<PAGE>   1
                                                                   Exhibit 10.35

                          FOURTH MODIFICATION AGREEMENT


         BY THIS FOURTH MODIFICATION AGREEMENT (the "Agreement"), made and
entered into as of the 28th day of July, 1999, BANK ONE, ARIZONA, NA, a national
banking association, as administrative agent for the Banks (as hereinafter
defined) (the "Administrative Agent"), and SIMULA, INC., an Arizona corporation
(the "Company"), all present and future Subsidiaries of the Company (with the
Company, the "Borrower"), in consideration of the mutual covenants herein
contained and other good and valuable consideration, the receipt and sufficiency
of which is hereby acknowledged, hereby confirm and agree as follows:

                                    RECITALS:

         A. Borrower, the Administrative Agent, the Issuing Bank and the "Banks"
named therein entered into that Senior Credit Agreement dated November 6, 1998
to provide financial accommodations to the Borrower as provided therein (as
modified from time to time, including without limitation by that Modification
Agreement dated as of February 12, 1999, that Second Modification Agreement
dated as of April 28, 1999 and that Third Modification Agreement dated as of
June 23, 1999, the "Senior Credit Agreement").

         B. Borrower and the Administrative Agent, with the consent of the Banks
and the Issuing Bank, desire to modify the Senior Credit Agreement as set forth
herein.

         C. All undefined capitalized terms used herein shall have the meaning
given them in the Senior Credit Agreement.

                                   AGREEMENT:

SECTION 1.  ACCURACY OF RECITALS.

         Borrower acknowledges the accuracy of the Recitals.

SECTION 2.  MODIFICATIONS OF LOAN DOCUMENTS; OTHER AGREEMENTS.

         2.1 The following definition in Section 1.1 of the Senior Credit
Agreement is hereby amended to read as follows:

                  "RLC Adjustment Date" means the earliest of (i) August 31,
         1999, (ii) the Discontinued Operations Sale Date, or (iii) the date on
         which the aggregate outstanding principal balance of the RLC is reduced
         to a balance below $20,000,000.00 by Borrower from funds obtained by it
         from a junior capital source after the date of the Second Modification
         Agreement between the Borrower and the Administrative Agent.
<PAGE>   2
         2.2 Section 3.2(e) of the Senior Credit Agreement is hereby amended to
read as follows:

                           (e) Rail Credit Fee. Company agrees to pay the
                  Administrative Agent for distribution to the Banks a monthly
                  fee (the "Rail Credit Fee") in advance on the first day of
                  each month, commencing May 1, 1999 if the Discontinued
                  Operations Sale Date has not yet occurred and continuing until
                  the Discontinued Operations Sale Date has occurred. The Rail
                  Credit Fee shall be (i) commencing May 1, 1999, $100,000.00
                  and (ii) commencing August 1, 1999, $150,000.00. The Rail
                  Credit Fee shall be distributed by the Administrative Agent as
                  follows: 80.0% to Bank One and 20.0% to Imperial.

         2.3 Section 7.19 of the Senior Credit Agreement is hereby amended to
read as follows:

                  7.19     Discontinued Operations.

                           (a) Upon its sale of the Discontinued Operations, the
                  Company shall apply the net proceeds from such sale to the
                  repayment of the Loans, first to the extent applicable the
                  RLC, second to the extent applicable the Term B Loan and third
                  to the extent applicable the Term A Loan.

                           (b) The Discontinued Operations shall be sold by the
                  Company no later than August 31, 1999. The terms of the sale
                  shall be approved by the Banks in writing. Any note and/or
                  other instrument delivered to the Company by the purchaser
                  evidencing any purchase carryback obligations to the Company
                  in connection with such sale shall be in a form that is
                  assignable to the Banks and, within two weeks after its
                  execution by the purchaser, shall be delivered to the
                  Administrative Agent with an assignment thereof acceptable to
                  the Bank, pledging it to the Banks to secure the Borrower's
                  Obligation hereunder.

                           (c) Unless the Banks otherwise agree in writing, the
                  Company shall not invest, directly or indirectly, more than an
                  $1,500,000.00 in cash in the Discontinued Operations during
                  the 1999 fiscal year.

         2.4 Each of the Loan Documents is modified to provide that it shall be
a default or an event of default thereunder if Borrower shall fail to comply
with any of the covenants of Borrower herein or if any representation or
warranty by Borrower herein or by any guarantor in any related Consent and
Agreement of Guarantors is materially incomplete, incorrect, or misleading as of
the date hereof.

                                      -2-
<PAGE>   3
         2.5 Each reference in the Credit Documents to any of the Credit
Documents is hereby amended to be a reference to such document as modified
herein.

SECTION 3.  RATIFICATION OF CREDIT DOCUMENTS AND COLLATERAL.

