Figure X. Two occupants of this aircraft received severe
spinal injuries. Note large tubular main spar
that passes through cockpit under seats.

Figure XI. Schematic of aircraft seat showing
meunting directly over large tubular
main spar with little space or structure
to attenuate vertical impact forces.

11



relayed from investigators or persons at the scene, request for assistance
from another person involved in investigating the accident and, to a great
extent, current interest in a specific questionable feature that may be found
in the crashed aircraft.

The usual method of investigating the accident consisted of becoming *
familiar with the circumstances of the accident as gathered by NTSB and FAA
investigators; documenting the scene, noting the flight path, obstacles,
impact points, distribution of wreckage, distribution of bodies, etc.; 2val-
uating the intactness o* cockpit/cabin area; examining each seat documenting
the direction and extent of deformation with the force of impact and the
sequence of multiple impacts, noting depressions in the instrument panel, the
distribution of blood and tissue that might verify the direction and force of
deceleration of the occupants; and reviewing and recording injuries as may be
revealed by autopsies or by hospital records. Frequently survivors and wit-
nesses were interviewad to gather their firsthand experiences with the acci-
dent. Photographs taken in all phases of the investigation were used to doc-
ument findings.

Previous reports (1,2) have presented various aspects of findings from
this accident investigation program. This report presents only selected
findings that bear on accidents whose investigation led to modifications in
the aircraft or pointed to certain questionable crashworthiness features.
Findings ard Discussion.

Shoulcer harness attachment brace.

Many aerial application aircraft have been built with a strong crashworthy
cockpit frame. Pilots are provided a seat, lapbelt, a shoulder harness,
an@ in many aircraft 4 large metallic roll on the panel to distribute impact
forces over the pilot's body should he strike it in an accident. Further,
the pilot usually wears a helmet. The crashworthiness of an aerial application
aircraft was illustrzated when, on fuel exhaustion, the aircraft stalled and
struck the ground in a 45° nosedown attitude. As illustrated in Figure Ia
there was moderately severe damage to the engine and hopper. The pilot, who
was wearing a lapbelt and shoulder harness, received lacerations above the
right eye and a fractured left arm and left knee. He had conspicuous shoulder
harness and lapbelt abrasions and contusions. A notable finding was the
forward bending of the shoulder harness attachment bracket and cracking of the
bond where the bracket had been welded to the cockpit structural bars (Figure
Tb). During deceleration the force of the pilot's body against the shoulder
harness apparently caused this deformation. Fracturing of the seat pedestal
and detachment of the seat. from the seat track probably increased the freedom
of the pilot's body, allowing it to come forward and strike the instrument
panel. The intact upper torso restraint probably prevented the pilot from
receiving lethal injuries.

A more severe impact in the same type aircraft occurred when an aircraft
contacted three power lines 35 feet above the ground and then traveled 170
feet before striking the ground. The plane tumbled end-~over-end 63 feet and
stopped upright. The aft portion of the fuselage and empennage were torn off



Figure VI. Pilot of aircraft (a) received head and chest
injuries as, on impact, he came forward into
windscreen, wirecutter, and instrument panel.

Cables used on shoulder harness and iapbelts all
failed (b). tronger cables and modified routing
apparently rectified the condition.

the pilot) and the cables were fixed to the frame of the aircraft. In both
places, over the pulley and where cables pass through the bulkhead, they were
sapjected to wear and considerable recurrent acute bending. In some opera-
tional aircraft, filaments of the wiltistrand cable were found to be broken.
This confiquration of the cable attachments for the shculder harness and lap-
belts appeared to subject them to wear and bending that weakened ana broke
strands of the cable. Thus, in the aircraft accident investigated, the cables
probably had been preweakened by wearing, bending and breaking of filaments;
and during the impact the pivot's body probably exerred force on the restraint
system sufficient to break the cables.

