March 2019 | Dios Kurniawan
The tragic crash of Ethiopian flight ET302 which happened only five months after Lion Air JT610 disaster last year has made me convinced that Boeing 737 MAX has inherent problems. Both accidents are clearly linked and grounding all 737 MAXs globally is the right decision, if not a little late.
The last time history saw the grounding of an entire fleet of a newborn airliner model was in 1954, which took place after two brand new DeHavilland Comet jetliners had crashed within only few weeks. It was later determined that an engineering design flaw was behind both accidents. The design was fixed but nobody trusted Comet anymore.’
DeHavilland Comet (source: BAE Systems website)
We are still long way from drawing a conclusion as the 737 MAX investigation is still ongoing, but the Maneuvering Characteristics Augmentation System (MCAS) which is the alleged culprit behind Lion Air’s crash last October, is again on the firing line.
MCAS was introduced by Boeing – somewhat quietly – as a result of the new engine placement and the increased overall engine size in the 737 MAX. Because the engines are installed slightly forward and above the wings, the new design changed aerodynamic characteristics of the plane, giving it a tendency to pitch the nose upwards during flight. The larger engine nacelle generates additional (unintended) lift in some circumstances. This problem does not exist in the classic 737 models.
The MAX engine nacelle generates additional lift in front of the Center of Gravity (drawing edited from: b737.org.uk)
MCAS prevents the pilots from pulling up the nose too much, especially during turns (in which the danger of losing lift significantly increases) by automatically adjusting the vertical stabilizer trim in the tail. In other words, Boeing 737 MAX is more prone to stall, and MCAS is there to compensate for it.
The new engine in MAX is larger, and is placed further from the wing (photo source: aviationpros.com)
The smaller engine in a classic 737 (photo source: airplane-pictures.net)
Boeing has been very silent with this new equipment, with many pilots saying that MCAS is not mentioned anywhere in the Pilot’s Manual.
Was MCAS the cause for the Ethiopian disaster? FlightRadar24 captured 6 minutes worth of data from the doomed Ethiopian flight (shown below) , and it showed that problem came into being almost immediately after the plane left the runway. The aircraft’s altitude, depicted in the blue line, saw erratic fluctuation less than two minutes after take off. This is similar to the Lion Air crash in October. In Lion Air’s case, pilots managed to keep the plane fly a little longer (12 minutes).
Airspeed and Altitude of The Ethiopian Flight (source: FlightRadar24)
Looking at the crash crater, the debris was concentrated in a relatively small area. It is a strong indication that the aircraft impacted the ground at high speed and at high angle. Something must have caused such a sudden nose-dive path. This is not a typical engine failure case, because even with both engines dead, the plane should still have enough forward momentum to keep it gliding for a while longer.
The crash crater indicates plane impacted the ground at high speed (photo source: CNN)
Again, this is consistent with the Lion Air crash.
MCAS is designed to engage when the aircraft is : 1) flying with auto pilot off; 2) flying with high AoA (angle-of-attack) or high bank angle; 3) not in landing or take off posture (flaps not extended). Under normal practice, pilots would fly the plane without auto pilot during take-off roll until it reaches approximately 3,000 feet of altitude, usually the first 2-3 minutes of the flight. Pilots must have retracted the flaps as well by that time. Therefore, all prerequisites for MCAS to engage had been fulfilled by the Ethiopian 737 flight before it crashed.
MCAS might have changed the stabilizer trim to push the nose down without being commanded by pilots because for whatever reason the computer thought the plane was going into a danger of stall. Pilots did not have enough time (or did not know how) to react properly and the plane nose-dived uncontrollably into the ground.
What might have triggered the MCAS to activate? MCAS could have received wrong data from onboard sensors that the AoA was too high or the bank angle too steep, or both.
There are a lot of similarities between the two fatal accidents, but the missing link is what might have caused the MCAS to activate – in the Lion Air case it was the AoA sensors that were supplying errorneous data, but in the Ethiopian case, it is still a mystery. This raises question in the design of MCAS, how the software reacts to incomplete or conflicting data.
Modern airliners are statistically very safe. However, today planes are increasingly dependent on onboard computer systems and software. Because software is getting more and more complex, testing a new software to find potential defects has as well become increasingly challenging. It is natural to say that we have doubt whether MCAS software has been tested thoroughly during certification process.
Boeing’s promise to release a “software upgrade” (which is actually a bug fix) not long after the Lion Air disaster highlights the fact Boeing knew there was a flaw in the MCAS. The question remains as to why the regulators did not instruct grounding of all 737 MAX right after Lion Air crash, simply awaiting for a second tragedy to happen?