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Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Close up photos of internal and external components Illustrated history and description of all variants of 737 Databases and reports of all the major 737 accidents & incidents History and Development of the Boeing 737 - MAX General flightdeck views of each generation of 737's Technical presentations of 737 systems by Chris Brady Detailed tech specs of every series of 737 A collection of my favourite photographs that I have taken of or from the 737 Press reports of orders and deliveries Details about 737 production methods A compilation of links to other sites with useful 737 content Study notes and technical information A compilation of links to major 737 news stories with a downloadable archive A quick concise overview of the pages on this site



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*** Updated 14 Nov 2021 ***

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Flight Controls - Roll and Speedbrakes

This is the first in of a 3-part series on Boeing 737 Flight Controls. This one covers roll control and also speedbrakes. Contents:Ailerons, Flight Spoilers & Speedbrakes, Ground Spoilers, Aileron - Spoiler Mechanics, Jams etc, Aileron – Spoiler Interaction, MAX Fly-By-Wire Spoilers.

Ailerons are powered by hydraulic systems A and/or B. If both hyd should fail, manual reversion is available from both control wheels. If the aileron system jams, the co-pilots wheel can be used to move the spoilers (hydraulically). There are balance tabs and balance panels on both ailerons.

The maximum roll control deflection which the autopilot can achieve is, by design, about 4.5° of aileron movement (a control wheel movement of about 20°), depending on the stimulus. The maximum aileron movement which can be achieved by the pilot is about 20°, which requires a control wheel input of about 82°. (Ref AAIB Bulletin No: 2/97)

Aileron trim moves the neutral position via the feel & centering mechanism. There are two aileron trim switches to prevent spurious electrical signals from applying trim. The fwd switch is for direction, the aft switch is simply an earth return. Use of aileron trim with the autopilot engaged is prohibited because of excessive roll when the a/p is disconnected.


The above series of photographs (737-300) show how the flight spoilers move with various combinations of aileron and speed brake. With speedbrake down, the spoiler simply rises on the down-going wing with aileron. With speedbrake applied, not only do the spoilers on the down-going wing rise but also the spoilers on the up-going wing fall. Notice that even with full speedbrake applied the spoilers still rise on the downgoing wing.

This property of the spoilers on both wings to respond to roll inputs is known as differential spoilers. It only occurs when speedbrake is used which is why the roll rate is increased when speedbrake is used. Boeing recommend that speedbrake is not used below 1000 feet for this reason.

NB In the bottom two photographs the speedbrake lever was only at the flight detent position but because the aircraft was on the ground the ground spoilers deployed. This is why if you have any sort of technical problem that might be due to a faulty air-ground sensor eg QRH "Gear Lever Will Not Move Up After Takeoff" you must not use speedbrake in case the ground spoilers deploy in-flight. You can see from the series of photographs just how much extra drag ground spoilers will give over flight spoilers.


737-400 Flight and ground spoiler deployment on landing


Spoilers / Speedbrakes

Flight spoilers (2 -1/2/3/4/500; 4 -NG's) augment the ailerons and are powered by hydraulic system A (inboard) & B (outboard). Spoilers will continue to operate with speedbrake deployed.

Ground spoilers are also from hydraulic system A.

Only the outboard flight spoilers are powered by hydraulic sys B

On landing, if armed, all spoilers will deploy when the thrust levers are at idle and any two wheels have spun up or right gear is compressed. If not armed, the speedbrakes will deploy when reverse thrust is selected.

Do not use speedbrakes in-flight if you have had to manually over-ride the gear to retract it. Because with the thrust levers closed, the ground spoilers will deploy as well.

The SPEEDBRAKE DO NOT ARM light in-flight does not preclude its use on the ground.  

There is no restriction on the speed at which the speedbrakes may be used (on airtests they are deployed at 360/M0.84); however the resultant position is a function of airspeed due to blowdown.

The SPEED BRAKE TEST buttons are only found on -1/200's and old -300's. They are used in conjunction with the speedbrake lever and antiskid system to illuminate the SPEED BRAKE ARMED and/or SPEED BRAKE DO NOT ARM lights (located next to the fuel gauges).

