ATA 27 Flight Controls

For activities in the lower nose compartment (Fwd pressure bulkhead) the white stabilo cable drum dripshield is often removed. It is mounted with a clamp to the front and two screws (hi-lock type) to the back. If the shield is removed the two screws stay behind in the frame, together with two harex blocks. They are mounted from above therefore it is easy to leave them this way. But because there are activities going on in this tight space it can happen that the screws are pushed up, where they will stay. Now the head of such a screw comes inside the stabilo-drum. The consequence is that if the stabilo wheel is moved by hand, the drum hits that screwhead with damage as a result. If the stabilo movement is done with the electric trimmotor, the damage will be much bigger. The cable drum then has to be replaced. And this could all be prevented if you remove those two screws. So next time remove the dripshield including the mounting screws!

2 - LE slat hydraulic lock reset procedure. 737-3/4/500

After a period in which a plane has been parked for a longer time without hydraulic pressure with slats and leading edge flaps in the extend position, it is possible that the LE flap/slat system is hydraulically locked by means of a closed retract fuse. This is a known phenomenon, causes by pressure differences in the actuators resulting in a leakage of hydraulic fluid along sticky seals. As a solution Boeing indicated that the fuse could be reset after waiting a while (according to the task 10 minutes, in reality frequently much longer), whereupon the LE flaps could be extended with stby power and then be retracted in the normal way. It is also possible to reset the fuse by loosening the lines on the fuse. On the boeing flight line, where the problem also frequently occurs, they have a much faster and very simple method for the fuse reset. THE TE flaps must be, after removing all hydraulic power, electrically cycled, which results in the resetting of the LE fuse through the leading edge control valve.

Release by alternate cycle:

Step by step:

1 - Switch off hydraulic systems A and B

2 - Disarm the hydraulic standby systems by pulling c.b's P6-11: stdby hyd pump normal and P6-12: stbdy hyd pump alt.

3 - Switch ALT FLAPS switch in ARM position

4 - Make a complete flap cycle with the toggle switch (pay attention to maximum operating time of the elec. motor)

5 - Switch ALT FLAPS switch in OFF position

6 - Push in c.b’s stby pump norm. and alt.

7 - Switch on hydraulic system A and B. (Note: the flaps position must agree with the flap handle position.)

8 - Make a complete flap cycle, using normal hydraulic power

9 - If after these steps a slat is still stucked, repeat the whole procedure.

3 - Extend/retract leading edges without moving trailing edge flaps. 737-3/4/500

During maintenance it is easy to move the leading edges (Slats and Kreuger flaps) without moving TE flaps (which is often not possible due to removed components). There is a standard procedure in the AMM (27-81-00) to de-activate the leading edges in the UP (retracted) position. This must always be carried out if this is necessary, e.g. for the engines, to work with thrust reversers opened. It is sometimes necessary to move the leading edges to EXTEND (halfway) or FULL EXTEND (entirely down).

This can be done by moving the LE control valve piston in the flap control box together with hydraulic pressure B available. However, the valve must be disconnected from the input rod, by removal of the attachment bolt.

-If the valve is moved fully in (inboard in the direction of the wheel well) then the leading edges will move to FULL UP (this is the de-activated situation as described in the AMM).

-If the valve is moved out (direction outboard) then the leading edges will move to FULL EXTEND.

-If the valve is moved halfway then the SLATS go to EXTEND. The Kreuger Flaps will fold out, since they have 2 positions only, up or entirely down.

Movement of the leading edges is now independent of the trailing edge flaps and the position of the flap-handle in the cockpit. Pay attention on the position of the flap-handle when switching on the B-pump! Always look to the INBOARD thrust reverser doors, if they are down.

4 - Flight Control & Spoiler Shutoff Valves: Close and check them! 737-ALL

If you are working with any of the flight controls, it is always better to switch OFF the 2 flight control switches and 2 flight spoiler switches in the cockpit on the P5 panel. Because of this all flight controls and flight-spoilers will be without hydraulic pressure even when hydraulic pressure A and/or B is present , by means of closing the shutoffvalves in the wheel well. For activities on the tail the 6 valves in the tail-compartment can be closed also, for extra security. This is possible only at -3/4/500 and NOT at -6/7/8/900!

However convince yourself always if the shutoff valves in the wheel well are closed indeed. Moving the switches on the P5 panel to OFF gives you no guarantee that the valves really close. This is because frequently you don’t look at the circuitbreakers of the valves. If these are pulled then nothing happens, and it gives you a false feeling of being safe. And if the de-activation-tools are placed on the valves, it is difficult to see in what position they are. It happened they where “de-activated” in OPEN position instead of closed ! Therefore always look in the wheel well to see the position of the valves . See pictures.

