<PAGE>

                                                                   Exhibit 10.21

<TABLE>
<S>                                 <C>                           <C>
AEROSONIC CORPORATION               FIRST COMMERCIAL BANK         Loan Number    6051361 / 00010
1212 N HERCULES AVE                 800 SHADES CREEK PARKWAY      Date           09/05/1999
CLEARWATER, FL 33765-1920           BIRMINGHAM, AL 35209          Maturity Date  09/05/2004
                                                                  Loan Amount $  1300092.00
    BORROWER'S NAME AND ADDRESS      LENDER'S NAME AND ADDRESS    (***)          00103
"1" includes each borrower above,   "You" means the lender, its   Fed. Tax ID    74-1668471
     jointly and severally.            successors and assigns.
</TABLE>

For value received, I promise to pay to you, or your order, at your address
listed above the PRINCIPAL sum of **ONE MILLION THREE HUNDRED THOUSAND NINETY
TWO DOLLARS** **AND ZERO CENTS** Dollars $1,300,092.00

[X] Single Advance: I will receive all of this principal sum on
                                                                ----------------
           . No additional advances are contemplated under this note.
      -----

[ ] Multiple Advances: The principal sum shown above is the maximum amount of
      principal I can borrow under this note. On
                                                 -----------------------------
              I will receive the amount of $               and future principal
      -------                               --------------
      advances are contemplated.

      Conditions: The conditions for future advances are
                                                         -----------------------

      --------------------------------------------------------------------------

      --------------------------------------------------------------------------

       [ ] Open End Credit: You and I agree that I may borrow under this Note,
           prepay this Note in whole or in part, and borrow again under this
           Note, so long as the aggregate unpaid principal amount owed under
           this Note at any time does not exceed the amount of the principal sum
           set forth above. My right to borrow is subject to all other
           conditions of This Note expires on                                .
                                              -------------------------------

       [ ] Closed End Credit: You and I agree that I may borrow in aggregate an
           amount not to exceed the principal sum shown above. Amounts I repay
           under this Note may not be re-borrowed later. My right to borrow is
           subject to all other conditions and expires on                      .
                                                          ---------------------

[X] INTEREST: I agree to pay interest on the outstanding principal balance from
       September 05, 1999 at the rate of 8.050000% per year until September 05,
       2004.

    Variable Rate: This rate may then change as stated below.

       [ ] Index Rate: The future rate will be N/A the following index rate: N/A
           N/A
           N/A

       [ ] No Index: The future rate will not be subject to any internal or
           external index. It will be entirely in your control.

       [ ] Frequency and Timing: The rate on this note may change as often as
           N/A.

              A change in the interest rate will take effect N/A.

       [ ] Limitations: During the term of this loan, the applicable annual
           interest rate will not be more than N/A % or less than N/A %. The
           rate may not change more than N/A % each N/A.

       Effect of Variable Rate: A change in the interest rate will have the
       following effect on the payments:

       [ ] The amount of each scheduled payment will change.

       [ ] The amount of the final payment will change.

       [ ]                                                                     .
           --------------------------------------------------------------------

ACCRUAL METHOD: Interest will be calculated on a ACTUAL # DAYS/360 -DAY YEAR
basis.

POST MATURITY RATE: I agree to pay interest on the unpaid balance of this note
owing after maturity, and until paid in full, as stated below:

       [X] on the same fixed or variable rate basis in effect before maturity
           (as indicated above).

       [ ] at a rate equal to                                                  .
                              -------------------------------------------------

[X] LATE CHARGE: I agree to pay a late charge on the portion of any payment not
    made within 10 days after it is due equal to 5% OF THE UNPAID AMOUNT, WITH
    A MINIMUM OF $25.00.

[X] ADDITIONAL CHARGES: In addition to interest, I agree to pay,
                                                                 ---------------
    the following charges which [X] are [ ] are not included in the principal
    amount above: $40.00 ADMINISTRATIVE FEES, $12.00 UCC FEE.

