By Bjorn Fehrm
March 27, 2018, © Leeham News.: Boeing presented the software fix to the MCAS problem in a press briefing ahead of meeting 200 Pilots and Regulators in Seattle today for a briefing around the 737 MAX update.
Here is what Boeing presented in its briefing to media:
Boeing has been developing an improved version of MCAS since the Lion Air accident. According to Vice President of Product Development and Product Strategy Mike Sinnett, the development started when Boeing understood the MCAS function could play a role in the accident.
“We always analyze operational events and accidents which are customer experience with our aircraft and we work on improvements as we understand how these can be made” said Mike Sinnett. We have been testing a new version of MCAS in our labs and simulators since months and we have conducted test flights with it, also with the FAA present. We have now flown our production 737 MAX test flights since two weeks with this improved software load.”
Here is what has changed regarding MCAS, said Boeing:
MCAS is designed to activate in manual flight, with the airplane’s flaps up, at an elevated Angle of Attack (AOA).
Boeing has developed an MCAS software update to provide additional layers of protection if the AOA sensors provide erroneous data. The software was put through hundreds of hours of analysis, laboratory testing, verification in a simulator and two test flights, including an in-flight certification test with Federal Aviation Administration (FAA) representatives on board as observers.
The additional layers of protection include:
These updates reduce the crew’s workload in non-normal flight situations and prevents erroneous data from causing MCAS activation.
The Angle of Attack signal, coming from the sensor to the ADIRU (Air Data Inertia Reference Unit) on each side of the aircraft will be compared before sending it on to the Flight Control Computer (FCC) to trigger the MCAS function.
If the AoA disagrees by more than 5.5° when the flap is retracted, MCAS will be disabled for the flight. To warn the pilots of the missing MCAS, an AoA disagree will be displayed on the Pilots Primary Flight Display (PFD), Figure 1.
MCAS will also be disabled and if the AoA Disagree displayed with the AoA differs more than 10° for over 10 seconds during flight.
When a valid Angle of Attack passes a set threshold close to stall, with the threshold dependent on speed and other data, the MCAS function will trigger. But it won’t repeat if the Pilot trims during or after the MACS intervention. It will just trim once nose down for every passing of the AoA threshold. Today’s function repeated over 20 times in the Lion Air case as the pilots trimmed against a faulty AoA signal.
Finally, the amount of trim the MCAS function is allowed to trim will not pass the limit where the aircraft is fully controllable in pitch even if the pilots do nothing to stop MCAS to trim nose down. Today’s function will run to max trim nose down if the pilots trim against but not long enough to compensate for the aggressive MCAS nose down trimming.
Boeing will now work to have the software update, which takes about one hour to install on the 737 MAX, approved by the world’s Airworthiness authorities. As the authorities are in the drivers’ seats for when the update gets approved to install, it’s impossible to say when the individual airline’s 737 MAX will fly again. It all depends when their local regulator approved the fix and allow the MAX to return to operation.
Before the 737 pilots are allowed to fly the updated 737 MAX, they will be obliged to pass a training curriculum which Boeing has produced and the aircraft’s manuals need updating. Here Boeing’s statements regarding training and changed manuals:
To earn a Boeing 737 type rating, pilots must complete 21 or more days of instructor-led academics and simulator training. Differences training between the NG and MAX includes computer-based training (CBT) and manual review.
Boeing has created updated CBT to accompany the software update. Once approved, it will be accessible to all 737 MAX pilots. This course is designed to provide 737 type-rated pilots with an enhanced understanding of the 737 MAX Speed Trim System, including the MCAS function, associated existing crew procedures and related software changes.
Pilots will also be required to review:
As of now, the MCAS is classified as a sub-function of the Speed Trim System which 737 pilots have been flying with since decades. Speed Trim enhances the pilots feel for the 737 at speed changes at low speeds. It trims in a schedule somewhat similar to MCAS but has some distinct differences:
In summary, Boeing now starts the long road of getting the improved MCAS function accepted and approved by all the countries Airworthiness authorities where 737 MAX airlines operate. It used to be a swift and smooth process; no longer. The dominant FAA, which other regulators followed without much discussion, has lost authority and this will take time to regain.