<PAGE>   1
                                                                    Exhibit 4.9a

                                    AMENDMENT
                                      NO. 2
                              (SEPTEMBER 15, 1999)

                               TO SUPPLEMENT NO. 2



                                       TO


                                    INDENTURE

                          DATED AS OF DECEMBER 17, 1993


                                  BY AND AMONG


                                  SIMULA, INC.
                                    AS ISSUER


                            THE SUBSIDIARY GUARANTORS
                                  AS GUARANTORS

                                       AND

                           BANK ONE TRUST COMPANY, NA
                                   AS TRUSTEE


              SERIES C 10% SENIOR SUBORDINATED CONVERTIBLE NOTES
<PAGE>   2
                                 AMENDMENT NO. 2

                               TO SUPPLEMENT NO. 2

         This amendment ("Amendment") is dated as of September 15, 1999, among
Simula, Inc., an Arizona corporation (hereinafter the "Company"), having its
principal office at 2700 North Central Avenue, Suite 1000, Phoenix, Arizona
85004; and Simula Safety Systems, Inc., Intaero, Inc., Airline Interiors, Inc.,
Artcraft Industries Corp., Simula Automotive Safety Devices, Inc., Simula
Technologies, Inc., International Center for Safety Education, Inc., all Arizona
corporations, and Simula Composites Corporation, a Delaware corporation
(hereinafter the "Subsidiary Guarantors"), having the same principal office as
the Company; and Bank One Trust Company, NA, having its principal office at
Corporate Trust Client Service Group, 100 East Broad Street, Columbus, Ohio
43271-0181, as trustee (hereinafter the "Trustee").

                                 R E C I T A L S

         Pursuant to Article 9 and Article 14 of the Company's indenture, dated
as of December 17, 1993 ("Indenture"), and Amended and Restated Supplement No.
2, dated as of September 12, 1996 ("Supplement No. 2"), the Company issued its
Series C 10% Senior Subordinated Convertible Notes due 1999 ("Notes").

         The Notes mature on September 15, 1999.

         The Company desires to give current holders of the Notes
("Noteholders") the option to extend the maturity date of their Notes until
October 31, 1999, in exchange for a payment of interest from the period
extending from September 15, 1999 until October 31, 1999 at a rate of 15% per
annum.

         All things necessary to make this Amendment a valid agreement of the
Company and the Subsidiary Guarantors, in accordance with their and its terms,
have been done.

         NOW THEREFORE, for and in consideration of the foregoing, it is
mutually covenanted and agreed, for the benefit of the holders of the Notes, as
follows:


                                       1
                        MODIFICATIONS TO SUPPLEMENT NO. 2


         1.1 Capitalized Terms. All capitalized terms not otherwise defined
herein shall have the meaning given in Supplement No. 2.

         1.2 Extension of Maturity. On or before September 15, 1999, a Holder
may elect to extend the Series C Notes Stated Maturity until October 31, 1999.
Accrued interest due and payable as of September 15, 1999 shall be paid to
Holder. Any Notes extended shall bear interest at the rate of 15% per annum
during the period extending from September 15, 1999 until October 31, 1999.

         1.3 No Further Modifications. With the exception of the foregoing, no
other modifications have been made to the terms of the Series C Notes as
described in Supplement No. 2, and all other provisions of Supplement No. 2
remain unchanged as stated therein.


                                       2
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                                        2


                             STEPS TO EFFECT CHANGE


         Authority Under Indenture. Pursuant to Article 9.2 of the Indenture,
the foregoing changes require the consent of Holders of not less than a majority
in principal amount of the Outstanding Notes by Act of said Holders and the
authorization by a Board Resolution. Both requirements have been met.

         IN WITNESS WHEREOF, SIMULA, INC., SIMULA SAFETY SYSTEMS, INC. INTAERO,
INC., AIRLINE INTERIORS, INC., ARTCRAFT INDUSTRIES CORP., SIMULA AUTOMOTIVE
SAFETY DEVICES, INC., SIMULA TECHNOLOGIES, INC., INTERNATIONAL CENTER FOR SAFETY
EDUCATION, INC. and SIMULA COMPOSITES CORPORATION have each caused this
Supplement to be signed in their respective corporate names by their respective
authorized officers and Bank One Trust Company, NA, Trustee, has caused this
Supplemental Indenture to be executed in its corporate name by one of its
authorized officers, all as of the date and year first above written.


