Flight Controls

How are flight controls connected to the power control units (PCUs)?

  • Pilot’s control wheels are connected via cables to the elevator, ailerons (CPTs control wheel) and spoilers (FOs control wheel on the 737-800) PCUs
  • The rudder pedals are connected via cables to main and standby rudder PCUs
  • The PCUs are powered by hydraulic systems A and B and move the primary flight control surfaces
  • Standby rudder PCU is powered by the standby hydraulic system in case of loss of system A or B
  • In case of loss of system A and B elevator and ailerons are controlled with manual reversion (cannot be controlled by the Standby hydraulic system) and rudder by the standby hydraulic system (cannot be controlled with manual reversion)

Note:

  • On the 737-8 / 737-8200 spoilers are fly-by-wire (i.e. FOs control wheel is not connected by cables to the spoiler PCU)
  • However, the Spoiler Control Electronic Unit receives signal to control the flight spoilers from the FOs control wheel

Which are the only fly by wire flight controls on the 737-8 / 737-8200?

  • Spoilers

How many flight spoilers on each wing?

  • 4

How many ground spoilers on each wing?

  • 2

How many leading edge flaps on each wing?

  • 2 Krueger leading edge flaps on the inboard part of each engine

How many leading edge slats on each wing?

  • 4 leading edge slats on the outboard part of each engine

When do flight spoilers automatically extend during the turn to assist ailerons in roll control?

  • When control wheel is moved by approximately more than 10° (approximately more than 1.6 units of aileron displacement)

How is roll control provided in case ailerons jam?

  • Through FOs control wheel which moves the spoilers
  • The CPTs control wheel does not move

How is roll control provided in case spoilers jam?

  • Through CPTs control wheel which moves the ailerons
  • The FOs control wheel does not move

How is pitch controlled if elevator jams?

  • Applying force on the control wheel will either clear the jam or activate a break out mechanism which separates the control columns and allows the non-jammed control column to control pitch (significantly higher forces required to control pitch – reduces total elevator travel but remains sufficient for landing)
  • Use stabilizer trim to help elevator in pitch control

Note:

  • With control wheel jammed, you may not be able to trim
  • As per NNC jammed or restricted flight controls, you put the stab trim override switch to override to allow trimming with a jammed control wheel (only checklist that requires you to use this switch)

When does the Mach trim system operate?

  • Automatically above M .615
  • Adjusts automatically elevator position to counteract the nose down pitching tendency aircraft experiences at high Mach numbers

How is stabilizer trim operated?

  • Electrically (transfer bus 2)
  • Can also be controlled manually through the manual trim handles

Note:

  • If needed to trim manually, make sure stab trim cutout switches are put to cutout first

Additional info:

  • With flaps extended trim moves 3 times faster with AP engaged (0.27 units / sec compared to 0.09 units / sec with flaps up) and 2 times faster with AP disconnected (0.4 units / sec compared to 0.2 units / sec)
  • To trim electrically, you need to press both trim switches of the control wheel
  • This is to provide redundancy against a runaway stabilizer condition

When will the FEEL DIFF PRESS light illuminate?

  • Significant hydraulic pressure difference (e.g. loss of system A or loss of system B)

Or

  • Significant air pressure difference (e.g. blocked elevator pitot tube)

When will the SPEED TRIM FAIL light illuminate?

  • Speed trim system has failed (on the 737-8 / 737-8200 light illuminates in case of failure of either the speed trim or the MCAS function)

Or

  • When illuminated on recall, it indicates a single FCC channel failure

When will the MACH TRIM FAIL light illuminate?

  • Mach trim system has failed or
  • When illuminated on recall, it indicates a single FCC channel failure

When will the STAB OUT OF TRIM light illuminate?