         The Credit Documents are ratified and affirmed by Borrower and shall
remain in full force and effect as modified herein. Any property or rights to or
interests in property granted as security in the Credit Documents shall remain
as security for the Loans and the obligations of Borrower in the Credit
Documents.

SECTION 4.  BORROWER REPRESENTATIONS AND WARRANTIES.

         Company and each Co-Borrower to the extent applicable represents and
warrants to the Banks:

         4.1 No default or event of default under any of the Credit Documents as
modified herein, nor any event, that, with the giving of notice or the passage
of time or both, would be a default or an event of default under the Credit
Documents as modified herein has occurred and is continuing.

         4.2 There has been no material adverse change in the financial
condition of Borrower or any other person whose financial statement has been
delivered to the Banks in connection with the Loans from the most recent
financial statement received by the Banks.

         4.3 Each and all representations and warranties of Borrower in the
Credit Documents are accurate on the date hereof.

         4.4 Borrower has no claims, counterclaims, defenses, or set-offs with
respect to the Loans or the Credit Documents as modified herein.

         4.5 The Credit Documents as modified herein are the legal, valid, and
binding obligation of Borrower, enforceable against Borrower in accordance with
their terms.

         4.6 Borrower is validly existing under the laws of the State of its
formation or organization and has the requisite power and authority to execute
and deliver this Agreement and to perform the Credit Documents as modified
herein. The execution and delivery of this Agreement and the performance of the
Credit Documents as modified herein have been duly authorized by all requisite
action by or on behalf of Borrower. This Agreement has been duly executed and
delivered on behalf of Borrower.


                                      -3-
<PAGE>   4
SECTION 5. BORROWER COVENANTS.

         Borrower covenants with the Banks:

         5.1 Borrower shall execute, deliver, and provide to the Administrative
Agent such additional agreements, documents, and instruments as reasonably
required by the Banks to effectuate the intent of this Agreement.

         5.2 Borrower fully, finally, and absolutely and forever releases and
discharges the Administrative Agent and the Banks and their present and former
directors, shareholders, officers, employees, agents, representatives,
successors and assigns, and their separate and respective heirs, personal
representatives, successors and assigns, from any and all actions, causes of
action, claims, debts, damages, demands, liabilities, obligations, and suits, of
whatever kind or nature, in law or equity of Borrower, whether now known or
unknown to Borrower, and whether contingent or matured, (i) in respect of the
Loans, the Credit Documents, or the actions or omissions of the Administrative
Agent or the Banks in respect of the Loans or the Credit Documents and (ii)
arising from events occurring prior to the date of this Agreement.

SECTION 6. CONDITIONS PRECEDENT.

         The agreements of the Banks and the Administrative Agent and the
modifications contained herein shall not be binding upon the Banks until the
Banks have executed and delivered this Agreement and the Administrative Agent
has received, at Borrower's expense, all of the following, all of which shall be
in form and content satisfactory to the Administrative Agent and shall be
subject to approval by the Administrative Agent:

         6.1 An original of this Agreement fully executed by the Borrower and
all Guarantors;

         6.2 Such resolutions or authorizations and such other documents as the
Administrative Agent may require relating to the existence and good standing of
each Borrower and Guarantor the authority of any person executing this Agreement
or other documents on behalf of each Borrower and Guarantor; and

         6.3 Payment of all the internal and external costs and expenses
incurred by the Administrative Agent and the Banks in connection with this
Agreement (including, without limitation, inside and outside attorneys,
appraisal, appraisal review, processing, title, filing, and recording costs,
expenses, and fees).

SECTION 7. INTEGRATION, ENTIRE AGREEMENT, CHANGE, DISCHARGE, TERMINATION, OR
           WAIVER.


                                      -4-
<PAGE>   5
         The Credit Documents as modified herein contain the complete
understanding and agreement of Borrower and the Banks in respect of the Loans
and supersede all prior representations, warranties, agreements, arrangements,
understandings, and negotiations. No provision of the Credit Documents as
modified herein may be changed, discharged, supplemented, terminated, or waived
except in a writing signed by the parties thereto.

SECTION 8. BINDING EFFECT.

         The Credit Documents as modified herein shall be binding upon and shall
inure to the benefit of Borrower and the Banks and their successors and assigns
and the executors, legal administrators, personal representatives, heirs,
devisees, and beneficiaries of Borrower, provided, however, Borrower may not
assign any of its right or delegate any of its obligation under the Credit
Documents and any purported assignment or delegation shall be void.

SECTION 9. CHOICE OF LAW.

         This Agreement shall be governed by and construed in accordance with
the laws of the State of Arizona, without giving effect to conflicts of law
principles.

SECTION 10. COUNTERPART EXECUTION.

         This Agreement may be executed in one or more counterparts, each of
which shall be deemed an original and all of which together shall constitute one
and the same document. Signature pages may be detached from the counterparts and
attached to a single copy of this Agreement to physically form one document.