PAYMENTS: I agree to pay this note as follows:

[ ] Interest: I agree to pay accrued interest
                                              ----------------------------------

[ ] Principal: I agree to pay the principal
                                            ------------------------------------

    ----------------------------------------------------------------------------

[X] Installments: I agree to pay this note in 60 payments. The first payment
    will be in the amount of $26500.00 and will be due October 05, 1999. A
    payment of $26500.00 will be due on the same day of each MONTHLY period
    thereafter. The final payment of the entire unpaid balance of principal and
    interest will be due September 05, 2004.

PURPOSE: The purpose of this loan is RN/6051361-0001/25.

FAIR CREDIT REPORTING ACT DISCLOSURE: At our discretion we may communicate
information solely about our transactions or experiences with you to persons
related to us by common ownership or affiliated by corporate control. We may
also, at our sole and absolute discretion, communicate other information to the
same persons, unless you direct us in writing at the following address within 15
days of the date hereof not to share such information:

                               Central Processing
                                  P.O. Box 120
                             Columbus, GA 31902-0120

ADDITIONAL TERMS:

<PAGE>

                                    SECURITY

SECURITY INTEREST: I grant you a security interest in all of the Property
   described below that I now own and that I may own in the future (including,
   but not limited to, all parts, accessories, repairs, improvements, and
   accessions to the Property), wherever the Property is or may be located, and
   all proceeds and products from the Property.

   [ ] Inventory: All inventory which I hold for ultimate sale or lease, or
       which has been or will be supplied under contracts of service, or which
       are raw materials, work in process, or materials used or consumed in my
       business.

   [X] Equipment: All equipment including, but not limited to, all machinery,
       vehicles, furniture, fixtures, manufacturing equipment, farm machinery
       and equipment, shop equipment, office and recordkeeping equipment, and
       parts and tools. All equipment described in a list or schedule which I
       give to you will also be included in the secured property, but such
       a list is not necessary for a valid security interest in my equipment.

   [ ] Farm Products: All farm products including, but not limited to:

       (a) all poultry and livestock and their young, along with their products,
       produce and replacements;
       (b) all crops, annual or perennial, and all products of the crops; and
       (c) all feed, seed, fertilizer, medicines, and other supplies used or
       produced in my farming operations.

   [ ] Accounts, Instruments, Documents, Chattel Paper and Other Rights to
       Payment: All rights I have now and that I may have in the future to the
       payment of money including, but not limited to:

       (a) payment for goods and other property sold or leased or for services
       rendered, whether or not I have earned such payment by performance; and

       (b) rights to payment arising out of all present and future debt
       instruments, chattel paper and loans and obligations receivable.
       The above include any rights and interests (including all liens and
       security interests) which I may have by law or agreement against any
       account debtor or obligor of mine.

   [ ] General Intangibles: All general intangibles including, but not limited
       to, tax refunds, applications for patents, patents, copyrights,
       trademarks, trade secrets, good will, trade names, customer lists,
       permits and franchises, and the right to use my name.

   [ ] Government Payments and Programs: All payments, accounts, general
       intangibles, or other benefits (including, but not limited to, payments
       in kind, deficiency payments, letters of entitlement, warehouse receipts,
       storage payments, emergency assistance payments, diversion payments, and
       conservation reserve payments) in which I now have and in the future may
       have any rights or interest and which arise under or as a result of any
       preexisting, current or future Federal or state governmental program
       (including, but not limited to, all programs administered by the
       Commodity Credit Corporation and the ASCS).

   [ ] Timber to be Cut: All timber to be cut on the real estate described
       below and all cutting rights with respect thereto, whether now owned or
       hereafter acquired, and all proceeds of the foregoing.