  • When the autopilot is not trimming the stabilizer correctly (can only illuminate with the autopilot engaged)

Note:

  • This light is inhibited during dual channel approaches below 800 ft RA. In case stabilizer is out of trim, the AP red warning light illuminates steady
  • On the 737-8 / 737-8200 light will also illuminate on the ground when groundspeed is below 30 kt if one FCC failed inflight (i.e. activation of the Cross FCC Trim Monitor system)

Additional info:

  • Cross FCC Trim Monitor system (737-8 / 737-8200) prevents against runaway stabilizer with single AP engaged and during manual flight
  • Activation of the Cross FCC Trim Monitor system will cause the related side AP to disengage if it was engaged (the other AP remains available and trim input remain available from the unaffected FCC)
  • SPEED TRIM FAIL light may illuminate or illuminate on recall

What is the function of the stab trim cutout switches?

  • Normal position: prevent trim inputs in the opposite direction of the control wheel (i.e. cannot trim nose down while pitching up and vice versa)
  • Cutout position: stops electric trim inputs

What is the function of the STAB TRIM OVERRIDE switch?

  • It allows trimming in the opposite direction of the control wheel (i.e. bypasses the function of the stab trim cutout switches in normal position)

Note:

  • The only checklist that would require you to use this switch is the jammed or restricted flight controls nnc (i.e. jammed elevator)

When will the spoilers extend automatically on landing? (With speedbrakes armed)

  • Both thrust levers in IDLE
  • Radio altitude < 10 ft for the 737-800 or Radio altitude < 6 ft for the 737-8 / 737-8200
  • Main landing gear wheels spin up > 60 kt
  • Any landing gear compresses (flight spoilers extend)
  • Ground spoilers extend when the right main landing gear compresses on the 737-800 or when both main landing gears compress on the 737-8 / 737-8200

When will the spoilers automatically extend in case of rejected takeoff? (With autobrakes set to RTO)

  • When thrust levers are moved to IDLE
  • Main landing gear wheel speed > 60 kt
  • Reverse thrust is selected

Note:

  • Although spoilers extend automatically when reverse thrust is selected and main landing gear wheel speed > 60 kt, speedbrakes should be manually selected prior to selecting reverse thrust

This is for two main reasons:

  • Reduces distance required for RTO and
  • Engines need to spool down first before selecting reverse thrust. If reverse thrust is selected immediately after closing the thrust and engines haven’t managed to spool down yet this may result in exceeding the thrust reverser design limits

What is the maximum position to use the speedbrakes in flight?

  • FLIGHT DETENT

Note:

  • FLIGHT DETENT position is achieved when the black triangle at the bottom of the speedbrake lever is aligned with the FLIGHT DETENT markings
  • Up position is used on the ground only to extend flight spoilers and ground spoilers to maximum position (if speedbrakes are armed, speedbrake lever moves automatically to UP position upon touchdown provided conditions mentioned above are met)

When will the SPEEDBRAKES EXTENDED light illuminate?

Inflight:

  • When speedbrake lever is beyond ARMED position and trailing edge flaps are more than 10 or radio altitude is less than 800 ft

On ground:

  • When speedbrake lever is in the DOWN position and ground spoilers are not stowed

Note:

  • On the 737-8 / 737-8200 it also illuminates if speedbrakes are extended and thrust is above idle for 15 seconds or thrust lever angle near go around thrust for 3 seconds

Which are the positions of the LE slats?

  • Up
  • Extended – TE flaps 1-5 on non-SFP aircraft or TE flaps 1-25 on SFP aircraft (737-800 SFP and 737-8 / 737-8200)
  • Fully extended – TE flaps 10-40 on non-SFP aircraft or TE flaps 30-40 on SFP aircraft (737-800 SFP and 737-8 / 737-8200)

Which are the positions of the LE flaps?

  • Up
  • Fully extended (TE flaps 1-40)

What is the reason for the flap gates at Flaps 1 and Flaps 15?

