Contact lenses and corrective flying spectacles in military aircrew--implications for flight safety

Abstract

Refractive devices used by aviators need to suit the aerospace environment or their failure can have serious implications. A relatively minor visual disability can result in loss of life and aircraft. We surveyed commonly occurring problems with the different types of refractive correction worn by Royal Air Force (RAF) aircrew over the previous 12 mo. We also asked if they had experienced any flight safety incidents (FSI) relating to their refractive correction.
A retrospective anonymous questionnaire survey was given to 700 active aircrew occupationally graded as requiring corrective flying spectacles (CFS) or contact lenses (CL) for flying.
63% (443) of the questionnaires were completed. CL were worn by 53% of aircrew; 71% of them used daily disposable CL. CFS were worn by the remaining 47% of aircrew, 14% of whom used multifocal lenses. Of CFS wearers, 83% reported problems including misting, moving, discomfort, and conflict with helmet-mounted devices (HMD). CL-related ocular symptoms were reported in 67% of wearers including cloudy vision, dry eye, photophobia, red eyes, excessive mucus formation, CL movement, itching, and grittiness. No CL-related FSI were reported over the previous 12 mo compared with 5% CFS-related FSI (p < 0.001). The graded performance of CL for vision, comfort, handling, convenience, and overall satisfaction was significantly higher than for CFS.
CFS are associated with problems in terms of comfort and safety. CL are well tolerated by aircrew, and deliver improved visual performance.

Full text

RESEARCH ARTICLE
Contact Lenses and Corrective Flying Spectacles in
Military Aircrew—Implications for Flight Safety
Andrew M. Partner, Robert A. H. Scott, Penny Shaw,
and William J. Coker
PARTNER AM, SCOTT RAH, SHAW P, COKER WJ. Contact lenses and
corrective flying spectacles in military aircrew—implications for
flight safety. Aviat Space Environ Med 2005; 76:661–5.
Background: Refractive devices used by aviators need to suit the
aerospace environment or their failure can have serious implications. A
relatively minor visual disability can result in loss of life and aircraft. We
surveyed commonly occurring problems with the different types of
refractive correction worn by Royal Air Force (RAF) aircrew over the
previous 12 mo. We also asked if they had experienced any flight safety
incidents (FSI) relating to their refractive correction. Methods: A retro-
spective anonymous questionnaire survey was given to 700 active
aircrew occupationally graded as requiring corrective flying spectacles
(CFS) or contact lenses (CL) for flying. Results: 63% (443) of the ques-
tionnaires were completed. CL were worn by 53% of aircrew; 71% of
them used daily disposable CL. CFS were worn by the remaining 47% of
aircrew, 14% of whom used multifocal lenses. Of CFS wearers, 83%
reported problems including misting, moving, discomfort, and conflict
with helmet-mounted devices (HMD). CL-related ocular symptoms were
reported in 67% of wearers including cloudy vision, dry eye, photopho-
bia, red eyes, excessive mucus formation, CL movement, itching, and
grittiness. No CL-related FSI were reported over the previous 12 mo
compared with 5% CFS-related FSI (p ⬍ 0.001). The graded perfor-
mance of CL for vision, comfort, handling, convenience, and overall
satisfaction was significantly higher than for CFS. Conclusion: CFS are
associated with problems in terms of comfort and safety. CL are well
tolerated by aircrew, and deliver improved visual performance.
Keywords: refractive correction, aviation environment, spectacles, eye-
glasses, contact lenses, questionnaire, survey.
R
EFRACTIVE PROBLEMS in aviators have more im-
plications than in the general population as a rela-
tively minor visual disability can have catastrophic ef-
fects. Refractive correction in aircrew historically has
been with corrective flying spectacles (CFS). Over the
past 20 yr high water content soft contact lenses (CL)
have also been successfully used for all Royal Air Force
(RAF) aircrew groups (2). The visual requirements of
aviators are changing with the increasing use of helmet-
mounted devices (HMD) and high performance air-
craft. These factors along with developments in visual
technology have increased the options for aircrew vi-
sual correction.
We previously surveyed the different types of refrac-
tive correction worn by RAF aircrew for flying and
assessed their subjective experience of comfort, in-flight
performance, and the occurrence of a flight safety inci-
dent (FSI) associated with CFS or CL (15). We found
that CFS were associated with significant problems in
terms of comfort and safety; these differed between
class of aircraft flown. FSI attributable to CFS were
noted in 27% of fast jet aircrew and 17% overall. The
incidents were related, for the most part, to lens misting
or sweat accumulation on the lens.
The majority of aircrew used CFS but showed a pref-
erence for CL if they had tried them. CL are well toler-
ated and give a greater field of view with no misting,
sweat degradation, or conflict with HMD. Only 65% of
aircrew CL wearers were under review by the RAF
optometry department and some were using unsuitable
CL types.
After the survey, we took steps to address the prob-
lems that were highlighted. A new CFS frame was
introduced to reduce discomfort and we improved the
ease of access to RAF optometry departments by hold-
ing outreach CL clinics at larger air bases throughout
the country. To close the audit loop, we repeated the
survey after 3 yr. The aim of this survey was to observe
changes in the refractive correction worn by RAF
aircrew during this period and to compare the subjec-
tive aircrew experience of comfort and in-flight perfor-
mance associated with CFS or CL.
METHODS
Our study was a retrospective anonymous question-
naire survey of 700 active aircrew occupationally
graded as requiring refractive correction for flying.
They were grouped according to the class of aircraft
flown—fast jet, heavy jet, C130, rotary aircraft, training
aircraft, and specialist support aircrew, e.g., sentry
aircrew, airmen aircrew. The questionnaire was clearly
stated as voluntary and anonymous and was arranged
From the Royal Centre for Defence Medicine, University Hospital
Birmingham, Selly Oak Hospital, Birmingham, UK (A. M. Partner,
R. A. H. Scott); the Department of Optometry, RAF Cranwell, Lin-
colnshire, UK (P. Shaw); and the Royal Air Force Centre for Aviation
Medicine, RAF Henlow, Bedfordshire, UK (W. J. Coker).
This manuscript was received for review in February 2005. It was
accepted for publication in April 2005.
Address reprint requests to: Flight Lieutenant Andrew M. Partner,
Royal Centre for Defence Medicine, University Hospital Birmingham,
Selly Oak Hospital, Raddlebarn Rd., Birmingham B29 6JD, UK;
amp195@yahoo.co.uk.
Reprint & Copyright © by Aerospace Medical Association, Alexan-
dria, VA.
661Aviation, Space, and Environmental Medicine • Vol. 76, No. 7, Section I • July 2005

