1 BOEING COMMERCIAL AIRPLANE GROUP FLIGHT OPERATIONS TECHNICAL BULLETIN NUMBER: 737-20-04 DATE: November 18, 2020 These bulletins provide information which may prove useful in airline operations or airline training. This information will remain in effect depending on production changes, customer-originated modifications, and Service Bulletin incorporation. Information in these bulletins is supplied by the Boeing Company and may not be approved or endorsed by the FAA at the time of writing. Applicable documentation will be revised as necessary to reflect the information contained in these bulletins. For further information, contact Boeing Commercial Airplane Group, Chief Pilot, Flight Technical, through the Service Requests Application (SR App) on the MyBoeingFleet home page. SUBJECT: Runaway Stabilizer Non-Normal Checklist (NNC) and Use of Manual Stabilizer Trim ATA NO: 27 APPLIES TO: 737 All REASON: To provide information on the appropriate implementation of the Runaway Stabilizer NNC and the use of manual stabilizer trim BACKGROUND INFORMATION The Runaway Stabilizer Non-Normal Checklist (NNC) is designed to allow the flight crew to stop a runaway stabilizer condition using built in safety functions in the B737 pitch control system. This Flight Operations Technical Bulletin (FOTB) is meant to describe the safety functions and further explain how each of the functions can stop a runaway stabilizer. Expanded information about the steps in the Runaway Stabilizer NNC will be provided with emphasis on the use of manual stabilizer trim. Boeing understands that operators can modify their own version of the Runaway Stabilizer NNC or use a Quick Reference Card (QRC). The intent of this FOTB is not to drive all operators to use the Boeing procedure but rather to expand on the reasoning for the steps in the NNC. The use of manual stabilizer trim is not common on the B737 but its use is referenced in certain Non-Normal Checklists such as the Runaway Stabilizer NNC. Boeing is providing in this FOTB suggested training scenarios for the use of manual stabilizer trim during non-normal conditions.
Transcript
1
BOEING COMMERCIAL AIRPLANE GROUP
FLIGHT OPERATIONS TECHNICAL BULLETIN
NUMBER: 737-20-04 DATE: November 18, 2020 These bulletins provide information which may prove useful in airline operations or airline training. This information will remain in effect depending on production changes, customer-originated modifications, and Service Bulletin incorporation. Information in these bulletins is supplied by the Boeing Company and may not be approved or endorsed by the FAA at the time of writing. Applicable documentation will be revised as necessary to reflect the information contained in these bulletins. For further information, contact Boeing Commercial Airplane Group, Chief Pilot, Flight Technical, through the Service Requests Application (SR App) on the MyBoeingFleet home page. SUBJECT: Runaway Stabilizer Non-Normal Checklist (NNC) and Use of Manual Stabilizer
Trim ATA NO: 27 APPLIES TO: 737 All REASON: To provide information on the appropriate implementation of the Runaway
Stabilizer NNC and the use of manual stabilizer trim BACKGROUND INFORMATION The Runaway Stabilizer Non-Normal Checklist (NNC) is designed to allow the flight crew to stop a runaway stabilizer condition using built in safety functions in the B737 pitch control system. This Flight Operations Technical Bulletin (FOTB) is meant to describe the safety functions and further explain how each of the functions can stop a runaway stabilizer. Expanded information about the steps in the Runaway Stabilizer NNC will be provided with emphasis on the use of manual stabilizer trim. Boeing understands that operators can modify their own version of the Runaway Stabilizer NNC or use a Quick Reference Card (QRC). The intent of this FOTB is not to drive all operators to use the Boeing procedure but rather to expand on the reasoning for the steps in the NNC. The use of manual stabilizer trim is not common on the B737 but its use is referenced in certain Non-Normal Checklists such as the Runaway Stabilizer NNC. Boeing is providing in this FOTB suggested training scenarios for the use of manual stabilizer trim during non-normal conditions.
