G12036
500  Flap Alternate Drive System Circuit 
Aug 15/67  Figure 10  27-51-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

D.  An electrical asymmetry system will stop operation of the flap drive system when a certain separation of left and right indicator pointers occurs. The separation closes a comparator switch in the indicator which operates the flap asymmetry relay, causing the hydraulic bypass valve to stop operation of the flap drive hydraulic motor.
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TRAILING EDGE FLAP SYSTEM - TROUBLE SHOOTING
1.  General
A.  Flap Drive System Stalling/Lockout In -Flight Problems
(1)  
In order to minimize occurrence of flap drive system stalling or lockouts in service, proper maintenance procedures, proper flap operating procedures, and system pilot reporting procedures are required. Flap components should be kept as clean as possible and lubricated at regular intervals to minimize friction. Drive system loads are greatly increased by excessive airspeed since loads vary as the velocity squared and overplacard speeds will always cause lockout. Therefore, flaps should be operated at speeds recommended in the 737 Pilots' Training Manual. Pilots reporting a flap drive lockout should record the following information to assist trouble shooting: (1) airspeed, (2) extension or retraction,

(3) flap setting, (4) whether flaps can be positioned electrically, (5) whether flaps can be reversed hydraulically, and (6) note any lateral trim change during lockout.

(2)  
Some component failures and jamming can increase drivescrew load but require multiple failures to increase loads into the lockout range. Other components are especially effective in causing lockout due to system geometry and transmission torque brake settings. The following items can cause lockout when acting alone (i.e., no other components failed or jammed):

(a)  
Power unit gearbox, angle gearbox, or right angle gearbox jam

(b)  
Flap drive transmission jam

(c)  
Flap drive ball nut failure

(d)  
Structural failure and/or interference

(e)  
Number 2, 3, 4, 5, 6 or 7 midflap carriage aft or forward flight roller jam

(f)  
Transmission output bearing jam at any location

(g)  
Flap transmission torque brake setting too low

(h)
Asymmetry shutoff due to drive system shearout or misrig of asymmetry system

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Oct 20/90  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  27-51-0 Page 101 

NOTE:  Flap jackscrews and carriages are identified from left to right across the wing. For example, flap jackscrew No. 1 is outboard screw of left outboard flap.
(3)  
The flap drive system is most susceptible to lockouts in the flaps 30 to 40 range where the highest airloads exist. If the lockout occurs in the flaps 0 to 25 range structural interference, drive gearing failure, jammed transmissions, foreign object damage or hydraulic system failure are the most probable causes. If a lockout occurs, proceed to check the airplane according to the trouble shooting chart under the trouble headings, "Flaps will not extend hydraulically or electrically in flaps 0 to 25 range inflight," and "Flaps will not extend hydraulically or electrically in flaps 25 to 40 range inflight."

(4)  
The items are listed in the trouble shooting chart in descending order of effectiveness for causing lockout but also weighted in order of probability of occurrence. If airline experience shows that a particular item is a chronic problem, that item should be checked first.

(5)  
If the flaps fail to extend hydraulically but will extend at the same airspeed electrically, the hydraulic system should be checked.

(6)  
Icing conditions during cold weather operation can cause lockout if procedures for operating during these conditions are not followed. See Cold Weather Maintenance in Chapter 12.

(7)  
If a lateral trim change is noted when lockout occurs, the probable cause may be failure in the inboard or outboard aft flap mechanisms, a drive failure which causes the flap asymmetry shutoff system to function, or an unsequenced foreflap.

B.  For trouble items not related to flap drive system inflight lockout, refer to appropriate trouble headings in trouble shooting chart.
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