Why do all avionic systems communicate in caps?

Most of these systems started out as teletype printers, back in the day. The teletype systems only had an all-caps character set.

Aviation is global, and we still need to interop with countries and airports that still use actual teletype. So the legacy protocols still remain, even though much of it is tunneled over IP links now.

Also: The keyboards used in both airplanes and in ATC facilities are often special reduced-footprint keyboards without shift/caps keys. (They're also often ABC layout rather than QWERTY.) So there's that, too.

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shrx 2 hours ago | parent [-]

Does anyone still use actual teletype?

	▲	tjohns 2 hours ago \| parent [-]
		Actual teletype machines? No. (Though flight strip printers used at ATC consoles are basically mini teletype printers.) However, AFTN (used for disseminating flight plans and weather between ATC facilities) still uses landline modems running TTY-originated protocols. In the US we've moved moved off AFTN in favor of AMHS/X.400, as have most countries - but my understanding is there's still some last holdouts that haven't fully switched over.

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upofadown 22 minutes ago | parent | prev | next [-]

Probably for the reason that the original monocase teletype machines did. Better absolute readability due to the larger characters. It tends to be very dense with little actual prose so it is better to reduce the possibility of mistaking one character for another.

Also, everyone is used to reading the upper case so they would have to spend some time learning what common things look like in lower.

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Symbiote 3 hours ago | parent | prev [-]

So they can be heard over the engine noise.