Open : the RED lever is at position 1 (to the RIGHT)

Closed: the RED lever is at position 2 (to the LEFT )

5 - Outboard flap tracks (1-2-7-8) fwd fitting must have play. 737-3/4/500

During inspections it appears that the fwd attach fitting of the outboard flaptracks (No. 1-2-7-8) are frequently declared unserviceable due to play. This play MUST be there and is even with new installed bushings a minimum of 0.018 "up to maximum 0.021". During an inspection the play can be as much as 0.029" max. which is 0.74 mm! And therefore the flaptrack can "rattle" on the front: No panic! See the limits in AMM 27-51-49 page 603 and 604 and figure below item 6 (bushing/bushing).

6 - Elevator mechanical control path inspection: where to look at. 737-6/7/8/900

Elevator Mechanical Control Path inspection A. Task 27-31-61-210-801: Explanation where to look at:

(1) Open the Forward Access Door, 112A to get access to the elevator mechanical control path.

(2) Do a general visual inspection of the forward elevator mechanical control path.

Look at: Control columns connection tube and fwd control quadrants.

(3) Close the Forward Access Door, 112A,

(4) Remove the Tailcone Access Door, 318BR to get access to the elevator mechanical control path.

(5) Do a general visual inspection of the aft elevator mechanical control path.

Look at: Aft control quadrants, input torque tube, output torque tube, control pushrods.

(6) Install the Tailcone Access Door, 318BR.

There is a seperate inspection card for the ctrl cables and for the single-element-dual-load path components.

7 - Inboard flaptrack coating damage, more information. 737-6/7/8/900

Visual inspection of the flap track can detect the loss of coating. One way to measure the depth of wear is using the intact area of coating as a baseline. Additionally, the CMM limits for the worn surfaces can be used as a general guideline for in-service wear limits made while the track is on the airplane. However, wear beyond the CMM limits should be reported to the aircraft manufacturer but does not necessarily mean that the track needs be removed from the airplane. Further information can be found in SL 737-SL-57-081 and Fleet Team Digest 737NG-FTD-57-01005. NOTE: The subject flap tracks do not require greasing of the roller surfaces. See illustrations for location. The inboard flaptracks can be replaced without removing the inboard flaps, with 2 man. Install a sling on the flap to make it weightless, then replace the track and afterwards check the skew sensor- and flap torquetube adjustments acc amm.

9 - Vibrating flight spoiler 737-3/4/500

During testing of a replaced aileron trim actuator we discovered a rumbling/vibrating flight spoiler #2.

The cause was play on one of the bearings located on the spoiler housing. This play was causing an output signal to the input lever of this spoiler actuator. Input and output signals are counteracting each other which results in this abnormal vibration of the flight spoiler. The solution to this is to replace the spoiler actuator or the bearings inside it. See picture.

11 - Elevator freeplay check procedure: difference between classic and next gen. 737-ALL

The elevator freeplay check on the 737 classic and 737 next gen looks almost the same in the manuals, even the freeplay limits are the same. But be aware that when you do this check on the next gen, you read the manual very carefully. On the next gen, when you have to push down on the elevator, you must first release it, and then make a mark for the freeplay. In all other cases you make a mark while pushing it up or down.

For 737 classic see amm 27-09-91 page 601 ff.

For 737 next gen see task 27-31-32-200-804.

See picture below for clarification.

12 - Manual movement of leading edge flaps/slats with an external pump. 737-3/4/500

When the aircraft has no electrical / hydraulic power available due to maintenance, it is still possible to move the leading edge flaps and slats fully up or down. This could be necessary for slat-adjustments or when the engine t/r's have to be opened.

All you need is an external (hand)pump filled with hydraulic fluid and 2 hoses.

The leading edge connections are shown in the first picture. The second picture shows how to connect the external pump on these connections in the wing root.

To extend all LE devices DOWN, put the pressure on the STANDBY PRESS line. To retract all LE devices UP, put the pressure on the PRESS line. It may be necessary to loosen the two lines in between (intermediate and full down lines) a little, to prevent a hydraulic lock.

(For Next Generation 737, see item 17 in this chapter).

13 - Flap alternate drive motor installation in any flap position. 737-3/4/500

When removing and installing a flap alternate drive electrical motor, the manual tells you to put the flap in the up position. See AMM 27-51-59. But often it is not possible to put the flaps in up position due to other maintenance activities, no elec/hydraulic power available etc.