2
BACKGROUND INFORMATION (continued) The information provided in the FOTB applies to all B737 airplanes. The series airplane groups are identified as follows:
• 737 refers to B-737-100/-200 • 737CL refers to B-737-300/-400/-500 • 737NG refers to B-737-600/-700/-800/-900 • 737 MAX refers to B-737-7/-8/-8200/-9/-10
Stabilizer Controls and Indicators ...................................................................................... 13
Stabilizer Trim Override Switch ...................................................................................... 13
Manual Stabilizer Trim ........................................................................................................ 14
Training on the Use of Manual Stabilizer Trim .................................................................. 17
Scenario 1: Initial Nose Down Out of Trim Condition ................................................ 17
Scenario 2: Initial Nose Up Out of Trim Condition..................................................... 18
Scenario 3: Acceleration – Deceleration While Using Manual Stabilizer Trim ........ 18
Control Column Actuated Stabilizer Trim Cutout Switch Demonstration......................... 19
Scenario 1: Nose Down Direction .............................................................................. 19
Scenario 2: Nose Up Direction ................................................................................... 19
Runaway Stabilizer Training .............................................................................................. 20
Dual Wire Short ........................................................................................................... 20
Short in the Main Electric Trim Switch ....................................................................... 20
Control Column Cutout Limits (737NG and 737 MAX) ..................................................... 21
4
Runaway Stabilizer NNC Condition Statement The condition statement explains that the flight crew can identify a runaway stabilizer condition by continuous uncommanded movement of the stabilizer. A runaway stabilizer condition can also be identified when the stabilizer moves in a manner that is not appropriate for flight conditions. For example, after takeoff the flight crew can expect repeated stabilizer trim movement as the Speed Trim function is providing speed stability augmentation. Similar stabilizer trim movement might not be appropriate while at cruise in constant airspeed. In addition, with the autopilot engaged, stabilizer trim movement can be expected during speed or vertical path changes and should not be interpreted as a runaway stabilizer condition. The following are descriptions of stabilizer trim movement that are not appropriate for flight conditions:
• During manual flight, stabilizer movement immediately after the main electric stabilizer trim switch is released.
• Stabilizer trim movement that cannot be stopped or reversed by use of the main electric stabilizer trim switch.
• Extended stabilizer trim movement without airspeed, altitude or thrust changes,
It is important for the flight crew to distinguish normal stabilizer trim operation from a runaway stabilizer condition as the Speed Trim System is designed to provide speed and pitch stability augmentation by commanding stabilizer trim movement. This determination can be made with a thorough understanding of the Speed Trim System (737CL, 737NG, 737 MAX). Flight crew action: When the flight crew recognizes that a runaway stabilizer condition occurs, at the direction of the pilot flying each crewmember takes action based on area of responsibility.
5
Holding Control Column During a runaway condition, holding the control column helps ensure that the flight crew maintains control of the airplane. In addition, holding the control column prepares the flight crew to resist any control column movement resulting from the disengagement of the autopilot. During this step, if the control column is displaced by a significant amount, the control column actuated stabilizer trim cutout switches can activate and pause stabilizer movement. Although activation of the column cutout switches may be the result of this step, it is not necessarily the objective.
The control column actuated stabilizer trim cutout switches are micro-switches located at the base of the control column that activate when the control column is moved beyond a threshold either forward or aft. When the micro-switches are activated, stabilizer trim commands from the main electric, autopilot and Speed Trim function are stopped. If the stabilizer trim commands stop when the control column is held firmly, relaxing the control column de-activates the micro-switches and allows the runaway stabilizer trim commands to continue. (737 MAX only) It is important to emphasize that when the micro-switches are activated, nose up Maneuvering Characteristics Augmentation System (MCAS) stabilizer trim commands are stopped. However, nose down MCAS stabilizer trim commands are not stopped.
Flight crew action: Firmly holding the control column is within the area of responsibility of the pilot flying.
6
Disengaging Autopilot, Using Control Column and Main Electric Stabilizer Trim
Disengaging the autopilot serves two purposes. First, disengaging the autopilot may stop the uncommanded stabilizer movement if it is resulting from a fault in the autopilot system. Second, disengaging the autopilot prepares the flight crew to maintain control of the airplane.
Once the autopilot is disengaged, it should not be re-engaged for the remainder of the flight. At this step in the NNC, control column and main electric stabilizer trim are available and should be used to maintain airplane control. It is important to note that even though the steps in the Runaway Stabilizer checklist are sequential, these steps can be accomplished simultaneously.
Note : The autothrottle disengagement step shown above is discussed separately in the next section.
