ET302 Accident Report

Looking at the accident report of Ethiopian Airlines Flight 302, it says the cut out switches were used. But the manual trim moved from 2.3-2.1? My reasoning for asking this is because the timeframe on this specific input is 5:40:42-5:43:11 which goes up to the point the crew re engaged the electric trim motors. It sounds like they’re being vague for a reason. Did the First Officer trim in the wrong direction and then tell his Captain “It’s not working”?

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    $\begingroup$ Hi Firefighter1 -- The stab moved 2.3 to 2.1, who or what moved it and why, should be in the final report when it comes out, but before then, the question is off-topic. Right now there's one sensible answer that says "I don't want to speculate", which is thankfully sensible, but it doesn't answer the question, which can't be answered at the moment. Voted to close. $\endgroup$ – ymb1 Sep 25 '19 at 13:30
  • $\begingroup$ @ymb1 While I agree with you that speculation about the pilot's actions is off-topic, all the various questions asked in the body are generic enough to be on topic (it seems the accident report was more of a motivation to ask them). Maybe the question title can be rephrased accordingly? $\endgroup$ – Bianfable Sep 25 '19 at 14:06
  • $\begingroup$ @Bianfable: The two paragraphs in the body are all about that accident, as far as I can see. I like to salvage questions whenever possible, but I don't see how here. OP can always ask another generic question about the 737's trim system if they wish. $\endgroup$ – ymb1 Sep 25 '19 at 14:09
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    $\begingroup$ @Firefighter1: No one said stop asking trim questions. Follow the link I provided for what is and is not on-topic. $\endgroup$ – ymb1 Sep 25 '19 at 14:43
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    $\begingroup$ Questions that can only be answered by speculation are defined as being off-topic here. Some forums allow participants to engage in speculation about aircraft accidents, but Av.SE does not. It's a choice about what type of forum is desired. This one is less about discussions and more about facts. Others take a different approach, and having both flavors available on the internet is, in my view, a good thing. $\endgroup$ – Ralph J Sep 25 '19 at 14:45

On the Boeing 737 the trim wheels are directly connected to the stabilizer jackscrew with cables. The force required to move the trim wheels depends on the load on the stabilizer. The more you are out of trim, the more force is required to move the stabilizer.

From the 737 NG FCOM (9.20.8 Flight Controls - System Description - Stabilizer Trim, emphasis mine):

Manual stabilizer control is accomplished through cables which allow the pilot to position the stabilizer by rotating the stabilizer trim wheels. The stabilizer is held in position by two independent brake systems. Manual rotation of the trim wheels can be used to override autopilot or main electric trim. The effort required to manually rotate the stabilizer trim wheels may be higher under certain flight conditions. Grasping the stabilizer trim wheel will stop stabilizer motion.

737 Trim Wheel and Jackscrew left: trim wheel with handle stowed (source), right: jackscrew in the tail (source)

The FCTM (8.17 Non-Normal Operations) explains the correct procedure for manual stabilizer trim (emphasis mine):

Runaway Stabilizer

Hold the control column firmly to maintain the desired pitch attitude. If uncommanded trim motion continues, the stabilizer trim commands are interrupted when the control column is displaced in the opposite direction.

Manual Stabilizer Trim

If manual stabilizer trim is necessary, ensure both stabilizer trim cutout switches are in CUTOUT prior to extending the manual trim wheel handles.

Excessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually.

Anticipate the trim changes required for the approach. Configure the airplane early in the approach. When reaching the landing configuration, maintain as constant a trim setting as possible. If a go-around is required, anticipate the trim changes as airspeed increases.

Note: this FCTM is from a 737 NG, not a MAX. The part about "trim commands are interrupted when the control column is displaced in the opposite direction" is no longer true when MCAS is active on the MAX, the rest is still valid though.

You can also see this procedure applied in this YouTube video by Mentour Pilot.

Regarding the Ethiopian crash: I don't want to speculate on what the crew did, but if

the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units

then it did indeed move in the wrong (AND - Aircraft Nose Down) direction.

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  • $\begingroup$ It's more or less the same trim system a J-3 Cub or Cessna 180 has, but with electric motors added. $\endgroup$ – John K Sep 25 '19 at 12:35
  • $\begingroup$ @RobertDiGiovanni The 737 stab trim units (I don't think it is in degrees) range from 0 (APL NOSE DOWN) to about 17 (APL NOSE UP). You can see them on the scale next to the trim wheel. $\endgroup$ – Bianfable Sep 25 '19 at 12:48
  • $\begingroup$ It may be helpful to see airspeed, trim and AOA data from from the entire event (from take-off roll). And the load sheet too. $\endgroup$ – Robert DiGiovanni Sep 25 '19 at 13:29
  • $\begingroup$ @RobertDiGiovanni It is (partially) in the accident report: see page 26 of the PDF. $\endgroup$ – Bianfable Sep 25 '19 at 13:34
  • $\begingroup$ Fully reading the report was helpful, see page 12/33 $\endgroup$ – Robert DiGiovanni Sep 25 '19 at 14:17

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