the management of air motion sickness in sa air force pilots · 2015. 10. 28. · 16 mg 3-4x/d,...

• October 2008 • The Specialist Forum

The management of air motionsickness in SA Air Force pilots

AbstractDespite being a highly selected popula-tion, not only in terms of aptitude, but interms of a comprehensive and stringentmedical selection process, candidatesundergoing the South African Air Force(SAAF) pupil pilots’ course (“pupes'course”) experience significant prob-lems with air motion sickness.The stringent medical selection processis to some extent offset by the fact thatthe course itself is significantly moredemanding than a private (or commer-cial) pilots' course, as it includes a sig-nificant amount of aerobatic and high Gmanoeuvres.

The incidence of problematic airmotion sickness on pupes' course is inthe region of 12 percent, and the per-centage of students who have to bewithdrawn from course due to refracto-ry air motion sickness is approximatelyfive percent. This makes it the mostprevalent single medical reason for poorprogress and non-completion of theSAAF pilot training.

Introduction: motion sickness theoryMotion sickness can be considered as part ofthe physiological vertigo syndrome and canoccur in any healthy person. It is due tounusual and unadapted stimulation of the sen-sory systems for static and dynamic spatialorientation.

The incidence is higher in children and females,with children below the age of two basicallyresistant. According to Brandt1 there are threetheories explaining the pathomechanism ofmotion sickness:1. The mismatch theory of perceptual conflict or

mismatch between actual and expectedmotion stimulation

2. The sensory conflict theory of intersensory(visual-vestibular) or intrasensory mismatch

3. Vestibular hyperexcitability due to migraineattacks and postoperative nausea and vomiting.

Approach to a SAAF pupil pilot experiencing air motionsicknessProblematic motion sickness usually manifestsitself during the Advanced General Flying(AGF) phase of the course, where aerobaticsand high G manoeuvres are introduced for thefirst time. At this stage, the student has loggedapproximately 40 flying hours, several of whichare solo, and has mastered basic flying skills(roughly equivalent to a Private Pilots’Licence).

This is the first time that the student is reallyexposed to the agility and high G capability ofthe Pilatus PC7 Mk II Astra, and it really is aneye opener. Almost without exception, the stu-dents experience varying degrees of “queasi-ness” during the initial sorties, but most habitu-ate quickly, and within a further five to 10 fly-ing hours, most of them are thoroughly enjoy-ing the responsiveness of the machine.

Approximately 12%, however, experiencemotion sickness to the point of emesis, or seri-ous enough that sorties cannot be completed.After two sorties where this has occurred, theSAAF requests medical intervention from theflight surgeon.

After an assessment to identify and manageany acute illness/treatable cause such as otitis,gastrointestinal conditions, self inflicted condi-tions (alcohol, poor eating habits, etc.), and psy-chological factors, the process of assisted habit-uation is initiated.

Dr RW BedfordMB ChB (Pret), BSc(Aerospace Medicine) CumLaude (Pret)

Dr LM HofmeyrMB ChB (Pret), MMed(Otorhinolaringology) (Pret)

Dr Rob Bedford is Chief Medical Officer at the Institutefor Aviation Medicine. Apartfrom his medical practice inaerospace medicine, he is anoperationally qualified SA AirForce (helicopter) pilot, makinghim currently the only fullyfledged flight surgeon in SouthAfrica.

Dr Louis Hofmeyr is Chief ENTSpecialist at 1 Military Hospital.He does regular sessions at theInstitute for Aviation Medicine,conducting specialist vestibularassessment on SA Air Force aircrew.

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Motion sickness can occur in any healthy person

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The Specialist Forum • October 2008 • Page 5

Small doses (starting at half the normal thera-peutic dose and titrated to the lowest effectivedose) of vestibular suppressants are prescribed,to be taken one hour before the planned sortie.Typical candidates for this regimen includecyclizine 25 mg, cinnarizine 12.5 mg, ormetoclopramide 5 mg. The intent is to suppressthe vestibular system to the point where emesisis avoided, but not to the point where habitua-tion is prevented. After five successful sorties,the dose is tapered slowly over a period of anadditional five sorties (cyclizine syrup is con-venient for accurate dose titration). An alterna-tive regime is to place the student on a two-week course of medication such as betahistidine16 mg 3-4x/d, flunarizine 5-10 mg/d, orphenytoin 100-200 mg/d. This is then taperedover a further two-week period. Agents withprominent sedating or extra-pyramidal adverseeffects (such as promethazine or prochlorper-azine) are not used at all.

