DTIC ADA494551: The Outcome of ATC Message Length and Complexity on En Route Pilot Readback Performance

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by  Defense Technical Information Center

DOT/FAA/AM-09/2
Office of Aerospace Medicine
Washington, DC 20591
The Outcome of ATC Message Length
and Complexity on En Route Pilot
Readback Performance
O. Veronika Prinzo
Civil Aerospace Medical Institute
Federal Aviation Administration
Oklahoma City, OK 73125
A.M. Hendrix
R. Hendrix
Hendrix & Hendrix
Roswell, NM 88201
January 2009
Final Report
OK-09-0434
NOTICE
This document is disseminated under the sponsorship
of the U.S. Department of Transportation in the interest
of information exchange. The United States Government
assumes no liability for the contents thereof.
___________
This publication and all Office of Aerospace Medicine
technical reports are available in full-text from the Civil
Aerospace Medical Institute’s publications Web site:
www.faa.gov/library/reports/medical/oamtechreports/index.cfm
Technical Report Documentation Page
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
DOT/FAA/AM-09/2
4. Title and Subtitle 5. Report Date
The outcome of ATC message length and complexity on en route pilot January 2009
readback performance 6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.
Prinzo OV,1 Hendrix AM,2 Hendrix R2

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)
1FAA Civil Aerospace Medical Institute 2Hendrix & Hendrix
P.O. Box 25082 Oklahoma City, OK 15 Circle Drive 11. Contract or Grant No.
73125 Roswell, NM 88201
12. Sponsoring Agency name and Address 13. Type of Report and Period Covered
Office of Aerospace Medicine
Federal Aviation Administration
800 Independence Ave., S.W.
Washington, DC 20591 14. Sponsoring Agency Code
15. Supplemental Notes
This work was performed under Task AM-B-06-HRR-516.
16. Abstract
Field data and laboratory studies conducted in the 1990s reported the rate of pilot readback errors and
communication problems increased as controller transmissions became more complex. This resulted in the
recommendation that controllers send shorter messages to reduce the memory load imposed on pilots by
complex messages. More than 10 yrs have passed since a comprehensive analysis quantified the types and
frequency of readback errors and communication problems that occurred in the en route operational
environment. Hence, a content analysis was performed on 51 hrs of pilot and controller messages that were
transmitted from five en route facilities in the contiguous United States between March and August 2006. This
report contains detailed and comprehensive descriptions of routine air traffic control (ATC) transmissions and
how ATC message complexity and message length affected pilot readback performance. The results show that
message complexity had a statistically significant effect on the production of errors of omission only, while
message length affected both the production of errors of omission and readback errors (substitution and
transposition errors). When pilots requested that controllers repeat their messages, often these messages included
the names of fixes, waypoints, and intersections, as well as the name of the next controlling sector or facility. Five
recommendations are made: (1) No more than three aviation topics should be included in an ATC transmission.
(2) A route clearance should be given as a stand-alone transmission. (3) The names of all fix, waypoint, location,
and other identifiers should be repeated, and if necessary, spelled outfollowing their first recitation. For example,
“CLEARED DIRECT COBAD THAT’S CHARLIE OSCAR BRAVO ALFA DELTA” or “CLEARED DIRECT COBAD C-O-B-
A-D.” (4) Slang should not be used or accepted as part of aviation phraseology. (5) Effort should be undertaken
to reduce excessive words/phrases — on, your, to, is, etc.
17. Key Words 18. Distribution Statement
Communications, ATC Communication, Document is available to the public through the
Defense Technical Information Center, Ft. Belvior, VA
Air Traffic Control
22060; and the National Technical Information

Service, Springfield, VA 22161
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price
Unclassified Unclassified 38
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized


CONTENTs
The Outcome of ATC Message Length and Complexity on En Route Pilot Readback Performance. . . .1.
METHOD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
. Materals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
. Subject.Matter.Experts.(SMEs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
. Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
. Sector.Descrptons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
RESULTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
. Routne.ATC.Communcaton. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
. Analyss.of.Plot.Faulty.Readback.Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
. Message.Length.and.Plot.Readback.Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Appendx.A
. Gude.to.the.Computaton.of.Complexty:.ATC.Instructon/Clearance.Avaton.Topcs. . . . . . . . .A1
Appendx.B
. Gude.to.the.Computaton.of.Complexty:.ATC.Advsory.Avaton.Topcs. . . . . . . . . . . . . . . . . . .B1
Appendx.C
. Gude.to.the.Classficaton.of.Plot.Readback.Errors:.Instructon/Clearance.Avaton.Topcs. . . . .C1
Appendx.D
. Readback.Error.Classficaton.Gude:.Advsory.Avaton.Topcs. . . . . . . . . . . . . . . . . . . . . . . . . . D1


EXECUTIVE sUMMARY
Ths.report.descrbes.and.summarzes.51.hrs.of.ar.traf- Readback.errors.ncreased.once.ATC.messages.ncluded.
