<PAGE>

                                                               EXHIBIT 10.10(c)

    AMENDMENT NO. 3 TO AMENDED AND RESTATED LOAN AND SECURITY AGREEMENT

     THIS AMENDMENT NO. 3 TO AMENDED AND RESTATED LOAN AND SECURITY AGREEMENT
(this "Amendment"), dated as of the 5th Day of April, 1998 is made by and among
QUESTECH, INC., a Virginia corporation ("QUESTECH"), QUESTECH SERVICE COMPANY, a
Virginia corporation formerly known as Engineering Resources, Inc. ("QTSC"),
QUESTECH PACKAGING, INC., a Virginia corporation ("QTPI"; QuesTech, QTSC, and
QTPI are referred to individually as a "Borrower" and collectively as the
'Borrowers"), and FIRST UNION NATIONAL BANK, a national banking association,
successor by merger to Signet Bank, a Virginia banking corporation (the
"Lender").

RECITALS

A. The Lender and the Borrowers entered into an Amended and Restated Loan and
Security Agreement dated as of June 3,1 996 (as amended through the date hereof,
the Agreement) pursuant to which the Lender has agreed to extend credit to the
Borrowers, and the Borrowers have agreed to obtain credit from the Lender, on
the terms and conditions set forth in such Agreement.

B. The Borrowers have requested that the Lender make certain modifications to
the Agreement, including increasing the Maximutn Amount, and the Lender has
consented to such request subject to the execution of this Amendment and the
satisfaction of the conditions specified herein.

C. The Borrowers and the Lender now desire to execute this Amendment to set
forth their agreements with respect to the modifications to the Agreement.

     Accordingly, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Lender and the Borrowers agree
as follows:

     SECTION 1. Definitions. Capitalized terms used in this Amendment and not
     defined herein are deemed in the Agreement.

     SECTION 2. Amendments to Agreement. The Agreement is hereby amended as
     follows:

     Amendments to Section 1. Section 1 of the Agreement is amended as follows:

          (a) Commitment Fee. The definition of the term Commitment Fee is
     replaced in its entirety with the following definition:

          "Commitment FEE" means the quarterly fee to be paid by the Borrowers
     to the Lender pursuant to Section 2.5 hereof in consideration of the
     commitment by the Lender to make Revolving Loans hereunder. The Commitment
     Fee due for each calendar quarter (or portion thereof) shall equal the
     product of the Commitment Fee Rate in effect for such quarter (or portion
     thereof) multiplied by the difference between $6,500,000.00 and the sum of
     the average daily principal balance of the Revolving Loans during such
     quarter (or applicable portion thereof) plus the average 


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<PAGE>


     daily face amount of all Letters of Credit outstanding during such quarter
     (or applicable portion thereof).

          (b) Maximum Amount. The definition of the term Maximum Amount is
     replaced in its entirety with the following definition:

          "Maximum Amount" means, at any time, the lesser of (a) the difference
     between $6,500,000.00 and the Letter of Credit Exposure as such time or (b)
     the then applicable Borrowing Base.

          (c) Revolving Note. The definition of the term Revolving Note is
     replaced in its entirety with the following definition:

          Revolving Note" means the promissory note in form and substance
     acceptable to the Lender in the original principal amount of $6,500,000.00
     (as it may be amended, modified supplemented or replaced from time to time)
     evidencing the obligation of the Borrower tO pay the principal amount of
     the Revolving Loans together with interest on the Revolving Loans.

     SECTION 3. Representations and Warranties of Borrowers. The Borrower,
     jointly and severally represents and warrants to the Lender that:

          (a) Each Borrower has the power and authority to enter into and to
     perform this Amendment, to execute and deliver all documents relating to
     this Amendment, and to incur the obligations provided for in this
     Amendment, all of which have been duly authorized and approved in
     accordance with the Borrower's corporate documents;

          (b) This Amendment, together with all documents executed pursuant
     hereto, shall constitute when executed the valid and legally binding
     obligations of the Borrowers in accordance with their respective terms;

          (c) Except with respect to events or circumstances occurring
     subsequent to the date thereof and known to the Lender, all representations
     and warranties made in the Agreement are true and correct as of the date
     hereof, with the same force and effect as if all representations and
     warranties were fully set forth herein;

          (d) Each Borrower's obligations under the Loan Documents remain valid
     and enforceable obligations, and the execution and delivery of this
     Amendment and the other documents executed in connection herewith shall not
     be construed as a novation of the Agreement or any of the other Loan
     Documents; and

          (e) As of the date hereof, no Borrower has any offsets or defenses
     against the payment of any of the Obligations.