The dilemma with pharmacological treatmentis that it is not legal to use any of these agentswhile piloting an aircraft (this counts for civil-ian pilots too!). The way that the military flightsurgeon gets around this is to stipulate that med-ication may only be used with a safety pilot inthe aircraft. Many of the early AGF sorties areflown with an instructor, but the problem comesin when a solo sortie is scheduled. The possibil-ity of a type-rated pilot like me “sitting in therear cockpit and pretending you’re not there” isan option, but the full value of such a sortie isthen never achieved. Much of the learning valueof solo sorties is vested in the pilot’s certainknowledge that whatever is done, the pilot hadbetter be able to recover, because there isnobody else who can take over control.

Any failure or relapse of this regime leads toprogression to the second tier. Based on theassumption at this stage that there is possiblysome vestibular pathology, I developed an in-aircraft vestibular assessment technique. Thereason for doing this was a practical one. Tosend a student from Langebaanweg to Pretoriafor specialised vestibular assessment was notonly more expensive than flying a diagnosticsortie, but it also caused an absence of a weekor more, causing the student to fall furtherbehind in the flying training. For a conditionwhere anxiety is a well-documented trigger,adding the thoughts to the individual’s mindthat he/she may be diagnosed with a conditionthat will terminate their training, not to mentionthe fact that they will have to work extremelyhard to catch up when they return, are stressorswhich are to be avoided if at all possible.

Diagnostic flying sortieThe following is an example of the report gen-erated from the in-aircraft vestibular assess-ment sortie. The narrative built into the reportserves to explain the manoeuvres performed.

SORTIE REPORT1. A motion sickness diagnostic sortie flown

on 11 June 2004:a. Aircraft: Astra 2041b. Commander: Lt Col (Dr) RW Bedfordc. Co-pilot: 2 Lt ******

2. The sortie was based on a 1.2 hourAdvanced General Flying sortie.

3. As anxiety is known to play an importantrole in motion sickness (including anxietyregarding doing procedures correctly andbriskly), all flying, radio work, and proce-dures were done by myself, except duringa specific exercise.

DEMONSTRATION4. The effect of sub-threshold roll was

shown, to demonstrate to the student theunreliability of the vestibular system. Asub-threshold (


that stage, the student had to very rapidly take control, recon-figure to the landing configuration, and complete the beforelanding checks, specifically where the approaching groundwas an unmistakable stimulus to do so expeditiously. At 200ft, I took control and did a go-around. From the way the stu-dent performed, it was evident that he was under considerablepressure, yet this did not progress to panic, nor did it elicitmotion sickness symptoms.

SEMI-CIRCULAR CANAL ASSESSMENT6. Isolated sensitivity of the lateral canals to slow rotation (6°/sec)

was tested. With the student’s eyes closed, and the head laterallyflexed 45° (to isolate the superior and posterior canals), a 45°banked turn was entered. This test revealed no hypersensitivity ofthe lateral canals. (See Diagram 1).

7. Isolated sensitivity of the lateral canals to fast rotation (180°/sec)was tested. With the student’s eyes closed, and the head flexed 90°(to isolate the superior and posterior canals), a 360° aileron rollwas performed. This test revealed no hypersensitivity of the later-al canals. (See Diagram 1).

8. Isolated sensitivity of the superior/posterior canals to slow rotation(6°/sec) was tested. With the student’s eyes closed, and the headrotated 45° to the right (to isolate the lateral canals, the right supe-rior canal, and the left posterior canal), a loop was performed. Thiswas repeated with the head rotated 45° to the left, to isolate the lat-eral canals, the left superior canal, and the right posterior canal.This test revealed no isolated hypersensitivity of the superior orposterior canals. (See Diagram 2).

9. Isolated sensitivity of the superior/posterior canals to fast rotation(180°/sec) was tested. With the student’s eyes closed, and the headrotated 45° to the left (to isolate the lateral canals, the right supe-rior canal, and the left posterior canal), a 360° aileron roll was per-formed. This was repeated with the head rotated 45° to the right,to isolate the lateral canals, the left superior canal, and the rightposterior canal. This test revealed no isolated hypersensitivity ofthe superior or posterior canals. (See Diagram 3).

