Figure 2-6.  Ground-based inspections include verification that “Xs” on the instrument display are displayed until the sensor activates. 2-5 deck reference guide is in the aircraft and assessable, system  driven removal of “Xs” over engine indicators, pitot/static  and attitude displays, testing of low level alarms, annunciator  panels, setting of fuel levels, and verification that the avionics  cooling fans, if equipped, are functional.  [Figure 2-6]  The  AFM/POH specifies how these preflight inspections are to  take place. Since an advanced avionics aircraft preflight  checklist may be extensive, pilots should allow extra time for  these aircraft to ensure that all items are properly addressed. Generally, the AFM/POH specifies a sequence for the pilot  to inspect the aircraft which may sequence from the cabin  entry access opening and then in a counterclockwise direction  until the aircraft has been completely inspected. Besides the  AFM/POH preflight assessment, the pilot must also develop  awareness for potential areas of concern, such as signs of  deterioration or distortion of the structure, whether metal  or composite, as well as loose or missing rivets or screws. Besides all items specified in the AFM/POH that must  be inspected, the pilot should also develop an awareness  for critical areas, such as spar lines, wing, horizontal, and  vertical attach points including wing struts and landing gear  attachment areas. The airplane skin should be inspected in  these areas as load-related stresses are concentrated along  spar lines and attach points. Spar lines are lateral rivet lines  that extend from one side of the wing to the other, horizontal  stabilizer, or vertical stabilizer. Pilots should pay close  attention to spar lines looking for distortion, ripples, bubbles,  dents, creases, or waves as any structural deformity may be  an indication of internal damage or failure. Inspect around  rivet heads looking for cracked paint or a black-oxide film  that forms when a rivet works free in its hole.  [Figure 2-7] Additional areas that should be scrutinized are the leading  edges of the wing, horizontal and vertical stabilizer. These  areas may be impact damaged by rocks, ice, birds, and or  Figure 2-7.  Example of rivet heads where black oxide film has  formed due to the rivet becoming loose in its hole.  Outer Wing Surfaces and Tail Section XPDR  UTC 19:24:36 GPS TERM E 6500 OAT 0°C ADF/DME MSG NAV1  NAV2  COM1 COM2 KDRO KFMN  DIS 16.3 NM BRG 212° GPS AP ALT 7680FT HDG TAS A I R S P E D F A I L A L T I T U D E F A I L V E R T S P E D F A I L ATTITUDE FAIL CRS 212° HDG 220° 0 D195I D212I A212I HDG UP 10 NM