By Seattle Times staff

Published Oct. 27, 2019

Inability to operate the manual trim on a 737 is a hot topic just now. During my 737 type conversion (3-400 series, UK, 1998) we were shown a techniqe for trimming manually even when aerodynamic loads were too large to allow normal movement of the wheel. An American Airlines flight manual for 737 Max pilots dated October 2017 said the thumb switches had less ability to move the nose than the manual wheel. Trim wheel is a relic of the Boeing.

The yoke mounted electric trim switches (theoretically) move the trim at the same speed. In comparison (I read on PPRuNe forum that) one turn of the manual wheel equals about 0.07 trim units, so requires 15 rotations per unit of trim. So MCAS trims down at 0.27 units per second, or 3.5 rotations of the manual wheel per second. When MCAS is activated in the cockpit and moves the horizontal stabilizer, a large wheel beside each pilot that’s mechanically connected to the stabilizer begins to spin. This is the manual trim. Sisk said the United 737 MAX differences bulletin already referred to situations where nose-down trim is automatically applied as airspeed decreases toward a stall.

Ethiopian Airlines Flight 302

Ethiopian 302

March 10, 2019

The flights were eerily similar, just minutes long, with pilots engaged in a terrifying battle against an automated safety system that ultimately sent the 737 MAX jets plunging to earth.

But the journeys of Lion Air 610 and Ethiopian Airlines 302 also contained some stark differences. The pilots in the second crash had the lessons of the first, with new guidelines from Boeing and the FAA. They tried to follow instructions for recovering the jet. It still wasn’t enough.

We’ve written before about competitive pressures that resulted in a rush to build the MAX, the flawed analysis behind the new, automated system called MCAS (Maneuvering Characteristics Augmentation System), the FAA’s shift to delegating certification tasks to Boeing, and internal pressures at Boeing to advance planes to completion.

Here, we examine the arc of the two tragic flights, tracking the movements of pilots and the details of flight-deck data through critical seconds.

Before takeoff

Problem 1

Even before takeoff with 189 people on board, the data on Lion Air 610 showed evidence of a problem. As the plane taxied, the two angle-of-attack (AOA) sensors on the nose of the jet recorded starkly different values. The left sensor was clearly wrong — the plane was still on the ground — but the aircraft didn’t recognize the discrepancy.

The angle of attack is the angle between the wing and the airflow. The sensors include vanes that align themselves with oncoming airflow. The angle of the vane changes as the plane’s nose rises or falls.

Problem 2

Those sensors are critical. While some airplanes, particularly those from Boeing rival Airbus, have three such sensors that can work with one another in the case of an erroneous value, Boeing’s 737 MAX has just two, and the aircraft used only one of those sensors to trigger a new automated system — MCAS — that would force the nose of the plane down if the sensor indicated a potential stall. Records also show that previous flights of this aircraft had problems with the angle-of-attack sensor.

The first minutes

Immediately after takeoff, the Lion Air plane began giving the pilots warnings as a result of the faulty sensor. The captain’s control stick began shaking, an indicator of a potential stall. The crew asked ground controllers to confirm their altitude and airspeed, due to disagreements on their instruments caused by the same faulty angle-of-attack sensor. They reported problems with the flight controls.

After climbing above 2,000 feet, the pilots retracted the flaps on the wings. The MCAS system, which the crew was unaware of, is designed to turn on once the flight crew retracts the flaps and disengages the autopilot. Seconds later, just a couple of minutes into the flight, the MCAS safety system engaged, pushing the nose down. The plane descended.

The pilots were able to regain control of their ascent by extending the wing flaps again, preventing MCAS from engaging. But it wouldn’t be long before the crew was ready to retract the flaps again.

On Ethiopian Airlines Flight 302, with 157 on board, there was no sign of a problem before takeoff. Flight data shows the angle-of-attack sensors tracking each other identically. That stayed consistent while the plane was on the ground, but it was about to change, just not in the same way that it did on Lion Air 610.

Shortly after takeoff, Ethiopian 302's angle-of-attack sensors recorded a sudden disagreement. It’s unclear what caused the fault, though the suddenness and severity of the change suggests a bird strike may have cut off one of the AOA vanes, which protrude from the front of the plane.

Problem 3

The plane’s left sensor, the one that apparently controlled the MCAS system, recorded an angle of attack at 74.5 degrees — far higher than would be conceivable. A stall would typically be a risk before 20 degrees.

