Blocked Frequency

[390cruise]

As a pilot of a large selection of light aircraft and a smaller selection of Boeing and Airbus aircraft all of which block all reception from the selected box when the push to talk is pressed. ( newer aircraft give warning tone after long continuous transmittion), I
am puzzled by ATC. It would appear that they think we hear their calls WHILE we transmitt !!!

At my base (STN) from time to time someone gets stuck on transmitt, after a few minutes of this horrible noise one hears
the controller ....... "check your transmitters" he says! (this call
will be heard by every aircraft EXCEPT the one with the open mike!! )

Chirp issue 73 mentions (on page 2 ) Blocked Frequency...
a loss of separation had occurred due to a stuck transmitt,
the pilot " had been requested to stop transmitting whilst a discrete frequency was brought into operation" the report went on to say "the pilot had apparently ignored this request and thus started the chain of errors that led to the loss of separation"

The CHIRP comment " some R/T transmitter defects prevent the flight crew from receiving incoming RTF transmissions"

My comment...if we are stuck on transmitt we cant hear you.!!!

[LXGB]

Hi 390,
I make the call anyway, just in case you've got it dialled up on another box. I might also be transmitting on more than one frequency simultaneously when I do it. It works, sometimes.

LXGB

[spekesoftly]

The initial part of the Chirp report states that the aircraft was intermittently blocking the frequency. Perhaps ATC tried to contact the pilot concerned when the frequency was not blocked?

[Gonzo]

Yes, we don't hold out much hope of you hearing us if you're still transmitting, but if there's a chance, why not?

It's certainly the case at my unit, but I can tell when I'm transmitting and someone else transmits over me. I think many ATCOs might presume this to be the case with aircraft radios.

[nibog]

I'm not sure about what other ATS units have, but in my unit if I transmit at the same time as an aircraft i recieve some very unpleasant feedback through my headset. It lets you know. I thought some aircraft might have similar systems??

Whenever I have Aircraft on Frequency with stuck mics, I'll always transmit a msg to check transmitters - Usually on more than one frequency. It works sometimes. We can also transmit voice on VOR frequency if all else fails.

But usually by then it's time to get out the Flare pistol.

[Uncommon Sense]

I think you are referring to a 'heterodyne' (sp?).

[HEATHROW DIRECTOR]

I'm afraid it is a fact that controllers are not given much training in the functioning of radio equipment and most are unaware that airborne transceivers mute the rx on tx. Another factor may be that ATC receivers do not (or did not!) mute on transmit and controllers are (were?) able to hear an aircraft transmit whilst they are (were) talking. It also provided a sidetone which was a genuine "received transmission" rather than some audio arrangement in the control desk. That could be the reason that nibog gets a heterodyne when an a/c transmit over him. A really excellent safety measure.. but.......

I have put alternatives in brackets because I believe that at LATCC and NERC there were moves to remove this facility. Again, a current controller may have more info..

[390cruise]

Gonzo and Nilbog

You are correct, I really believe many ATCO think we can receive
while we transmitt (My original point!!). I am glad I brought up
the subject.

Spekesoftly

You made a good point, it did say 'intermittently'

In an ever busier world there must be a strong case for
all multi radio aircraft to listen out on guard at all times.
You will never sell that idea until we stop all non urgent
use of this frequency including the pain of sunny weekend
days ....the practise pan!!

[Spitoon]

To add to HD's response, certainly in the UK, ATC have separate tx'ers and rx's and so can still hear what is going on on the frequency whilst they transmit (if the equipment is set up that way) - this is the source of the sidetone. GA transceivers seem to completely mute all rx when the tx PTT is pressed - don't know whether bigger aircraft are different.

As LXGB says, sometimes it works.

It's not uncommon when there's an intermittent stuck transmitter, especially if the controller has worked out which aircraft it is, for the controller to manage to get through to the crew when the tx'er is not selected only for the crew to push the button again and say sorry .........

[Vlad the Impaler]

390 Cruise,
Speaking as one of those Stansted controllers I believe that we are all aware of the functionality of our radio equipment. I have certainly uttered the above phrase on many occasions, after the frequency has cleared. You would be amazed how many times it clears only to be blocked again ten seconds later by the same tx. At least a mention of it on the frequency may alert the perpetrator.
Every little helps and a blocked ground frequency at half past six in the morning is a royal pain in the arse.

[forget]

Heathrow Director.

You say "I have put alternatives in brackets because I believe that at LATCC and NERC there were moves to remove this (off air sidetone) facility".

Here's the background, as I understand it.