fic.control.(ATC).communcaton.that.occurred.durng. two.or.more.ATs ..The.most.common.readback.errors.
normal,.day-to-day.operatons.at.five.dfferent.en.route. nvolved.alttude.and.alttude.restrctons,.followed.by.
centers.between.March.and.August.2006 ..On.average,. changes.n.rado.frequency,.route/poston.clearances,.and.
an.arcraft.requested.and.receved.ar.traffic.servces.every. altmeter.settngs ..Some.of.the.readback.errors.nvolved.
2.mn.4.s,.wth.approxmately.10.messages.exchanged. slang ..That.s,.an.ATC.nstructon.to.“contact.center.on.
from.ntal.contact.to.the.transfer.of.communcatons .. one.one.nner.pont.zero,”.was.occasonally.read.back.as.
The.most.frequently.transmtted.nstructons.nvolved. “nneteen.zp”.or.“nneteen.nothng .”.What.s.worrsome.
changes.n.alttudes.and.swtchng.plots.to.another.rado. for.avaton.safety.s.that.other.plots.may.not.understand.
frequency .. these.types.of.readbacks.(especally.plots.who.are.less.
Unlke.the.findngs.reported.by.Cardos.(1993).where. proficent.n.avaton.Englsh.or.makng.ther.first.flghts.
78%.of.the.ATC.messages.nvolved.one.or.two.peces.of. nto.the.U .S .) .
nformaton.(e .g .,.“contact.center.on.one.one.two.pont. A.common.practce.among.plots.s.to.acknowledge.
three.four”.=.1.pece.of.nformaton,.whle.“contact.[name]. controller.nstructons/advsores.wth.ether.ther.call.
center.on.one.one.two.pont.three.four”.=.2.peces.of. sgn.only.or.ther.call.sgn.plus.roger/wlco ..Also.com-
nformaton),.we.found.that.less.than.1%.of.the.control- mon.s.a.response.such.as.“Down to two five zero”.
lers’.messages.had.only.one.or.two.peces.of.nformaton. (when.acknowledgng.an.nstructon.to.reduce.speed.
(of.the.two.examples,.the.first.had.a.complexty.value.of. or.assgnment.of.a.new.alttude).and.partal.readback.
6,.and.the.second.had.a.complexty.value.of.7) ..In.ths. (respondng.to.only.one.of.two.or.more.nstructons) ..
report,.84%.of.the.controllers’.transmssons.contaned. These.examples.llustrate.plot.readbacks.that.mght.leave.
only.one.avaton.topc.(AT).(e .g .,.alttude,.headng,. the.controller.n.doubt.of.plot.understandng.and.may.
speed),.whle.messages.wth.two.ATs.occurred.n.14%. thus.adversely.affect.safety .
of.the.other.transmssons . Possbly.to.avod.the.rsk.of.ether.readng.back.the.
We.also.report.a.10 .2%.ncrease.n.full.readbacks.and.a. contents.of.an.ATC.transmsson.meant.for.another.ar-
3 .4%.decrease.n.partal.readbacks ..Moreover,.plots.who. craft.(stolen.transmsson).or.readng.back.ts.contents.
provded.a.full.readback.also.ncluded.the.complete.call. ncorrectly,.plots.asked.the.controller.for.ether.a.full.
sgn.n.approxmately.64%.of.ther.responses ..The.fact. or.partal.repeat.of.the.orgnal.message ..Of.these.112.
that.call.sgns.were.excluded.n.10%.of.ther.readbacks. plot.requests,.five.nvolved.a.call.sgn.msspoken.by.the.
and. .2%.of.the.spoken.call.sgns.were.unntellgble.leaves. controller.to.whch.the.plot.wanted.confirmaton.of.
room.for.mprovement ..Unfortunately,.both.reports.show. the.message’s.recpent ..A.closer.look.at.requests.showed.
the.number.of.unacknowledged.ATC.transmssons.at. that.67 .5%.had.ether.a.fix/ntersecton.or.faclty.name.
slghtly.more.than.3% . ncluded.as.part.of.ther.content.—.and.many.of.these.