into three sections (General, CFS, and CL). Questions
were forced choice, performance grading, or subjective.
Both the CFS and CL groups were asked whether they
had experienced a FSI. A FSI is a situation during the
flight where the respondent felt their refractive device
endangered the safe passage of the aircraft or its load.
The results were collated on a database for statistical
analysis and the Fischer exact test was used to compare
data groups.
RESULTS
Of the 700 questionnaires sent, 443 (63%) were com-
pleted and returned. The average age of the aircrew
was 39 yr (range 21– 60 yr); 97% were male. The average
experience in the aircraft flown was 9.1 yr (range 2
mo–39 yr). CL had been tried by 255 (58%) at some
stage during their flying career and, at the time of
questioning, CL were used by 233 (53%); the remaining
210 (47%) used CFS (Fig. 1).
Distance correction alone was used by 380 subjects
(86%). The remaining 63 required a reading correction
in the form of a multifocal lens. Multifocal lenses allow
you to focus through different prescriptions for differ-
ent distances through the same lens. There were 34
(54%) who wore D segment bifocals, 12 (19%) who wore
executive bifocals, 16 (25%) who wore varifocals, and 1
who wore (2%) trifocals. Outside flying duties, 10 of the
63 aircrew using multifocal CFS wore varifocals instead
of bifocals, 10 wore ‘half moon’ readers only, 2 wore
executive bifocals instead of D segment bifocals, 2 wore
D segments instead of executive bifocals, and 7 wore CL
rather than CFS.
Corrective Flying Spectacles
A significant problem with CFS over the preceding 12
mo was reported by 83% of aircrew. There were 44%
who reported misting of the lens, 36% who reported
CFS movement, 63% who reported CFS discomfort,
10% who reported poor vision, 5% who reported bifocal
problems, and 25% who reported conflict with their
HMD. The proportion of the aircrew types experiencing
misting, CFS movement, and discomfort by aircraft
type is represented in Fig. 2. A FSI was reported by 5%
of aircrew wearing CFS over the previous 12 mo.
Contact Lenses
Of the 255 aircrew who had tried CL, 233 (91%)
continued to use them for flying. The RAF optometric
CL fitting and review service was used by 222 (95%),
the remainder arranged their own review. During fly-
ing duties, 165 (71%) wore daily disposable CL, 27
Fig. 1. Percentage of aircrew using CL or CFS according to their different aircrew groups.
Fig. 2. Percentage of each aircrew group experiencing the common problems associated with CFS wear over the previous 12 mo.
AIRCREW REFRACTIVE DEVICES—PARTNER ET AL.
662 Aviation, Space, and Environmental Medicine • Vol. 76, No. 7, Section I • July 2005

