And does anybody have a reference to where this might be in the books?

Thanks for your input. CEJM

Junkflyer
22nd Jan 2008, 18:36
Can't speak for the Airbus, but the classic 74 has two HF's that share the same coupler controller which can lock up if both are set to the same frequency. It is stated in the systems section of the AOM. We always tune the first radio away from the freq before trying the second.

Wodrick
22nd Jan 2008, 21:42
For an Airbus (any) with a dual installation, then it's fairly easy.

Airbus use a common antenna, the fin leading edge, certainly for 300 to 340. Where a dual installation is fitted then interlock is enabled, i.e. transmission is only allowed on one HF at once, the other being inhibited on a first come first served basis. An additional feature of HF interlock is that the non paired receiver is also inhibited. so both on the same frequency should not be an issue. This is normal HF practice.

For Boeings it can get more complex with differing installations. 70, 72, and 73 all used common antennas, the "HF probes" on the 70 & 72 and back to the Fin leading edge on the 73. In a dual installation all would have interlock. This is valid for the 5,6, & 7 also which use the fin (note sure about the 77 but visually seems a good guess)

The 747 classics (2/3) in most I have seen had a dual probe antenna installation from each wingtip trailing edge. I have seen these both with and without interlock, those without seeming to manage quite happily with the separation distance between the wingtips.
I have no knowledge of the 400 but expect that its back to the fin.

These things vary much from installation to installation so I have tried to reply in general terms but expect the howitzer. :)

13370khz
24th Jan 2008, 23:31
Boeing Ops Manual for B747-148 (long gone !)


HF Comms
There are two complete HF communication sytems ............
............. Both systems may be keyed simultaneously.

crispey
24th Jan 2008, 23:40
In hamradio operations with two HF sets together physically on your bench you would certainly tune one well away from the other if you intend transmitting on one.Same aerial or not.Maybe a/c radios are better insulated.

I keep being told HF is going out of fashion.No?I still hear a lot going on.