     SECTION 4. Waiver of Claims. As a specific inducement to the Lender without
     which the Borrowers acknowledges the Lender would not enter into this
     Amendment and the other documents executed in connection herewith, each
     Borrower hereby waives any and all claims that it may have against the
     Lender, as of the date hereof, arising out of or relating to the 


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     Agreement or any other Loan Document whether sounding in contract, tort or
     any other basis.

     SECTION 5. Conditions of Effectiveness. This Amendment shall become
     effective when, and only when, the Borrowers have executed and completed
     this Amendment and a Revolving Note in form and substance acceptable to the
     Lender, have delivered such original, executed documents to the Lender, and
     have reimbursed the Lender for the Lender's costs and expenses incurred in
     connection with this Amendment.

     SECTION 6. Miscellaneous.

          6.1 Reference To Agreement and Note. Upon the effectiveness of this
     Amendment, each reference in the Agreement to "this Agreement" and each
     reference in the other Loan Documents to the Agreement, shall mean and be a
     reference to the Agreement as amended hereby and each reference in the
     Agreement and the other Loan Documents to the "Note" shall mean and be a
     reference to the applicable Note executed by the Borrowers and delivered to
     the Lender pursuant to Section 5 hereof.

          6.2 Effect on Loan Documents. Except as specifically amended above,
     the Agreement and all other Loan Documents shall remain in full force and
     effect and are hereby ratified and confirmed. Without limiting the
     generality of the foregoing, all Collateral given to secure the Obligations
     of the Borrowers under the Agreement and the other Loan Documents prior to
     the date hereof does and shall continue to secure all Obligations of the
     Borrowers under the Agreement, as amended hereby and the other Loan
     Documents, and, except as provided in the Agreement and the other Loan
     Documents, no such Collateral shall be released until all conditions to
     such release as contained in the Loan documents are satisfied or all
     Obligations are satisfied and completely discharged.

          6.3 No Waiver. The execution, delivery and effectiveness of this
     Amendment shall not operate as a waiver of any right, power or remedy of
     the Lender under any of the Loan Documents, nor constitute a waiver of any
     provision of any of the Loan Documents.

          6.4 Costs, Expenses and Taxes. The Borrowers, jointly and severally,
     agree to pay on demand all costs and expenses of the Lender in connection
     with the preparation, reproduction, execution and delivery of this
     Amendment and the other instruments and documents to be delivered
     hereunder, including the reasonable fees and out-of-pocket expenses of
     counsel for the Lender with respect thereto.

          6.5 Governing Law. This Amendment shall be governed by and construed
     in accordance with the laws of the Commonwealth of Virginia, without giving
     effect to conflict of law provisions.



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<PAGE>



     IN WITNESS WHEREOF, the Borrowers and the Lender have caused this Amendment
to be signed by their duly authorized representatives under seal all as of the
day and year first above written.

                          QUESTECH, INC., a Virginia corporation
ATTEST:


M. P. Rivera              By V. L. Salvatori
-----------------------      -----------------------------------------------
(Asst.) Secretary            Vincent L. Salvatori, Chairman
[corporate seal]


                          QUESTECH SERVICE COMPANY, INC., a Virginia
                          corporation

ATTEST:


M. P. Rivera              By: V. L. Salvatori
-----------------------      -----------------------------------------------
(Asst.) Secretary             Vincent L. Salvatori, Chairman
[corporate seed]


                          QUESTECH PACKAGING, INC., a Virginia
                          corporation

ATTEST:


M. P. Rivera              By: V. L. Salvatori
-----------------------      -----------------------------------------------
(Asst.) Secretary             Vincent L. Salvatori, Chairman
[corporate seed]




                         FIRST UNION NATIONAL BANK, a national banking
                         association


                         By:  Robert Lee Reed
                            -----------------------------------------------
                              Robert Lee Reed, Assistant Vice President


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<PAGE>



Customer #
Note #
                                 REVOLVING NOTE

$8,000,000.00                                             28 April, 1998
                                                  ----------------------
                                                  Falls Church, Virginia

