B737-800 crew reports a flight control B low pressure light illuminated in cruise and QRH procedure ...

Narrative:

Flight control B low pressure light illuminated in cruise. Followed QRH procedure and placed flight control B switch to stby rud. B system quantity and pressure were normal. While autopilot a was engaged at the time; there were no apparent flight control issues; although there was increased oscillation in yaw. The procedure offers no additional steps or information. When we placed the switch in stby rud; the feel diff press; stby rud on and yaw damp lights illuminated; which seemed appropriate for the procedure but were not mentioned in the QRH. I consulted dispatch and maintenance control. Maintenance control advised that flight control operation would be normal; however we would not have use of 2 flight spoilers. This is not mentioned in the QRH. Based upon this information and the increased yaw oscillation; we declared an emergency and diverted. With the anticipation of possible extra braking being needed; we requested arff meet the aircraft. Landed without incident. When arff advised that the brakes appeared normal; we terminated the emergency and taxied to the gate without further incident.

Original NASA ASRS Text

Title: B737-800 crew reports a Flight Control B Low Pressure Light illuminated in cruise and QRH procedure required placing Flight Control B Switch to STBY RUD. After consulting with Maintenance Control crew elects to divert to an airport with a longer runway.

Narrative: Flight Control B Low Pressure Light illuminated in cruise. Followed QRH procedure and placed Flight Control B Switch to STBY RUD. B system quantity and pressure were normal. While autopilot A was engaged at the time; there were no apparent flight control issues; although there was increased oscillation in yaw. The procedure offers no additional steps or information. When we placed the switch in STBY RUD; the Feel Diff Press; Stby Rud On and Yaw Damp lights illuminated; which seemed appropriate for the procedure but were not mentioned in the QRH. I consulted Dispatch and Maintenance Control. Maintenance Control advised that flight control operation would be normal; however we would not have use of 2 flight spoilers. This is not mentioned in the QRH. Based upon this information and the increased yaw oscillation; we declared an emergency and diverted. With the anticipation of possible extra braking being needed; we requested ARFF meet the aircraft. Landed without incident. When ARFF advised that the brakes appeared normal; we terminated the emergency and taxied to the gate without further incident.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1054344
Time
Date	201212
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Component
Aircraft Component	Aeroplane Flight Control
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Events
Anomaly	Aircraft Equipment Problem Less Severe

B737-800 crew reports a flight control B low pressure light illuminated in cruise and QRH procedure required placing flight control B switch to stby rud. After consulting with maintenance control crew elects to divert to an airport with a longer runway.