10. Sagittal conjugate sensitivity of the superior/posterior canals toslow rotation (6°/sec) was tested. With the student’s eyes closed,and the head neutral (to isolate the lateral canals only), a loop wasperformed. This test revealed no conjugate hypersensitivity of thesuperior/posterior canals in pitch. (See Diagram 4).

11. Sagittal conjugate sensitivity of the superior/posterior canals tofast rotation (180°/sec) was tested. With the student’s eyes closed,and the head rotated 90° to the left (to isolate the lateral canalsonly), a 360° aileron roll was performed. This test revealed noconjugate hypersensitivity of the superior/posterior canals inpitch. (See Diagram 5).

12. Coronal conjugate sensitivity of the superior/posterior canals toslow rotation (6°/sec) was tested. With the student’s eyes closed,and the head rotated 90° to the left (to isolate the lateral canalsonly), a loop was performed. This test revealed no conjugatehypersensitivity of the superior/posterior canals in roll. (See Dia-gram 6).

13. Coronal conjugate sensitivity of the superior/posterior canals tofast rotation (180°/sec) was tested. With the student’s eyes closed,and the head neutral (to isolate the lateral canals only), a 360°aileron roll was performed. This test revealed no conjugate hyper-sensitivity of the superior/posterior canals in roll. (See Diagram 7).

The managementof air motion sickness in SA AirForce pilots

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Diagram 1

Diagram 2

Diagram 3

Diagram 4

Diagram 5

Diagram 6

Diagram 7



OTOLITHIC ORGAN ASSESSMENT14. The aircraft was bunted to -1Gz for 5 sec-

onds. This elicited immediate and severenausea (although no vomiting).

ADDITIONAL ASSESSMENT15. After the student’s nausea had subsided, I

instructed him to take control, and fly somebasic manoeuvres. During this phase, thefollowing was noted:• When perceiving an incorrect flight

parameter (e.g. high attitude), the cor-rection was made in a sudden (jerky)fashion, inducing refractory but rapidonset Gz impulses. This elicited nau-sea in the student.

• The student was overactive on the rud-ders (e.g. balance ball way out to theleft during a right turn). This highlyunnatural out-of-balance flight quiteunderstandably elicited nausea.

16. During the return to the airfield at 1600 ft,the small amplitude but rapid cycle/rapidonset Gz impulses caused by turbulenceelicited nausea in the student.

FINDINGS17. No specific semicircular canal hypersensi-

tivity was elicited.18. There is no indication that anxiety plays a

role in the student’s motion sickness.19. Negative Gz per se, as well as short dura-

tion, rapid onset Gz impulses elicit nausea,suggesting a possible otolithic organ prob-lem, but the possibility also exists that thestudent has never experienced suchvestibular stimulation before, and is simplyunaccustomed to it.

20. The student’s motion sickness iselicited/exacerbated by some incorrect fly-ing techniques.

RECOMMENDATIONS21. The student is to continue with the normal

flying program, but:• Low doses of vestibular suppressants

will be used initially, but only with asafety pilot. The medication will betapered according to a fixed schedule.

• Instructors are to concentrate onavoiding unnecessary negative Gz.

• Instructor and student are specificallyto concentrate on rudder inputs tomaintain balanced flight and smoothcontrol inputs.

• Turbulence encountered during lowlevel routings to the flying sectors willserve as an opportunity to habituatethe student.

22. Failing these measures, the student willhave to undergo specialised vestibularassessment and/or centrifuge habituationat the Institute for Aviation Medicine.

(This particular individual successfullycompleted pharmacologically assistedhabituation.)

For those individuals where the diagnosticsortie indicates that no specific pathology ispresent, they are re-enrolled for another ses-sion of pharmacologically assisted habitua-tion. To date all such individuals have contin-ued to habituate successfully. The individualswhere the diagnostic sortie indicates thatpathology is probable (the sortie has shownremarkable accuracy in identifying thisgroup), are sent to the Institute for AviationMedicine for specialist vestibular assessment.