You can remove and install the flap alternate drive motor in ANY flap position because this motor has no internal control switches. The control is regulated by the flap switches in the flap control unit.

14 - Outboard flap assy sling installation attachment points, a possible solution. 737-6/7/8/900

When removing an outboard flap assy on the next generation, the maintenance manual gives you another picture for the sling attachment points than the picture in the illustrated tool and equipment manual.

Apart from the different sizes between the sling attachment bolts and the attachment bolts in the flap, it can be confusing and causing delays in maintenance when it is not clear what method to use and what size bolts to use. Until this is rectified by the aircraft manufacturer, you can use the attachment points pictured below. They proved to be satisfactory during removal and installation of the flap assy. Of course you use this tip at your own risk, but using common sense at this moment is better then waiting and gambling between the two different pictures from the official manuals. By the way, the aircraft manufacturer will be informed asap about this, if not already done.

possible solution on next 3 pictures below:

15 - Speedbrake handle pointer usage errors; a simple tip but often used faulty. 737-ALL

The speedbrake handle is often set in a position (flight detent or up) where the metal notch on the side of the handle is used as an indicator/pointer. This is not the correct pointer, but it is very easy to make this mistake since the real pointer, the arrow on the backside of the handle, is hardly visible when moving the handle while the person is seated in one of the cockpit seats. It leads to complaints from pilots (!) and engineers, that the handle cannot be moved to fully up position, or that it does not move to full up during autospeedbrake movement after touchdown. This tip seems to be too easy, but this error happens so many times that a notification on this website is on its place.

See the pictures for clarifications.

16 - Stabilizer not moveable because of strong tailwinds. 737-ALL

BACKGROUND: The crew started the After Landing checklist when the aircraft was parked
and the engines stopped. During the After Landing check, the crew moved the horizontal
stabilizer to the "Aircraft Nose Down" (AND) position. When the stabilizer came to 4 units, it
stopped (correctly) and the Stabilizer Trim Motor could not move up or down electrically. The
problem could not be solved by telephone so Technicians flew to the station. They started the
hydraulic pumps and set the FLAP to the NOT UP position. Then the Stabilizer Trim Motor
operated correctly.

NOTE: The movement of the Stabilizer Trim Motor depends on the Flap position.
When the Flap is:
UP - Stabilizer Trim movement is approximately 4 -17 units.
NOT UP - Stabilizer Trim movement is approximately 0 -17 units.

PROBLEM: When the stabilizer moved up (AND), a strong tailwind also moved the elevator
up (Aircraft Nose Up) (ANU). This caused the column cutout switches to disconnect the
electrical power to the Stabilizer Trim Motor.

WHAT HAPPENED: The Elevator Control Column was in the ANU position and the stabilizer
was commanded to the AND position. . Because the two commands oppose each other, the
column cutout switches disconnected the Stabilizer Trim Motor.

ACTION 1: If you think that this problem has occurred,
Start the EMDP, system A or B
The Elevator Control Column moves to the neutral position
Set the FLAP to the NOT UP position
Use the Stabilizer Trim Switches to move the Stabilizer up and down.

ACTION 2: Also, to see if the Stabilizer Trim motor operates correctly, set the STAB TRIM
OVERRIDE switch to OVERRIDE (This bypasses the column cutout switches). Use the
Stabilizer Trim Switches to move the Stabilizer up and down.

REFER TO: SDS 27-41

NOTE: This event was on a B737-6/7/8/900 but could even occur on the B737-3/4/500
because the system is similar. The B737-3/4/500 has only one control cutout switch.

17- Manual movement of leading edge flaps/slats with an external pump. 737-6/7/8/900

All you need is an external (hand)pump filled with hydraulic fluid and 2 hoses.

The pictures show you how to connect the external pump on which connection in the wing-root.

These connections are found under panel 511 AT (LH wing) and 611 AT (RH wing).

(For 737 Classic, see item 12 in this chapter)

18 - Temporary solution for heavy aileron input forces. 737-ALL

When there are pilot complaints of heavy aileron input forces, or when the aileron steering wheel does not come back to neutral very smoothly, often the bearings inside the LH aileron input quadrants in the wheel well are dirty or corroded and may be saturated with moisture, causing them to freeze-up in the air.

The only permanent solution is to replace the complete input assy.