The sensor’s value remained stuck. But even though the sensor was no longer showing even slight fluctuations and was stuck at an inconceivable value, MCAS continued considering it valid data.

Problem 4

This set off a cascade of warning indicators in both cockpits — airspeed, altitude and stall — for the pilots to consider. The MAX was supposed to come with an indicator to tell the pilots that there was a disagreement between the two angle-of-attack sensors. However, due to a software glitch that Boeing discovered back in 2017 but hadn’t fixed, that light wasn’t actually functional for most customers, including both Lion Air and Ethiopian Airlines.

MCAS kicks in

Problem 5

After ascending to 5,000 feet, the Lion Air pilots retracted their flaps again. The MCAS system soon reactivated, turning the plane’s nose down. Flight data shows the pilots manually countering the MCAS system, using electrical thumb switches on the control column to bring the nose of the plane back up.

But instead of shutting off and ceding control to the pilots, MCAS repeatedly engaged. Over the next seven minutes, MCAS pushed the plane down more than two dozen times. By using the electric trim power, accessible by thumb switches on the yoke, and by pulling back on the control column, the pilots were able to move the nose back up.

Problem 6

While MCAS moved the jet nose down with substantial power, Boeing had determined that pilot training was not necessary for the system and MCAS was not included in the pilot manual.

Boeing believed pilots would consider repeated nose-down commands to be similar to a runaway stabilizer, for which there is a cockpit checklist. But the pilots appear to have not interpreted their readings that way and fixated on other issues: They asked ground controllers to help determine their altitude because those readings were haywire due to the faulty sensor.

Problem 7

Five seconds after the Ethiopian pilots disengaged the autopilot, MCAS took control. The system moved the trim down 2.5 units, sending the plane into descent. As first reported in The Seattle Times, FAA officials believed, based on Boeing’s initial system safety assessment of the MAX aircraft, that MCAS was only supposed to be able to move the trim by 0.6 units, not 2.5.

Problem 8

In addition, because MCAS repeatedly engaged, it effectively acted with unlimited power. The pitch of the jet is marked on a scale that’s visible on the stabilizer trim wheel beside the throttles. On this scale, 0 is maximum nose-down pitch and 4.7 is level flight. The green band on the wheel shows the range of where the pitch should be in normal flight. At one point in the Ethiopian 302 flight, MCAS pushed the stabilizer to just 0.4 units, a severe nose-down pitch.

Then, just 35 seconds after the sudden takeover of MCAS, the pilots appeared to have relied on Boeing guidance on how to shut it off. The first officer called out “stab trim cutout” two times. The captain agreed, so the crew flipped the two “Stab Trim” cutout switches (seen at bottom right) to block electric movements of the horizontal tail.

The fight to save the plane

After more than five minutes of battling the plane, the repeated efforts by the Lion Air pilots broke down. They had been talking with ground controllers about their direction and plans, then the second-in-command asked for a new direction to apparently avoid some weather. Just 11 seconds later, the pilot asked for help determining the altitude again.

Data from the report shows that the captain successfully countered MCAS more than 20 times. However, the data suggests the first officer then took over, perhaps to let the captain figure out what was happening. The plane then responded to two further nose-down commands with only short flicks of the thumb switches, which failed to fully counter the movement. That sent the plane’s trim further down.

Problem 9

Separate data released to the Indonesian parliament shows the pilots desperately pulling the control column back. On the previous 737 model and other Boeing planes, a pilot pulling back hard on the column would trigger a cutout switch that would halt automatic commands in the opposite direction. In order to make MCAS work, Boeing had to disable that feature when the new flight control system was active.

While the Ethiopian Airlines pilots had cut off the electric power that moved the horizontal tail, the tail was still in a position to pitch the nose down. The pilots pulled on the control column and continued ascending thanks to the elevator tabs on the trailing edge of the horizontal tail.

The raised elevator tabs at the back and the leading edge of the horizontal tail swiveled upward form a sort of V shape that creates opposing nose-up and nose-down forces.

Boeing’s previous 737 model had two separate cutout switches, one specifically for electric power to the automatic controls, the other for electrical power to the manual thumb switch on the control column. However, the option to operate these separately was never included in the manuals. And on the MAX, the two switches had identical functions. One primary and one backup, either one cut out all electric power, automatic and manual. Standard procedure was to flip both at once.

