Ethiopian Airline Crash - Boeing Advice To 737 MAX Pilots Was Flawed
By Moon of Alabama
Apr 6, 2019 - 5:30:40 AM
The preliminary report on the March 10 Ethiopian Airline crash shows that the advice given by the FAA and Boeing to 737 MAX pilots was incomplete. The pilots followed the advice but it was phyisically impossile for them to bring the plane back into a stable flight.
In October 2018 a brand new Boeing 737 MAX airplane, flown by the Indonesian Lion Air airline, crashed into the sea shortly after take off. 189 people died. An investigation found that Boeing had added a 'maneuver characteristics augmentation system' (MCAS) to the MAX that could directly influence the stabilizer, a primary flight control surfaces, but based its decisions on the input of only one sensor. When the sensor failed the system went wild and destabilized the airplane.
Neither the pilots nor the airlines knew about the system. The regulators. which certified the plane as safe to fly, were misinformed about it. They had directed Boeing to include the new system into training material for the pilots which Boeing, for commercial reasons, did not do.
After the Lion Air crash the Federal Aviation Administration issued an Airworthiness Directive 2018-23-51 which adviced 737 MAX pilots how to handle an MCAS failure.
The FAA told 737 MAX pilots to use the Stabilizer Trim Cutoff switches to interupt the power supply for the system's actuator, a motor driven jackscrew in the back of the airplane. The pilots should then use the manual trim wheels in the cockpit, which move the jackscrew and stabilizer via steel cables, to righten the aircraft.
On March 10 a 737 MAX flown by Ethiopian Airline crashed shortly after take off. 157 people died. Radar data and debris found showed that the cause was likely a similar MCAS failure as had happened on the Indonesian Lion Air flight.
All 737 MAX planes were grounded with the U.S. being the last country to order it.
Some U.S. pilots, as well as some commentators here, publicly blamed the darker skin pilots for not using the simple procedure the FAA had put out: "Why didn't they just flip the switches? Stupid undertrained third-world dudes."
It now turns out that the well trained and experienced pilots on the Ethiopian Airline flight did exactly what Boeing and the FAA told them to do. From the Ethiopean Airlines press release(pdf):
The preliminary report clearly showed that the Ethiopian Airlines pilots who were commanding Flight ET 302/10 March have followed the Boeing recommended and FAA approved emergency procedures to handle the most difficult emergency situation created on the airplane. Despite their hard work and full compliance with the emergency procedures, it was very unfortunate that they could not recover the airplane from the persistence of nose diving.
The procedure Boeing and the FAA advised to use was insufficient to bring the aircraft back under control. It was in fact impossible to recover the plane. The possibility of this to happen was discussed in pilot fora and on specialized websites for some time.
The MCAS system moves the front of the stablizer up to turn the nose of the airplane down. The plane then decends very fast. The aerodynamic forces (the "wind") pushing against the stabilizer gets so strong that a manual counter-trim becomes impossible.
Avionics engineer Peter Lemme details the physics involved in this.
via Seattle Times - full picture
With the 737MAX cutout switches, MCAS runaway is stopped by throwing both switches, losing electric trim altogether. In this case, the flight crew must rely on manual trim via turning the trim wheel/crank. As discussed above, the manual crank can bind up, making flying much more difficult.
Bjorn Fehrm, a senior engineeer and pilot now writing at Leeham News, came to a similar conclusion:
[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.
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.
On the FAA's Airworthiness Directive Fehrm writes:
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.
An detailed analysis of the flight recorder data as documented in the preliminary crash report confirms the conclusions:
The high speed of 340kts indicated airspeed and the trim at 2.3 units causes the Stabilator manual trim to jam, one can't move it by hand. The crew is busy trying to hand trim the next two minutes but no trim change is achieved.
via Leeham News - bigger
The pilots then do the only thing possible. They reengage the electric stabilizer trim to righten the aircraft.
But the aggressive MCAS, trimming with a speed 50% higher than the pilot and for a full nine seconds, kicks in at 8 with a force they didn't expect. Speed is now at 375kts and MCAS was never designed to trim at these Speed/Altitude combinations. Dynamic pressures, which governs how the aircraft reacts to control surface movements, is now almost double it was when last MCAS trimmed (Dynamic pressure increases with Speed squared).
The Pilots are thrown off their seats, hitting the cockpit roof. Look at the Pitch Attitude Disp trace and the Accel Vert trace. These are on the way to Zero G and we can see how PF loses stick pull in the process (Ctrl Column Pos L). He can barely hold on to the Yoke, let alone pull or trim against.
His reduced pull increases the pitch down further, which increases the speed even more. At 05.45.30 the Pilots have hit the seats again (Accel Vert trace and Ctrl Columns force trace) and can start pulling in a desperate last move. But it's too late. Despite them creating the largest Control Column movement ever, pitch down attitude is only marginally affected.
The pilots and their passengers lose the fight:
It's easy to say "Why didn't they trim then?". Because they are going down at 20 degrees nose down (which is a lot, a normal landing approach is 3°) and at 400kts. Then you just pull for all you have. And the aircraft is not reacting to the largest Control Column displacement since takeoff. This makes them pull even harder, the aircraft is unresponsive and they are fighting for theirs and all the passenger lives.
A diligent safety anlysis would have predicted this outcome. Neither Boeing nor the FAA seems to have done such after the first 737 MAX crashed. They provided an Airworthiness Directive with procedures that were insufficiant to correct the system induce misbehavior.
Moreover their description of the MCAS was incomplete. It is only now known that the MCAS trims the stabilizer at a speeed of 0.27 units (degrees) per second while the pilots electric trim moves the stabilizer at only 0.18 units per second:
"It's like a Tasmanian devil in there," says Dennis Tajer, a 737 pilot and communications chair for Allied Pilots Association, which represents American Airlines' pilots.
If MCAS keeps tripping, and if pilots do not shut off electric trim entirely, the result is what Tajer describes as a two-steps-back, one-step-forward scenario, with MCAS maintaining an edge.
"The MCAS knows but one speed, which is 0.27, which is the most-aggressive speed," Tajer says. "If you look at the balance sheet on it, MCAS is winning, and you are losing."
The insufficient advice to pilots given after the first crash only adds to the long list of criminal mistakes Boeing made and which the FAA allowed to pass.
Today the Washington Post reports of another software defect which the FAA demands to have fixed:
Boeing confirmed to The Washington Post that it had found a second software problem that the Federal Aviation Administration has ordered fixed - separate from the anti-stall system that is under investigation in the two crashes and is involved in the worldwide grounding of the aircraft.
That additional problem pertains to software affecting flaps and other flight-control hardware and is therefore classified as critical to flight safety, said two officials with knowledge of the investigation.
The criminals at Boeing again offer no explanation and play down the issue:
In a statement, Boeing called the additional problem "relatively minor" but did not offer details of how it affects the plane's flight-control system. "We are taking steps to thoroughly address this relatively minor issue and already have the solution in work to do that," it said.
What other 'features' were secretly implemented into the 737 MAX without sufficiant analysis about their side effects and consequences?
Previous Moon of Alabama posts on the 737 MAX crashes:
Posted by b on April 5, 2019
writings by members of AbundantHope are copyrighted by
AbundantHope - All rights reserved