Approxmately.71%.of.the.plot.responses.contaned. messages.nvolved.route.clearances.and.the.transfer.of.
faulty.readbacks.—.67 .4%.contaned.one.or.more.er- communcatons ..The.names.of.locaton.dentfiers,.
rors.of.omsson.that.could.be.attrbuted.to.ncreases.n. fixes,.ntersectons,.navgatonal.ades,.etc ..are.crtcal.
the.complexty.of.ATC.messages ..Changes.n.message. to.navgaton;.thus,.plots.were.prudent.n.requestng.
complexty.alone.had.no.statstcally.sgnficant.effect.on. ether.the.repetton.or.confirmaton.of.prevously.heard.
the.producton.of.readback.errors.(e .g .,.the.substtuton. dentfiers .
or.transposton.of.numbers.n.the.controller’s.orgnal. Based.on.our.findngs,.we.recommend.that:
transmssons) .. 1).No.more.than.three.ATs.should.be.ncluded.n.an.
Readback.errors.comprsed.the.remanng.4 .0%.of.the. ATC.transmsson ..2).Route.clearances.should.be.gven.
plots’.faulty.readbacks.—.78%.resulted.from.a.combna- separately.as.stand-alone.transmssons ..3).The.names.
ton.of.an.error.of.omsson.plus.one.or.more.substtutons. of.all.fixes,.wayponts,.locatons,.and.other.dentfiers.
(89%).or.transpostons.(11%) ..Unlke.message.complexty,. should.be.repeated,.and.f.necessary,.spelled.out.fol-
message.length.affected.both.the.producton.of.errors.of. lowng.ther.first.rectaton ..For.example,.“CleareD
omsson.and.readback.errors ..There.were.more.errors.of. DireCt CobaD – Charlie-osCar-bravo-alfa-Delta,”.
omsson.as.ATC.message.length.ncreased.from.short.(1.AT),. or.“CleareD DireCt CobaD – C-o-b-a-D CobaD .”.4).
to.moderate.(2ATs),.and.long.(3.or.more.ATs) ..Redundant. Slang.should.not.be.accepted.by.ATC.as.part.of.a.
nformaton.mght.be.elmnated.to.ncrease.the.economy. plot.readback .
of.capacty.lmtatons.of.verbal.workng.memory ..
v

T O ATc m L c
he uTcOme Of essAge engTh And OmpLexiTy
e R p R p
On n OuTe iLOT eAdbAck eRfORmAnce
Many attempts to communicate are nullified by saying too much.
—.Robert.Greenleaf
Founder,.Modern.Servant.Leadershp.Movement
More.than.10.years.have.lapsed.snce.a.comprehensve. Prnzo.et.al ..(2006).attempted.to.remove.as.much.
analyss.was.undertaken.to.quantfy.and.report.the.types. of.the.subjectve.component.as.possble.when.countng.
and.frequences.of.plot.readback.errors.and.ar.traffic. the.complexty.present.n.communcaton.elements ..As.
control.(ATC).communcaton.problems.that.occur.n.the. noted.n.Prnzo.(1996),.communcaton.elements.are.
operatonal.envronment ..In.partcular,.t.s.mportant.to. the.fundamental.unt.of.meanngful.verbal.language ..In.
determne.whether.the.findngs.from.the.prevous.decade. avaton,.communcaton.elements.are.dentfied.accord-
(Cardos,.1993;.Cardos,.Brett,.&.Han.1996;.Prnzo,. ng.to.ther.functonalty.(Address/Addressee,.Courtesy,.
1996;.Prnzo.&.Brtton.1993).reman.representatve.of. Instructon/Clearance,.Advsory/Remark,.Request,.and.
routne.communcatons.n.2006 .. Non-Codable).and.are.restrcted.wth.regard.to.ther.
Ths.s.the.second.of.two.reports.that.nvestgated.the. avaton.topc.(AT:.alttude,.headng,.speed,.traffic,.route,.
role.of.ATC.message.complexty.and.message.length.on. etc .:.Prnzo,.Brtton,.&.Hendrx.1995) .
operatonal.communcatons.between.controllers.and. For.Prnzo.et.al ..(2006),.the.complexty.value.of.a.
plots ..In.the.first.report,.Prnzo,.Hendrx,.and.Hendrx. communcaton.element.s.defined.by.each.word.or.set.of.