Problem 10

Having hit the cutout switches, the only way the Ethiopian crew could adjust the horizontal tail was to do it manually, by physically turning a wheel beside the pilot that is connected by cables to the back of the plane. The first officer tried but reported that it wasn’t working.

Boeing has described the shutoff of the trim as simple, but with the swiveled horizontal tail pushing nose down and the pilot-controlled elevator tabs on the back pushing nose up, the airflow can create substantial force that makes it physically challenging for pilots to move the wheel and swivel the trim manually.

Problem 11

Boeing’s emergency directive, issued after the Lion Air crash, included a line near the bottom that mentions a way to avoid high forces making the manual trim wheel hard to move. It says that pilots “can” use the electric trim switches on the control column to neutralize those forces before hitting the cutout switches.

But that was not emphasized as a necessary step to avoid disaster. It appears that the Ethiopian pilots, eager to stop MCAS quickly and avoid the fate of the Lion Air flight, jumped immediately to the cutout switches without the vital step of first using the thumb switches to ease the forces on the horizontal tail. That’s why they couldn’t move it manually.

The final seconds

Just 31 seconds before the Lion Air crash, the pilot was still well aware of his surroundings, asking ground controllers to prevent other planes from flying near their altitude. When the controller asked their preferred altitude, the pilot responded: “five thou.” That was 19 seconds before the crash.

Problem 12

The pilot pulled on the control column with all possible force. But with the column only controlling the elevator tabs at the back of the horizontal tail, he was unable to fully counter the nose-down pitch from the MCAS swiveling of the larger portion of the horizontal tail.

The airspeed rose.

The swiftness of the descent increased.

The pilots lost control.

Even in the doomed plummet, they yanked the control column in desperation. At 6:31:54 a.m., Lion Air Flight 610 nose-dived into the Java Sea at about 500 mph.

The Ethiopian Airline pilots inexplicably hadn’t throttled back the engines, which were still at takeoff power. As a result, the speed of the plane rose well above 400 mph — higher than the maximum design speed of the airplane. That triggered overspeed warnings in the cockpit that include an alarming clicking sound called a “clacker.” At that speed, the forces on the tail would have made it impossible to move manually.

The pilots were likely using sizable physical effort to keep the plane level by pulling back on the control column. They then made a call: Their efforts to keep the airplane pitch were not enough. They decided to re-engage the electric controls. With the electric power to the tail restored, they used the thumb buttons to trim the nose up — but, for some reason, not substantially, just two short flicks. One expert theorized that the plane’s high speed may have caused an alarming shudder when they moved the trim.

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For a few seconds, they eased the descent. But with the electric power restored, MCAS now re-engaged.

The plane abruptly turned downward again with a force that would have pitched passengers off their seats. The plane dove steeper, faster.

Just 23 seconds after MCAS had re-engaged, Ethiopian Airlines Flight 302 — traveling at a speed approaching 600 mph — smashed into the ground at 8:43:43 a.m. The compact, deep crater in the earth indicated a near-vertical impact.

SINGAPORE (Reuters): U.S. and European regulators knew at least two years before a Lion Air crash that the usual method for controlling the Boeing 737 MAX’s nose angle might not work in conditions similar to those in two recent disasters, a document shows.

The European Aviation and Space Agency (EASA) certified the plane as safe in part because it said additional procedures and training would “clearly explain” to pilots the “unusual” situations in which they would need to manipulate a rarely used manual wheel to control, or “trim,” the plane’s angle.

Those situations, however, were not listed in the flight manual, according to a copy from American Airlines seen by Reuters.

The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said.

It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s center console rather than an electric thumb switch on the control yoke.

EASA and the U.S. Federal Aviation Administration (FAA) ultimately determined that set-up was safe enough for the plane to be certified, with the European agency citing training plans and the relative rarity of conditions requiring the trim wheel.

In the deadly Lion Air crash in October, the pilots lost control after initially countering the Maneuvering Characteristics Augmentation System (MCAS), a new automated anti-stall feature that was pushing the nose down based on data from a faulty sensor, according to a preliminary report from Indonesian investigators released in November.

The flight conditions were similar to those described in the EASA document, a source at Lion Air said. The source said that training materials before the crash did not say the wheel could be required under those conditions but that Boeing advised the airline about it after the crash.

