By Bjorn Fehrm
Note: The article offered a Video as an example of the effects of not trimming to neutral before hitting Cut-Off. After reviewing the effects of the video in the context of the article, MentourPilot decided we should leave the text but not include the video any longer. MentourPilot’s YouTube channel is an independent initiative to offer the public correct and positive information of all the work which is behind their safe travel in the skies. In the sensitive situation of an ongoing investigation of the crash of ET302, where not all facts are on the table, he concluded after discussing the impression the video left with a colleague, it was not in line with his intent of setting up the channel. In the haste of the change we took this as his company not wanting the video aired, it was not. As facts in the ET302 case comes forward we will discuss with MentourPilot if this situation changes.
April 03, 2019, © Leeham News: The crew of Ethiopian Airlines ET302, which crashed with 157 people on board, used the prescribed Stabilator Trim Cut-Out switches to stop MCAS, according to an article by Wall Street Journal today. Yet still, they crashed. We’ve had the information this could indeed be the case for several days, but we didn’t want to speculate in such a sensitive matter.
The Wall Street article cites information coming from the investigation. By it, we can now reveal how it’s possible the aircraft can crash despite using the Cut-Out switches. To verify, we ran it all in a simulator together with MentourPilot Youtube channel over the last days.
The aeronautical world was stunned when ET302 went down in what seemed a case similar to Lion Air JT610. “Why didn’t they use the trim Cut-Out switches?” was the reaction from longtime pilots. “Every pilot in the world knew what to do, weren’t they briefed?”
The information around the ET302 crash, which comes forward piece by piece, points to an almost identical chain of events to JT610. It also points to the pilots being briefed and, to the knowledge we have available, following procedures including the procedures for a wild running MCAS.
How could ET302 then crash? The FAA and Boeing issued an Airworthiness Directive 2018-23-51 on 8th of November 2018 on what Emergency checklist to use to avoid another MCAS caused crash? The crew should execute the “Trim runaway” emergency checklist.
So why didn’t it work, if the crew indeed followed this AD? Here’s why:
Two weeks ago I wrote: the flying with a full nose down Horizontal Stabilator trim was possible on a 737. The pilot could control the aircraft with his elevator control via the Yoke, even against a full nose down MCAS trim. This was verified in a 737 simulator by a US airline. At the time, I asked what the flown speed was? The answer I got was; the typical speeds after Taking Off.
In subsequent discussions with MentourPilot, a YouTube channel with millions of viewers, I was informed this would probably not be true for higher speeds. MentourPilot’s experience when examining hundreds of pilots as Type Rating Examiner for the 737 was the elevator got exceedingly heavy at higher speeds and manual trimming at the slightest miss-trim of the Stabilator from neutral Yoke forces was very difficult.
He also pointed out the high speeds observed in ET302’s FlightRadar24 trace (Figure 1) were logical. It’s a consequence of following the Emergency checklist for “IAS disagree” (IAS is Indicated Airspeed, i.e. the dynamic air pressure experienced by the aircraft) after takeoff.
The combination of the preceding checklist followed by an MCAS Trim Runaway checklist could create a situation where manual trimming after a Trim Cut-Out would be difficult to impossible and would require non-checklist actions.
Combined with the speed which follows from an “IAS disagree” Emergency checklist procedure the Pilot Monitoring (PM) could have problems to move the trim while Pilot Flying (PF) would fight to hold the Yoke against the elevator forces. At a larger miss-trim, the situation is unattainable.
The excessive manual trim forces have been confirmed by an airline pilot which has done 737 test flights after elevator maintenance, where manual trimming needs to be checked. At a miss-trimmed Stabilator, you either have to re-engage Electric trim or off-load the Stabilator jackscrew by stick forward, creating a nose-down bunt maneuver, followed by trim.
Stick forward to trim was not an option for ET302, they were at 1,000ft above ground. According to The Wall Street Journal, the ET302 crew re-engaged electrical trim to save the situation, to get the nose up. It was their only chance. But too late. The aggressive MCAS kicked in and worsened the situation before they could counter it.
We refrain from speculating more on the limited information we have available. What exactly happened in ET302 will be revealed by the preliminary report, which should be issued within days.
We now know that if the Wall Street article and the rumors are correct, the Trim Cut-Out switches were used by the ET302 crew.
We also know the execution of the Trim Runaway checklist as described in the Emergency Airworthiness Directive (AD) did not describe all the consequences of a trim Cut-Out after an MCAS cycle. Here the text from the AD:
In the event an uncommanded nose down stabilizer trim is experienced on the 737-8/-9, in conjunction with one or more of the indications or effects listed below, do the existing AFM Runaway Stabilizer procedure above ensure that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight?
An erroneous AOA input can cause some or all of the following indications and effects:
Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT.
Nowhere is it described the trim could be impossible to move if the Cut-Out switches were cut at the slightest miss-trim at the speeds flown. And there is no warning on when to move the Cut-Out switches, the checklist says “Cut, then trim manually.” This is not the whole truth.
What exactly happened will be known once the preliminary report is there. Today we know the crowing from Western pilots, “Typical third world crews,” was not called for. Anyone who has tried a correctly set up MCAS situation in a simulator is more muted.
PS. Regarding blowback of the Elevator, can this come into play as I wrote about here? With the information added by MentourPilot the situation seems to get unattainable before a blowback comes into play. We will know more once the ET302 preliminary report is here. DS