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How often does it actually happen?

Also, is it fine to ask them to slow down if it’s hard to understand, for example, due to a heavy accent?

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    $\begingroup$ One would certainly hope it happens 100% of the time where there's any chance of confusion or misinterpretation. Reality is far less than that, then bad things happen. $\endgroup$
    – Ian W
    May 18 at 2:41
  • $\begingroup$ I just ask my audio panel. But it happens plenty and as long as its not a super busy Class B Airspace the controllers dont really mind. $\endgroup$
    – Dave
    May 19 at 2:37
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    $\begingroup$ Isn't the real trick that the pilot repeats the essential information? If so, the ATC would realize that the readback is not correct and trigger the repeat itself. Of course when the pilot could understand nothing, he/she can't repeat properly. $\endgroup$
    – U. Windl
    May 20 at 6:22

4 Answers 4

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How often is hard to quantify, but in general I would suggest the answer is all the time.

ATC is there to help you, as a service to you as a pilot. Sometimes out of necessity things are said quickly and it is the nature of the beast that sometimes some information is missed, or unclear. NEVER feel unable to ask for repeated information, or clarification if the information is unclear or ambiguous. Better to take up some extra airwaves than make a mistake!

If the information was delivered too fast and/or some was missed, be specific with what information was missed. This is preferable to asking for all the information to be repeated. A typical scenario might be

ATC: G-ABCD, Runway 22 in use, QNH <unintelligible>, taxi holding point Delta 2 via Apron, Alpha and Delta, giving way to the Cessna now entering the Apron.

G-ABCD: Runway 22, taxi to holding point Delta 2 via Apron, Alpha and Delta giving way to traffic on the apron, say again QNH, G-CD

ATC: G-CD, QNH 1015

G-ABCD: QNH 1015 G-CD

As you can see from the above example, the readback of received information is important, and ask for clarification of the missed items only.

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In one study undertaken in France, 3% of transmissions are requests for confirmation, and a third of those are "say again". The reasons are mentioned in the quotation below:

Errors and misunderstandings: these can be directly traced to voice channel characteristics (i.e. its volatile nature, national accents, or poor quality of the audio signal). Such events have a direct impact on the efficiency of R/T use, if not on safety. As an example, one erroneous pilot read back was recorded per hour (mainly wrong frequency during transfer of communications), and led to an explicit correction. On average, the Vocalise samples contain almost 6 requests for repetition or confirmation originating from pilots per hour, and 2 from controllers, approximately accounting for 3% of the total R/T occupancy (up to 14 such pilots requests can even be found in a single one-hour sample). Among these, 30% are simple "say again" requests, and another 30%, requests for repetition or confirmation of route information.

— Graglia, L., B. Favennec, and A. Arnoux. "Vocalise: Assessing the impact of data link technology on the R/T channel." 24th Digital Avionics Systems Conference. Vol. 1. IEEE, 2005.

See Jamiec's answer for how to efficiently request such repeats.

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    $\begingroup$ I went on a trip to S America some years ago, to do a ferry back to N Am, my 1st and only time flying down there. The ATC comms were like talking to the drive thu speaker, made worse by Spanish accents, made worse by different and random phraseology, and topped off by controllers who acted like typical uppity public officials in developing countries, and scolded you in a nasty fashion for mistakes. A LOT of say agains + wincing. Even the big airports didn't have ATIS so the ATIS data was jumbled in with your arrival clearance. $\endgroup$
    – John K
    May 17 at 16:15
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    $\begingroup$ +1 for answering the question with some objective numbers. $\endgroup$ May 18 at 6:53
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Also is it fine to ask them to slow down if its hard to understand for example due to heavy accent?

The ICAO standard phraseology manual includes the phrase SPEAK SLOWER meaning "Reduce your rate of speech".

Source: ICAO Annex 10 - Aeronautical Telecommunications - Volume II - Communication Procedures including those with PANS status (viewable here)

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  • $\begingroup$ Perhaps answer it more directly? $\endgroup$ May 19 at 12:52
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How often can it happen or how often should it happen are different questions from what you asked. You asked how often it does happen. I’d disagree from the “all the time” answer. In fact, in my experience, considering overall operations, it happens rarely. That’s not real quantifiable, so I’ll say that when I was flying a lot, there were days when I heard it maybe once or twice in a day. There were also days when I heard it more often. It just depends. However, if you’re from the South (U.S.) where we speak fairly slowly and your trip takes you up to the NYC area where they speak a bit more rapidly and with a different accent than in the South, you may find yourself needing to ask the controller to “say again” or “confirm heading two-seven-five after Alpha”, etc. There have already been some good suggestions in other answers, but the bottom line is your main job is flight safety. If you didn’t completely understand controller instructions, it’s not at all safe to continue until you do understand. Do whatever you need to do to get there (i.e., to a point of understanding).

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    $\begingroup$ +1 for the last sentence alone. The alternative to "Also is it fine to ask them to slow down if its hard to understand for example due to heavy accent?" is potentially killing hundreds of people! $\endgroup$ May 18 at 8:18
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    $\begingroup$ Yes indeed. Too many worry more about how it’ll make them look than about its effect on safety. $\endgroup$ May 18 at 12:33
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    $\begingroup$ I'm thinkin the "all the time" is ment to read as "it's not rare at all. I heard it daily, then again my airfield has a lot of training flights. There should be no stigma to transmitting say again, repeat all..., words twice or speak slower. Surely if you do that all the time at LHR or JFK you'll get the telephone number, but as you stated, safety first. $\endgroup$
    – Jpe61
    May 19 at 16:30
  • $\begingroup$ @Jpe61: Indeed. More generally, at least in American usage, saying something happens "all the time" implies that no explanation or excuse is expected. For a pilot to say "Acme 123, I'm afraid I didn't catch the flight level because the flight attendant dropped my coffee" would probably be a bigger annoyance than having the pilot simply transmit "Acme 123, Repeat flight level". $\endgroup$
    – supercat
    May 20 at 17:54

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