Boeing 737 Memory Items – Runaway Stabilizer

JohanAero

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:

Runaway Stabilizer

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

Autopilot (if engaged)...........................................Disengage
     Do not re-engage the autopilot.
     Control airplane pitch attitude manually with control column and main
     electric trim as needed.

Autothrottle (if engaged)........................................Disengage
     Do not re-engage the autothrottle.

If the runaway stops after the autopilot is disengaged:

                                 ----

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 could 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.

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.

1 thought on “Boeing 737 Memory Items – Runaway Stabilizer

  1. Good review of memory items, and the system as well.
    The only missing ponit ( under my opinion) is do not mention the “aerodynamic stab trim runaway” concept.
    Otherwise, looking the system diagram, nobody can understand the last step of the memory items.
    Nice web. Congrats.

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