These observations were reported to others in the Federal Aviation
Administration and published in General Aviation Inspection AIDS (3), a
bulletin that calls technical matters to the attention of inspectors in the
field. The manufacturer was also informed of the finding in this accident.
Subsequent to the publication of this notice for inspectors, the manufacturer
issued a service bulletin by which stronger and different cables were put in
the aircraft and the configuration of the lead-in through the buikhead was
modified so as to eliminate most of the bending. Since the modification,
there have been no accidents in which we have observed the cables to have
broken and an examination of operational aircraft in the field has revealed
no flaws in the cables.

Side-mounted seat.

One aircraft has the inertia reel mounted iow on the back of the seat.
The single shoulder strap passes forward through an opening in the upright



Although the pilot may have received fatal injuries during any of several
motions in the deceleration or tumbling of the aircraft, it appears that at
one peint in the crash sufficient force was exerted on the shoulder harness
to fail the attachment bracket.

The failure of the shoulder harness bracket was reported to the manu~
facturer who then modified the shoulder harness attachment orace using a
longer piece of metal, wrapping it further around the structural bars and
welding it securely (Figure III). The vesult was that the strength of the
shoulder harness attachment was improved. In subsequent accidents of this
aircraft we have not seen a failure of this impreved shoulder harness
attachment.

Figure Ili. Modified shoulder harness attachment brace
has longer meta? wrapped further around and
more securely welded to cockpit structural
members.

Shouider_ harness.

Another modification in the restraint system in this series of aerial
application aircraft resulted from crash injury investiaation of a single
accident. The aircraft struck at a 65° nose and rightwing down attitude,
resulting in marked damage to the engine and hopper areas and upper displace-—
ment of the cockpit canopy. The pilot was wearing a lapbelt and shoulder
harness (loosely), but not a crash helmet. The accident was immediately
fatal to the pilot. No autopsy was performed but it was observed that there
were extensive skull fractures with blood and spinal fluid coming from the
left ear canal, multiple lacerations of the face, fracture of the right fore-
arm and burns on each hand. An indentation on the extreme right of the
instrument panel with tissue debris present was consistent with having been
struck by the head of the occupant. Restraint system attachments and webbings
remained intact except ‘hat the right shoulder strap separated where it was
sewn to the lapbelt (Figure IV). There was fracturing of the legs on the seat.
It appeared that the pilot, during deceleration, was thrown forward and to the
right. The failure of the right shoulder strap, where it was attached t> the
right limb of the lapbelt, probabiy permitted the pilot to travel forward enough
to strike his head on the right side of the instrument panel and cockpit frame.

4

taeda vce bee

we.


Figure I. In this accident damage was limit2d and cockpit was
intact (a). However, shoulder harness attachment
brace showed v-shaped deformation (b) due to force
of pilot's bedy against restraint during crash.

(Figure Ila). The cockpit structure remained intact. The accident was
fatal to the pilot and, although no autopsy was performed, the coroner

Figure II. Same type aircraft as in figure I. Damage is more
severe but cockpit is intact (a). Shoulder harness
attachment brace has been pulled off sites where
welded to cockpit structural bars

possibly

reported that death was due to multiple injur:es a
the neck. The pilot was wearing lepbelt, shoulder
The lapbelt was intact. The shoulder harness attachme
welded to. the aft cockpit structural bars, had failed
to separate from the structural bars (Ficure IIb).

Arness 2



j

:
j

a b

Figure V. Inertia reel was reported not to have held in
accident of moderate impact. Separation between
threads (a) suggested longitudinal folding of strap
upon itself on reel. Such folding can interfere with
reel-latching mechanism (b). Modification by use of a
guide on reel housing should prevent this occurrence.

Again, as a result of informal conferences, the aircraft and inertia reel
manufacturers agreed to modify the reel by placing a more precise shoulder
harness strap guide on the reel housing rather than depending upon the more
oval-shaped strap guide previously used. This modification should prevent the
folding of the shoulder strap and obviate this as a cause of the inertia reel
failing to latch. There has been no accident experienc: to evaluate the
effectiveness of this change. The reliability of the latching mechanisms
and the overall strength of inertia reels are subjects of continuing crash-
worthiness investigations.

