B737 Flight Controls: Pitch and Stabilizer Trim

The Boeing 737 controls pitch with two surfaces: two elevators, moved by the pilots through cables and hydraulic power control units (PCUs), and a movable horizontal stabiliser, which trims the aeroplane. The pilot feels artificial forces in the column, produced by a feel computer, and the stabiliser can be moved by the pilots' trim switches, by the autopilot, by hand with the trim wheels, or automatically by the speed trim system. The Mach trim system instead adjusts the elevators relative to the stabiliser.

None of this is fly-by-wire, but several automatic functions act on the stabiliser, and the 737 MAX added one more, MCAS, after its larger engines changed the aeroplane's handling at high angle of attack. An uncommanded trim movement, the runaway stabiliser, is one of the type's recall procedures. This article describes the 737 NG and states where the MAX differs. The general principles are covered in trim systems and powered flying controls and artificial feel.

Elevator control and manual reversion

Cables connect both control columns to the elevator PCUs, which are powered by hydraulic systems A and B, and a torque tube joins the two elevators. Either hydraulic system can operate all the primary flight controls on its own. If both are lost, the elevators, like the ailerons, can still be operated manually: this is manual reversion (see B737 hydraulic system).

If a control column jams, an override mechanism allows the two columns to be separated. Force applied against the jam breaks out either the captain's or the first officer's column, and whichever column then moves freely gives adequate elevator control. On the 737 MAX, the fly-by-wire spoilers add an elevator jam landing assist function.

Elevator feel and centring

Because the pilot does not move the elevators directly, the elevator feel system gives the column artificial feel. Its elevator feel computer simulates aerodynamic forces from airspeed, taken from the elevator pitot system, and from stabiliser position. It uses system A or B pressure, whichever is higher, and the forces reach the columns through the elevator feel and centering unit (the feel and centering unit), which also returns the column to neutral.

The amber feel differential pressure light (FEEL DIFF PRESS) shows excessive differential pressure in the elevator feel computer. It is armed with the trailing edge flaps up and inhibited with them extended. Erroneous activation of the elevator feel shift module can cause it.

The elevator feel shift (EFS) module is part of stall identification. During a stall it raises the system A pressure supplied to the feel and centering unit, roughly doubling the column forces, so that the pilot feels the stall approach in the column. It is inhibited on the ground, below 100 ft radio altitude and with the autopilot engaged, but if it is already active when the aeroplane descends through 100 ft it stays active until the angle of attack falls below about the stick shaker threshold. Nothing on the flight deck shows whether the EFS is armed or active.

Stabiliser trim system

The pilots and the autopilot move the stabiliser in three ways:

• Main electric stabiliser trim. Pushing both stabiliser trim switches on a control wheel trims the stabiliser electrically in the selected direction and disengages the autopilot if it is engaged.
• Autopilot trim. With the autopilot engaged, it trims the stabiliser automatically.
• Manual trim. Cables from the two stabiliser trim wheels on the control stand move the stabiliser directly. The wheels override every other trim input and rotate whenever the stabiliser is moving, and grasping a wheel stops the stabiliser.

Main electric and autopilot trim have two speeds: high with the flaps extended and low with the flaps retracted. Two independent brake systems hold the stabiliser in position. Stabiliser position is read in stabiliser trim units, and each method has its own authority:

Manual trim has the widest range, and the trim wheels remain available when every electric channel has been cut out.

Trim cutout and override switches

Several switches limit what the electric trim can do:

• Control column actuated stabiliser trim cutout switches. When the column is moved against the direction of trim, they stop main electric and autopilot trim, so that pulling against a nose-down trim, or pushing against a nose-up trim, interrupts it.
• Stabiliser trim override switch. At OVERRIDE it bypasses the column cutout switches and restores power to the control wheel trim switches.
• STAB TRIM cutout switches. The MAIN ELECT switch at CUTOUT deactivates the control wheel trim switches. The AUTOPILOT switch at CUTOUT deactivates autopilot trim and disengages the autopilot. Both are at NORMAL for flight, and the autopilot cannot be engaged unless the AUTOPILOT switch is at NORMAL.

Take-off green band

The stabiliser trim green band on the trim indicator marks the allowable range of trim settings for take-off. Before start the crew set the take-off trim and check it is in the green band. If take-off is attempted with the trim outside the range, the take-off configuration warning sounds an intermittent horn. On the flight control surface position indicator, the centre mark of the elevator scale shows the elevator's neutral position on the ground with the stabiliser trimmed in the green band; the mark is set for nominal take-off conditions.