Vestibular assessmentAt the Institute for Aviation Medicine, the can-didates receive a comprehensive vestibularassessment by an ENT specialist with experi-ence in aviation medicine and a special interestin balance disorders. This includes fullvideonystagmography (VNG). Over 90% of thecandidates who were identified by the diagnos-tic sortie with probable pathology have beendiscovered to have pathological VNG abnor-malities which make them unfit for furtherSAAF flying training. In cases where the diag-nostic sortie has not been used for pre-screen-ing, only approximately 30% have disqualify-ing pathology.

By implication, the candidates who havereached this point have refractory motion sick-ness without any specific vestibular or otherpathology. One last possibility exists for thesecandidates (maybe)...

Centrifuge habituation (see Box 1)A very effective method of vestibular habitua-tion using the human centrifuge was pioneeredat the Institute for Aviation Medicine by Dr FLvan der Laan. The human centrifuge was pur-chased in 1970, and was at that time state of theart. It has a 4 m radius of rotation, and the Gz(see Box 2) vector is passively maintained by achair which is free to tilt inside the gondola cap-sule. The chair is weighted at the bottom, andthe resultant g vector (see Box 3) tilts the bottomof the chair outwards, keeping the sum of the gforces (centrifugal and gravitational) in approx-imate alignment with the Gz axis.


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The occupant (equipped with the necessary airsickness bags) is exposed to low supra-physio-logical (i.e. greater than 1 and less than 2.5) Gzforces for one hour per weekday, for a period offour weeks. The principle is to subject the occu-pant to extended periods of vestibular stimula-tion which is maintained just below the emesisthreshold, in order to provide a maximal stimu-lus for vestibular habituation. Dr van der Laan

described a series of formal head movementsduring the centrifugation in order to provideconstant stimulation to the entire vestibular sys-tem. In theory, after the initial rotational accel-eration is completed, it takes 20 to 25 secondsfor the endolymph in the semicircular canals to“catch up” to the canal walls, after which thereis no movement of the endolymph relative tothe canals, and vestibular stimulation ceases.


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Box 2. Aircraft axis designation

By convention, the three axes around which an aircraft can rotate are designated as follows:The aircraft pitches (nose towards the canopy[“up”] or towards the undercarriage [“down”])about the y axis, rolls (one wing towards thecanopy [“up”]/other wing towards the undercar-riage [“down”]) about the x axis, and yaws (nosetowards the left or right wing-tip) about the z axis.This seemingly strange nomenclature is usedbecause “up, down, left and right” are very rela-tive concepts in an agile aircraft which may at anygiven time be, for example, vertical, or inverted.

The most important and commonly experienced g force during flying is +Gz: namely the inertial force whichacts in the z axis and pulls blood downwards/away from the head. Excessive +Gz is the cause of G inducedLoss Of Consciousness (G-LOC).

Box 1. Basic centrifuge theory

Circular motionIn order to travel in a circular path, an object with a velocity (v)must constantly accelerate toward the centre of the resulting circle, hence the term centripetal acceleration. The radius of thecircular path depends on the square of the velocity and the centripetal acceleration, according to the formula:

r = v2/aBecause the object is now moving in a circle, it is convenient tomeasure the velocity in terms of rotational velocity (ω) ratherthan tangential velocity (v). The SI unit for ω is the radian, whichis based on the ratio between the radius and the circumference of a circle. A radian is defined as thefull circumference (360°)/2π, which yields a result of approximately 57.3°. After rotating through 1 radian, the object has moved the same distance as 1 radius, making the conversion of v to ω very sim-ple, according to the formula:

ω = v/rIf an object moving at v = 10 m.s-1, is forced to follow a circular path with a 2 m radius, after 1 secondit will still have travelled a distance of 10 m (five times the radius), and the rotational velocity is therefore 5 rad.s-1.

Centripetal acceleration (and the resultant “centrifugal” inertial acceleration vector which is equal inmagnitude, but opposite in direction according to Newton’s third law), is dependent on the radius ofrotation and the square of the rotational velocity. Thus:

a = rω2 a = Centripetal acceleration (m.s-2)r = Radius of rotation (m)

ω = Rotational velocity (rad.s-1)Because the direction of the centripetal acceleration is constantly changing to maintain the circulartrack, a body, even while in stable circular motion, is continuously accelerating.



