UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13G

                    Under the Securities Exchange Act of 1934
                               (Amendment No. 6)*

                       Environmental Tectonics Corporation
                                (Name of Issuer)

                          Common Stock, Par Value $0.05
                         (Title of Class of Securities)

                                    294092101
                                 (CUSIP Number)

                                  December 31,
                                    2004____
             (Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

      |X|   Rule 13d-1(b)
      |_|   Rule 13d-1(c)
      |_|   Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).



                                  SCHEDULE 13G

--------------------                                           -----------------
CUSIP No.  294092101                                           Page 2 of 4 Pages
--------------------                                           -----------------

--------------------------------------------------------------------------------
      1
                      NAME OF REPORTING PERSON
                      SS. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

                        Emerald Advisers, Inc.
--------------------------------------------------------------------------------
      2
                      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP 
                        (See Instructions)

                      (a) _______
                      (b) _______

--------------------------------------------------------------------------------
      3               SEC USE ONLY

--------------------------------------------------------------------------------
      4               CITIZENSHIP OR PLACE OF ORGANIZATION

                         Pennsylvania

--------------------------------------------------------------------------------
                          5      SOLE VOTING POWER
  NUMBER OF SHARES
 BENEFICIALLY OWNED               688,705 shares
 BY EACH REPORTING
    PERSON WITH
                      ----------------------------------------------------------
                          6      SHARED VOTING POWER

                                 0 shares

                      ----------------------------------------------------------
                          7      SOLE DISPOSITIVE POWER

                                 1,258,368 shares

                      ----------------------------------------------------------
                          8      SHARED DISPOSITIVE POWER

                                 0 shares

--------------------------------------------------------------------------------
      9               AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH 
                       REPORTING PERSON

                      1,258,368 shares

--------------------------------------------------------------------------------
      10              CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES 
                        CERTAIN SHARES (See Instructions)

--------------------------------------------------------------------------------
      11              PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

                      16.47%

--------------------------------------------------------------------------------
      12              TYPE OF REPORTING PERSON (See Instructions)

                      IA

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


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Schedule 13G                                                         Page 3 of 4

Item 1.

(a)   The name of the issuer is Environmental Tectonics Corporation.

(b)   The address of the issuer's principal executive office is County Line
      Industrial Park, Southampton, PA 18966.

Item 2.

(a)   The name of the person filing is Emerald Advisers, Inc.

(b)   The address of the principal office of the person filing is 1703 Oregon
      Pike, Suite 101, Lancaster, Pennsylvania 17601.

(c)   The state of organization is Pennsylvania.

(d)   The title of class of security is common stock, par value $0.05 per share.

(e)   The CUSIP number is 294092101.

Item 3.

The person filing is an Investment Adviser registered under Section 203 of the
Investment Advisers Act of 1940.

Item 4.

(a)       The amount beneficially owned is 1,258,368 shares of common stock.

(b)       The percent of class is 16.47 % based on 7,640,686 shares of common
          stock outstanding as of August 27, 2004, as reported in the Issuer's
          Form 10-Q for the period ended August 27, 2004.

(c)(i)    The number of shares as to which Emerald Advisers, Inc. has sole
          voting power is 688,705.

(c)(ii)   The number of shares as to which Emerald Advisers, Inc. has shared
          voting power is 0.

(c)(iii)  The number of shares as to which Emerald Advisers, Inc. has sole
          dispositive power is 1,258,368.

(c)(iv)   The number of shares as to which Emerald Advisers, Inc. has shared
          dispositive power is 0.

Item 5.

This statement is NOT being filed to report the fact that as of the date hereof
the reporting person has ceased to be the beneficial owner of more than five
percent of the class of securities. [CONFIRM]


                                       3


Schedule 13G                                                         Page 4 of 4

Item 6.

Other persons have the right to receive and/or the power to direct the receipt
of dividends from, or the proceeds from the sale of, such securities.

Item 7.

Not applicable. [CONFIRM]

Item 8.

Not applicable. [CONFIRM]

Item 9.

Not applicable. [CONFIRM]

Item 10.

By signing below, I certify that, to the best of my knowledge and belief, the
securities referred to above were acquired in the ordinary course of business
and were not acquired for the purpose of and do not have the effect of changing
or influencing the control of the issuer of such securities and were not
acquired in connection with or as a participant in any transaction having that
purpose or effect.

                                    SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.

                                        January 11, 2005                        
                                        Date


                                        /s/ Kenneth G. Mertz II                 
                                        ----------------------------------------
                                        Signature


                                        Kenneth G. Mertz II / President
                                        ----------------------------------------
                                        Name/Title


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