Incident: Sunwing B738 near Dublin on Sep 7th 2012, stab trim failure (http://avherald.com/h?article=455b8af4&opt=0)
Could someone explain in simple english what happened as its no clear to me from the article.
Callsign Kilo
14th Sep 2012, 11:58
There are two electical trim circuits on the 737. One for the main trim (pilot controlled) and one for the autopilot. Both are connected to the stabiliser. As the aircraft changes attitude and speed the stabiliser will change position, largely in order to maintain the effectiveness of the elevator which helps maintain the aircraft's pitch. Typically a pilot will do this through sensing 'feel' or feedback to the column. Every time a pitch, thrust or speed change is made, the stabiliser should be retrimmed. With the autopilot engaged, trimming of the stabiliser is automatic. This crew would have recognised that the autopilot wasn't trimming the stabiliser, most likely from the illumination of the 'STAB OUT OF TRIM' Light and exercised the Non Normal Checklist /QRH. One of the initial steps is to disengage the autopilot (to remove the autopilot trim circuit from the stab) and hand control to the main trim circuit via pilot input. Failing this, both electric trim circuits are disengaged via the 'STAB TRIM CUT OUT SWITCHES' Itis then the responsibility of the crew to trim the aircraft manually via the cable system attached to the trim wheels running to the stabiliser.
The PAN was most likely called due to the problem being related to the flight controls and due to the increased workload experienced by the crew. The crew would have most likely experienced such a scenario in the simulator at some stage. A pretty good job done by all accounts.
Hope this helps?