Cables attaching restraints.

The pilot of an aerial application aircraft inadvertently dipped his
xight wing in some wheat and, on recovering, struck a parked cotton trailer
with the left wing tip. The aircraft hit the ground and came to rest inverted.
The cockpit remained intact (Figure Via) but the seatbelt attachment cables
and the shoulder harness cable failed (Figure VIb), allowing the pilot to come
forward. His head struck the metallic wirecutter in front of the windscreen
and his chest made a broad impact on the panel. The pilot's helmet sustained
a large vertical cleavage corresponding to the impact with the wirecutter and
the pilot suffered a corresponding laceration of the forehead.

The crash injury investigator could not understand why all three cables
would fail in such an accident. He began to look at other aircraft in service
and study the cables. In this aircraft the cable to the shoulder harness
coursed over a pulley and down behind the pilot to an inertia reel. The lap-
belt attached to cables that passed through an opening in a bulkhead (eround





tia Nt IT ata asa ITA DE Kin

Figure VIII. Pilot in this seat received head injuries in
@ survivable accicent. Note bending of axles
and fracturing of roller from axles. Modification
to prevent seat breakdown was attachment of bar
on seat to engage seat track.

The particulars of this accident were reviewed on an informal basis with
engineering representatives of the manufacturer. Following this, the manu-
facturer issued a service bulletin which called for a strengthening of the
seat back latching mechanism and for a bar to be bolted onto the seat frame
so that if the seat rollers bent or were broken, the seat would collapse
onto the seat track and the bar projecting out each side of the seat would
engage the upper lip of the track anc prevent the seat from collapsing into
the floor. Shortly after the service bulletin was issued, the FAA issued an
airworthiness directive (4) mandating the changes as prescribed ‘. the serv-
ice bulletin.

Since this modification, we have seen accidents in which the seat, in
spite of the fact that the modification was present, broke down and became
free in the cabin (Figure IX). Close examination of the bar that was added
to prevent collapse of the seat, failed to indicate that it significantly
eng. od on the side tracks as it was designed to do. We further noted that
in .ch accidents the axles of the rollers tend to be bent several inches
from the roller itself. This finding suggests that the axle is pulled
laterally during the impact as it ‘s acutely bent. Lateral movement probably
occurs because the fuselage, which is oval in shape, during impact undergoes
tcp to bottom deformat:ons, increasing the lateral dimension. Such deforma-
tion is readily discerned in high-speed motion pictures of aircraft fuselages
undergoing drop tower testing conducted by the National Aeronautics and Space
Administration. It thus appears that in this aircraft with side-mounted seats,
the fuselage on impact deforms, causing the spreading apart of the sidetracks.
This spreading extends the axles of the seat rollers laterally in their
retainers. The force of the occupant is borne on the extended axle. Further,
the bar which was placed on the seat to engage the track, appears to be too

9

AER


seat back and attaches to the lapbelt. The strength of the shoulder harness
depends upon the strength of the seat-back latch to keep the seéc upright
during an impact. If the seat back were to break forward the shoulder har-
ness would be of reduced value. Furthermore, the seat is mounted on rollers
that extend from the seat frame to tracks on the sidewall and a center console
(Figure VII). There are no legs or pedestals under this seat. The seat and
upper torso restraint arrangement is such that the forward decelerative forces
of the occupant transmitted to the shoulder harness must be borne by the seat
attachment.

Figure VII. Scnematic of side-mounted seat.

In an accident of an aircraft of this type carrying four persons, an
infant was killed, a child received only minor injuries, and the pilot and his
wife were seriously injured. The pilot received serious head injuries when his
head struck the instrument panel. The pilot’s seat (Ficure VilIa) was found
loose in the cabin and the axles of all four rollers were either severely
bent or the rollers were broxen off the axles. The seat appeared to have
broken downward under the impact loading.