(12%) wore twice disposable weekly CL, 38 (16%) wore
monthly disposable CL, 2 (1%) wore silicone hydrogel
extended wear CL, and 1 (0.5%) wore rigid gas-perme-
able CL.
All subjects were compliant with approved cleaning
regimens. Only the 2 subjects who wore extended wear
CL regularly slept overnight in them, although 71 (30%)
with other types of CL slept overnight in them from
time to time. No incidents of CL-related corneal ulcers
were reported.
CL-related ocular symptoms during flight were re-
ported in 67% of aircrew. Dry eyes were the most
common side effect, occurring in 104 (45%). Other prob-
lems included grittiness in 55 (24%) airmen, CL move-
ment in 49 (21%) airmen, red eyes in 37 (16%) airmen,
itching in 29 (12%) airmen, cloudy vision in 26 (11%)
airmen, excessive mucus formation in 4 (2%) airmen,
and photophobia in 1 (0.5%) airman. No FSI related to
CL wear were reported over the previous 12 mo, sig-
nificantly less than with CFS (p ⬍ 0.001).
Performance of CFS and CL
The graded performance of CFS as good or excellent
was 74% for vision, 13% for comfort, 25% for handling,
22% for convenience and 22% for overall satisfaction.
For CL it was graded as good or excellent in 91% for
vision, 88% for comfort, 67% for handling, 78% for
convenience, and 88% for overall satisfaction. The per-
formance of CL was rated as good or excellent signifi-
cantly more than CFS, for all parameters (p ⬍ 0.001). A
performance summary is represented in Table I.
DISCUSSION
Compared with the previous RAF aircrew refractive
survey, there has been a marked increase in the use of
CL from 31% to 53% (Fig. 3). The superior visual per-
formance of CL suggests operational and safety advan-
tages that outweigh the disincentive of personal expen-
diture on CL (only CFS are issued at public expense).
Exposure to CL use in aircrew was increased after the
previous survey and 91% continued to use them as their
preferred choice of refractive correction for flying du-
ties, especially fast jet aircrew.
While 5% of CFS-wearing aircrew (equating to 52 per
thousand aircrew per year) reported an FSI that endan-
gered the flight or resulted in a near miss, it is of note
that no actual incident of loss or endangerment of RAF
aircraft has been attributed to defective CFS, either
wholly or in part. In civilian aviation, Nakagawara et al.
attempted to report a prevalence of FSI where refractive
devices used by the pilots were deemed a contributing
factor (10). Unfortunately, the available resources to
link refractive correction to a FSI are limited, and many
investigations highlighting human error are restricted
by liability and employment concerns. Indeed, since
80% of all aviation accidents result from human error,
one would expect vision, arguably the most important
special sense in control of the aircraft, to be a significant
TABLE I. VISUAL PERFORMANCE OF CL AND CFS WITH PERCENTAGE ATTAINED FOR EACH CATEGORY
(1 ⫽ VERY POOR, 2 ⫽ POOR, 3 ⫽ NEITHER GOOD NOR BAD, 4 ⫽ GOOD, AND 5 ⫽ EXCELLENT).
Factors
Grade
12345
CL CFS CL CFS CL CFS CL CFS CL CFS
Vision 0 2.3 1.3 5.8 7.7 17.6 34.6 46.5 56.4 27.6
Comfort 0 25.4 1.3 35.6 10.3 25.9 39.7 11.3 48.7 1.6
Handling 1.3 7.4 5.1 24.5 26.3 43.3 39.7 22.2 27.6 2.7
Convenience 4.5 14.4 1.9 27.3 16.0 36.7 31.4 17.9 46.2 3.9
Overall Satisfaction 0 9.7 1.9 31.0 9.6 37.3 46.2 20.0 41.7 2.1
Fig. 3. Change of contact lens usage between the first (2000) and second (2003) survey according to aircrew group.
AIRCREW REFRACTIVE DEVICES—PARTNER ET AL.
663Aviation, Space, and Environmental Medicine • Vol. 76, No. 7, Section I • July 2005

