Tetraethyl lead (TEL) is added to fuel to increase its octane or performance rating.
b. Tricresyl Phosphate (TCP) is added to prevent spark plug fouling. Only those speciic piston engine aircraft
showing excessive continuous spark plug lead fouling may be serviced with TCP additive fuels.
(1) TCP is stored and issued under NSN 6810-00-597-5775, supplied in 200 ml (cc) cans, and is a Federal
Speciication TT-T-656, Type III product. Each 200 ml can of type III will treat one 55-gallon drum of aviation gasoline.
Do not use Tricresyl Phosphate, Federal Speciication TT-T-656. The concentration of phosphorus
in TT-T-656 is not correct for use in A VGAS.
(2) TCP transforms lead deposits to phosphorus compounds, thus altering the electrical and temperature
characteristics of the deposits which reduce spark plug fouling and preignition. TCP only helps relieve spark plug
lead fouling in reciprocating engines using highly leaded fuels, especially in those engines operating on a lean mix-
ture. It does not help when an engine is operating under rich fuel mixture conditions. TCP will not help in the
resolution of problems involving engine power loss, excessive oil pumping, carburetor icing, valve-sticking, or the
elimination of deposits. TCP is not a cure-all.
(3) The following procedures will be used to blend TCP with AVGAS. To treat a full 55 gallon drum of AVGAS,
pour the entire contents of one 200 ml can of MIL-T-9188 type III TCP into the drum. The drum then will be rolled
a minimum of 100 feet or agitated long enough to insure thorough mixing. Under no circumstances will TCP be
introduced directly into aircraft fuel tanks because of the possibility of improper blending.
The majority of problems resulting from the use of TCP can be traced to incomplete mixing with fuel.
(4) The use of ine wire platinum spark plugs and TCP is helpful in lessening the problems caused by the
use of leaded AVGAS.
TCP is a toxic chemical. If TCP comes in contact with the skin, affected areas should be washed
immediately with soap and water. Precautions should be observed to prevent inhalation of TCP