fire can be put out with a typical carbon dioxide (CO$_{2}$) fire  extinguisher. Aim the extinguisher nozzle at the base of the  flame and spray in a sweeping motion to have the agent  fall over the flames to displace the oxygen and smother the  fire. Dry chemical fire extinguishers rated for fuel can also  be used. These leave behind a residue that requires cleanup  that can be extensive and expensive. Do not use a water-type  extinguisher. Fuel is lighter than water and could be spread  without being extinguished. Additional precautions used to  prevent fire are discussed below in the fueling–defueling  section of this chapter. Maintaining aircraft fuel systems in acceptable condition to  deliver clean fuel to the engine(s) is a major safety factor in  14-50 This checklist must be completed prior to start of wet fuel cell entry and/or at shift change PRIOR to work assignment for the  continuation of tank work started by a previous shift. Wet fuel cell entry location Figure 14-92.  Fuel tank checklist entry.  Area or building:  Stall:  Airplane:  Tank: Shift: Date: Supervisor: 1. Airplane and adjacent equipment properly grounded. 2. Area secured and warning signs positioned. 3. Boost pump switches off and circuit breakers pulled and placarded. 4. No power on airplane: battery disconnected, external power cord disconnected from airplane, and external power  recepacle placarded. 5. Radio and radar equipment off (see separation distance requirements). 6. Only approved explosion-proof equipment and tools will be used for fuel cell entry (lights, blowers, pressure and test  equipment, etc.). 7. Ensure requirements listed on aircraft confined space entry permit are complied with, including appropriate personal  protective equipment: OSH class 110 respirator at a minimum, approved coveralls, cotton cap and foot coverings, and eye protection. 8. Trained attendant and confined space logsheet required for all wet fuel cell entries. 9. Aerators checked for cleanliness prior to use. 10. Sponges available for residual fuel mop out. 11. All plugs in use have streamers attached. 12. Mechanical ventilation (venturis or blowers) installed to ventilate all open fuel cells. Note: Ventilation system must remain in operation at all times while fuel cells are open. If ventilation system fails or any ill effects, such as dizziness, irriation, or excessive odors, are noted, all work shall stop and fuel cells must be evacuated. 13. Shop personnel entering cells and standby observers have current “fuel cell entry” certification cards. Certification requires the following training: • Aircraft confined space entry safety • Respirator use and maintenance • Wet fuel cell entry 14. Fire department notified. Meter Reading 15. Oxygen reading (%):  By: 16. Fuel vapor level reading (ppm):  By: 17. Combustible gas meter (LEL) reading:  By (FD): I confirm that all entry requirements were met prior to any entry. Signature of supervisor or designee  Date Fuel System Servicing