A3. The time required for a fuze to detonate after impact or a preset time is known as the functioning time. A4. The distance along the trajectory that a bomb travels   from   the   releasing   aircraft   in   an unarmed  condition  is  known  as  the  safe  air travel (SAF). A5. The  two  basic  classes  of  fuzes  are  electrical and mechanical. A6. The force used to initiate the mechanical fuze is like the hammer and primer used to fire a rifle. A mechanical force drives a striker into a sensitive detonator. A7. An  electrical  impulse  initiates  an  electrical fuze. A8. The three special safety features incorporated into fuzes are detonator safe, shear safe, and delay arming features. ARMING DELAY TIMES.—Arming delay times are inscribed into the face of the forward nose retaining ring. A white indexing line is scribed on the knurled delay setting knob below the arming vane. The white indexing line must be matched to one of the indicated arming times to select the desired arming delay. To select the required arming delay time, depress the setting index locking pin and rotate the knurled arming delay setting knob until the white indexing line is aligned with the desired arming delay time stamped on the nose retaining ring. The 2- and 4-second arming times are for use with retarded weapons, and are only set by removing the stop screw located next to the setting index locking pin.  Never try to reinstall the stop screw when either of these two settings are used. The stop screw may be reinstalled at any delay setting of 6 seconds or more. IDENTIFICATION OF ARMED FUZES.— There  are  three  conditions  of  the  M904  fuze—safe, partially  armed,  and  fully  armed.  You  can  verify  the fuze conditions by looking through the two observation windows in the fuze body (fig. 1-1). To check the fuze condition, hold the fuze vertically and look through the windows perpendicular to the fuze body. Look at table 1-2.  It  shows  you  what  you  would  see  through  the observation windows of the M904E3/4 fuze at various time settings and fuze conditions. Also, check the M904E4 to make sure the thermal sleeve  is  firmly  bonded  to  the  fuze  collar  and  is  not cracked. NOTE:   If the safe condition of any fuze is in doubt,   explosive   ordnance   disposal   (EOD) personnel should be notified immediately. 1-5 Condition Time Setting Upper Window Lower Window Safe 18 Seconds White number "18" on green background. Vacant or dark in color. 6 Seconds White  number  "6"  on green background. Partially Armed 18 and 6 Seconds Green background with no numbers visible. (If   numbers appear   at   other   than "18"    or    "6"    second setting  or  if  numbers do  not  match  settings, fuze is partially armed.) Vacant or dark in color. Armed Any setting.   (Time setting    cannot    be changed.) *Red with black letter "A." (Some    green may   show   at   top   of window.) *Red with black Letter "A." Table 1-2.—Indications for Determining Conditions of M904E3/4 Nose Fuzes