B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 193
Aircraft Equipment Problem Less Severe
Flight Crew Became Reoriented; General Maintenance Action
Aircraft
While flying at cruise altitude at night; the Pilots noticed that the standby attitude gyro was showing 15 degrees left wing down; while the two primary attitude displays were showing 'wings level.'  As the flight proceeded; the standby attitude gyro precession in pitch (20 degrees nose up and down) and precession in roll (15 degrees left and right). From the first time the Pilots noticed the standby attitude gyro behaving abnormally until we arrived at the gate (approximately 45 minutes); the 'orange Gyro Failure flag' never appeared to warn the Pilots of the failure.  The Captain/Maintenance made appropriate logbook entries.  Maintenance removed and replaced the standby attitude gyro; and the next flight was conducted without any problems with the standby attitude gyro.  Tonight we were able to reference the two primary attitude displays and outside ground cues in order to maintain the proper aircraft attitude with ease.  However; in the unlikely event the two primary attitude references failed; AND the standby attitude gyro failed without the proper 'orange Gyro Failure flag(s);' the Pilots would not know for certain which attitude references were providing valid information and which ones were not providing valid information.  Is the B-737-700 standby attitude gyro designed to display an 'orange Gyro Failure flag' when gyro fails? If not; it would certainly be of great assistance during night; IMC flight with multiple attitude display failures.
B737-700 Captain experiences a standby attitude indicator failure in cruise without displaying any failure flags.