                                     SIMULA, INC., an Arizona corporation



                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                               Bradley P. Forst, Executive Vice
                                               President and Secretary


                                     BANK ONE TRUST COMPANY, NA
                                     as Trustee


                                     By    /s/  David B. Knox
                                           ------------------------------------
                                                Trust Officer


                                       3
<PAGE>   4
                                     SUBSIDIARY GUARANTORS:


                                     SIMULA SAFETY SYSTEMS, INC.,
                                     an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary




                                     INTAERO, INC., an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary



                                     AIRLINE INTERIORS, INC.,
                                     an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary



                                     ARTCRAFT INDUSTRIES CORP.,
                                     an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary



                                     SIMULA AUTOMOTIVE SAFETY DEVICES, INC.
                                     an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary


                                       4
<PAGE>   5
                                     SIMULA TECHNOLOGIES, INC.,
                                     an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary



                                     INTERNATIONAL CENTER FOR SAFETY
                                     EDUCATION, INC., an Arizona corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary



                                     SIMULA COMPOSITES CORPORATION,
                                     a Delaware corporation


                                     By    /s/ Bradley P. Forst
                                           ------------------------------------
                                           Bradley P. Forst, Assistant Secretary



                                       5
<PAGE>   6
STATE OF           Arizona          )
                                    ) ss.
County of          Maricopa         )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst, to me
personally known and known to me to be the same person who executed the within
and foregoing instrument, who, being by me duly sworn, did depose, acknowledge
and say that he is the Executive Vice President and Secretary of SIMULA, INC.,
one of the corporations described in and which executed the foregoing
instrument; that said instrument was executed on behalf of said corporation by
authority of its Board of Directors; and he acknowledged the execution of said
instrument to be the voluntary act and deed of said corporation by it
voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)






STATE OF           Ohio             )
                                    ) ss.
County of          Franklin         )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared David B. Knox to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say: That he/she is a Trust Officer of BANK ONE TRUST COMPANY, NA, one of the
corporations described in and which executed the foregoing instrument; that said
instrument was executed on behalf of said corporation by authority of its Board
of Directors; and he/she acknowledged the execution of said instrument to be the
voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Marie E. Jackson
                                                     --------------------------
                                                     Notary Public
(Seal)


                                       6
<PAGE>   7
STATE OF           Arizona          )
                                    ) ss.
County of          Maricopa         )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of SIMULA SAFETY SYSTEMS, INC., one of
the corporations described in and which executed the foregoing instrument; that
said instrument was executed on behalf of said corporation by authority of its
Board of Directors; and he acknowledged the execution of said instrument to be
the voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)






STATE OF        Arizona             )
                                    ) ss.
County of       Maricopa            )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of INTAERO, INC., one of the corporations
described in and which executed the foregoing instrument; that said instrument
was executed on behalf of said corporation by authority of its Board of
Directors; and he acknowledged the execution of said instrument to be the
voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                      /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)


                                       7
<PAGE>   8
STATE OF           Arizona          )
                                    ) ss.
County of          Maricopa         )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of AIRLINE INTERIORS, INC., one of the
corporations described in and which executed the foregoing instrument; that said
instrument was executed on behalf of said corporation by authority of its Board
of Directors; and he acknowledged the execution of said instrument to be the
voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)






STATE OF           Arizona          )
                                    ) ss.
County of          Maricopa         )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of SIMULA TECHNOLOGIES, INC., one of the
corporations described in and which executed the foregoing instrument; that said
instrument was executed on behalf of said corporation by authority of its Board
of Directors; and he acknowledged the execution of said instrument to be the
voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)


                                       8
<PAGE>   9
STATE OF             Arizona       )
                                   ) ss.
County of            Maricopa      )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of ARTCRAFT INDUSTRIES CORP., one of the
corporations described in and which executed the foregoing instrument; that said
instrument was executed on behalf of said corporation by authority of its Board
of Directors; and he acknowledged the execution of said instrument to be the
voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)






STATE OF             Arizona        )
                                    ) ss.
County of            Maricopa       )

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of SIMULA AUTOMOTIVE SAFETY DEVICES,
INC., one of the corporations described in and which executed the foregoing
instrument; that said instrument was executed on behalf of said corporation by
authority of its Board of Directors; and he acknowledged the execution of said
instrument to be the voluntary act and deed of said corporation by it
voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)


                                       9
<PAGE>   10
STATE OF          Arizona )
                          ) ss.
County of         Maricopa)


         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of INTERNATIONAL CENTER FOR SAFETY
EDUCATION, INC., one of the corporations described in and which executed the
foregoing instrument; that said instrument was executed on behalf of said
corporation by authority of its Board of Directors; and he acknowledged the
execution of said instrument to be the voluntary act and deed of said
corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/ Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)






STATE OF            Arizona )
                            ) ss.
County of           Maricopa)

         On this 15th day of September, 1999, before me, a Notary Public in and
for said county and state, personally appeared Bradley P. Forst to me personally
known and known to me to be the same person who executed the within and
foregoing instrument, who, being by me duly sworn, did depose, acknowledge and
say that he is the Assistant Secretary of SIMULA COMPOSITES CORPORATION, one of
the corporations described in and which executed the foregoing instrument; that
said instrument was executed on behalf of said corporation by authority of its
Board of Directors; and he acknowledged the execution of said instrument to be
the voluntary act and deed of said corporation by it voluntarily executed.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal this
15th day of September, 1999.


                                                     /s/  Elaine Micucci
                                                     --------------------------
                                                     Notary Public
(Seal)

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