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What is meant by “Five by Five”?

Posted by on April 9, 2009 12 Comments Category : Flight Instructor Blog

tower150

Have you ever wondered what is meant by the term “Five by Five?”  You’ll hear this sometimes as a response when an aircraft or tower is asking for a radio check:

“Citation XYY, how do you hear this transmitter?”

“5 by 5″

Well, I’ve heard this term used countless times and it was a recent transmission that finally motivated me to do some research.  I wanted to find out exactly what is meant by that term “5 by 5″ when I heard an aircraft respond to a radio check by saying:

“I hear you 3 by 5.”

Ok, so what transmission quality is represented by the “3″ and what is represented by the “5″.

Well, thanks to my local tower controllers, I learned that the first number is for signal strength and the second number is for readability. Signal strength and readability are measured on a five point scale with 5 being the highest value possible and 1 being the lowest.  So when you say to the controller (or pilot) “5 by 5″, it is literally another way of saying that the transmitter you hear is “loud and clear.”  If you say “3 by 5″ it’s like saying, “Your coming in kind of weak, but I can still make out what your saying”

So now you too know the meaning of the term “Five by Five”.

12 Comments



  1. Sarah on Apr 10, 2009

    Interesting. I’d always heard it the other way around, the first number being readability and the 2nd strength. From olden days radio, the system was called the “RST”, “readability/strength/tone” 5x5x9 scale. The “tone” refers to dah-dit-dah-dah tone.

    Here: http://en.wikipedia.org/wiki/RST_code

    Moot.. anyway. 99.9% of the time the response is “5×5″.



  2. Danny V on Apr 10, 2009

    In Canada, this is how we are taught to do radio checks. “# by #” is the response format we expect when we ask for a radio check.

    The scales are as follow (in the format # – readability scale – strength):
    1 – unreadable – bad
    2 – now & then – poor
    3 – with difficulty – fair
    4 – readable – good
    5 – perfect – excellent

    So a “3 by 5″ means (as you said) “with difficulty but signal strength is excellent.”

    Keep up the great job!



  3. Paul on Apr 10, 2009

    So readability and strength seems to be the answer, not strength and quality. Thanks for the correction.



  4. Jeremy on Apr 10, 2009

    There seems to be a lot of confusion on which number is which. Remember that on AM radio, which includes aircraft bands, signal strength and volume (loudness) are equivalent. So you didn’t fully correct your blog post, because if the reading were “3 x 5″ and the first number is readability, that would mean a maginally readable signal but perfectly loud/strong. For example it might be full of crackles and pops, or a bad microphone, or background noise, or something like that – but still fully loud and strong. A weakish signal that is otherwise readable would be ’5 x 3′ after your correction.

    There is a lot of disagreement as to which number is which – I’ve seen dozens of references to both. After all, the catchphrase “loud and clear” would mean that strength comes first and readability second, right? I actually think this is the more common method, not the method Sarah suggests, but it’s certainly ambiguous.

    To resolve this ambiguity problem, in Australia the required response to a radio check question is something like, “strength 5, readability 4″.

    By the way, the strength is supposed to be on a scale of 1-10. A strength of 5 is normal – if it were something higher it would mean you were overpowering the transmitter and overloading receiving speakers or something. Most people forget this though as modern radios are pretty good about not doing that, and the only issue is weaker than normal signals. Readability is indeed a 1-5 scale.

    Hope this helps,
    Jeremy



  5. Paul on Apr 11, 2009

    Jeremy,

    Your right on several accounts. The RST scales have strength on a 1-10 scale, not 1-5.

    I’m going to do a little more digging.

    On a rather funny note, I have something to admit. When I asked my local controller for his source for his answer, you know what he said? WIKIPEDIA!!!

    So apparently, I need to do a better job of checking my sources!



  6. Paul on Apr 11, 2009

    Ok. I have reverted to my original post content (Goolge is going to LOVE me). The reason for this is that I think it can be interpreted either say. I think Jeremy has the right answer that when asked for a radio check, the appropriate response should be (on a 1-5 scale) “signal strenth 5, readability 3″ or you can reverse it, as long as you give the term before the value. So, that’s my answer until I see a written authoritative source that applies to aviation (FAA or ICAO) and then I’ll of course change it…



  7. Paul on Apr 12, 2009

    Paul,

    your blog currently states “first number is for signal strength and the second number is for signal strength”, which I’m assuming is a mistake after you changed it.

    As a military signaller and later military pilot, I was always taught 5 by 5 as in loud and clear, meaning strength and readability is the correct way around. Hope that helps.



  8. Paul on Apr 12, 2009

    Paul. Yes, thanks for that catch. It has been updated now to the original content which is in agreement with your meaning of 5 by 5 which is strength then readability.

    Geesh..I’m struggling a little with this post.



  9. John Elliot on Jun 18, 2009

    all this comes from old time phraseology, 5 by 5 is still LOUD & CLEAR, same answer to QSA QRK as the question.
    In Peru we still use terms such as QDM QDR QAB QTA Etc as normal ATC lingo.
    From an old timer.
    John



  10. Jerry on Aug 24, 2011

    So I don’t remember where I read this, but I read that five by five was just a slang term used to mean loud and clear was used by either a British or French pilot years and years ago (spelling loud, “Loude”) thus the term mearly refrenced the words. Since I have never seen a scale describing the clarity vs strength in any official flight publication (FAR/AIM, FIH etc) I like the story I read. Whenever I hear guys on the radio say “3×5″ gives me a geek cringe.



  11. Tony on Nov 22, 2013

    I’m a ground guy in charge of the comms on a team that controls ord drops. You’ll be happy to know that we simplified it even further by just turning it into, “Five-by”. Meaning, 5×5. We just know that it means comms won’t be the reason we can’t drop today. I was actually talking to another guy on the team about this and we figured it correlated to, “loud and clear”. So I’m here. It’s nice to see the expertise come together from so many places to discover the roots of what otherwise would be just slightly more than daily jargon. The pilots we’ve talked to haven’t really known the whole tale around this term either. Thanks to everyone for the insight.



  12. abe lincoln on Apr 01, 2014

    from an old flight engineer, I think this is where it all might have started from.

    how is this for an “Authoritative sources”

    https://archive.org/details/Fm24-6

    check out this old radio operators handbook from WW2. on page 11 it explanse what it means, and when to do and not to do a check, this was back in the day when they used mores, and only limited voice.

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