Because of the way it is mounted it appeared that the value of the shoulder
harness would depend, not only on the strength of the seat back latching mech-
anism, but upon the seat remaining in position. A breakdown of the seat from
the track could allow the oassenger, even though wearing a shoulder harness,
to rotate forward into structures in front of him. The finding that the rollers
were stripped from their axies suggested that this seat did not support the
occupant in chis crash.

Sag FP nn 3 eae



EE IOS

Figure IV. Threads tore where shoulder harness was sewn
to lapbelt. Modification consisted of wrapping
shoulder harness around lapbelt and sewing through
the three layers of restraint webbing.

This failure of the attachment of the shoulder strap to the lapbelt was
reported to the manufacturer in an informal conference and the manufacturer
made a modification of the restraint system, wrapping the shoulder harness
strap around the lapbelt and sewing it on both sides. Modifications or
exchanges of the restraint systems were made in this model of aircraft. In
investigations of subsequent accidents we have seen no failures of the
shoulder harness where it was sewn to the lapbelts.

Inertia reel.

Further observations in accidents involving aerial application aircraft
have revealed failures in the inertia reel in some cases. In one accident
this involved failure of the ‘atching mechanism to completely engage, result-
ing in the spooling out of the shoulder harness, allowing the pilot's head to
strike the instrument panel. In other accidents the inertia reel housing
fractured and the reel failed to restrain the pilot. In an accident inves-
tigated by others, but reported to us, a pilot was injured by the apparent
failure of the inertia reel to latch and held. The inertia reel was sent
to us for examination. The common strap of the double shoulder harness
nermally is taken up on the spool of the reel. In this instance the strap
had some separation of the webbing so that it appeared to have been rolled
or folded upon itself while being taken up on the reel (Figure V). It :
appeared that the oval guide, an extension of the reel cover, was not suffi- :
cient to prevent the strap from rolling upon itself and thereby increasing
the bulk on the spool. In so doing it could override and possibly interfere
with the reel latching mechanism. Indeed, with a new reel, if the strap is
led in from the side, as one in the pilot's seat would do repeatedly in
removing the harness and letting it retract, the shoulder harness common
strap can fold upon itself as it is taken up on the spool. Such folding can
obstruct the reel-locking mechanism.



IMPROVING THE CRASHWORTHINESS OF GENERAL
AVTATION AIRCRAFT BY CRASH INJURY INVESTIGATIONS

Introduction.

Statistics compiled by the National Transportation Safety Board (NTSB)
show that a yearly average of 3911 accidents occurred in “small fixed-wing
aircraft" (under 12,500 1b) for the 6-year period, 1973 through 1978. Of
these, 663 accidents, or 17 percent, involved fatal injuries. In the fatal
accidents, a yearly average of 1303 persons were killed, amounting to two
aircraft occupants per accident. Studies have shown that the human body
can withstand sizeable impact forces if such forces are properly distrib-
uted. Thus, it would appear that significant loss of life ane seriousness of
injuries could be lessened in survivable crashes if occupants of aircraft
were better cushioned against the decelerative impact forces experienced in
acrash. The most important improvements in the crashworthiness of light
aircraft can be made by restraining occupants to afford them better protec-
tion against impact. In order to implement changes, aircraft accidents should
be studied for crashworthiness.

One way to determine the crashworthiness of a vehicle is to investigate
accidents as they happen, record information relative to the severity of the
crash, document the integrity of the vehicular structures, note the function
of the restraint systems, and study injuries received by the occupants. For
over a decade, an ongoing biomedical and crash injury field investigative
research program has been conducted at the Civil Aeromedical Institute (CAMI).
A portion of this activity has been to correlate the types of injuries sus-
tained by aircraft occupants with the severity of the impact, the deformation
of the aircraft cabin, impact of occupants against aircraft structure, and
the function of the restraint systems. The function and adequacy of restraint
systems have been cf particular concern in these research activities because
modifications to afford better protection to occupants often can be made at
relatively little expense to manufacturers or aircraft owners. Indeed, some
specific changes made by manufacturers as a result of this investigative
research have improved the crashworthiness of the involved aircraft and prob~-
ably saved lives.