The control column is the primary method of controlling airplane pitch and should be used as needed to set the desired airplane pitch attitude. As the control column is moved to set the desired pitch attitude, the control column actuated stabilizer trim cutout switches can activate and stop the runaway stabilizer movement. Use thrust levers to ensure airspeed remains in the desired range for the phase of flight. Use main electric stabilizer trim to reverse any runaway stabilizer trim movement. This reduces control column forces and reduces workload if manual stabilizer trim is subsequently needed. If the runaway stabilizer condition has adversely affected airplane pitch attitude, sustained use of main electric stabilizer trim input may be needed to first correct the pitch attitude and then to relieve the control column forces. Flight crew action: Disengaging the autopilot is within the area of responsibility of the pilot flying. Using control column and main electric stabilizer trim are within the area of responsibility of the pilot flying.
7
Disengaging Autothrottle
If the autothrottle is engaged, it should be disengaged.
Disengaging the autothrottle is not an attempt to stop the runaway stabilizer. This step is instead standard Boeing policy for airplanes that are not equipped with a Fly-By-Wire system with thrust change compensation. Leaving the autothrottle engaged can make it difficult to control the airplane as the autothrottle responds to thrust requirements resulting in pitch changes. Flight crew action: Disengaging the autothrottle is within the area of responsibility of the pilot flying.
8
If the Runaway Stops
If the runaway stops after the autopilot is disengaged, do not re-engage the autopilot or autothrottle. The NNC is complete. It is important to differentiate a runaway stabilizer condition that is stopped when the autopilot is disengaged vs when the control column actuated stabilizer trim cutout switches stop the runaway. It is possible that when the flight crew reaches this step in the NNC, the runaway is stopped due to the control column actuated stabilizer trim cutout switches. In this scenario the flight crew should consider the runaway as continuing after the autopilot is disengaged.
The flight crew should then reference the Runaway Stabilizer NNC to ensure all steps have been completed. Flight crew action: If the runaway stops after the autopilot is disengaged and when the airplane’s flight path is under control, the pilot flying calls for the Runaway Stabilizer NNC. The pilot monitoring reads the Runaway Stabilizer NNC. For specific guidance on how to read the NNC, refer to the Non–Normal Checklist Use section of the QRH in Checklist Instructions, Section 2.
9
STAB TRIM Cutout Switches
If the runaway continues after the autopilot is disengaged, the flight crew then places both STAB TRIM cutout switches to the CUTOUT position.
Note : Before moving the STAB TRIM cutout switches to CUTOUT, use electric stabilizer trim to reverse any runaway stabilizer trim commands and relieve control column forces. Sustained electric stabilizer trim input may be needed. This also reduces workload if manual stabilizer trim is needed.
(737 All) This step is designed to isolate main electric trim or autopilot trim failures by disconnecting these systems from the stabilizer motor. (737CL, 737NG and 737 MAX) This step also stops any Speed Trim System stabilizer trim commands, including MCAS commands (737 MAX only). If the Speed Trim System senses a need to trim but is not able to because the STAB TRIM cutout switches in the CUTOUT position, the SPEED TRIM FAIL light can illuminate. Flight crew action: Moving both STAB TRIM cutout switches to CUTOUT is within the area of responsibility of the pilot monitoring.
10
Grasp and Hold Stabilizer Trim Wheel
If the runaway continues even after placing both STAB TRIM cutout switches to the CUTOUT position, the flight crew needs to grasp and hold the stabilizer trim wheel.
While extremely improbable, at this point in the NNC, the likely cause of the runaway stabilizer is a failure of the jackscrew brake system. Grasping and holding stops the stabilizer runaway from continuing to the system’s mechanical stops. It is important to emphasize that if the flight crew releases the trim wheel, the runaway will likely continue. If the jackscrew brake system has failed, the stabilizer will likely run in the nose down direction due to aerodynamic forces. Moving the control column aft to overcome the nose down stabilizer movement will likely aggravate the situation as aerodynamic forces increase due to elevator movement. After completing this step, the flight crew should then reference the Runaway Stabilizer NNC to ensure all steps have been completed, and continue the NNC by doing the remaining reference steps. Flight crew action: Grasping and holding the stabilizer trim wheel is within the area of responsibility of both pilots. The pilot flying can direct the pilot monitoring to do this step. When the airplane’s flight path is under control, the pilot flying calls for the Runaway Stabilizer NNC. The pilot monitoring reads the Runaway Stabilizer NNC.
