#B #Advisory 

USDepartment . 

of Tronsportotm 

Federal Aviatbn Circular - 

Mminirtfation 

Subject: USE OF AUTOMOBILE GASOLINE IN 

AGRICULTURAL AIRCRAFT 

Date: 8/24/M 

Initiated by: Ap+2c)O 

AC No: 21-21 

Change: 

1 l PURPOSE. This advisory circular (AC) sets forth acceptable amditims 

under which autmbile gasoline (autogas) may be used in restricted category 

agricultural aircraft mered by Pratt and Whitney R-985 and R-1340 radial . 

engines, and being used in agricultural operations under Federal Aviation 

Regulations (FAR) Part 137. 

2 CANCELLATION l AC 21-8, Aircraft Airworthiness: Restricted Category: 

Cirtification of Aircraft with Uncertificated or Altered Engines or Propellers, 

‘dated May 21 I 1969, is canceled. 

3 REFERENCES l FAR 21.25; Civil Air Regulations (CAR) Part 8; Civil 

Akonautics Manual (CAM) 8.10; and-AC 20033B, Technical Information Regarding 

Civil Aeronautics Manuals 1, 3, 4a, 4b, 5, 5, 7, 8, 9, 13, and 14. - 

4 BACKGROUND. Under current FA4 policy in AC 20033B and CAM 8.10-4(d) ( 2) I it 

ii permissible to use uncertificated engines in restricted category agricultural 

aircraft, under operating limitations in addition to those prescribed in * 

FAR 91.39. The use of autogas in a type certificated engine, without FAA 

approval, 

it; 

trlould be an unapproved major alteration of the type design, rendering 

in effect, to be decertificated. This AC provides guidance and amdit ions 

,acceptable to the FAA under which restricted category agricultural aircraft may 

be operated If autogas is to be used in certain type &tificated engines .&at 

may be installed in such aircraft. . 

5 l APPLICAESILITY. The provisions of this AC apply only to restricted category 

aircraft equipped with Pratt and Whitney R-985 or R-1340 engines that are used 

in agricultural aircraft operations under FAR Part 137. 

6 PROCEDURES. Any person operating an aircraft that meets the applicability 

provision of paragraph 5 of this AC may use autogas for operation of that 

aircraft under the following mnditions: 

a. An FAA Form 337, Major Repair and Alteration, must be mmpleted for each 

aircraft, including identification of the pxerplant in. Item 4 of the form, 

and indicating “alteration” in Item 5. 

s b On the reverse side of the FAA ,Form 337, in Item 8, the followinq 

entrik must be made, with each of the blank spaces filled in as appropriate: 
AC 21-21 0/24/84 

"The procedures and mdif ications mvered by this FAA Fbrm 337 

have been found acceptable to the Administrator in Advisory 

Circular 21-21, Use of Autombile Gasoline in Agricultural 

Aircraft, for the purrpose of compliance with FAK43.13. The 

aircraft and engine identified in Items 1 and 4 on this form 

mav be approved for return to service by a person authorized 

by&FAR 43.7 when the following. renditions have been met: 

A. The following placard is displayed on the instrument panel: 

NOIKE: THIS AIRCRAFT WY BE OPERATEDWITH AUIWUI'IVE \ \ 

. FUEL OR MIXTURES OF Au?wJK)TIVE AND AVIATIaJ FUEL. 

B. The following placards are displayed in the m&pit in clear 

view of the pilot: 

THIS AIRUWFT IS NOT ELIGIBLE FOR AN APPEUVAL 

FOR OPERATIm CWER =ESTED AREAS UNDER FAR 137.51 

OR FOR A WIVER OF ANY LIMITATIONS IN FAR 91,39(d). 

ENGINE SERIAL NUMBER INSTALLED IN THIS 

AIRCRAFT AND OPERATED WITH AUTOMOTIVE TYPE FUEL IS 

NOT ELIGIBLE FDR INSTALLATION IN AN AIRCRAFT HAVING 

A STANDARD AIRKUU'HINESS CERTIFICA?E UNTIL IT HAS 

BEEN SUEUE~X'ED '.I0 A mJOR OVERHAUL. 

C. The following placard is permanently affixed in the specific& 

section of the engine logbook(s): + \ 

THIS ENGINE, SERIAL NUMBER t HAS BEEN 

OPERATED WITH AUTOMOTIVE m FUEL AND IS NOT 

ELIGIBLE FOR INSWLLATION IN A NORMAL C4TEZUW 

AIKRAFT HAVING A SI'JWDARD AIRbKXWINESS CERTIFICATE 

UNTIL IT HAS BEm SURJECYED ?o A M?UOR OVERHAUL. 

D. A placard is added at or near the fuel filler aver to read:- 

AUTO FUEL - 87 OCTANE MI:I!WM. 

E. The autmtive fuel used does not mntain alcohdl additives. 

F. The following certification mxt be ompleted by a clertif icated 

pilot properly rated for the aircraft (reference FAR 137.19): 

I certify this aircraft/engine mmbination has been flight 

checked through all anticipated agricultural tm maneuvers 

using 87 octane (R+M/2 method) automotive fuel and the 

engine has performed without evidence of malfunctioning and 

within the limitations specified in the w Certificate 

Data Sheets: Aircraft TC !W. ; Engine TC No'. t -- 

Pilot Name Certificate No. . l 

-- 

. Par 6 
% A copy of this FAA F&m 337 must be affixed in a prominent 

pia= in the logbook for the aircraft and in the l-book for the 

engine, in addition to the maintenance record entriks required 

hy FAR 43.9. 

H. This approval will be invalidated if the engine identified in 

Item 4’is removed from the aircraft and replaced with another engine. 

If the replacement engine is to be operated with autogas, conditions 

A through G must be Et for the replacement engine and a new FAA 

Form 337 must be ampleted.” 

Director of Aimorthiness 

Par 6