In this report we use selected examples of crashworthiness and@ crash
injury findings to illustrate how such investigations have been used to
effect changes. Some other specific examples cf unresoived crashworthiness
findings are illustrated.

Methods.

The investigation of general aviation aircraft accidents for crash
injury correlations has been carried out by investigators at CAMI, not number-
ing cover four persons at any time in the last decade. The decision to
investigate an accident depended on any of a number of factors such as the
proximity of the accident to CAMI, the type of aircraft, funds available
for travel, personnel available to investigate the accident, information



application aircraft as illustrated in Figure XII. The six people in this
aircraft all received moderate to severe injuries. The pilot and person in
the copilot position had facial injuries and spine fractures. The forward
four seats were partially or completely separated from the seat tracks and

the pilot's and copilot's seats had fractured seat pedestals as shown in the
figure. Cast seat legs and parts are strong and meet all FAA requirements
but in accidents wren the failure point is reached they appear to break expos-
ing the occupant to possible high peak decelerative forces in secondary impacts
rather than deforming under the occupant and attenuating the energy of impact.
At present we do not know how pervasive failures of seats are in general
aviation aircraft or how such failures add to the severity of injuries. This
is a matter of continuing crashworthiness investigations.

Figure XII. On impact, the occupants of this aircraft
received serious injuries. Cast alloy seat
pedestals were found to be fractured as can
be seen in photograph.

Summary.

In summary, it is apparent that occupants of aircraft that crash could,
in many cases, have the severity of their injuries reduced by well-designed
seats and restraint systems. Further, it is clear that one can learn of the
weak points in restraint systems by studying their effectiveness and failures
in accidents. Significant findings brought to the attention of manufacturers
can bring about changes to improve the crashworthiness of aircraft, thereby
reducing injury and saving lives. Crash injury investigations are, therefore,
an important and fruitful part of the investigation of many accidents.

12

Ce SS Eee


ACKNOWLEDGEMENT

The author gratefully acknowledges the contributions to this report made
by several Crash Injury Investigators from the Civil Aeromedical Institute,
particularly James M. Simpson and Terry F. Wallace, who investigated
aircraft accidents and participated in technical discussion with manu-
facturers’ representatives and others.

Further, Paula M. Grape contributed significantly by assisting in the
compiling and preparation of the material for this report.



Technical Report Documentotion Page

12 Sanam aver. fid-A 123 BIG

(Ae Report No. |; 4 2. Goverment Accession No. 3. Recipient's Cotalog No.

I‘ ET Title ond Subtitle = i Report Date eeeeeae
a “IMPROVING THE CRASHWORTHINESS OF GENERAL AVIATION LO MAY asl vai IS
Ce. ‘AIRCRAFT BY CRASH INJURY INVESTIGATIONS, + [8 Bettorming Orgonizanon Code

~ . |
8. Performing Organization Report No.
[7 Authors) — oat
} mal William RY Kirkham
—_ 9. Performing Organization Nome and Address VO. Work Unit No. (TRAIS)
FAA Civil Aeromedical Institute
P.O. Box 25082 . 1. Contract or Gront No.
Oklahoma City, Oklahoma 73125 : |
13. Type of Report and Period Covered
12. Sponsoring Agency Name ond Address :
Office of Aviation Medicine
Federal Aviation Administration
800 Independence Avenue, S.W. soring Agency Code
Washington, D.C. 20591
15. Supplementary Notes poe
\ This work was performed under Task AM-B-81-TOX-23.

