[SystemSafety] Fwd: Re: Chicago controller halts Delta jet's near-miss....

Sun Jun 21 10:36:51 CEST 2015

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On 2015-06-21 01:28 , Les Chambers wrote:
> It seems to me that the ATC - Pilot voice protocol is missing a step. ... In concept, a safer 
> protocol might look like this: ATC: You are cleared for takeoff Pilot: My understanding is
> that I am cleared for takeoff ATC: Your understanding is correct

This misses crucial information, namely addressing, which is critical in a multiuser broadcast
context. A clearance is preceded by a call sign, and an ACK is succeeded by the call sign. Call
signs may be abbreviated, which can lead to confusion when the abbreviations are close, and when
transmissions are stepped on, which might have been the case in the incident in question. So let's
correct for call signs, and translate into the standard ATC-aircraft controlled language. What you
suggest is:

> [1] ATC: [call sign] Cleared for takeoff [2] CRW: Cleared for takeoff [call sign] [3] ATC: 
> [call sign] Affirmative

Steps 1 and 2 are required. Step 3 is not; if Step 2 is not correctly executed, then Controller
will respond:

> [1] ATC: [call sign] Cleared for takeoff [2] CRW: Cleared for takeoff [other call sign] [3] 
> ATC: <other call sign> Negative [other call sign]

or

> [1] ATC: [call sign] Cleared for takeoff [2] CRW: Cleared for takeoff [other call sign] [3] 
> ATC: [other call sign] Negative [other call sign]; <[call sign] Cleared for takeoff>

which, if you analyse it, works just as well, and is more efficient. (the "<...>" indicates an
optional expression.) Don't forget that this may be interspersed with other transmissions, for
example

> [2'] CRW: Cleared for takeoff [call sign]

in which case the option will not be exercised.

There is no good reason for a controller to ACK a correct readback, and it would complicate
matters cognitively when transmissions take up almost all the air time, which often happens at a
major airport.

Further, complete expression as whole phrases doesn't illustrate the resilience of the language in
the face of partial obscuration, which is an important feature.

Cushing (op. cit. antea) provided a grammar for such communications in his book. Cushing is a
linguist, but his grammar was partially incorrect and also structurally more complex than need be.
Twelve-thirteen years ago, some people working with me fixed it. See Review of the Cushing
Grammar, by Martin Ellermann and Mirco Hilbert at
http://www.rvs.uni-bielefeld.de/publications/Papers/hillermann-critique.pdf and Building a Parser
for ATC Language, by the same authors, at
http://www.rvs.uni-bielefeld.de/publications/Papers/hillermann-critique.pdf

PBL

Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany
Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de