   [ ] The secured property includes, but is not limited by, the following:

If this agreement covers timber to be cut, minerals (including oil and gas),
fixtures or crops growing or to be grown, the description of the real estate is:

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

[ ] If checked, file this agreement on the real estate records. Record owner
    (if not me)
                ----------------------------------------------------------------

--------------------------------------------------------------------------------

The Property will be used for a [ ] personal [X] business [ ] agricultural
[ ]                                      purpose.
    ------------------------------------

                   ADDITIONAL TERMS OF THE SECURITY AGREEMENT

GENERALLY - This agreement secures this note and any other debt I have with you,
now or later. However, it will not secure other debts if you fail with respect
to such other debts, to make any required disclosure about this security
agreement or if you fail to give any required notice of the right of rescission.
If property described in this agreement is located in another state, this
agreement may also, in some circumstances, be governed by the law of the state
in which the Property is located. All references to "this note" or "this
agreement" or "this loan" shall mean this Universal Note and Security Agreement.

OWNERSHIP AND DUTIES TOWARD PROPERTY - I represent that I own all of the
Property, or to the extent this is a purchase money security interest I will
acquire ownership of the Property with the proceeds of the loan. I will defend
it against any other claim. Your claim to the Property is ahead of the claims of
any other creditor. I agree to do whatever you require to protect your security
interest and to keep your claim in the Property ahead of the claims of other
creditors. I will not do anything to harm your position.

     I will keep books, records and accounts about the Property and my business
in general. I will let you examine these records at any reasonable time. I will
prepare any report or accounting you request, which deals with the Property.

     I will keep the Property in my possession and will keep it in good repair
and use it only for the purpose(s) described on page 1 of this agreement. I will
not change this specified use without your express written permission. I
represent that I am the original owner of the Property and, if I am not, that I
have provided you with a list of prior owners of the Property.

     I will keep the Property at my address listed on page 1 of this agreement,
unless we agree I may keep it at another location. If the Property is to be used
in another state, I will give you a list of those states. I will not try to sell
the Property unless it is inventory or I receive your written permission to do
so. If I sell the Property I will have the payment made payable to the order of
you and me.

     I will pay all taxes and charges on the Property as they become due. You
have the right of reasonable access in order to inspect the Property. I will
immediately inform you of any loss or damage to the Property.

     If I fail to perform any of my duties under this security agreement, or any
mortgage, deed of trust, lien or other security interest, you may without notice
to me perform the duties or cause them to be performed. Your right to perform
for me shall not create an obligation to perform and your failure to perform
will not preclude you from exercising any of your other rights under the law or
this security agreement.

PURCHASE MONEY SECURITY INTEREST - For the sole purpose of INSURANCE - I agree
to buy insurance on the Property against the risks and for the amounts you
require and to furnish you continuing proof of coverage. I will have the
insurance company name you as loss payee on any such policy. You may require
added security if you agree that insurance proceeds may be used to repair or
replace the Property. I will buy insurance from a firm licensed to do business
in the state where the property is located. The firm will be reasonably
acceptable to you. The insurance will remain in force until the Property is
released from this agreement. If I fail to buy or maintain the insurance (or
fail to name you as loss payee) you may purchase it yourself.

WARRANTIES AND REPRESENTATIONS - If this agreement includes accounts, I will not
settle any account for less than its full value without your written permission.
I will collect all accounts until you tell me otherwise. I will keep in trust
for you the proceeds from all the accounts and any goods which are returned to
me or which I take back. I will not mix them with any other property of mine. I
will deliver them to you at your request. If you ask me to pay you the full
price on any returned items or items retaken by myself, I will do so.

     If this agreement covers inventory, I will not dispose of it except in my
ordinary course of business at the fair market value for the Property, or a
minimum price established between you and me.

     If this agreement covers farm products I will provide you, at your request,
a written list of the buyers, commission merchants or selling agents to or
through whom I may sell my farm products. In addition to those parties named on
this written list, I authorize you to notify at you sole discretion any
additional parties regarding your security interest in my farm products. I
remain subject to all applicable penalties for selling my farm products in
violation of my agreement with you and the Food Security Act. In this paragraph
the terms farm products, buyers, commission merchants and selling agents have
the meanings given to them in the Federal Food Security Act of 1985.