  • This is because for go around with 2 engines (and the landing flaps set to 30 or 40) the flaps are initially retracted to 15 whereas for go around with 1 engine inoperative (and the landing flaps set to 15) the flaps are initially retracted to 1 and the gates prevent inadvertent retraction to lower flap settings

[Updated] What is the function of FSEU? (Flap Slat Electronic Unit)

  • Provides flap load relief to prevent flaps from structural damage in case of overspeed. For non-SFP aircraft flap load relief function is provided for flaps 30 and 40 and for SFP aircraft (737-800 SFP and 737-8 / 737-8200) for flaps 10-40. If speed goes 1 kt above flap placard speed for the current flap setting, FSEU automatically retracts flaps to the previous flap setting and re-extends them automatically once speed goes 4 kt below flap placard speed (e.g. if flaps 30 are selected and speed goes above 176 kt FSEU will retract flaps to 25 and re-extend flaps to 30 once speed goes below 171 kt)
  • Monitors TE flaps for asymmetry or skew or uncommanded motion. If asymmetry or skew is detected FSEU stops flap movement and you see a needle split on the flap position indicator
  • Monitors LE devices for incorrect position or uncommanded motion. LE slats 2-7 are also monitored for skew condition (the outboard LE slats 1 and 8 not monitored for skew)

Note:

  • FSEU flap load relief function is based on the CPTs IAS
  • In case of CPT airspeed unreliable, if speed is higher than flap placard speed it wouldn’t allow to extend flaps to desired position
  • What you can do, is select the higher flap setting than desired
  • For example, if you want to land with flaps 30 and CPT IAS (unreliable) shows 190 kt, then by selecting flaps 40 the flaps would move to flaps 30
  • Be aware however that if speed decreases 4 kt below F40 flap placard speed the flaps will move to 40, so be ready to select flaps back to 30 as speed decreases below 171 kt (i.e. 4 kt below F30 placard speed)
  • Flap load relief is not available when flaps are electrically extended (alternate flap extension)

What is the difference between asymmetry, skew condition and uncommanded motion?

  • Asymmetry: Flap panel on one wing is not aligned with the same flap panel on the other wing
  • Skew: When the inboard and outboard part of a flap or slat panel do not operate at the same rate during extension or retraction, causing the panel to twist
  • Uncommanded motion: When two or more flaps or slats move away from their commanded position or move in the opposite way from their commanded position

Note:

  • Both TE flaps asymmetry and skew conditions show as needle slip on the flap position indicator and require completion of the trailing edge flaps asymmetry NNC (for asymmetry the flap position indicator shows the actual position of the TE flaps whereas for skew condition the flap position indicator shows a 15° angle split with the skewed side TE flap needle retracting to form the 15° angle)
  • In case of TE flaps uncommanded motion, the position of the TE flaps on the flap position indicator disagrees with the position of the flap lever. In this case action the trailing edge flaps disagree NNC
  • Remember though that for any flap malfunction, the trailing edge flaps asymmetry NNC takes precedence (e.g. if you have both an asymmetry and flap position disagrees with flap lever, then action the trailing edge flaps asymmetry NNC)
  • Do not extend the TE flaps electrically with a TE flap asymmetry or skew condition as there is no asymmetry or skew protection

QRH info:

Trailing edge flap asymmetry

  • Set the flap lever to the nearest detent that is equal or less than the smallest flap position on the flap position indicator (e.g. if needle shows between 5 and 10 you put flap lever to 5)
  • Moving the flap lever has no effect on the flap position as the FSEU has stopped flap movement
  • It just adjusts the speed tape according to the actual flap position and also determines your Vref for landing (or if you need to proceed to Trailing edge flaps up landing NNC to extend the LE devices)
  • Do not use FMC fuel predictions as FMC does not account for extra drag created by the flaps/slats
  • Use the formula distance / groundspeed x fuel flow to estimate fuel burned until landing (e.g. if you need to fly 60 NM, fuel flow is 3 tons / hr and groundspeed is 240 kt then until landing you will burn 60 / 240 x 3000 = 750 kg approximately)
  • If flap lever is set to up, then NNC directs you to Trailing edge flaps up landing NNC where you will extend the LE devices to fully extended position by momentarily selecting ALTERNATE FLAPS position switch DOWN
  • If LE devices do not go to the fully extended position, you will be directed to All flaps up landing NNC
  • Consider burning fuel to reduce touchdown speed (max tyre speed limit is 195 kt for the 737-800 and 204 kt for the 737-8 / 737-8200) taking into consideration extra fuel consumption in case of potential go around and extra fuel needed to reach an alternate airport if needed
  • Take into account airport elevation and temperature as these will affect your groundspeed upon landing as well
  • In case of TE flaps up landing or All flaps up landing, plan for a long final as you will need to maintain up speed until established on final approach to ensure you have the 25° + 15° overshoot margin
  • Once established on final approach you can reduce speed to approach speed
  • In case roll is encountered due to asymmetry (e.g. control wheel deflected left or right) and after accomplishing the primary rudder trim technique (as explained in FCTM) it results in excessive rudder trim, then accomplish the alternate rudder trim technique (as explained in FCTM)
  • This will require you to trim the rudder to get bank pointer centered
  • Remember that to trim the aileron you need to have the autopilot disengaged!
  • Notice how many units of aileron deflection you have with the AP engaged, disconnect the AP and trim the aileron in the direction of the low side of the control wheel until you don’t need any force on the control wheel to keep wings level (maintain constant heading)
  • If you trimmed correctly the aileron, it means that the aileron units you have to keep wings level is the same with the aileron units indicated before you disconnected the AP
  • This is the new aileron neutral position
  • After you trimmed the aileron you can re-engage the AP