contributing factor. However, the National Transporta-
tion Safety Board found only 17 out of a total 46,462 (⬍
0.0004%) FSI were attributed to refractive correction. If
this figure only represents FSI in the air transport sec-
tor, we would expect this number to be higher in the
very different military aviation environment. This im-
plies that either the RAF aircrew FSI have not been
properly reported or that aircrew concerns have not
stood up to objective scrutiny.
CFS problems have decreased from 97% to 83% com-
pared with the previous survey (15). Preferential use of
CL over CFS may have contributed to this reduction. In
addition, specific measures to improve CFS comfort
over the nose and ears have resulted in a significant
drop in discomfort problems from 84% to 63%. CFS
conflict with HMD remained at approximately 25% in
both surveys. It has previously been reported that there
is an increased incidence of aircraft accidents due to
spatial disorientation with HMD (especially night vi-
sion devices) and conflict with CFS may add to the
problem (1).
CL wearers reported fewer problems during flight
than CFS wearers (67% vs. 83%) and only 11% of these
were related to poor vision. Near correction was re-
quired for 16% of aircrew. The majority of presbyopic
aircrew (54%) used D segment bifocals for flying. These
incorporate a small reading segment devoting the ma-
jority of the field of view to distance vision. The near
vision requirements in the air appeared to differ from
those on the ground with 49% choosing a different near
vision lens type outside flying duties. To help CL wear-
ing in presbyopic aircrew, we have developed a half-
moon reader with a frame that is suitable for aircrew
use (13). These are intended for intermittent use, espe-
cially in heavy jets and multi-engine transport aircraft
where aircrew have opportunity to remove their spec-
tacles during flight. They are, however, incompatible
with HMD usage.
Efforts to make the RAF optometric CL fitting and
follow-up service more accessible have increased atten-
dance from 65% to 95%. In the general population, soft
contact lenses now represent 95% of new fits and 80% of
refits; 55% of soft CL wearers prefer monthly dispos-
ables and 45% prefer daily disposables (9). Daily dis-
posable CL use in aircrew has increased from 35% to
71% over the past 3 yr. They offer good visual perfor-
mance and comfort while being hygienic and conve-
nient (11,14). Silicone hydrogel extended wear CL were
successfully used in two aircrew members and repre-
sent one in six soft CL refits in the general population.
They are a new type of soft CL with sufficient oxygen
transmission to prevent corneal edema seen with con-
ventional CL if the lenses are worn overnight. Users of
CL are at increased risk of microbial keratitis if they fail
to wash their hands prior to insertion or over-wear their
CL. Blepharitis and dry eyes also increase the risk of
infection and there has been research demonstrating a
change in the normal ocular biota in the lids and con-
junctiva favoring extended wear CL users, something to
consider when treating CL-related infections (16). Cur-
rently extended wear CL are licensed for up to 30 d
continuous wear, but are associated with allergic reac-
tions in some individuals and should, therefore, be
prescribed to aircrew with caution (4).
Other CL-related ocular symptoms of dryness, pho-
tophobia, red eyes, excessive mucus formation, CL dis-
placement, and itching during flight all reduced from
our previous survey. The most common CL-related
symptom, dry eye, reduced from 73% to 45%. This
symptom is more prevalent among aircrew and proba-
bly relates to a combination of dehydration in a high
altitude environment associated with a reduced blink
rate in CL users. Appropriate CL fitting and advice by
optometrists experienced in the management of aircrew
is likely to have contributed to this improvement. A
similar experience of successful CL usage was reported
in United States Air Force and Marine Corps aircrew
(3,8).
Corneal refractive surgery has the potential to pro-
duce the benefits of CL wear without the side effects.
The U.S. Army, Navy, and Air Force perform pho-
torefractive keratectomy (PRK) on certain personnel
under audited conditions with a relatively low com-
plication rate and good visual results (5,12). Post-
operative night vision abnormalities and reduced
contrast sensitivity have not been significant prob-
lems. Simulated cockpit studies after PRK found that
while appreciation of low-contrast targets decreased,
the operation did not decrease head-up-display read-
ability. The reduced low-contrast sensitivity does not
appear to be clinically or operationally significant.
High altitude or G forces do not affect post-operative
vision. In summary, flight performance after PRK has
been found to improve with better aircraft detection,
better use of night vision goggles, improved instru-
ment appreciation, and higher aircraft carrier landing
scores. More recently, Levy et al. recommend further
research into the appropriateness of laser in situ kera-
tomileusis in aircrew following an uneventful recov-
ery in an Israeli fast jet pilot (6).
CONCLUSION
This survey highlights the increased use of CL by the
aircrew population over the past 3 yr. For many, CL use
has advantages in terms of comfort and visual perfor-
mance over CFS. Refractive correction in aircrew needs
to address the unique visual environment and offer a
range of suitable and safe options, which possibly in-
cludes corneal refractive surgery (7,17).
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