(2006).descrbe.and.summarze.the.controller-plot. words.transmtted.by.ATC.to.the.flght.deck.that.contans.
communcaton.process.that.takes.place.durng.normal,. a.new.pece.of.nformaton.crtcal.to.the.understandng.of.
day-to-day.operatons.n.the.termnal.radar.approach. that.communcaton.element ..Identfyng.the.amount.of.
control.(TRACON).envronment ..In.ths.report,.we.apply. nformaton.n.ATC.utterances.begns.by.parsng.a.mes-
the.same.theoretcal.approach.and.methodology.to.the. sage.nto.ts.communcaton.elements,.then.definng.each.
analyss.of.operatonal.communcatons.n.the.en.route. communcaton.element.accordng.to.ts.speech.act.and.
envronment.usng.dgtal.audo.tapes.(DATs).provded. AT,.assgnng.a.complexty.value.to.each.communcaton.
by.five.Ar.Route.Traffic.Control.Centers.(ARTCCs) .. element,.and.finally,.summng.the.complexty.values.to.
In.ther.2006.report,.Prnzo.and.colleagues.took. arrve.at.the.level.of.complexty.for.each.message ..They.
the.concept.of.message.complexty.that.was.orgnally. developed.several.user.gudes.for.determnng.the.message.
proposed.by.Cardos.(1993).for.the.ARTCC.doman,. complexty.for.the.TRACON.(Prnzo.et.al .,.2006).and.
modfied.by.Cardos.et.al ..(1996).for.the.TRACON. En.Route.(see.Appendces).envronments .
doman,.further.refined.t.and.provded.a.theory-drven. Ther.scorng.scheme.assumes.that.communcaton.
ratonale.for.computng.message.complexty ..In.partcular,. elements.that.contan.more.nformaton.should.be.as-
Wasow.(1997).suggested.that.an.utterance’s.complexty. sgned.larger.values ..Ths.assumpton.holds,.partcularly.
can.be.derved.from.the.amount.of.nformaton.expressed. for.alttude.nstructons ..For.example,.the.numercal.values.
n.ts.consttuents.as.measured.by.the.number.of.words,. present.n.alttude.nstructons.such.as.“three.thousand.
syntactc.nodes,.or.phrasal.nodes.n.the.consttuent .. five.hundred,”.“one-zero.thousand,”.and.“four.thousand”.
They.further.cte.the.semnal.work.of.Mller.(1956).and. are.lkely.to.mpose.quantitatively.dfferent.loads.on.work-
Baddeley.(1987,.2000).and.Baddeley.and.colleagues. ng.memory ..In.partcular,.t.takes.longer.to.pronounce.
(1974,.1975),.who.provde.a.strong.theoretcal.bass.for. ‘three.thousand.five.hundred’.than.‘four.thousand’.(e .g .,.
capacty.and.processng.components.of.verbal.workng. artculatory.loop.proposed.by.Baddeley,.2000).and.utlzes.
memory ..Mller.proposed.that.verbal.workng.memory. more.capacty.because.t.contans.more.words.(Mller,.
s.lmted.n.the.amount.of.nformaton.that.can.be.ac- 1956) ..In.fact,.Morrow.and.Prnzo.(1999).found.that.
tvely.processed.(7±2),.whle.Baddeley.added.a.temporal. when.seral.reproducton.s.requred,.numercal.content.
constrant.—.external.nformaton.encoded.as.mental. that.utlzes.more.resources.may.be.partally.or.completely.
representatons.must.be.actvely.rehearsed;.otherwse,. omtted.or.lead.the.plot.to.request.a.repetton .
they.begn.to.decay.n.about.2.s.or.are.overwrtten.by. In.ths.report,.we.descrbe.and.summarze.the.
ncomng.nformaton .. .controller-plot.communcaton.process.that.takes.place.
1
durng.normal,.day-to-day.operatons.n.the.en.route. how.we.evaluated.communcaton.elements and.assgned.
envronment ..We.also.apply.the.same.theoretcal.approach. complexty.values ..Each.anchor.word.s.also.assgned.a.
and.methodology.that.was.developed.for.the.TRACON. value.=.1;.n.ths.case,.they.are.the.words.“contact”.and.
envronment.to.the.analyss.of.operatonal.communca- “pont .”.(2).Cardos.evaluated.voce.tapes.from.eght.
tons.n.the.en.route.envronment.(see.Prnzo.et.al .,.2006) .. dfferent.ARTCCs.durng.hgh.and.moderate.traffic.