Trailing edge flaps disagree

  • In this case if TE flaps are less than 15, you can extend the TE flaps to 15 using alternate flap extension because you don’t have skew or asymmetry (in case of asymmetry or skew as indicated by needle split you would be directed to the Trailing edge flaps asymmetry NNC)

What indications you get in case of a LE device in incorrect position, uncommanded motion or LE slat skew condition is detected?

  • LE FLAPS TRANSIT light illuminated
  • Related flap or slat light is illuminated on the aft overhead panel green, amber or may not be illuminated at all, depending on the position of the LE device

Note:

  • After takeoff, make sure you don’t accelerate to more than 230 kt with this light illuminated (e.g. VNAV may accelerate to higher speed)
  • If this light illuminates, check the overhead panel for the actual position of the leading edge devices
  • Do not change the flap lever position which triggered the light until you are directed by the QRH (e.g. if malfunction occurs on final, do not retract the flaps in case of go around)
  • For example, if there is a problem with extending the LE devices from extended to the fully extended position and you retract the flaps, then the light would extinguish however when you try to extend the flaps it would re-appear again

Additional info:

  • As per QRH, if only one leading edge device light is illuminated speed needs to be limited to 300 kt or 0.65 whichever is lower

What is the function of the autoslat system?

  • Autoslat system is designed to extend the leading edge slats from the extended to the fully extended position i.e. flaps 1-5 for non-SFP aircraft and 1-25 for SFP aircraft (737-800 SFP and 737-8 / 737-8200) if the aircraft approaches a stall condition
  • This is to delay flow separation at the outboard part of the wing and improve aircraft’s stall handling characteristics
  • After angle of attack is reduced sufficiently below the critical angle of attack, they retract back to the extended position (LE FLAPS TRANSIT light does not illuminate during autoslat operation)

What does the SPEEDBRAKE DO NOT ARM light indicate?

  • Automatic speedbrake deployment is inoperative
  • Also illuminates after landing with wheel speed < 60 kt and speedbrake lever not down

Note:

  • Speedbrakes can still be manually deployed after touchdown

What does the AUTOSLAT FAIL light indicate?

  • Autoslat system has failed
  • If illuminated on recall, it indicates single SMYD channel failure

How can you operate the TE flaps and LE devices in case of loss of system B?

  • TE flaps are extended electrically and LE devices are extended to the fully extended position through the standby hydraulic system (LE devices cannot be retracted in this case)

Note:

  • Alternate flap extension does not provide asymmetry or skew protection
  • As per NNC, to extend the TE flaps electrically (Transfer bus 2) you put the ALTERNATE FLAPS master switch to ARM
  • Then the TE flaps are extended by holding down the ALTERNATE FLAPS position switch
  • The leading edge devices move to the fully extended position once the switch is pressed down
  • Simultaneously move the flap lever to the desired flap position
  • This does not have an effect on flaps but drives the speed tape and gives the flap maneuvering speed, flap placard speed (bottom of upper red/black bar) and flap placard speed for next flap setting (bottom of upper amber band)
  • Before matching the speed with the selected flap setting, wait until flaps reach the desired position to match the speed. This is because TE flap movement is slow when extending flaps electrically (takes 2 minutes from flaps up to flaps 15) and could end up in a low speed situation if not waiting for the flaps to reach position first
  • After releasing the switch, wait at least 15 seconds before pressing it again
  • To retract the flaps, select the ALTERNATE FLAPS position switch to UP (switch is held up – doesn’t need continuous pushing). This is to aid in flap retraction in case of go around
  • However, after retracting flaps, remember to select the switch back to OFF

Note:

  • After releasing the ALTERNATE FLAPS position switch wait 15 seconds before pressing it again
  • After extension of flaps from up to 15 and then back to up (e.g. in case of go around), wait 5 minutes before moving the switch again

Additional info:

  • ALTERNATE FLAPS master switch is red to indicate irreversible action as the LE devices cannot be retracted once extended (same applies for the IDG switches and CVR erase switch)
  • The reason for extending TE flaps to 15 and and not 30 or 40 is go around performance (slower flap retraction)
  • In case of go around, speed bug moves according to the actual flap position (e.g. 737-800: 172 kt for F15, 182 kt for F5 etc.) and not the position of the flap lever. However move the flap lever as normal when retracting flaps as this will adjust the speed tape (flap maneuvering speeds, flap placard speed etc.)

[Updated] What does Flight Control LOW PRESSURE light indicate?

  • Hydraulic pressure from the related system (A or B) to the primary flight controls is low
  • Accomplish the FLIGHT CONTROL LOW PRESSURE NNC

Note:

  • When selecting FLT CONTROL switch to STBY RUD ensure STBY RUD ON light illuminates
  • CMD on the affected side if engaged disengages and cannot be re-engaged. Select the CMD of the unaffected side
  • There’s no MEL item for this, so it’s a no-go

Additional info:

  • In case of flight control B low pressure, yaw damper will trip off after moving FLT CONTROL switch to STBY RUD. As per YAW DAMPER NNC, do not make a flaps 40 landing

How many yaw dampers does the aircraft have?

  • 2 (Primary and Standby)
  • Primary yaw damper is powered by hydraulic system B (normal operations)
  • Standby yaw damper is powered by the standby hydraulic system (manual reversion)

Up to how many degrees does yaw damper move the rudder to provide turn co-ordination and gust damping and prevent Dutch roll?

  • 2-3°

Additional info:

  • In case of inoperative yaw damper, do not make a F40 landing

What is the purpose of the Speed Trim System on the 737-800 or Speed Trim function of the Speed Trim System on the 737-8 / 737-8200?

  • To return the aircraft to target speed using the stabilizer (most commonly during low speed-high thrust conditions such as takeoff and go around)
  • The system operates in manual flight only
  • If the aircraft is flying lower than target speed the speed trim system will automatically trim nose down to return back to target speed
  • If the aircraft is flying higher than target speed the speed trim system will automatically trim nose up to return back to target speed

Note:

  • This function is available only with:
    • Autopilot disengaged
    • 10 seconds after takeoff
    • 5 secs after releasing the control wheel trim switches
    • Speed between 100 kt and M 0.68
    • Trim needed (i.e. not on desired speed)

Additional info:

  • In order for the speed trim system to trim, it needs at least 5 seconds after releasing the control wheel trim switches
  • If trimming occurs within this 5 second period, it indicates runaway stabilizer (or activation of MCAS on the 737-8 / 737-8200)

What is the purpose of the MCAS on the 737-8 / 737-8200?