         DATED as of the date first above stated.

                                   SIMULA, INC., an Arizona corporation



                                   By:         /s/ Donald Townsend
                                        --------------------------------------
                                   Name:       Donald Townsend
                                        --------------------------------------
                                   Its:        Treasurer
                                        --------------------------------------
                                                                         COMPANY


                                      -5-
<PAGE>   6
                                   SIMULA SAFETY SYSTEMS, INC., formerly
                                   known as Simula Government Products, Inc., an
                                   Arizona corporation



                                   By:           /s/ Donald Townsend
                                        --------------------------------------
                                   Name:         Donald Townsend
                                        --------------------------------------
                                   Its:          Assistant Treasurer
                                        --------------------------------------

                                   SIMULA AUTOMOTIVE SAFETY DEVICES,
                                   INC., an Arizona corporation,
                                   a/k/a ASD-Simula


                                   By:           /s/ Donald Townsend
                                        --------------------------------------
                                   Name:         Donald Townsend
                                        --------------------------------------
                                   Its:          Treasurer
                                        --------------------------------------

                                   SIMULA TRANSPORTATION EQUIPMENT
                                   CORPORATION, an Arizona corporation



                                   By:           /s/ Donald Townsend
                                        --------------------------------------
                                   Name:         Donald Townsend
                                        --------------------------------------
                                   Its:          Assistant Treasurer
                                        --------------------------------------

                                   AIRLINE INTERIORS, INC., an Arizona
                                   corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:           Donald Townsend
                                        --------------------------------------
                                   Its:            Assistant Treasurer
                                        --------------------------------------


                                                                             -6-
<PAGE>   7
                                   ARTCRAFT INDUSTRIES CORP., an Arizona
                                   corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                   COACH AND CAR EQUIPMENT
                                   CORPORATION, an Arizona corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                   VIATECH, INC., a Delaware corporation


                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                   SIMULA TECHNOLOGIES, INC., an Arizona
                                   corporation


                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Treasurer
                                        --------------------------------------



                                      -7-
<PAGE>   8
                                   SIMULA AUTOMOTIVE SAFETY DEVICES
                                   LIMITED, a company organized under the laws
                                   of the United Kingdom



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

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



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Treasurer
                                        --------------------------------------
                                   INTAERO, LTD., an Arizona corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------
                                                                    CO-BORROWERS


                                   BANK ONE, ARIZONA, NA, a national banking
                                   association


                                   By:            /s/ Steve Reinhardt
                                        --------------------------------------
                                   Name:          Steve Reinhardt
                                        --------------------------------------
                                   Its:           Vice President
                                        --------------------------------------
                                                            ADMINISTRATIVE AGENT


                                                                             -8-
<PAGE>   9
CONSENT AND AGREEMENT OF GUARANTORS


         With respect to the Fourth Modification Agreement, dated July 28, 1999
("Agreement"), between SIMULA, INC., an Arizona corporation (the "Company"), all
present and future Subsidiaries of the Company (with the Company, the
"Borrower") and BANK ONE, ARIZONA, NA, a national banking association as
administrative agent for the Banks (as defined in the Agreement)
("Administrative Agent"), the undersigned (individually and, if more than one,
collectively "Guarantor") agrees for the benefit of the Banks as follows:

         1. Guarantor acknowledges (i) receiving a copy of and reading the
Agreement, (ii) the accuracy of the Recitals in the Agreement, and (iii) the
effectiveness of (A) the Continuing Guarantees as modified herein, and (B) any
other agreements, documents, or instruments securing or otherwise relating to
the Continuing Guarantees, (including, without limitation, any arbitration
resolution and any environmental certification and indemnity agreement
previously executed and delivered by the undersigned), as modified herein. The
Continuing Guarantees and such other agreements, documents, and instruments, as
modified herein, are referred to individually and collectively as the "Guarantor
Documents."

         2. Guarantor consents to the modification of the Credit Documents and
all other matters in the Agreement.

         3. Guarantor fully, finally, and forever releases and discharges the
Banks and the Administrative Agent and their successors, assigns, directors,
officers, employees, agents, and representatives from any and all actions,
causes of action, claims, debts, demands, liabilities, obligations, and suits of
whatever kind or nature, in law or equity, that Guarantor has or in the future
may have, whether known or unknown, (i) in respect of the Loans, the Credit
Documents, the Guarantor Documents, or the actions or omissions of the Banks and
the Administrative Agent in respect of the Loans, the Credit Documents, or the
Guarantor Documents and (ii) arising from events occurring prior to the date
hereof.

         4. Guarantor agrees that all references, if any, to the Notes, the
Senior Credit Agreement, the Security Documents, and the Credit Documents in the
Guarantor Documents shall be deemed to refer to such agreements, documents, and
instruments as modified by the Agreement.

