B737 Flaps Asymmetry

[avioniker]

On the NG aircraft:
Asymmetry Detection: The FSEU compares the data from the left and right TE flap position transmitters to monitor the position of the TE flaps. If they are different by more than 9 resolver degrees, there is an asymmetry condition. If the TE flaps are in an asymmetry condition, the FSEU sends electrical power to the bypass valve. This stops the hydraulic operation of the TE flaps. During an asymmetry condition, the needles on the flap position indicator show the actual data from the flap position transmitters.

The difference is very small and can not be seen by looking at the flaps or the indicator. The lockout criteria is roughly 3/4th revolution difference on the torque tubes as compared between left and right wings and the transmitters have an 800:1 turn ratio.

If one flap segment doesn't extend symetrically with the corresponding flap segment on the opposite wing there are sensors at each end of each flap segment (8 total) which will cause the flap skew sensing section of the FSEU to shut down the system and it will cause the flap needles to indicate a 15deg difference between left and right flaps. Once again you won't be able to see a difference just looking at the flaps.

There is one other time when the indicator will indicate a 15 deg split. That is when the exitation voltage to the position transmitters is lost. The system will once again be locked out in this situation because an assymetry can not be detected.

Any other needle split is an indication malfunction and the flap system should shut down as in an assymetry fault.

The system is so sensitive because the flaps have been redesigned on the NG aircraft. They are considered high lift before drag generating devices whereas on the previous versions of the 737 they were considered to have a higher drag element than lift. For roll trim problems the flaps are the first thing looked at in 9 out of 10 instances.

This is a somewhat lengthy but still simplified explanation. I hope it's adequate...