Click here for 737MAX FBW spoiler system

The above photo (-3/4/500 series) shows how the ground spoilers move more than the flight spoilers and appear to have just two deflection angles. On the 737-NG the story is a little more complex and spoiler movement varies as follows:

Speedbrake lever angle Detent Flight Spoiler Position Ground Spoiler Position
    Nos. 2,3,10&11 / 4,5,8 &9 Nos. 6&7 / 1&12
0 DOWN 0 0
4 ARMED 0 0
5-8 - Start to move 0
29 - Between 0 and 20/22.5 52 / 60
35.5 FLIGHT 19.5 / 24.5 52 / 60
48 UP (no detent) 33 / 38 52 / 60




The rudder is moved by a PCU powered by hyd sys A and/or B. If A and/or B fails a standby PCU can be powered from the standby hydraulic system. There is no manual reversion for the rudder.

From Jan 2003 (l/n 1268) all 737's were delivered with the enhanced rudder control system. This can be recognised in the flight deck by a new STBY RUD ON light in the STANDBY HYD column (see below).

For an in-depth explanation of the rudder system click here.


Yaw Damper

The 737 is positively damped in combined lateral-directional oscillations, which in plain English means that if you set up a Dutch Roll the aircraft will gradually stop oscillating. So the yaw damper is not required for dispatch, however it is fitted for passenger comfort. It is powered by hydraulic system B.

The 1/200 series had a dual yaw damper system because at the design stage, from experience of the 707 and 727, it was expected that the 737 would not be so naturally, positively yaw damped. So two were fitted to allow dispatch in case one failed. As it happened, none were required. The 1/200's have a yaw damper test switch to the right of the indicator. Note that aircraft with the RSEP installed, the yaw damper test switch is inoperative (shown right).

The NG's saw a return to a dual system by having a standby yaw damper, note that only the main yaw damper inputs are shown on the indicator.

The yaw damper system can move the rudder a maximum of 2 degrees (-1/200), 3 deg (-3/4/500), 2 deg (NG flap up), 3 deg (NG flap down), either side of the trimmed position. Yaw damper inputs are not fed back into the rudder pedals, which is why there is an indicator.  However from April 2010 the indicator will not be fitted to new aircraft.


737-1/200 Rudder trim knob

737-3/400 old blade style rudder trim knob

Rudder Trim

The rudder trim knobs have evolved over the years. The -1/200 series had the large knob from 707 days when there was more hand flying and asymmetric thrust before the days of autothrottle.

The 737-300 featured a new smaller blade shaped knob but this was changed to the present round knob following the crash of a US Air 737-400 which overran La Guardia with full left rudder trim set (NTSB/AAR-90/03). It was believed that a jump seat occupant may have inadvertently moved the knob with his foot when it was resting on the aft electronic panel. A raised shield was also fitted around the aft end of this panel to prevent inadvertent movement of the trim controls.

See this SAIB issued by the FAA following an incident to a 737 over Japan following the inadvertent application of rudder trim.



The control column moves the elevators using hyd A and/or B. If both hyd should fail, manual reversion is available from both control columns. If the elevator system jams, the stabilizer (trim) should still be available. There are balance tabs and balance panels on both elevators.

Notice how the camber of the stabiliser is like an inverted aerofoil, demonstrating that the tailplane produces a downforce.

The FEEL DIFF PRESS light will illuminate if either hydraulic system or an elevator pitot fails. The elevator feel system will continue to work on the remaining hydraulic system.

Stab Trim

Pitch trim is applied to the stabilizer. Trim can be applied by electric trim switches, autopilot or a manual trim wheel. Electric and autopilot trim may be disengaged by cutout switches on the control stand in the event of a runaway or other malfunction (See here for details of this procedure).

Moving the control column in the opposite direction to electric trim will stop the trim, unless the STAB TRIM switch is set to OVERRIDE. This function could be used to control the pitch of the aircraft with trim say in the event of a jammed elevator.

The trim authority varies according to aircraft series and method of trim. The full range is only available with the manual trim wheel, but if at an extreme setting, electric trim can be used to return to the normal range. There are two electric trim switches on each control column, the right is for the direction and the left is an earth return for protection against spurious electrical signals.