  • The 737-8 / 737-8200 has bigger engines which are also more forward compared to the 737-800
  • This creates a nose up pitching moment at high angles of attack when compared to the 737-800
  • To aid in pitch stability at high angles of attack (approaching a stall) and counteract this nose up pitching moment, the 737-8 / 737-8200 has the MCAS system (Maneuvering Characteristics Augmentation System) which is designed to trim nose down incrementally as the aircraft approaches a threshold angle of attack (i.e. approaching stall) and make the handling characteristics of the 737-8 / 737-8200 similar to the 737-800

Note:

  • In order for MCAS to activate both alpha vanes must agree within 5.5° otherwise speed trim system and MCAS are deactivated and will remain inoperative for the rest of the flight. In this case SPEED TRIM FAIL light illuminates
  • When activated, MCAS takes a snapshot of the stab trim position at the time of activation and limits nose down stabilizer trim to no more than 2.5 units at low Mach numbers or 0.65 units at high Mach number
  • This is so that the maximum nose down trim added is not more than what could be counteracted by the pilots using the elevator only
  • Note that pulling aft the control column does not interrupt MCAS, however electric trimming does interrupt MCAS nose down commands. Pushing forward the control column interrupts MCAS nose up trim
  • As angle of attack decreases below threshold angle of attack, MCAS automatically reverses trim and trims nose up towards the stab trim position at which the MCAS was activated (snapshot)
  • If trim reaches maximum nose down command limit SPEED TRIM FAIL light illuminates and speed trim system remains inoperative for the rest of the flight
  • MCAS can only activate once for each high angle of attack event. This means that for a second MCAS activation, angle of attack would need to be reduced below threshold angle of attack and then increase above threshold again
  • If angle of attack is above threshold before retracting flaps to up, MCAS does not activate
  • In order to activate angle of attack would first need to be reduced below threshold with the flaps up and then increase above threshold angle of attack again

Additional info:

  • MCAS can only activate with:
    • Flaps up
    • Autopilot disengaged
    • Angle of attack above threshold angle of attack (both alpha vanes should agree within 5.5°)
    • At least 10 seconds after takeoff
    • Mach number below 0.84

What is the difference between MCAS and Speed Trim function on the 737-8 / 737-8200?

  • MCAS enhances pitch stability whereas Speed Trim enhances speed stability
  • They are both part of the Speed trim system on the 737-8 / 737-8200
  • If one of them fails, the other is inhibited as well and the SPEED TRIM FAIL light illuminates

[New] What is the purpose of the Elevator Feel Shift module? (EFS)

  • It increases control column forces during a stall, pushing the control column forward
  • It activates at a higher angle of attack than the stick shaker

Note:

  • EFS helps with stall prevention/recognition through the elevator whereas STS through the stabilizer
  • EFS can only be activated during manual flight and RA > 100 ft (stays active below 100 ft RA if activated before)

What is the function of LAM (Landing Attitude Modifier)?

  • Flaps 30-40: Maintain the same nose landing gear contact margins with the 737-800 (as on the 737-8 / 737-8200 nose gear is 20.3 cm / 8 inches longer) by symmetrically raising the spoilers when speed is Vref + 10 (spoilers start to extend) to Vref + 20 (maximum spoiler deflection)
  • Flaps 15-30: with the thrust levers in idle, spoilers deploy to help aircraft both decelerate and maintain the path similar to the 737-800

What is the function of MLA (Maneuver Load Alleviation)?

  • Partially retracts spoilers when load factor is more than 1.3 G or less than 0.3 G

What is the function of EDS (Emergency Descent Speedbrakes)?

  • Raise the flight spoilers to higher angle and increase rate of descent in case of emergency descent
  • System activates when cabin altitude warning is active, altitude is above 30000 ft and speedbrake lever is in flight detent
  • System deactivated when cabin altitude warning is no longer active or when speedbrake lever is stowed

What is the function of EJLA (Elevator Jam Landing Assist) on the 737-8 / 737-8200?

  • Raises spoilers to a pre-set position and then extends or retracts spoilers based on push or pull of the control column (i.e. if control column is pushed spoilers extend higher to increase rate of descent whereas if control column is pulled spoilers retract to reduce rate of descent)
  • This system would be activated as part of the Jammed or Restricted Flight Controls NNC in case of jammed elevator
  • System requires autopilot to be disengaged, flaps 1 or greater and ELEVATOR JAM LANDING ASSIST switch ON

What does the SPOILERS light indicate on the 737-8 / 737-8200?

  • One or more spoiler pairs are inoperative