Lke.the.TRACON.report,.the.purpose.of.ths.report.s. stuatons,.and.we.examned.voce.tapes.representatve.
to.(1).provde.current.nformaton.regardng.routne.en. of.hgh.traffic.stuatons.from.five.dfferent.ARTCCs ..(3).
route.communcaton.practces,.(2).document.the.types. Cardos.examned.48.hrs.of.voce.tapes,.and.we.examned.
of.transmssons.that.are.exchanged.between.plots.and. 51.hrs.of.voce.tapes ..
the.certfied.professonal.controllers.who.provde.them. Ths.report.s.also.smlar.to.the.Cardos.(1993).
wth.en.route.ATC.servces,.and.(3).record.communca- and.Prnzo.et.al ..(2006).reports.n.that.they.provded.
ton.problems.by.type.and.frequency.of.occurrence ..Lke. descrptons.and.descrptve.statstcs.related.to.three.
the.TRACON.report,.we.restrcted.the.analyss.of.ATC. general.types.of.nformaton:.Routne.ATC.Commun-
messages.to.nstructons/clearances,.altmeter.settngs,.(or. catons,.Plot.Faulty.Readback.Performance,.and.Plot.
both),.and.the.plots’.responses.to.these.messages .. Requests.for.Repeat.of.an.ATC.Transmsson ..Lke.Car-
Ths.report.s.smlar.to.Cardos’s.(1993),.n.that.(1). dos.(1993).and.Prnzo.et.al ..(2006),.complexty.values.
each.report.examned.voce.tape.samples.from.Chcago,. were.determned.for.the.transmssons.that.contaned..
Los.Angeles,.New.York,.and.Oakland.ARTCCs;.(2).the. a).nstructon/clearance.speech.acts.that.nvolved.head-
sampled.ARTCCs.had.a.relatvely.hgh.proporton.of. ng,.headng.modfier,.alttude,.alttude.restrcton,.speed,.
foregn.carrers;.and.(3).the.communcaton.samples. approach/departure,.frequency,.holdng,.route/poston,.
ncluded.both.hgh-.and.low-alttude.sectors .. and.transponder.ATs,.and.b).advsory.speech.acts.that.
It.dffers.from.the.Cardos.report.n.the.followng. nvolved.the.altmeter.AT ..
ways:.(1).Cardos.measured.message.complexty.“as. In.ths.report,.separate.analyses.were.performed.for.
the.number.of.separate.elements.contaned.n.a.sngle. routne.ATC.communcatons.and.plot.requests.for.
transmsson ..Each.word,.or.set.of.words,.the.control- repeat ..Routne.ATC.Communcatons.are.related.to.(1).
ler.sad.that.contaned.a.new.pece.of.nformaton.to. types.of.nformaton.ncluded.n.ATC.messages,.(2).ATC.
the.plot.and.was.crtcal.to.the.understandng.of.the. message.length.and.complexty,.(3).plot.responses.to.ATC.
message.was.consdered.to.be.an.element .”.[p3] ..For. messages,.and.(4).plot.use.of.call.sgns.n.readbacks ..The.
example,.“….contact.Mnneapols.Center.118 .8”.was. analyss.of.Plot.Faulty.Readback.Performance.nvolves.
consdered.one.pece.of.nformaton.f.the.plot.was. (1).plot.errors.of.omsson,.(2).plot.readback.errors,.(3).
already.on.a.Mnneapols.Center.frequency.and.as.two. message.complexty.and.plot.readback.performance,.and.
peces.of.nformaton.f.the.plot.receved.t.whle.com- (4).message.length.and.plot.readback.performance ..The.
muncatng.wth.a.dfferent.ARTCC ..We,.on.the.other. analyss.of.Plot.Requests.for.Repeat.of.an.ATC.transms-
hand,.treated.“ . . ..Contact.Mnneapols.Center.118 .8”.as. son.were.further.subdvded.nto.four.types.of.requests:.
one.communcaton.element.wth.a.complexty.value.=. (1).requests.for.the.repeat.of.a.specfic.avaton.topc,.
6 ..Our.ratonale.was.that.each.dgt.comprsng.the.rado. (2).requests.for.the.repeat.of.an.entre.transmsson,.(3).
frequency.mposes.a.memory.load.that.could.result.n.the. requests.for.confirmaton/verficaton.of.a.specfic.ava-
plot.erroneously.readng.back.the.wrong.faclty/functon;. ton.topc,.and.(4).requests.for.confirmaton.that.the.
omttng,.transposng,.or.substtutng.any.(or.all).of.the. transmsson.was.for.them ..
dgts.n.the.rado.frequency;.or.both ..Fgure.1.shows.
Figure 1. Assigning a Complexity Value to an Aviation Topic.
2