The STAB TRIM light was only fitted to the 1/200 series.

Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas, pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition.

Mach trim is automatically applied above M0.615 (Classics onwards), M0.715 (-1/200) to the elevators. This provides speed stability against Mach Tuck; i.e. as Mach increases, the centre of pressure moves aft and the nose of the aircraft tends to drop.

Maneuvering Characteristics Augmentation System (MAX only) will apply nose down stabilizer trim during high AoA flight when the flaps are up and the A/P is not engaged. See MCAS page for more details following the two MAX accidents.

Elevator Jam Landing Assist (MAX only). This will give limited changes to the vertical flight path from the spoilers to assist the approach and landing if the normal elevator system jams. The control panel is located on the Aft Overhead Panel, even if it is switched on it will only be active when the flaps are 1 or greater. When in use, the spoilers rise to a preset position; they then extend or retract as the elevator column is pushed or pulled to increase or decrease the rate of descent.

Trivia: On the 737-100 to 500, the stab trim control wheels should be mounted with their white marks 90 +/- 15 degrees apart from the other wheel, so that in the event that the trim wheel handles need to be used, one handle will be in an accessible position (AMM 27-41-64, Page 402). This is not so for the 737-NG (27-41-61, Page 401).

Leading Edge Devices

Are comprised of 4 Krueger flaps inboard of the engines and 6 slats outboard of the engines. The LE flaps are extended whenever the TE flaps are not up. The slats extend from 1 to 5 and fully extend when beyond 5. Slat numbers 1 & 6 (the outboard slats) move a few degrees less than slats 2 to 5 when at full extend, causing the leading edge to look slightly disjointed in this configuration, this is normal.

The NG's have an extra outboard slat on each wing giving 8 in total. They have the same sequencing as the classics.




737-200Adv / Classic LED Panel

On the early -1/200 series, only outboard slats (1 & 6) had a full extend position; this is used when the TE flaps are beyond 25. The -200Adv has the same arrangement as the classics

737-1/200 basic LED Panel

Notice how only the outboard slats go to full extend.

The normal power for the LEDs is hydraulic system B (System A on the -1/200). If this fails the LEDs may be extended but not retracted with the standby hydraulic system using the ALTERNATE FLAPS switch on the flight controls panel. Note this will drive the LED's to the FULL EXTEND position only.

The autoslat system will fully extend the slats for stall protection whenever flap is selected and the slats are not already at the full extend position (ie flap 1 to 5). If system B pressure is lost, system A can pressurise system B fluid for autoslat via the PTU. The LE FLAP TRANS light is inhibited during autoslat operation.

737-200Adv Slats

737 NG Slats


Each slat has:

1) A single hydraulic ram actuator, which is normally powered by hyd system B. The actuator is powered (to the full extend position only) by the standby hydraulic system when the ALTERNATE FLAPS control is selected to DOWN.

2) Two main tracks (outer) and two auxiliary tracks (inner) to help hold the slats in the three positions.

3) A bleed air duct (brown) for wing anti-ice and exhaust holes in the underside of the slat. Note NG's do not have wing anti-ice on their outboard slats.


Note the LE flaps only have actuators, ie no tracks or anti-ice.

Side view of partially extended number 1 Krueger flap on a 737-300


There are four LE Krueger flaps, two inboard of each engine. They are either fully extended or retracted (the photo shows a flap in transit). When the flap is retracted, the folding nose section rotates and is stored under the wing.

The LE flaps do not have anti-ice.

737-200 Basic Krueger Flaps

737-200 Adv Krueger Flaps


The 737-200Adv incorporated major wing improvements over the basic -200, including new leading edge flap sequencing and an extension of the inboard Krueger flaps from the landing lights to the fuselage. This remains just about the only way to tell a 737-200 from a 737-200Adv externally.


The NG's have three small aerodynamic fences (vortilons) on the underside of the leading edge slats to restrict the spanwise flow of air.


