A Southwest Airlines Boeing Co. 737 aircraft. Image Credit: Bloomberg

Aviation authorities originally certified the Airbus A320, the world’s second-best-selling airliner, to fly using a 16-bit computer chip that is laughably anaemic by today’s standards. The Intel 80286 was in charge of the most critical functions of the A320 — translating the pilot’s inputs to climb, descend, and turn — and yet it would barely turn on a smartphone today ... let alone allow you to scroll through a social media feed.

So how could a Boeing computer programme tasked only with preventing aerodynamic stalls — a much more limited task than the A320’s early brain — be at the centre of an investigation into two deadly crashes of the cutting-edge Boeing 737 Max? As investigators look for similarities between the Ethiopian Air crash and the Lion Air disaster in November, the use of computers and software in aviation is under intense scrutiny.

More modern chips have replaced the Intel 80286 in most A320s flying today, but little about how Airbus uses them has changed. Whenever a pilot (human or automatic) wants to bank the A320’s wings or climb to a higher altitude, the computer transfers the command digitally to the control surfaces in the wings and tail. This fly-by-wire system can also recognise and discourage commands that might cause a stall or other hazards to safe flight.

By contrast, the versions of the Boeing 737 currently flying have physical links between their control columns and the surfaces. Computers do not interpret a pilot’s wishes in the same way as on the A320. Inputs from the cockpit are delivered to the wings and tail with hydraulic cables, not ones and zeros.

While it’s true that software flies the A320, the Airbus fly-by-wire system is a marvel of robustness and simplicity, in no small part because it was designed to work with the constraints of 1980’s-era computer hardware. Some experts have even suggested that the successful ditching of an A320 in New York’s Hudson River in 2009 would have been much more difficult without the A320 software’s vigilance about preventing a stall.

Flying by wire also offers Airbus a unique opportunity to make continuous improvements to the aeroplane, much as software developers do for smartphone apps. The most significant improvement to the A320 came in 2010, when Airbus tweaked the fly-by-wire characteristics and made other changes so it could fit more fuel-efficient engines.

The result, called the A320neo for “new engine option,” proved so successful with airlines that Boeing decided to do the same thing to its 737, and the 737 Max was born.

But because the 737 does not fly-by-wire, software improvements to allow for new engines on the same airframe had to be created from scratch, not revised. Boeing believed that it could do so without the need to extensively retrain 737 pilots, and the US Federal Aviation Administration agreed. As the Seattle Times reports, the process that led to the certification of the 737 Max and its new software, the Manoeuvring Characteristics Augmentation System (MCAS), was rife with inaccuracies and relaxed regulatory oversight.

Official causes of the Lion Air and Ethiopian crashes have yet to be determined, but one thing is clear: hundreds of A320neo planes are currently flying worldwide while the Boeing 737 Max is grounded in almost every country as a result of the crashes. As investigators sift through evidence, Boeing is racing to fix the MCAS, and it is expected to announce a software fix this week.

If the MCAS is indeed a culprit in the twin tragedies in Ethiopia and Indonesia, it would not be a sign that software isn’t safe enough to fly aeroplanes. But it would suggest that not every aeroplane can take a software update without a major re-evaluation of how it flies.

A new smartphone app doesn’t deserve intense regulatory scrutiny, but fitting new engines to an aeroplane by tweaking its software certainly does. The Airbus A320neo proves this is possible, and now it is Boeing’s turn to reassure passengers by re-evaluating its approach to digital flying.

Tom Brant writes about computers and other personal technology for