16. Abstrect

An accident investigative research program has correlated injuries to aircraft occu-
pants with the accident severity and structural changes in the crash. Findings
brought to the attention of aircraft manufacturers have led to specific aircraft
being made more crashworthy. Following the finding of a failure in a shoulder har-
ness attachment the manufacturer strengthened the attachment brace. The way a shoul-
der harness was joined to a lapbelt was modified as a followup to failure of the
attachment in an accident. Noted fractures cf lapbelt and shoulder harness cable
tiedowns led to the use of stronger cables and modification of the installation.
Other accident findings resulted in a shoulder strap guide being placed on an
inertia reel and a side-mounted seat being modified.

Described also are three seat-related features wich, although meeting FAA standards,
may, during the dynamics of a crash, lack desirable energy attenuation. These are:
(#an installation in which the seat is mounted directly over the wing spar so that
there is little provision for attenuation of vertical impact forces on seat occu-
pants, ¢& a side-mounted seat which has been found to have broken from its mounting
in several accidents, even after a modification, and, (4 seats with cast alloy legs
and pedestals, that, in accidents, have been found to fracture and come loose from
the seat tracks.

These findings illustrate the value of making crash injury correlations in general
aviation accidents.

17. Key Words 18. Diawibonon Statement

Crashworthiness Document is available to the public

Crash injury through the National Technical Information
Restraint systems Service, Springfield, Virginia 22161
Injury correlations

19. Security Clessil. (of this report) BD. Security Clossif, (of this page) Tl. No. of Pages | 22. Price
Unclassified Unclassified 13

Form DOT F 3700.7 (8-72) Reproduction of completed page outhorized

O8FLLL



aha

Figure IX. In this accident, seats, even though modified,
broke down, probabiy adding to injuries of
occupants. Note degree of bending of axles
on rollers.

short to adequately engage the top of the track during lateral deformation or
spreading of the fuselage.

Other crashworthiness features.

An additional crashworthiness feature involves an aircraft in which the
wain spar is a large aluminum -ibe that passes through the cockpit. This
hollow tube serves also as a fuel cell. The two-seat aircraft has both seats
mounted side by side on a small metallic track directly on top of the main spar.
The bottom of the seat is plywood and the seat cushion consists of 2 inches
of foam rubber. This arrangement appears to offer little attenuation of
impact forces by the seat. During impact, forces, particularly vertical forces,
would tend to be transmitted through the main spar to the pelvis and the axial
spine of the occupant.

Tie aircraft in Figure X was in a stall-spin accident and, the fuselage
remained relatively intact, yet there were severe <rinal injuries to the two
occupants. One can see that the seat mountirg, as depicte" in Figure xI,
places the occupant directly over the main spar, In another acc. “-nt of this
type aircraft, vertical loading sufficient to fail the landing gear st. -*
resulted in fracture of the pelvis and compression fractures of lumbar veri.>rae.

A number of general aviation aircraft are equipped with seat frames, logs,
and pedestals cast of magnesium-aluminum alloy. It has been observed in acci-
dents that the legs and the pedestals of such seats are frequently fractured.
Indeed, in a review of accidents of one aerial application aircraft employing
this type seat (2), it was found that in 14 of the 18 accidents investigated,
the seats left their tracks and in 14 of the 18 accidents one or more seat
legs or pedestals were fractured. The cited report reviewed findings in an
aerial application aircraft because this type of aircraft was specifically
designed to be crashworthy. However, the problem extends beyond the aerial

10

 CdaisteD Viaw sbe 20) onde

ri
hue.


REFERENCES

Swearingen, J. J.: General Aviation Structures Directly
Responsible for Trauma in Crash Decelerations. FAA Office
of Aviation Medicine Report No. 71-3, 1971.

Kirkham, W. R., J. M. Simpson, T. F. Wallace, and P. M. Grape:
Aircraft Crashworthiness Studies: Findings in Accidents Involving
an Aerial Application Aircraft. FAA Office of Aviation Medicine
Report No. 80-3, 1980.

General Aviation Inspection AIDS, Federal Aviation Administration,
AC No. 20-7M, November, 1975.

FAA Airworthiness Directive 77-16-09, August 4, 1977.

13

i arsenal >.