TM 1-1500-204-23-3 4 Inspect for lack of adhesion by applying air at a maximum of 100 psi with an air gun placed approximately 1/2 inch from the sealant.  If loose sealant is found, cut through the sealant and strip by pulling away from the structure. Strip  sealant  until  it  breaks  rather  than  pulls  away  from  the  tank  structure.    Take  extreme  care  not  to  damage  the structure or the corrosion control coating. 5 Check for loss of luster, discoloration, chalking or loss of topcoat. 6 Inspect for loss  of  elasticity  by  firmly  pressing  sealant  with  a  blunt  metal  punch  of  not  less  than  3/16  inch diameter.  The sealant is good if it gives and returns to its original position; it is defective if the sealant breaks and holds its pressed position. 7 Inspect for loose, cracked, or missing fasteners. NOTE Visible  defects  in  sealant  or  the  structure  are  not  necessarily  the  source  of  a  true  leak. Continue visual inspection until the entire suspected leak area has been carefully inspected. Mark all defects. (b) Cells.  Inspect fuel cells for wear and damage.  Refer to tables 2-5 and 2-6 for serviceable limits. (3) Maintenance.  Maintenance for fuel tanks and cells includes inspection for damage and leak testing.  Tank sumps must be drained prior to flight to remove any contaminants.  Fuel tank strainers must be cleaned and inspected on a  regular  basis.    Fuel  tank  pumps  must  be  replaced  as  required.    All  repairs  should  be  made  in  accordance  with  the applicable aircraft maintenance manuals. (4) Purging.  Fuel cells may be purged and preserved by either of the following methods.  The method to be used will be determined by the availability of preserving oil, equipment, number of aircraft involved, cost of preserving agent, availability of C02 or nitrogen gas, time, manpower, and skills available to perform the operation. (a) Primary method.  If an adequate supply of lubricating oil, MIL-L-6081, grade 1010, and the necessary defueling equipment is available, purge and preserve the fuel cells as follows: WARNING · The aircraft and all equipment used in performing the purging operation must be properly grounded.  This includes defueling equipment, work stands, purging equipment, and any powered  or  pneumatic  devices.    Ungrounded  equipment  may  produce  static  electricity which can ignite fuel vapors. · Work stands shall be equipped with a personnel static discharge plate of copper or zinc. The  plate  shall  be  attached  in  such  a  position  so  that  personnel  can  contact  the  plate before coming in contact with the aircraft.  High static electrical charges are created by the contact  and  separation  of  unlike  substances,  or  by  any  sort  of  motion  of  persons  or material.  These charges are a constant source of danger when generated in the presence of fuels or flammable vapors. WARNING Fuel  tanks  shall  not  be  drained  near  the  end  of  the  working  day  and  then  allowed  to  stand empty overnight.  This action could make a perfect set of conditions for producing explosive vapors.    The  critical  fuel-air  ratio  could  develop  a  residue  fuel  drains  down  the  sides  of  the tank, forms puddles, and evaporates into the air of the tank. 1 Drain fuel cells. 2 The flashpoint of empty fuel cells may be reduced by pouring approximately five gallons of lubricating oil, MIL-L-6081, grade 1010, into each cell. NOTE A larger amount of flushing oil may be required to flush large fuel cells which are installed in the  aircraft  or  multiple  cells  with  one  fuel  opening  and  interconnecting  fuel  lines.    The important thing to remember is that the bottom of each fuel cell must be flushed.  Reduction of  the  flashpoint  in  purging  operations  will  reduce  the  amount  of  lubricating  oil  necessary when an assembly line operation is set up. 2-19