REMEDIES - I will be in default on this security agreement if I am in default on
any note this agreement secures or if I fail to keep any promise contained in
the terms of this agreement. If I default, you have all of the rights and
remedies provided in the note and under the Uniform Commercial Code. You may
require me to make the secured property available to you at a place which is
reasonably convenient. You may take possession of the secured property and sell
it as provided by law. The proceeds will be applied first to your expenses and
then to the debt. I agree that 10 days written notice sent to my last known
address by first class mail will be reasonable notice under the Uniform
Commercial Code. My current address is on page 1 . I agree to inform you in
writing of any change of my address. You may demand immediate payment of the

<PAGE>

                          ADDITIONAL TERMS OF THE NOTE

DEFINITIONS - As used on pages 1 and 2, "[X]" means the terms that apply to this
loan. "I," "me" or "my" means each Borrower who signs this note and each other
person or legal entity (including guarantors, endorsers, and sureties) who
agrees to pay this note ( together referred to as "us"). "You" or "your" means
the Lender and its successors and assigns.

APPLICABLE LAW - The laws of the United States and, to the extent not
inconsistent therewith, the laws of the state of Alabama. Any term of this
agreement which is contrary to applicable law will not be effective, unless the
law permits you and me to agree to such a variation. If any provision of this
agreement cannot be enforced according to its terms, this fact will not affect
the enforceability of the remainder of this agreement. No modification of this
note or any agreement securing this note is effective unless the modification is
in writing and signed by you and me. Time is of the essence in this agreement.

PAYMENTS - Each payment of principal and interest I make on this note will first
reduce the amount I owe you for charges which are neither interest nor
principal. The remainder of each payment will then reduce accrued unpaid
interest, and then unpaid principal. If you and I agree to a different
application of payments, we will describe our agreement on this note. I may
prepay a part of, or the entire balance of this loan without penalty, unless we
specify to the contrary on this note. Any partial prepayment will not excuse or
reduce any later scheduled payment until this note is paid in full (unless, when
I make the prepayment, you and I agree in writing to the contrary).

INTEREST - Interest accrues on the principal remaining unpaid from time to time,
until paid in full. If I receive the principal in more than one advance, each
advance will start to earn interest only when I receive the advance. The
interest rate in effect on this note at any given time will apply to the entire
principal sum outstanding at that time. Notwithstanding anything to the
contrary, I do not agree to pay and you do not intend to charge any rate of
interest that is higher than the maximum rate of interest you could charge under
applicable law for the extension of credit that is agreed to in this note
(either before or after maturity). If any notice of interest accrual is sent and
is in error, we mutually agree to correct it, and if you actually collect more
interest than allowed by law and this agreement, you agree to refund it to me.

INDEX RATE - The index will serve only as a device for setting the interest rate
on this note. You do not guarantee by selecting this index, or the margin, that
the interest rate on this note will be the same rate you charge on any other
loans or class of loans you make to me or other borrowers.

POST MATURITY RATE - For purposes of deciding when the "Post Maturity Rate"
(shown on page 1) applies, the term "maturity" means the date of the last
scheduled payment indicated on page 1 of this note or the date you accelerate
payment on the note, whichever is earlier.

SINGLE ADVANCE LOANS - If this is a single advance loan, you and I expect that
you will make only one advance of principal. However, you may add other amounts
to the principal if you make any payments described in the "PAYMENTS BY LENDER"
paragraph on page 2.

MULTIPLE ADVANCE LOANS - If this is a multiple advance loan, you and I expect
that you will make more than one advance of principal. If this is closed end
credit, repaying a part of the principal will not entitle me to additional
credit.

SET-OFF - I agree that you may set off any amount due and payable under this
note against any right I have to receive money from you.