11
Manually Trim Stabilizer
The flight crew now only has the ability to manually trim the stabilizer by using the stabilizer trim wheel.
Increased air loads can result from high airspeeds, pulling or pushing the control column. A two pilot effort may be used to correct an out of trim condition. In extreme cases it may be necessary to relieve the air loads to allow manual trimming. Air loads can be relieved by reducing airspeed. Anticipate the trim changes required for speed and configuration changes. Do not re-engage the autopilot or autothrottle. Refer to use of manual trim section of this FOTB for additional information. Flight crew action: Manually trimming the stabilizer is within the area of responsibility of both pilots. The pilot flying can direct the pilot monitoring to assist with this step.
12
Deferred Items
The NNC then continues with the Deferred Items section. The steps in the Deferred Items section essentially contain the normal Descent, Approach and Landing Checklists. The difference is the addition of the Landing Configuration and Trim guidance.
The flight crew should configure the airplane early on the approach in order to establish an in-trim condition. When reaching the landing configuration, maintain an in-trim setting. If a go-around is required, anticipate the trim changes as airspeed increases. Flight crew action: Manually trimming the stabilizer is within the area of responsibility of both pilots. The pilot flying can direct the pilot monitoring to assist with this step. When all of the deferred items have been completed, the pilot monitoring calls the Runaway Stabilizer NNC complete.
13
Stabilizer Controls and Indicators Stabilizer Trim Override Switch
(737CL, 737NG and 737 MAX) While the Stabilizer Trim Override Switch on the Aft Electronic Panel is not used during a runaway stabilizer condition, the switch is being discussed here to ensure understanding of its functionality. The use of the switch is only called for in the Jammed or Restricted Flight Controls NNC.
The purpose of the Stabilizer Trim Override Switch is to bypass the control column actuated stabilizer trim cutout switches in the event one or both of the micro-switches fail. In this case, opening the guard and placing the switch in the OVRD position allows the flight crew to bypass the micro-switches on the control column and restore the use of electric stabilizer trim.
14
Manual Stabilizer Trim
If manual stabilizer trim is needed, it is important to emphasize that the STAB TRIM cutout switches should be placed in the CUTOUT position prior to extending the stabilizer trim wheel handles. Personal injury can occur if the stabilizer trim wheel moves due to electric trim inputs while the stabilizer trim wheel handles are extended.
The stabilizer trim wheel handles are installed so that the two handles are offset by 90 degrees. This increases the pilots mechanical advantage during manual stabilizer trimming allowing the pilots to apply more force to the trim wheel handles.
A change of one stabilizer trim unit requires approximately 10 turns of the stabilizer wheel for the 737 and 737CL, and 15 turns for the 737NG and 737 MAX.
When the STAB TRIM cutout switches are moved to CUTOUT, the pilots must trim the airplane manually using the manual stabilizer trim wheel. Prior to moving the STAB TRIM cutout switches to CUTOUT, the pilots should use main electric stabilizer trim to reduce column forces. This will minimize the amount and the force required to trim the stabilizer manually. Manual stabilizer is intended for small trim corrections from an in-trim condition. For example, during a runaway stabilizer condition, the flight crew should use main electric stabilizer trim to ensure the airplane is in trim prior to placing the STAB TRIM cutout switches to CUTOUT. This reduces the amount of manual stabilizer trim needed to maintain an in-trim condition as well as flight crew workload.
If starting from an in-trim condition and manual stabilizer trim is needed, the pilot flying can make the needed trim adjustments or ask the pilot monitoring to make the adjustments. Good flight crew coordination is important. If starting from an out of trim condition, one or two pilot effort may be used to return the airplane to an in-trim condition. Depending on how far the airplane is out of trim and the force required to move the stabilizer trim wheel, the flight crew should coordinate which of the following techniques should be used.
The stabilizer trim is labeled in the airplane nose down and airplane nose up directions. When coordinating manual trim, the flight crew should reference nose down or nose up directions.