Boeing declined to comment on the EASA document or its advice to Lion Air, citing the ongoing investigation into the crash.

Ethiopia’s Transport Ministry, France’s BEA air accident authority and the FAA have all pointed to similarities between the Lion Air crash and an Ethiopian Airlines disaster this month. But safety officials stress that the Ethiopian investigation is at an early stage.

‘NOT PHYSICALLY EASY’

The crashes have also heightened scrutiny of the certification and pilot training for the latest model of Boeing Co’s best-selling workhorse narrowbody, now grounded globally.

In the EASA document, the regulator said simulations showed the electric thumb switches could not keep the 737 MAX properly trimmed under certain conditions, including those of the Lion Air and Ethiopian Airlines crashes, according to the Indonesian preliminary report and a source with knowledge of the Ethiopian air traffic control recordings.

The trim system adjusts the angle of the nose. If the nose is too far up, the jet risks entering a stall.

Additional procedures and training needed to “clearly explain” when the manual wheel might be needed, according to the document. The EASA spokesman said that was a reference to the Boeing flight crew operations manual.

An American Airlines Group Inc flight manual for 737 MAX pilots dated October 2017 said the thumb switches had less ability to move the nose than the manual wheel.

The manual, which is 1,400 pages long, did not specify the flight conditions in which the wheel might be needed.

The trim wheel is a relic of the Boeing 737’s 1960s origins and does not appear in more modern planes like the 787 and Airbus SE A350. It is not often used, several current and former 737 pilots told Reuters.

“It would be very unusual to use the trim wheel in flight. I have only used manual trim once in the simulator,” said a 737 pilot. “It is not physically easy to make large trim changes to correct, say, an MCAS input. You – or more than likely the other pilot – have to flip out a little handle and wind, much like a boat winch.”

The EASA document said that after flight testing, the FAA’s Transport Airplane Directorate, which oversees design approvals and modifications, was concerned about whether the 737 MAX system complied with regulations because the thumb switches could not control trim on their own in all conditions.

FAA declined to comment on the European document. A trim-related “equivalent level of safety” (ELOS) memorandum listed in its 737 MAX certification document is not available on the FAA website. The agency declined to provide it to Reuters.

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CONFUSING SIGNALS

The night before the Lion Air crash, different pilots on the same plane faced a similar problem with MCAS and tried to use electric trim to counteract it, according to the preliminary report from Indonesian investigators.

After the third time MCAS forced the nose down, the first officer commented that the control column was “too heavy to hold back” to counter the automated movements, the preliminary report said.

Former FAA accident investigator Mike Daniel said that to prevent stalls, the control column was designed to require more force for a pilot to pull back than to push forward.

Boeing on Wednesday said software changes to MCAS would provide additional layers of protection, including making it impossible for the system to keep the flight crew from counteracting it.

On the 737 MAX, Boeing removed the “yoke jerk” function that enabled pilots to disable the automated trim system with a hard pull on the control column rather than hitting two cut-out switches on the center console.

In a blog post on his personal website, former Boeing engineer Peter Lemme said that could make things harder for a pilot in a crisis.

“In the scenario where the stabilizer is running away nose down, the pilot may only fixate on pulling the column back in response,” he said. “They may not be mentally capable to trim back or cutout the trim – instead they just keep pulling.”

Ultimately the crew the evening before the Lion Air crash stopped the automated nose-down movement with the cut-out switches and used the wheel to control trim for the remainder of the flight, the preliminary report said.

That was the proper procedure to deal with a runaway stabilizer, according to Boeing.

However, current and former pilots told Reuters that the way the trim wheel and other controls behaved in practice compared with in training may have confused the Lion Air crews, who were also dealing with warnings about unreliable airspeed and altitude.

“MCAS activation produces conditions similar to a runaway trim, but the training is not done with a stick shaker active and multiple other failures, which make the diagnosis much more difficult,” said John Cox, an aviation safety consultant and former commercial pilot. The stick shaker alerts pilots to a potential stall by vibrating the control column.

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Reuters this month reported that an off-duty pilot in the cockpit on the night before the Lion Air crash spotted the runaway stabilizer problem, according to two sources familiar with the matter.

Boeing on Wednesday said changes to the MCAS software would help “reduce the crew’s workload in non-normal flight situations.”