     "Right to receive money from you" means:

     (1)  any deposit account balance I have with you;

     (2)  any money owed to me on an item presented to you or in your possession
          for collection or exchange; and

     (3)  any repurchase agreement or other nondeposit obligation.

     "Any amount due and payable under this note" means the total amount of
which you are entitled to demand payment under the terms of this note at the
time you set off. This total includes any balance the due date for which you
properly accelerate under this note.

     If my right to receive money from you is also owned by someone who has not
agreed to pay this note, your right of set-off will apply to my interest in the
obligation and to any other amounts I could withdraw on my sole request or
endorsement. Your right of set-off does not apply to an account or other
obligation where my rights are only as a representative. It also does not apply
to any Individual Retirement Account or other tax-deferred retirement account.

     You will not be liable for the dishonor of any check when the dishonor
occurs because you set off this debt against any of my accounts. I agree to hold
you harmless from any such claims arising as a result of your exercise of your
right to set-off.

DEFAULT - I will be in default if any one or more of the following occur: (1) I
fail to make a payment on time or in the amount due; (2) I fail to keep the
Property insured, if required; (3) I fail to pay, or keep any promise, on any
debt or agreement I have with you; (4) any other creditor of mine attempts to
collect any debt I owe him through court proceedings; (5) I die, am declared
incompetent, make an assignment for the benefit of creditors, or become
insolvent (either because my liabilities exceed my assets or I am unable to pay
my debts as they become due); (6) I make any written statement or provide any
financial information that is untrue or inaccurate at the time it was provided;
(7) I do or fail to do something which causes you to believe you will have
difficulty collecting the amount I owe you; (8) any collateral securing this
note is used in a manner or for a purpose which threatens confiscation by a
legal authority; (9) I change my name or assume an additional name without first
notifying you before making such a change; (10) I fail to plant, cultivate and
harvest crops in due season; (11) any loan proceeds are used for a purpose that
will contribute to excessive erosion of highly erodible land or to the
conversion of wetlands to produce an agricultural commodity, as further
explained in 7 C.F.R. Part 1940, Subpart G, Exhibit M.

REMEDIES - If I am in default on this note you have, but are not limited to, the
following remedies:

     (1)  You may demand immediate payment of all I owe you under this note
          (principal, accrued unpaid interest and other accrued unpaid charges).
     (2)  You may set off this debt against any right I have to the payment of
          money from you, subject to the terms of the "SET-OFF" paragraph
          herein.
     (3)  You may demand security, additional security, or additional parties to
          be obligated to pay this note as a condition for not using any other
          remedy.
     (4)  You may refuse to make advances to me or allow purchases on credit by
          me.
     (5)  You may use any remedy you have under state or federal law.
     (6)  You may make use of any remedy given to you in any agreement securing
          this note.

     By selecting any one or more of these remedies you do not give up your
right to use later any other remedy. By waiving your right to declare an event
to be a default, you do not waive your right to consider later the event a
default if it continues or happens again.

COLLECTION COSTS AND ATTORNEY'S FEES - I agree to pay you all reasonable costs
you incur to collect this debt or realize on any security. This includes, if the
amount financed is more than $300.00, your reasonable attorneys' fees of up to
15% of the unpaid debt if you refer collection of this note to an attorney who
is not your salaried employee. This provision also shall apply if I file a
petition or any other claim for relief under any bankruptcy rule or law of the
United States, or if such petition or other claim for relief is filed against me
by another.

WAIVER - I give up my rights to require you to do certain things. I will not
require you to:

     (1)  demand payment of amounts due (presentment);
     (2)  obtain official certification of nonpayment (protest); or
     (3)  give notice that amounts due have not been paid (notice of dishonor).

     I waive any defenses I have based on suretyship or impairment of
collateral. To the extent permitted by law, I also waive all personal property
exemptions in the property securing this loan.