LE Vortilon

The new NG flight controls panel

737-1/200 flight controls panel

From Jan 2003 (l/n 1268) all 737's were delivered with the Rudder System Enhancement Program (RSEP) installed. This can be recognised in the flight deck by a new STBY RUD ON light in the STANDBY HYD column. This indicates that the standby rudder PCU is pressurised. It may be pressurised either automatically (loss or system A or B etc) or manually (through the FLT CONTROL switches).


Trailing Edge Flaps

Are triple slotted (-3/4/500) / double slotted (-NG) and are normally powered by hyd sys B.

With an asymmetry condition on the NGs, the flaps/slats electronics unit (FSEU) will detect an asymmetry at the flap position transmitters at a level which is almost undetectable by eye. In any series of 737, if an asymmetry is detected, hydraulic power is removed. The NG FSEU will also detect a flap skew, if detected the flap position indicators will display a 15 degree split.

If hydraulic system B is lost, flaps can be moved electrically with ALTERNATE FLAPS. There is no asymmetry protection with alt flaps and the LE flaps and slats can be extended but not retracted. The duty cycle limitations are for a complete extension and retraction e.g. if flaps moved from 0 to 15 and back to 0, then must wait 5mins before using alternate flap again.

A flap load limiter (-3/4/500) / flaps/slats electronics unit (-NG) will automatically retract the flaps from 40 to 30  (-3/4/500) / also 30 to 25 (-NG) if the limit speed is exceeded. The flaps will extend again when speed is reduced. This feature is on all aircraft even though the FLAP LOAD RELIEF light is only fitted to a few.

The basic 737-1/200's also had a green LE FLAPS FULL EXT light

.Flap Position Indicator

Trivia: Although the flap placard limit speeds are different for each 737NG variant, the structural limit speed for the flaps is equal to the placard speeds (175k F30, 162k F40) for the heaviest variant (737-800/900). The Flap Load Relief trigger speeds (176k F30, 163k F40) are set to allow all variants to fly to the structural limit speed without system activation. Setting lower flap placard speeds for the 600 and 700 variants allows for greater service life of flap components due to the larger margins to the structural design speed.

737 TE Flap Positions


Triple slotted trailing edge flaps at the 40 position on a 737-400.


Flap track fairings and outboard landing light on a 737-500.

Short-field Performance Enhancement Program

The Short Field Performance improvement package was developed in 2005/6 to allow GOL airlines to operate their 737-800s into the 1,465m (4,800ft) Santos Dumont airport. The modifications enable weight increases of approx 4,700kg (10,000lbs) for landing and 1,700kg (3,750lbs) for take-off from short runways. It includes the following changes:

  • Flight spoilers are capable of 60 degree deflection on touchdown by addition of increased stroke actuators. This compares to the current 33/38 degrees and reduces stopping distances by improving braking capability.
  • Slats are sealed for take-off to flap position 15 (compared to the current 10) to allow the wing to generate more lift at lower rotation angles.
  • Slats only travel to Full Ext when TE flaps are beyond 25 (compared to the current 5). Autoslat function available from flap 1 to 25.
  • Flap load relief function active from flap 10 or greater.
  • Two-position tailskid that extends an extra 127mm (5ins) for landing protection. This allows greater angles of attack to be safely flown thereby reducing Vref and hence landing distance.
  • Main gear camber (splay) reduced by 1 degree to increase uniformity of braking across all MLG tyres.
  • Reduction of engine idle-thrust delay time from 5s to 2s to shorten landing roll.
  • FMC & FCC software revisions.

The SFP package has now become an option on all 737-800s (known as 737-800SFPs) and standard on the 737-900ER. Some of the features may also be fitted to the 600/700 series. The first SFP was delivered 31st June 2006. To date over 250 aircraft have been ordered with this package as either factory build or retrofit.


A trailing edge flap modification from AeroTech has been available for the 737-2/3/4/500 series since 2004. The modification slightly extends and lowers the aft segments of the trailing edge flaps thereby increasing wing area, camber and importantly lift-to-drag ratio. The original modification had one fixed position for the aft flap segments. However it is now adjustable/tunable to fit a wide spectrum of flight operations. The cruise angle of attack is reduced by approximately 0.5 degree which reduces induced drag. There are no changes to operational procedures and every pilot that has flown this mod (FAA, test pilots, and line pilots) says that the aircraft handles better. Fuel savings vary with series and route structure but may average up to 4.3%.



