- Boeing addresses MCAS system providing multiple enhanced protection
- MCAS will now only be activated if both sensors agree
- Additional improvements made to the 737 MAX
- Orders placed for the 737 MAX and over 4,000 deliveries to be made
Following the grounding of the Boeing 737 MAX in 2019, Boeing have performed a number of enhancements to the 737 MAX aircraft.
Aircraft Completion News takes a look at what has changed following the Boeing overhaul of the aircraft.
The main change is MCAS.
MCAS [Maneuvering Characteristics Augmentation System] describes software that is built into the Boeing 737 MAX flight control computer – its purpose to provide stall identification or protection.
MCAS works by directly engaging the horizontal stabilizer.
As an automated corrective measure, the the previous version of MCAS was given full authority to bring the aircraft nose down, and could not be overridden by pilot resistance against the control wheel as on previous versions of the 737 MAX.
The stated goal of MCAS, according to Boeing, is to provide consistent aircraft handling characteristics at elevated angles of attack in certain unusual flight conditions only and hence make the 737 MAX perform similarly to its immediate predecessor, the 737NG.
Moreover, the system was intended to help the aircraft emulate the handling characteristics of the earlier 737 Next Generation [737-800 NG] and therefore meet Boeing’s internal objective of provides consistent airplane handling characteristics in a very specific set of unusual flight conditions – therefore minimizing training requirements for pilots already qualified on the 737NG.
I.e. Customers could purchase the aircraft and be operationally flying it with typed approved pilots quicker, by being able to use the same type rating and have 737 interchangeably of flight crew.
The software function now relies on two sensors, activated only once and never overriding the pilot’s ability to control the airplane and operates in unusual flight conditions only.
Summary of Boeing provided multiple MCAS enhanced protections:
- Measurements from two Angle of Attack (AOA) sensors will be compared.
- Each sensor will submit its own data to the airplane’s flight control computer.
- MCAS will only be activated if both sensors agree.
- MCAS will only be activated once.
- MCAS will never override the pilot’s ability to control the airplane using the control column alone.
Boeing and the FAA do not describe MCAS distinctly not an anti-stall system – MCAS together with the elevator feel shift (EFS) functions could be considered as stall identification and stall protection systems.
Boeing’s former CEO Dennis Muilenburg said “[MCAS] has been reported or described as an anti-stall system, which it is not. It’s a system that’s designed to provide handling qualities for the pilot that meet pilot preferences”.
According to the Joint Authorities Technical Review [JATR], MCAS “used the stabilizer to change the column force feel, not trim the aircraft”.
Though there are two sensors on the MAX only one of them is used at a time to trigger MCAS activation on the 737 MAX. Any fault in this sensor, perhaps due to physical damage, creates a single point failure: the flight control system lacks any basis for rejecting its input as faulty information.
What else has changed?
A number of additional improvements have been performed on the 737 MAX that do not directly relate to the Maneuvering Characteristics Augmentation System (MCAS) as part of Boeing’s continuous product improvement.
1. Developed software updates to address theoretical horizontal stabilizer issue
During testing, the Boeing team simulated what would happen in the event of various potential fault conditions. These identified a theoretical combination of faults that could lead to a runaway stabilizer condition. Although this condition has never occurred during the 200 million hours of flight operations on any 737 aircraft, new software was developed, tested and certified to ensure that it can never happen. Updated software will be loaded on all airplanes before they return to service.
2. Determined solution to modify some wiring to meet regulators’ requirements
The regulators’ comprehensive review process included a robust examination of the aircraft’s horizontal stabilizer control system. During the review the Boeing team determined that some of the wiring associated with the system wasn’t separated as far apart as required. All MAX aircraft will be modified to meet this requirement before returning to service. In some cases, Boeing will perform this task for their customers; in others, we’ll provide them with all of the technical documentation and materials they need to do the work themselves.
3. Checking stored airplanes for Foreign Object Debris (FOD)
FOD can be an unintentional byproduct of the airplane production process. During routine maintenance on airplanes in storage, we found some instances of FOD. Boeing immediately inspected all of the stored airplanes for FOD and shared inspection recommendations and detailed instructions with customers storing their own aircraft. Boeing also enhanced their training and procedures to reduce the likelihood of FOD, along with checking all aircraft before delivery or return to service.
4. Updated software to address remote possibility of autopilot disengagement
Extensive testing and analysis identified a remote possibility that the autopilot could disengage without a pilot command. Flight deck alerts and warnings were already in place that would alert the crew to this issue, which has never actually occurred. New software was developed, tested and certified to prevent this remote possibility from ever occurring. Updated software will be loaded on all 737-8 and 737-9 aircraft before they return to service.
What else has been of consideration?
Increased Angle of Attack (AOA) integrity
One regulator requested that Boeing also consider future action to further increase AOA integrity. The Boeing team are currently looking at the best way to develop and implement that functionality.
Additional crew alerting testing
Following return to service, Boeing and key regulators will continue to engage in studying the human factors associated with the crew alerting features on new models of the 737. Boeing’s planning for this initiative is in the early stages.
Current order numbers and backlog
- Over 4,000 737 MAX total unfilled aircraft orders
- Boeing logged orders for 82 new aircraft in February and had 51 cancellations.
- The manufacturer delivered 22 planes including 18 737 Max jetliners.
- The Covid-19 pandemic continues to weigh on sales of new planes.
Boeing has over 4,000 737 MAX aircraft as unfilled orders [4,013 as of mid-April 21′], and 3,240 categorized as backlog.
The Revenue Recognition Accounting Standard ASC 606 imposes criteria which requires Boeing to recognize backlog as beyond the existence of the contract to deliver.
Overall, Boeing sold 82 aircraft in February 2021 and logged 51 cancellations, marking the first time since November 2019 that monthly sales outpaced scrapped orders.
The Covid-19 pandemic continues to weigh on sales of new aircraft as customers try to manage cash flow. and spend largely try to conserve cash wherever they can. Some customers, however, are starting to prepare for a recovery and are actively buying new planes; especially when prices are favorable.
BBJ MAX Orders
A total of 21 BBJ MAX orders have been placed with Boeing, with 3-orders placed post the March 2019 grounding and 1 order in March of this year for a single 737 MAX VIP Completions Boeing Business Jet.
Boeing also has a number of unidentified customer single and small digit order quantities placed for the MAX placed since October 2019.
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Note: For more on Boeing 737-MAX Q&A relating to changes to he Boeing 737 MAX please visit: Boeing 737 MAX Updates