But as there are not always spare parts, you can temporarily fix this problem by lubricating the bearings with some WD40 or a similar product. Then cover the bearings with some grease 16 or 7 to prevent ingestion of moisture and dirt. This action will almost certainly make the steering input light and smoothly for a short period of time, just enough to plan the a/c for a maintenance visit to replace the parts.

Of course do this according to your own company policies and rules.

See both pictures of these input quadrants and the spots where to lubricate them.

19 - Speedbrake light ON with flaps down on winglet aircraft. 737-6/7/8/900

When the speed brake light comes on when flaps are lowered on winglet aircraft, we found that the autostow actuator (in the pedestal behind the lower DU, not below the floor) is faulty or not rigged.

This is applicable for NG's with winglets prior to line number 778----- From 778 and on no autostow actuator required. Note: The auto stow actuator is also installed on all 737-700's.

Thanks to someone from SWA.

See picture below for the location of this actuator:

20 - Aileron and rudder trim out of limits? Check this first. 737-ALL

When pilots complain about rudder trim or aileron trim needed to keep the a/c level (more than 1 unit), first check the cable tension of the aileron wing cables ABSA and ABSB in LH and RH wing. Very often the cable tension of these cables is far too low, causing a sloppy aileron (control) due to the low cable tensions. The cable tensions can drop as far as 50% from the required level ! Especially with flaps not up this can cause trim problems. Make sure you adjust the aileron positions and cable tensions exactly iaw amm:

737 Next Gen: task 27-11-00-820-801

737 classic: task 27-11-00-825-001

Also check the neutral position of the rudder compared with the rudder trim-indication in the cockpit. Often these positions are not exactly matched, causing a false rudder trim complaint. ( Of course the rudder system must be rigged exactly to zero iaw the amm. ) But it happens that the trim indicator has some deviations compared with the real rudder trim position. A quick and simple procedure is to adjust the zero-indication of the trim indicator on the bottom of the indicator-module.

After checking and adjusting the aileron cable tensions and the rudder trim indication, the complaints are often solved.

21 - Slat remains extended due to actuator malfunction. 737-6/7/8/900

When a slat remains extended after a retract command is given, it is possible that the affected actuator is failing due to internal leakage. See also 737NG-FTD-27-02007 and picture below as a nice example.

22 - Inboard aft flap actuating mechanism bellcranks chafed. 737-6/7/8/900

During 8C-checks on NG we have several cases of chafed bellcranks on inboard aft flap actuating mechanism. Description and pictures below:

On the B-737 NG you can find damaged bellcranks on aft flap actuating mechanism in INBD main flap during internal visual inspection. Damage is caused by the aft flap actuating mechanism control cable fork. Repairing of bellcranks rests in damage removal with 1 inch radius blend ratio and with 63 um roughness, then you must measure remain of bellcrank material thickness, when is there found thickness less than 6,223mm you must report bellcrank to the Boeing. After measuring perform NDT with HFEC and do the flap peening on reworked area, then apply surface treatment. Against next chafing between fork and bellcrank you must insert teflon washers to the bellcrank.

Little tip to bellcranks removal – when you loose the control cable, count turn of turnbuckle and write down to the paper tape attached to the turnbuckle. When you install bellcranks back to the flap you can tight the turnbuckle by the same amount of turn and then only check the adjusting of aft flap actuating mechanism and cables tension. Thanks to Josef from Czech Republic.

23 - Spoiler mixer bearing wear due to sand contamination. 737-6/7/8/900

Due to repetitive speed brake actuator malfunctioning (found burns) on 737NG of

our fleet, and also a hardness of speed brake mechanism, an inspection has

been carried out of the spoiler mechanism, which lead us to incriminate the

spoiler mixer, therefore it has been sent to the workshop, the inspection

finds that:

1. A sand quantity inside the spoiler mixer (our fleet often operates in

the desert area's).

2. Due to the sand the bearing (A) shown in pictures, was

found jammed therfore causing a frictional wear against its housing as shown

in picture (3).

3. Auto speed brake actuator malfunctioning is the consequence of the hardness

of speed brake mechanism.

Many thanks for Makhloufi for this input.

24 - Flap track fairing support loose rivets. 737-3/4/500

During T/E OUTB FLAP components inspection on B-737 3/4/500 you can find loose rivets on FLAP TRACK FAIRING FWD supports. It is not visible at first look, you can discover this irregularity by pressing upper plate opposite to the lower plate and check the gap between plate and web or you can find that the rivets have play. Some photos have been attached to explain this. Thanks to Josef, Czech Republic.