     FOR VALUE RECEIVED, QUESTECH, INC., a Virginia corporation ("QuesTech"),
QUESTECH SERVICE COMPANY, a Virginia corporation formerly known as Engineering
Resources, Inc. ("QTSC"), QUESTECH PACKAGING, INC., a Virginia corporation
("QTPI") and QT INFORMATION SCIENCES CORPORATION, a Virginia corporation
("QTISC"; QuesTech. QTSC, QTPI and QTISC are referred to individually as a
"Borrower" and collectively as the "Borrowers"), hereby jointly and severally
promise to pay to the order of FIRST UNION NATIONAL BANK, a national banking
association, successor by merger to Signet Bank, a Virginia banking corporation
(the "Lender"), at North Tower, 5th Floor, 7799 Leesburg Pike, Falls Church,
Virginia, or such other location as the holder hereof may in writing designate,
the principal sum of Eight Million and no/100 Dollars ($8,000,000.00) or such
lesser amount as shall equal the aggregate unpaid principal amount of the
Revolving Loans made by the Lender to the Borrowers in accordance with the Loan
Agreement in lawful money of the United States of America in immediately
available funds, on the Termination Date and to pay interest on the unpaid
principal amount of the Revolving Loans, at such office, in like money and
funds, for the period commencing on the date of each Revolving Loan until such
Revolving Loan shall be paid in full, at the rate or rates per annum and on the
dates provided in the Loan Agreement. The Borrowers may borrow, prepay without
penalty and reborrow hereunder in accordance with the provisions of the Loan
Agreement.

     This Revolving Note is the Revolving Note referred to in the Amended and
Restated Loan and Security Agreement dated as of June 3, 1996 among the Lender
and QuesTech. QTSC and QTPI (as amended, modified, supplemented and replaced
from time to time, including that certain Amendment No. 4 to Amended and
Restated Loan and Security Agreement dated the date hereof, the "Loan
Agreement"), and evidences Revolving Loans made by the Lender thereunder.
Capitalized terms used in this Revolving Note which are not otherwise defined
herein have the respective meanings assigned to them in the Loan Agreement.

     The Lender is hereby authorized by the Borrowers to maintain records of the
amount of each Revolving Loan made by the Lender in accordance with the Loan
Agreement, the date such Revolving Loan is made, and the amount of each payment
or prepayment of principal, interest, or other charges made in respect of such
Revolving Loan received by the Lender. The Borrowers agree that, absent manifest
error, the amounts so evidenced in such records shall constitute conclusive
evidence of the amount owed hereunder.

     Upon the occurrence and continuation of an Event of Default, the principal
hereof and accrued interest hereon may be declared to be, or may become,
forthwith due and payable in the manner, upon the conditions and with the effect
provided in the Loan Agreement.

     The Borrowers and every guarantor and endorser hereof hereby waive
presentment, demand, notice of dishonor, protest and all other demands and


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<PAGE>



notices (other than as required by Section 8 of the Loan Agreement) in
connection with the delivery, acceptance, performance and enforcement of this
Revolving Note.

     This Revolving Note is given in substitution for, and not in novation of,
that certain Promissory Note given by QuesTech, QTSC and QTPI to the Lender in
the principal amount of Six Million Five Hundred Thousand and no/100 Dollars
($6,500,000.00) dated April 5, 1998.

     IN WITNESS WHEREOF, the Borrowers have caused their corporate names to be
signed by their duly authorized officers under seal as of the date and year
first above written.

                              QUESTECH, INC. a Virginia corporation



ATTEST:                       By:   V. L Salvatori
                                   -------------------------------
                                   Vincent L. Salvatori, Chairman

M. P. Rivera
----------------------
Secretary
[corporate seal]

                              QUESTECH SERVICE COMPANY, a Virginia
                              corporation



ATTEST:                       By:   V. L Salvatori
                                   --------------------------------
                                   Vincent L. Salvatori, Chairman

M. P. Rivera
----------------------
Secretary
[corporate seal]

                              QUESTECH PACKAGING, INC., a Virginia
                              corporation



ATTEST:                       By:   V. L Salvatori
                                   ------------------------------
                                   Vincent L. Salvatori, Chairman

M. P. Rivera
----------------------
Secretary
[corporate seal]



                                        6
<PAGE>





                              QT INFORMATION SCIENCES
                              CORPORATION, a Virginia corporation



ATTEST:                       By:   V. L Salvatori
                                   -------------------------------
                                   Vincent L. Salvatori, Chairman

M. P. Rivera
-----------------------
Secretary
[corporate seal]



COMMONWEALTH OF VIRGINIA )
                         ) to-wit:
COUNTY OF FAIRFAX        )

     The foregoing instrument was subscribed sworn to, and acknowledged before
me, the undersigned notary, public on this 28 day of April, 1998, in the
jurisdiction aforesaid, by Vincent L. Salvatori, the Chairman of QuesTech, Inc.,
QuesTech Service Company, QuesTech Packaging, Inc. and QT Information Sciences
Corporation, in behalf of, and as the duly authorized act of, each
such corporation.



                                         M. D. Benford
                                  ---------------------------------
                                        Notary Public [SEAL]

My commission expires:   7/30/2000
                        --------------





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