In practice, however, this does not happenexactly according to the book. Having been aregular centrifuge occupant over the past 15years, I soon discovered from personal expe-rience that the slightest (and unavoidable)movement of the head during even steadystate centrifugation produces powerfulvestibular stimulation – powerful enough tochallenge even my completely normal andadroit (I’ve never experienced motion sick-ness in any aircraft, even during some prettyextreme manoeuvres) vestibular system. Inaddition to this, the supra-physiological Gforces are constantly stimulating the otolithicorgans. For these reasons, the formal headmovements are no longer employed – with nodiscernible deterioration in the efficacy of theprocedure.

Typical efficacy of centrifuge habituation isin the region of 80%. The centrifugationallows the subject to habituate enough to beable to tolerate flying sorties without incapac-itating motion sickness, and with regular fly-ing (it is attempted to get the student to flyevery weekday for at least eight weeks afterthe centrifuge habituation), further habitua-tion is completed in the aircraft. Althoughsome loss of habituation is to be expected ifthe pilot goes for a long period without flying,to date no cases of relapse have been recordedfollowing successful centrifuge habituation.

The 1970 centrifuge at the Institute hasexceeded its design life, and has suffered fail-ures which have caused it to be condemnedfor human occupancy. This means that thecentrifuge option is currently unavailable, andcandidates, who may otherwise have beensuccessfully habituated in the centrifuge, arebeing lost.

The Institute is in the process of acquiring anew centrifuge. Current fifth generation cen-trifuges differ in several aspects from theexisting first generation centrifuge. For pre-cisely the phenomenon of “adverse” vestibu-lar stimulation during high G training, therotation arms are much longer (at least 8metres) in order to minimise vestibulareffects.

The other important difference is that thegondola attitude is now actively (computer)controlled. This allows the computer to fur-ther minimise vestibular stimulation, not onlyby precisely aligning the gondola to thedesired (usually Gz) axis, but also utilisinggondola pitch adjustment techniques duringacceleration and deceleration. From personalexperience in the latest centrifuges, these newtechniques do dramatically reduce thevestibular stimulation associated with cen-trifugation, but in my opinion, the remainingvestibular stimulation is likely to be sufficientfor vestibular habituation.


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Box 3. The concept of g (G)

Note: The SI unit for the earth’s gravitation is (lowercase) g. Uppercase G is an entirely different gravi-tational constant. However, in aviation, the convention in naming an inertial g vector is uppercase G,followed by the axis in which it acts, i.e. x, y or z. The direction of the vector is indicated by a + or - signpreceding the G, hence the full nomenclature +Gz, -Gx etc.One “g” (earth’s gravitational acceleration) is equal to 9.80665 m.s-2, so to produce 1 g of centripetalacceleration with a 4 m radius of rotation:

ω2 = a/rω2 = 9.8/4ω2 = 2.45ω =√2.45ω = 1.57 rad.s-1

(ω ≈ 90°/sec)By the same calculation, for 1 g of centripetal accelerationwith an 8 m radius, ω ≈ 60°/sec.

The earth’s gravitational acceleration is 1 g towards the centre of the earth (‘down’). If the centrifugeproduces a 1 g inertial vector away from the centre of rotation, the total g experienced by the occupantis a sum of these two vectors, namely √2 (≈ 1,41) g at an angle of 45° to the vertical (head nearer theinside).

Most modern centrifuges use this as the “dynamic baseline” from which all high G training starts, andreturns to.



The way forwardIt is the intention to do even more stringentvestibular assessment during initial SAAFpilot selection, including vestibular adroitnesstesting (VAT) and full videonystagmography.

Habituation in the new centrifuge is alsoplanned, probably using centrifugation at the“dynamic baseline” of the new centrifuge.Time will tell whether the planned procedurein the new centrifuge is as effective as the pre-vious regimen, or whether modifications willbe required to the procedure in order to com-pensate for the described differences betweenthe old and the new centrifuge. It is certainlyan avenue where the authors intend to conductfurther research.

Reference1. Brandt T. Motion sickness. In: Brandt T. Ver-

tigo: its multisensory syndromes. 2nd ed.London: Springer-Verlag; 2002. p. 485-96.


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Figure 1. The SAAF training aircraft, the Pilatus PC7 MkII, during the vertical phase of a stall turn.

Even more stringent vestibularassessment is intended

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the management of air motion sickness in sa air force pilots · 2015. 10. 28. · 16 mg 3-4x/d,...

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