25 - Slat track replacement fast procedure. 737-6/7/8/900

When you have to replace a LE slat track, for any reason, you don't find the procedure in the AMM. The AMM states only the removal/installation of a complete slat assy (including tracks). The track installation on the slat is stated in the CMM of the slat assy itself. So a seperate track change is a combination of both. This procedure describes a handy and short way to replace a track, without removing the slat from the wing. See CMM 57-43-02 for the slat / track assy installation procedure.

1-extend the slats to FULL extend.

2-open the LE panels around the track to be replaced (to reach both sides of the endstop on the track)

3-remove the bolt/nut which hold the track on the slat.

4-slide the track upwards into the "UP" position.

5-remove the small composite panel under the track attachment lug on the slat, by removing 4 hilocks.

6-slide the track downwards and remove the endstops from the rear end of the track, note the correct position of the adjustable endstops!

7-remove the attachment bolt from the slat ACTUATOR rod end from the slat.

8-lift the slat by hand OUT and UP, while holding the loose track.

9-now you can slide the track underneath the slat out of its position.

10-slide the new track (lightly lubricated) into its position, while you still hold the slat up.

11-re-assemble all parts in the following step order > 7 - 6 - 4 - 5 - 3 - 2

12-lubricate the track and rollers iaw AMM chapter 12.

See pictures for clarification of some of the steps:

26 - Balancing flight controls after a paint job. 737-ALL

For balancing flight controls on/off the aircraft, a nice document is written to understand more about this complex situation you may get into when painting an aircraft or flight control.

Read this document in English here. (also available in Dutch ).

Also very helpful: Flight control balance weight calculator, to calculate balance weights after a paint job.

27 - Elevator feel and centering unit broken springs. 737-6/7/8/900

When the pilot reports lower than normal elevator feel forces in the control column, the very first thing to check are the centering springs inside the elevator feel and centering unit. Inside this unit are 2 springs, a smaller inside a larger one. The large spring, but sometimes also the small inner one, might be broken. This gives a low elevator feel for the pilot. They might report it as "sloppy elevator" or so.

Its not unusual according the aircraft manufacturer. See FTD 737NG-FTD-27-05004, (MBF account needed) and CMM 27-31-95 Item 400. See picture of the location of the springs in the tail compartment.

28 - Elevator tab control mechanism inspection info. 737-6/7/8/900

The aircraft manufacturer has recently received a report from an operator where the failure of the aft attach lugs on the left elevator tab control mechanism resulted in unwanted elevator freeplay vibration during flight. Investigation revealed that the fractured aft attach lugs on the elevator tab control mechanism allowed freeplay of the aft end of the uncontrolled motion of the tab control
mechanism, which allowed freeplay of the forward end of the elevator tab control rods.

This message shows photos of the location of the tab control mechanism attach lugs and also shows the parts to inspect. The inspection should be accomplished per the ref Maintenance Task Cards. One addition to the inspection procedure is to add a step to include a „finger feel check“ of the lug bearing spacer.

The first SB 737-27A1296 states to perform detailed inspections of the bearings in the elevator control tab mechanism aft attach lugs. Look for gaps and loose parts. If there are gaps or loose parts, replace the elevator control tab mechanism.

The second SB 737-27A1296 does a more enhanced inspection than the inspection in SB 737-27A1296 and continues with repeat inspections until the elevator tab control mechanism is replaced. The more enhanced inspection includes using a feeler gage at specific locations to check for gaps. The inspections in this SB will detect a loose bearing in the aft attach lugs of the elevator tab control mechanism. The pictures below are for information and extra awareness of this inspection. Also a failed mechanism is shown on one of the pictures.

29 - Cable tensioning tool. 737-ALL

Whenever new aileron bus cables are installed, they need to be double-tensioned first; about 300 lbs (Classic) and over 400 lbs (NGs)- temperature variations aside. It can be quite difficult to rotate the turnbarrel whilst maintaining anti-rotation control of the two cables at each end.

The tool is made from a MS21914-6 hydraulic plumbing 'B' nut. A slot is cut down the side to allow the nut to fit onto the cable and a hole drilled to match the size of hole in the turnbarrel.

The nut is simply fitted onto the cable, slid onto the turnbarrel and then secured with a split pin, drill shank, etc. A 11/16" open-end spanner is used on the nut.
The same principal can be used for use on spoiler cable turnbarrels, using smaller nuts.

I wouldn't be surprised if some tool maker made a mass-produced version. But there you go...most of us just want to keep it simple and help others, with no intention of commercial gain! Thanks to Andreas from Australia for this input !

Click on the pictures to see a large version of them.

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