15
Manual Stabilizer Trim (continued)
1. Pilot flying and trimming with one hand.
2. Pilot flying not trimming and pilot monitoring trimming with one hand.
3. Pilot flying and trimming with one hand and pilot monitoring trimming with one hand.
16
Manual Stabilizer Trim (continued)
4. Pilot flying not trimming and pilot monitoring trimming with two hands.
5. Pilot flying and trimming with one hand and pilot monitoring trimming with two hands.
6. Pilot flying not trimming and pilot monitoring trimming with two hands, one on each handle.
17
Training on the Use of Manual Stabilizer Trim
While the use of manual stabilizer trim is not common on the B737, its use can be needed in certain non-normal conditions. Boeing is providing the following suggested training to increase a flight crew’s understanding of manual stabilizer trim and the ability of a flight crew to manually use stabilizer trim to return the airplane to an in-trim condition.
Scenario 1: Initial Nose Down Out of Trim Condition
• Establish straight and level flight at 250 knots and 10,000 feet. • Ensure airplane is properly in trim. • Move the Stabilizer Trim Override Switch on the Aft Electronic Panel to OVRD.
o This bypasses the control column cutout switches to allow use of main electric stabilizer trim.
• Disengage the autopilot and autothrottle. • Use main electric stabilizer trim to move the stabilizer 2 units in the nose down direction
while maintaining level flight with the use of the control column. • Move both STAB TRIM cutout switches to the CUTOUT position. • Use the manual stabilizer trimming techniques described above to return the airplane to in-
trim condition while maintaining level flight.
CAUTION - Ensure both stabilizer trim wheel handles are stowed.
• Move both STAB TRIM cutout switches to the NORMAL position. • Move the Stabilizer Trim Override Switch on the Aft Electronic Panel to NORM. • Engage the autopilot and autothrottle.
18
Training on the Use of Manual Stabilizer Trim (continued)
Scenario 2: Initial Nose Up Out of Trim Condition
• Establish straight and level flight at 250 knots and 10,000 feet. • Ensure airplane is properly in trim. • Move the Stabilizer Trim Override Switch on the Aft Electronic Panel to OVRD.
o This bypasses the control column cutout switches to allow use of main electric stabilizer trim.
• Disengage the autopilot and autothrottle. • Use main electric stabilizer trim to move the stabilizer 2 units in the nose up direction while
maintaining level flight with the use of the control column. • Move both STAB TRIM cutout switches to the CUTOUT position. • Use the manual stabilizer trimming techniques described above to return the airplane to in-
trim condition while maintaining level flight.
CAUTION - Ensure both stabilizer trim wheel handles are stowed.
• Move both STAB TRIM cutout switches to the NORMAL position. • Move the Stabilizer Trim Override Switch on the Aft Electronic Panel to NORM. • Engage the autopilot and autothrottle.
Scenario 3: Acceleration – Deceleration While Using Manual Stabilizer Trim
• Establish straight and level flight at 250 knots and 10,000 feet. • Ensure airplane is properly in trim. • Disengage the autopilot. • Move both STAB TRIM cutout switches to the CUTOUT position. • Accelerate to 300 knots while maintaining 10,000 feet. • Decelerate to 250 knots while maintaining 10,000 feet. • Use the manual stabilizer trimming techniques described above to maintain an in-trim
condition while maintaining level flight.
CAUTION - Ensure both stabilizer trim wheel handles are stowed.
• Move both STAB TRIM cutout switches to the NORMAL position.
19
Control Column Actuated Stabilizer Trim Cutout Switch Demonstration Scenario 1: Nose Down Direction
• Establish straight and level flight at 250 knots and 10,000 feet. • Ensure airplane is properly in trim. • Disengage the autopilot and autothrottle. • PF uses control column to maintain altitude. • PM applies sustained nose down main electric stabilizer trim.
o Note : Continue to use sustained nose down main electric stabilizer trim throughout the demonstration.
• PF uses control column to maintain altitude which will activate the control column actuated stabilizer trim cutout switches and stop the PM’s nose down input.
• PF uses aft/forward control column to activate/deactivate the control column actuated stabilizer trim cutout switches.
• Both PF and PM observe functionality. • PM releases main electric stabilizer trim. • PF returns airplane to in trim condition.
Scenario 2: Nose Up Direction
• Establish straight and level flight at 250 knots and 10,000 feet. • Ensure airplane is properly in trim. • Disengage the autopilot and autothrottle. • PF uses control column to maintain altitude. • PM applies sustained nose up main electric stabilizer trim.
o Note : Continue to use sustained nose up main electric stabilizer trim throughout the demonstration.