Speed limit If only 1 leading edge device remains extended.


Speed limit If more than 1 leading edge device remains extended.


Do not deploy speedbrakes in flight at radio altitudes less than: 1000ft
Holding in icing conditions with flaps extended is prohibited.
Do not use speedbrake when flaps are beyond 15.
Max flap extension altitude 20,000ft

This is taken from the Boeing Airliner magazine:

"Several operators have asked Boeing why the Airplane Flight Manual has a limitation restricting the use of flaps above 20,000 feet. The reason for the limitation is simple; Boeing does not demonstrate or test (and therefore does not certify) airplanes for operations with flaps extended above 20,000 feet.

There is no Boeing procedure that requires the use of flaps above 20,000 feet. Since flaps are intended to be used during the takeoff and approach/land phases of flight, and since Boeing is not aware of any airports where operation would require the use of flaps above 20,000 feet, there is no need to certify the airplane in this configuration."

Wing Design

OK the wing is not strictly a flight control but since plenty of readers want to know about the wing here goes. (All dimensions in meters)

  Originals Classics NG's
Span 28.35 28.88 34.32
Gross Area (m) 102.00 105.4 124.58
Aspect Ratio 8.83 8.99 9.45
Taper Ratio 0.339 0.240 0.159
Root Chord (basic) 4.71 4.71 7.877
Mean Aerodynamic Chord 3.80 3.73 3.96
Tip Chord 1.60 1.13 1.251
Thickness/chord - Root 14.00 14.00 ?
Thickness/chord - Average 12.89 12.89  ?
Thickness/chord - Tip 11.50 ?  ?
Dihedral () 6 6 6
Incidence at root () 1 1 1
Chord Sweep () 25.02 25.02 25.02


The wings are an aluminium alloy, dual-path, fail-safe, two-spar structure. Shear loads are carried by the front and rear spars, bending loads are carried by the upper and lower skin panels. These four surfaces form the wing box which also serves as an integral fuel tank. The spars are reinforced by vertical stiffeners and the skin panels are reinforced by spanwise "Z" or "J" section stringers.

The original wing was designed to be short to keep weight to a minimum, but be large enough to carry fuel for the designed range. It also had to have a short chord to accommodate the engine which was to be flush mounted rather than on a pylon because of ground clearance. It also had much less sweep than its predecessors (25º compared to 35º on the 727) because speed was not considered important for a short range jet. This is why we plod along the airways at M0.74 classics, M0.78 NG's.

Having the engines wing-mounted also helped alleviate the bending moment of the wings from the lift, which allowed the spar weights to be reduced. Unfortunately this was initially overdone as a prototype managed to buckle the rear wing spar during high speed tests at 34% above normal operational loads. The spars were strengthened and this was incorporated into all production aircraft.

The basic wing remained the same on all 1/200's until the 3/4/500's which had 0.2m wingtip extensions and a slightly modified aerofoil section (taken from the 757/767) for the LE slats. This gave a 4% improvement in maximum L/D, a 0.02 Mach increase at maximum L/D, and a 3% reduction in block fuel at 1500nm range. The new slats went from engine pylon to wingtip and gave an average chord increase of 4% over the whole wing and gave the classics similar approach speeds to the originals. The wingtip extension came about when a flutter boom was being designed to eliminate flutter between the wing and the new powerplant & pylons. As it happened the boom was not needed but the wingtip extension was retained and with the new slats gave the classics an extra 4000ft altitude capability.

The 737-1/2/3/4/500 aerofoil is made up of four specially designed Boeing aerofoil sections splined into one smooth surface as follows:

Short field take-off performance was achieved by Krueger flaps and slats. Short field landing performance came from the triple slotted flaps. Both of these gave the 737 good low speed handling and allowed it to operate into many fields that had not previously been able to take jets.

The NG wing has a whole new airfoil section, 25% increase in area, 107" semi-span increase, 17" chord increase, raked wing-tip and a larger inspar wingbox with machined ribs. Approach speeds have been further reduced and the altitude capability increased by another 4000ft to FL410.

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