Speed trim and Mach trim

The speed trim system (STS) is a speed stability augmentation system. It moves the stabiliser to improve the handling at low weight, aft centre of gravity and high thrust, when the autopilot is not engaged. It operates:

• with its Mach gain fully enabled between 100 KIAS and Mach 0.60, fading out to zero by Mach 0.68;
• from 10 seconds after take-off;
• from 5 seconds after the trim switches are released;
• with the autopilot not engaged and a trim requirement sensed.

The Mach trim system works above Mach 0.615. As speed increases, the Mach trim actuator repositions the elevator feel and centering unit, adjusting the elevators relative to the stabiliser and moving the column's neutral position.

The amber SPEED TRIM FAIL and MACH TRIM FAIL lights show a failure of the system. If one lights only during a MASTER CAUTION recall and goes out when the system is reset, a single flight control computer (FCC) channel has failed.

MCAS on the MAX

The 737 MAX's LEAP-1B engines are larger than the NG's CFM56-7B and sit higher and further forward. Near the stall and in high-g manoeuvres they would make the column lighter than the certification rules allow. The manoeuvring characteristics augmentation system (MCAS), part of the MAX's speed trim system, restores the required pull force by adding nose-down stabiliser trim. The NG has speed trim but no MCAS.

MCAS works only in manual flight, with the autopilot and control wheel steering not engaged, and with the flaps up; since the return-to-service flight control computer software (P12.1.2) it activates only when the angle of attack rises from below to above a threshold that varies with Mach. It trims at 0.27° per second, the speed trim rate with flaps down, and a single command moves the stabiliser by at most 2.5°, at low Mach. The return-to-service changes include:

• Each FCC compares both angle of attack inputs. A split above 5.5° for a set time disables MCAS and speed trim for the rest of the flight, and SPEED TRIM FAIL stays lit. This split monitor is active only once the flaps are up.
• The electric trim switches pause MCAS, and releasing them no longer resets it: the total nose-down trim cannot exceed one MCAS command until the angle of attack has dropped well below the threshold.
• A maximum command limit disables MCAS and speed trim for the rest of the flight if the stabiliser moves too far from its position at activation, preserving pitch control with the column alone.
• AOA DISAGREE, now standard, shows on both PFDs when the vanes differ by more than 10° for more than 10 seconds.

Unlike main electric trim, an MCAS command bypasses the aft column cutout switch, so pulling the column does not stop it. Both STAB TRIM cutout switches at CUTOUT stop and disable it. The original design relied on one angle of attack sensor at a time and could command repeated nose-down trim; the FAA's review links it to the Lion Air JT610 accident, and the MAX was grounded in March 2019, after the second accident, ET302. EASA later found that MCAS is needed for full compliance with the stall and pitch control rules, but that its loss does not prevent safe flight and landing.

Runaway stabiliser and out-of-trim alerts

A runaway stabiliser is uncommanded, continuous trim movement; the revised checklist also covers stabiliser movement that is wrong for the conditions. The recall items are:

1. Control column: hold firmly.
2. Autopilot (if engaged): disengage, and do not re-engage.
3. Autothrottle (if engaged): disengage.
4. Control column and thrust levers: control pitch attitude and airspeed.
5. Main electric trim: reduce control column forces.
6. If the runaway continues: both STAB TRIM cutout switches to CUTOUT.
7. If it still continues: grasp and hold the stabiliser trim wheel.

The FAA Flight Standardization Board (FSB) report asks training to stress trying main electric trim before cutout; once both switches are at CUTOUT only the trim wheels remain. Manual trimming becomes harder as airspeed rises, so the runaway stabiliser and stabiliser trim inoperative checklists note that reducing airspeed makes it easier, and training covers the manual trim techniques Boeing defines.

The amber STAB OUT OF TRIM light works on the 737 NG only with the autopilot engaged and shows that the autopilot is not trimming the stabiliser properly. On the 737 MAX it can also light on the ground below 30 kt after the cross-FCC trim monitor tripped during the flight, and take-off is then forbidden. On a dual channel approach, a stabiliser out of trim below 800 ft radio altitude turns the autopilot disengage lights steady red. At 400 ft in a dual autopilot ILS approach the stabiliser is trimmed an extra amount nose up, so if the autopilots then disengage, forward column force may be needed.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.