• PF uses control column to maintain altitude which will activate the control column actuated stabilizer trim cutout switches and stop the PM’s nose up input.
• PF uses forward/aft control column to activate/deactivate the control column actuated stabilizer trim cutout switches.
• Both PF and PM observe functionality. • PM releases main electric stabilizer trim. • PF returns airplane to in trim condition.
20
Runaway Stabilizer Training
Runaway stabilizer training has been and will continue to be part of 737 training. However, Flight Control Computer (FCC) software P12.1.2 introduced a Cross-FCC Trim Monitor on the 737 MAX. The Cross-FCC Trim Monitor allows each FCC to monitor the other FCC’s stabilizer trim commands. In the event the Cross-FCC Trim Monitor activates, the FCC sends a signal to the other FCC to inhibit its stabilizer trim commands, this includes autopilot, CWS, and Speed Trim System commands. This functionality essentially eliminates the probability of a runaway stabilizer condition on the 737 MAX.
In order to provide a training scenario that allows the flight crew to do the Runaway Stabilizer NNC, two runaway stabilizer conditions have been added to 737 MAX simulators. A dual wire short outside of the FCC and a short in the main electric trim switch.1
Dual Wire Short This malfunction, while extremely improbable, occurs outside of the FCC but along the FCC’s command path by simulating a dual wire short. While it occurs outside of the FCCs, the FCCs are able to detect the runaway through voltage feedback causing the SPEED TRIM FAIL, FLT CONT system annunciator and MASTER CAUTION lights to illuminate. In addition, the STAB OUT OF TRIM light illuminates on landing when below 30 knots ground speed. Boeing has issued a Simulator Data Bulletin (SDB-737-008) in order to provide the same runaway stabilizer condition for 737NG simulators. The 737NG exhibits similar flight deck effects with the exception of the STAB OUT OF TRIM light. On the 737NG, the SPEED TRIM FAIL, FLT CONT system annunciator and MASTER CAUTION lights can illuminate if there is a Speed Trim System trim command in the direction opposite of the runaway.
Short in the Main Electric Trim Switch This malfunction is also extremely improbable and simulates a short of one of the main electric trim switches which causes the trim do move in the direction selected by the instructor. This malfunction allows the opposite main electric trim switch to stop but not reverse the runaway. The runaway continues when the opposite main electric trim switch is released. This occurs due to the opposing trim commands between the two main electric trim switches. One is commanding the runaway due to the wire short and the other is attempting to reverse it.
1 Refer to Master Malfunction List for the 737 NG/MAX Training Simulator - D611A067
21
Control Column Cutout Limits (737NG and 737 MAX)
Column Function Forward Value Aft Value Maximum Column Deflection 13.75° 14.75° Main Electric Stabilizer Trim Column Cutouts 2.8° 3.7° FCC Trim Column Cutouts 3.3° 5.5°
FCC Trim Software Column Cutouts (New, 737 MAX Only)
4.6° 7.2°
22
Control Column Cutout Limits (737NG and 737 MAX) (continued)
Maximum Column Deflection – This is the maximum control column travel in the forward and aft directions.
Main Electric Stabilizer Trim Column Cutouts – When the control column is deflected beyond the forward value, use of nose up main electric stabilizer trim is inhibited. When the control column is deflected beyond the aft value, use of nose down main electric stabilizer trim is inhibited.
FCC Trim Column Cutouts – When the control column is deflected beyond the forward value, nose up stabilizer trim commands from the FCC are inhibited. This includes autopilot and Speed Trim System (Speed Trim function and MCAS function). When the control column is deflected beyond the aft value, nose down stabilizer trim commands from the FCC are inhibited. This includes autopilot and Speed Trim function. MCAS nose down stabilizer trim commands are not inhibited.
FCC Trim Software Column Cutouts (737 MAX Only) – When the control column is deflected beyond the forward value, nose up stabilizer trim commands from the FCC are inhibited. This includes autopilot and Speed Trim System (Speed Trim function and MCAS function). When the control column is deflected beyond the aft value, nose down stabilizer trim commands from the FCC are inhibited. This includes autopilot and Speed Trim function. MCAS nose down stabilizer trim commands are not inhibited. Main electric stabilizer trim commands are not inhibited in either direction.