         5. Guarantor reaffirms the Guarantor Documents and agrees that the
Guarantor Documents continue in full force and effect and remain unchanged,
except as specifically modified by this Consent and Agreement of Guarantors. Any
property or rights to or interests in property granted as security in the
Guarantor Documents shall remain as security for the Continuing Guarantees and
the obligations of Guarantor in the Continuing Guarantees.

         6. Guarantor agrees that the Credit Documents, as modified by the
Agreement, and the Guarantor Documents, as modified by this Consent and
Agreement of Guarantors, are the legal,
<PAGE>   10
valid, and binding obligations of Borrower and the undersigned, respectively,
enforceable in accordance with their terms against Borrower and the undersigned,
respectively.

         7. Guarantor agrees that Guarantor has no claims, counterclaims,
defenses, or offsets with respect to the enforcement against Guarantor of the
Guarantor Documents.

         8 Guarantor represents and warrants that there has been no material
adverse change in the financial condition of any Guarantor from the most recent
financial statement received by Lender.

         9 Guarantor agrees that this Consent and Agreement of Guarantors may be
executed in one or more counterparts, each of which shall be deemed an original
and all of which together shall constitute one and the same document. Signature
and acknowledgement pages may be detached from the counterparts and attached to
a single copy of this Consent and Agreement of Guarantors to physically form one
document.

         DATED as of the date of the Agreement.

                                   SIMULA SAFETY SYSTEMS, INC., formerly
                                   known as Simula Government Products, Inc.,
                                   an Arizona corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------


                                   SIMULA AUTOMOTIVE SAFETY DEVICES,
                                   INC., an Arizona corporation,
                                   a/k/a ASD-Simula


                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Treasurer
                                        --------------------------------------



                                      -2-
<PAGE>   11
                                   SIMULA TRANSPORTATION EQUIPMENT
                                   CORPORATION, an Arizona corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                   AIRLINE INTERIORS, INC., an Arizona
                                   corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                   ARTCRAFT INDUSTRIES CORP., an Arizona
                                   corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                   COACH AND CAR EQUIPMENT
                                   CORPORATION, an Arizona corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------


                                                                             -3-
<PAGE>   12
                                   VIATECH, INC., a Delaware corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------


                                   SIMULA TECHNOLOGIES, INC., an Arizona
                                   corporation

                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Treasurer
                                        --------------------------------------


                                   SIMULA AUTOMOTIVE SAFETY DEVICES LIMITED,
                                    a company organized under the laws of
                                   the United Kingdom



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------


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



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:          Donald Townsend
                                        --------------------------------------
                                   Its:           Treasurer
                                        --------------------------------------


                                                                             -4-
<PAGE>   13
                                   INTAERO, LTD., an Arizona corporation



                                   By:            /s/ Donald Townsend
                                        --------------------------------------
                                   Name:       Donald Townsend
                                        --------------------------------------
                                   Its:           Assistant Treasurer
                                        --------------------------------------

                                                                       GUARANTOR


                                                                             -5-
<PAGE>   14
CONSENT OF THE BANKS


Re:      Simula, Inc. and its Subsidiaries

         The undersigned:

         (a) is a Bank named in that Senior Credit Agreement dated November 6,
1998 between Simula, Inc. and its Subsidiaries (the "Borrower"), Bank One,
Arizona, NA, a national banking association, as administrative agent for the
Banks (the "Administrative Agent"), the Issuing Bank, and the Banks; and

         (b) consents to that Fourth Modification Agreement dated July 28, 1999
entered into between the Borrower and the Administrative Agent.

                                   BANK ONE, ARIZONA, NA, a national banking
                                   association


                                   By:            /s/ Steve Reinhart
                                        --------------------------------------
                                   Name:          Steve Reinhart
                                        --------------------------------------
                                   Its:           Vice President
                                        --------------------------------------

                                                       "Issuing Bank" and "Bank"
<PAGE>   15
CONSENT OF THE BANKS


Re:      Simula, Inc. and its Subsidiaries

         The undersigned:

         (a) is a Bank named in that Senior Credit Agreement dated November 6,
1998 between Simula, Inc. and its Subsidiaries (the "Borrower"), Bank One,
Arizona, NA, a national banking association, as administrative agent for the
Banks (the "Administrative Agent"), the Issuing Bank, and the Banks; and

         (b) consents to that Fourth Modification Agreement dated July 28, 1999
entered into between the Borrower and the Administrative Agent.

                                   IMPERIAL BANK, California banking
                                   corporation, successor by merger to
                                   IMPERIAL BANK banking corporation



                                   By:            /s/ Stephen Wallis
                                        --------------------------------------
                                   Name:          Stephen Wallis
                                        --------------------------------------
                                   Its:           Senior Vice President
                                        --------------------------------------

                                                                          "Bank"