The old steam driven ATC Transmitter Receiver switching worked exactly as it should. The receiver was always live, with its own antenna remote from the transmitting antenna. When you transmitted, and you heard your own sidetone, you had absolute confidence that everything, from Mic to antennas, was working properly. This arrangement, sometimes, allowed you to hear an aircraft transmitting at the same time. Only sometimes, because the relative signal strengths at the receiving antenna will decide whether you hear one transmission, or both.

Enter the age of 'digital' voice switching, with NERC as an example.

When the system was going through acceptance tests the controllers complained of a serious echo in their headsets - their lips were moving but their headphone sidetone was coming in much later. Very annoying.

Don’t ask me why so called 'digital' ATC systems produce these delays - it’s a mystery to me - but slack design comes to mind. When was the last time you heard an echo on a 12,000 mile 'digital' telephone link?

Anyway, back to the NERC problem. Rather than fix the problem, a simple bodge was decided on. Wire the microphones direct to the headsets. Problem solved? Hardly.

With the new arrangement controllers will hear what they think is sidetone.

It’s not. It's the result of a low grade bodge to fix a problem that should never have been allowed to happen. Unlike the 'old' system, the transmitters and receivers could have been nicked overnight, and the controller still thinks he's talking to someone.

In fairness to NERC I believe the original spec called for off-air sidetone so why the bodge was ever allowed is another mystery.

[HEATHROW DIRECTOR]

Thanks forget for the explanation, which is exactly as I understood it (my ancient memory seizes up sometimes). I think it's a shame that a "bodge" has removed a very useful facility. Probably another case of a system being engineered by people who have little clue about the "sharp-end".

[forget]

Heathrow Director. I'd call off-air sidetone safety essential, not just nice to have. Here's a clue from CAP 670. CAA 1998.

5.2.11. The operator shall be provided with a degree of assurance that Air-Ground Communications transmissions have been successful. NOTE: The normally accepted method of implementing this is to provide off-air sidetone to the operator’s headset derived from either the radio receiver associated with the transmitter for that radio channel, or from a separate receiver. It is recognised that the future widespread implementation of digital telecommunications systems may mean that off-air sidetone cannot be implemented in all situations. In which case alternative ways of indicating the successful transmissions may be used.

…………………

A case of world-class sloping shoulders, and let me guess what happened.

The new fangled 'digital' people made such a mess of the design and installation that they had to convince the CAA of inherent shortcomings peculiar to 'digital'. Codswallop! Electricity and radio waves travel at the speed of light - which is bloody fast!

But slow it all down by converting analogue voice to digital and, back again several times, and the speed of light apparently slows down. Result - echo echo echo. A proper job would give you off-air sidetone.

[TheOddOne]

forget et al,

I couldn't agree more. I'd say that practically every time I take a drive around the airfield, monitoring Ground as I'm required to, I overhear a 'stepped on' instruction from ATC where the controller is aware that the instruction has been interrupted and is able to quickly correct the situation.

I think I've been told that it's not just the 'digital' nature of the NERC setup that causes problems but also that the Tx frequency is slightly offset at some repeater transmitters which exist in order to gurarantee reception in the area covered. This offset exists to prevent 'beat' or phase distortion that would occur with two txs on exactly the same freq but different distances from the receiver.

Yes, I agree that it's a terrible 'bodge'. Great heavens, it's only a bit of audio and amplitude modulated VHF, which was well established during the 2nd world war! Surely we can do better in this day & age!

I guess it'll all get washed away soon with the advent of instructions relayed direct to the a/c from the ATC computer.

While we're on the subject of jammers, the vast majority of these at our place are a/c not vehicles and are mostly caused by the use of mics in cockpits instead of headsets. It isn't loose ptt switches, but carelessly stowed mics that seem to cause the problems, on the flight decks that I've visited to sort it out, anyway. I thought there was a rule against use of mics below fl100 with PT flights in the UK?

Cheers,
TheOddOne

[HEATHROW DIRECTOR]

<<instructions relayed direct to the a/c from the ATC computer. >>

Problem is, there ain't no ATC computer capable of doing the actual controlling! (BTW, I agree with everything about sidetone - and multicarrier transmission systems have been in use for 50 years so that is not a valid reason.)

[Bern Oulli]

Had an aircraft with intermittent sticky transmit. Long tx, short breaks. I think it was on the ground somewhere on the airfield. Waited for the break and called him, "Golf blah blah you have a stuck transmit switch DO NOT ACKNOWLEDGE". What happens? "Golf blah blah roger, sorry....hisssssssssssssssssssssss".