OBLIGATIONS INDEPENDENT - I understand that I must pay this note even if someone
else has also agreed to pay it (by, for example, signing this form or a separate
guarantee or endorsement). You may sue me alone, or anyone else who is obligated
on this note, or any number of us together, to collect this note. You may
without notice release any party to this agreement without releasing any other
party. If you give up any of your rights, with or without notice, it will not
affect my duty to pay this note. Any extension of new credit to any of us, or
renewal of this note by all or less than all of us will not release me from my
duty to pay it. (Of course, you are entitled to only one payment in full.) I
agree that you may at your option extend this note or the debt represented by
this note, or any portion of the note or debt, from time to time without limit
or notice and for any term without affecting my liability for payment of the
note. I will not assign my obligation under this agreement without your prior
written approval.

CREDIT INFORMATION - I agree and authorize you to obtain credit information
about me from time to time (for example, by requesting a credit report) and to
report to others your credit experience with me (such as a credit reporting
agency). I agree to provide you, upon request, any financial statement or
information you may deem necessary. I warrant that the financial statements and
information I provide to you are or will be accurate, correct and complete.

SIGNATURES: I AGREE TO THE TERMS OF THIS NOTE (INCLUDING THOSE ON PAGES 1 AND
2). I have received a copy on today's date.

          CAUTION - IT IS IMPORTANT THAT YOU THOROUGHLY READ THE CONTRACT BEFORE
          YOU SIGN IT.

AEROSONIC CORPORATION


BY: /s/ ERIC J. McCRACKEN EVP/CFO
---------------------------------
ERIC J. McCRACKEN EVP/CFO

<PAGE>

<TABLE>
-----------------------------------------------------------------------------------------------
<S>                             <C>                           <C>
AEROSONIC CORPORATION           FIRST COMMERCIAL BANK         Loan Number 6051361 / 00010
1212 N HERCULES AVE             800 SHADES CREEK PARKWAY
CLEARWATER, FL 33765-1920       BIRMINGHAM, AL 35209          Date        09/05/1999
BORROWER'S NAME AND ADDRESS     LENDER'S NAME AND ADDRESS
"I" includes each borrower      "You" means the lender, its   Mat.Date    09/05/2004
above, jointly and severally.   successors and assigns.
                                                              Loan Amount $1,300,092.00

                                                              Renewal of  6051361 / 00001
-----------------------------------------------------------------------------------------------
</TABLE>

I hereby authorize and request the following disbursement from the loan
referenced above:

a.   Amount given to me directly                    $
                                                     ---------------------
b.   Amount paid on my account (#  6051361-00001 )  $1,300,040.00

c.   To Lender     ADMIN FEE                        $       40.00

          Amounts paid to others on my behalf:

d.   To Property Insurance Company                  $
                                                     ---------------------
e.   To Credit Life Insurance Company               $
                                                     ---------------------
f.   To Disability Insurance Company                $
                                                     ---------------------
g.   To Public Officials                            $
                                                     ---------------------
h.   ORIGINATION                                    $
                                                     ---------------------

Comments:
          ----------------------------------------------------------------

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--------------------------------------------------------------------------
Loan Officer: A TODD BEARD

i.   CREDIT REPORT FEE                              $
                                                     ---------------------
j.   UCC FEE                                        $                12.00

k.                                                  $
     --------------------------------------------    ---------------------
l.                                                  $
     --------------------------------------------    ---------------------
m.                                                  $
     --------------------------------------------    ---------------------
n.                                                  $
     --------------------------------------------    ---------------------
o.                                                  $
     --------------------------------------------    ---------------------
p.                                                  $
     --------------------------------------------    ---------------------
q.                                                  $
     --------------------------------------------    ---------------------
r.

s.
     ---------------------------------------------------------------------
t.   AEROSONIC CORPORATION

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                                                              (page 1 of 1)

(C) 1985 BANKERS SYSTEMS, INC., ST. CLOUD, MN 56301 FORM DA 8/13/85