Say Again

SLOP1126 · Week 1

The design of a language that cannot be misheard

Ordinary English is extraordinarily good at its job.

That is the problem.

What English does well

  • Redundant — lose half of it, recover the meaning
  • Self-repairing — a puzzled look and you say it differently
  • Expressive — negotiate meaning you had no words for

None of these survive the next slide.

Simplex

One carrier. One speaker.

Press to transmit and you cannot hear.

So you cannot know you were stepped on.

Neither can the other station.

The receiver closes when the key opens

One channel carrying one transmission. The transmitting ground station's own receive path is struck through — pressing to transmit closes its receiver — while the aircraft at the other end hears it and has no way to answer until the carrier drops.KEYED — AND DEAFHEARING

And then

  • No face. No raised eyebrow. No back-channel.
  • Band-limited: roughly 300 Hz to 3 kHz.
  • The listener is often tired, often not a native speaker, often busy.
  • So are you.
  • One shot. A clearance acted on wrongly is an aeroplane somewhere it should not be. Now.

Which leaves this much of a voice

The energy in a speaking voice plotted against frequency on a log axis from 100 Hz to 8 kHz, with the part between 300 Hz and 3.4 kHz picked out as what an aviation radio channel passes and the rest dropped back as what it discards.100 Hz3001k3.4k8keverything you get

The response was not “speak better English”

It was to build a much smaller language and make it mandatory.

Four moves

  1. Closed vocabulary — a few hundred phrases, one meaning each
  2. Fixed word order — callsign, instruction, constraint
  3. Mandatory readback — build an acknowledgement out of the message
  4. Reserved words — forbidden except in one exact context

Every one of the four gives up expressiveness on purpose.

Worse at almost everything. Harder to get wrong at one thing.

What a transmission looks like

TWR      Speedbird one two three, wind two four zero degrees one two knots,
         runway two seven left, cleared to land.
BAW123   Cleared to land, runway two seven left, Speedbird one two three.

Read it as a pair to be checked against each other — not as two people talking.

The claim this course makes

Phraseology is usually taught as a list that has always been there.

It hasn’t.

Every phrase was chosen, by identifiable people, out of alternatives, on evidence that was sometimes good and sometimes a single accident.

So for every rule

  • What problem was it for?
  • What was the alternative?
  • What evidence decided it?
  • Does the evidence still hold?

That last one is live. Week 10 is a technology that removes mishearing entirely — and takes something else with it.

The accident record

Not here for drama.

It is the only evidence available about where a language engineered against misunderstanding still lets misunderstanding through.

Thursday

You transmit. Not next week.

  • The shape of a call
  • The round-robin — everybody takes the pilot seat
  • Being stepped on, deliberately

Say again