Boeing 737 Memory Items – Runaway Stabilizer

When something goes wrong while flying on the Boeing 737, we use the QRH (Quick Reference Handbook). We just have to find out the correct NNC (Non Normal Checklist) and then we just read it and execute it.

For some of these checklists, we have memory items, meaning we have to know by heart some actions.

I have listed 9 memory items:

• Aborted Engine Start
• Airspeed Unreliable
• APU Fire
• Cabin Altitude Warning / Emergency Descent
• Engine Overheat
• Engine Fire, Severe Damage or Separation
• Engine Limit or Surge or Stall
• Loss of Thrust On Both Engines
• Runaway Stabilizer

Runaway Stabilizer

Control column.................................................Hold firmly

Autopilot (if engaged)...........................................Disengage

Autothrottle (if engaged)........................................Disengage

Control column and thrust levers..........................Control airplane
                                               pitch attitude and airspeed

Main Electric
Stabilizer trim.............................................Reduce control
                                                             column forces

If the runaway stops after the autopilot is disengaged:

     Do not re-engage the autopilot or autothrottle.

                                 ----

If the runaway continues after the autopilot is disengaged:

     STAB TRIM CUTOUT
     switches (both)................................................CUTOUT

     If the runaway continues:
          Stabilizer
          trim wheel........................................Grasp and hold

The runaway stabilizer is probably the most famous of the memory items today for Boeing 737 pilots, sadly because of the Lion Air and Ethiopian Airlines Boeing 737 MAX crashes.

The stabilizer and the elevators are assembled together so when the stabilizer moves, the elevators follow the same way. The elevators move independently from the stabilizer. That’s what we do when we push or pull the control column. We basically fly the aircraft with the elevators but depending on the phase of the flight, when too much force is felt, then we need to trim the stabilizer. When the autopilot is engaged, this is done automatically.

The stabilizer is powered electrically with a single motor. It’s also linked to the trim wheel on both sides of the thrust levers. Inside the wheel, it is possible to extend a handle and then move the stabilizer manually. To do that, we need to make sure both stabilizer trim cutout switches are in CUTOUT.

The checklist for a runaway stabilizer is pretty simple but again we need to recognize the problem quickly to recover. Whatever the reason, if the trim wheel starts rotating without stopping, then we can suspect a runaway stabilizer. We first hold firmly the control column, then we disconnect the autopilot. This should stop the runaway. If it continues, then we put the stab trim switches to CUTOUT. If again, it doesn’t stop, we have to grasp and hold the trim wheel, extend the levers and trim manually.

We can expect 2 kinds of runaway: Electric and Aerodynamic.

Electric runaway is when the wheel starts spinning like crazy. Disengaging the autopilot might stop the runaway. If not, we put the stab trim switches to CUTOUT. If it doesn’t stop, then it means it’s an aerodynamic runaway caused by the failure of both brake systems. Each brake can maintain the position of the stabilizer. In case of a dual brake failure, if there is no action from the crew to stop the wheel, the stabilizer will go crazy until it reaches the mechanical stop.

The elevators are hydraulically powered. From the control column, cables are connected to the PCU (Power Control Unit) which actuates the elevators with hydraulic systems A and B.

Here is a video of the stabilizer jackscrew that is present in the tail and allows the stabilizer to move upward or downward.