NZALPA Senior Technical Officer Dave Reynolds considers the recent concerns of the 737 MAX 8 and what might lay behind the recent tragic accidents.
What caused Boeing 737 Max 8 to nose dive after take-off? This is the question asked by aircraft investigators following the fatal Ethiopian Airlines ET302 accident last month. It was also an unanswered question leading to the type’s worldwide grounding.
A focus for the Federal Aviation Authority (FAA) soon became the aircraft’s (then) little known Maneuvering Characteristics Augmentation System (MCAS). The system was essentially fitted to make flying the MAX similar to earlier ‘Next Generation’ (NG) variants despite the MAX’s more powerful ‘LEAP’ engines. These engines, when flown at low speeds and at high angles of attack when clean caused a pitch up movement as a result of their extra thrust and forward mounting.
The addition of the MCAS system working with the aircraft Flight Control Computer (FCC) countered this unwanted pitch up.
“The logic of introducing MCAS was sound. However, not making the crew aware of it was not.”
However, the system had a very questionable architecture. A system that can effectively take control of the aircraft, based on the output of a single angle of attack (AOA) sensor, feeding a computer unable to discern if inputs from that sensor are spurious, beggars belief given all the hard lessons we have learnt around a single point of failure in aviation.
So, where are we now with MCAS? First, of course, these tragic events now mean that there will be no-one who flies the type, or should I say flew the type, who is unaware of MCAS!
It seems that following some pretty extensive reworking of the system by Boeing, the FCC software has now been updated to improve longitudinal stability ‘narrowing the band’ in which MCAS needs to operate. Where previously only one AOA sensor was used, both sensors are now utilised, with MCAS now able to sense a faulty sensor or system fault.
MCAS no longer repeatedly pushes the aircraft nose down every five seconds when operating. Related to this and very crucially - pilot elevator input also now exceeds MCAS stabiliser authority so that pitch control is assured at all times. In addition, MCAS operation-related airspeed and altitude faults are now annunciated together with an ‘AOA disagree’ to pilots. This, then, is to place these into context as being associated with an MCAS/AOA fault.
But isn’t there something else that needs to be addressed here? Something more fundamental?
Why were the crew unaware of the system – one which clearly had the ability to take control of the aircraft from them?
There has been much talk in recent years of the ‘dumbing down’ of technical content and the shortening of pilot type training courses. Technology to an extent has driven this, but also, without doubt, so has cost. A reduction in pilot numbers means that an airline wants to get its assets trained as quickly as possible and out flying aircraft.
Are the MAX8 losses also a manifestation of this?
<< General Manager's note Grief can hit in various ways >>