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What airlines need to do before they can fly Boeing 737 MAX?

While the Federal Aviation Administration (FAA) has formally un-grounded the Boeing 737 MAX, a lot has to be done before the jet takes to the skies commercially.

Following 20 months of grounding, the FAA cleared the 737 MAX to return to the skies on November 18, 2020.  Aviation authorities across the globe temporarily suspended the aircraft’s type certificate after two fatal crashes in Indonesia and subsequently in Ethiopia. While the FAA was pretty much the last one to do so, it is the first to un-ground the aircraft as well.

But hold thy horses, as the 737 MAX has quite the way to go before it can fly passengers on commercial flights within the United States, or anywhere else for that matter.

Maintenance procedures

Between May 2017, when Malindo Air became the launch customer of the type, and March 2019, Boeing built and delivered 387 aircraft to customers across the globe. The largest number of 737 MAX aircraft was delivered to North America, including Mexico, where 166 of the global fleet has landed before the grounding. China remains a very important market as well, where Boeing delivered 81 MAXs – the second-largest market for delivered aircraft behind the US.

Prior to operating the aircraft, airlines will have to complete quite a few tasks. One of the most crucial ones is to complete the tasks in the Aircraft maintenance manual (AMM). For example, the AMM for a Boeing 737 Classic, an older generation of the aircraft, points out 44 references on how to put back the aircraft into serviceable condition after storage. On average, around 40 man-hours are required to make a stored aircraft airworthy once again, according to the British Women Pilots’ Association (BWPA).

For the Boeing 737 MAX, the final step of returning the aircraft to service requires an Operational Readiness Flight, according to Service Bulletin 737-00-1028. However, due to the background of the groundings, the procedure has a lot of added complexity.

Software and wire changes

The major work for airlines will be the installation of the revised software that would change the way that the Maneuvering Characteristics Augmentation System (MCAS) works. The controversial system, which has been at the forefront of attention throughout the grounding period, will be changed drastically.

Prior to the grounding, the MCAS relied on data from one Angle of Attack (AoA) sensor, without the redundancy of the other sensor located at the nose of the Boeing 737. The automatic system could also shoot off multiple times, slowing down the aircraft and result in an unrecoverable stall of the aircraft via the multiple nose-down movements.

Now, the MCAS will rely on data from both sensors. If the data is erroneous and disagrees by 5.5 degrees, the Speed Trim System (STS), including the MCAS, will be disabled for the remainder of the flight. Pilots will also be notified of the disagreement via the AoA DISAGREE alert and the disablement of the STS. Furthermore, the MCAS will only be able to fire off once per one event, rather than multiple times. The limited magnitude of the nose-down movement will “preserve the flight crew’s ability to control the airplane’s pitch by using only the control column,” read the FAA’s Airworthiness Directive (AD). An extra layer of redundancy is added by the Flight Control Computers (FCC) cross-monitoring each other, including the fact that it will now detect erroneous commands, including trim stabilizer commands.

The US-based authority also imposed the requirements for operators to change the horizontal stabilizer trim wire routing installations in order to prevent a short-circuit in the wiring, which could cause stabilizer trim runway.

All in all, the FAA estimates that installing new software on the FCC, the revised MAX Display System (MDS) and the stabilizer wiring changes would cost up to $10,760 per aircraft, including parts and labor.

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