On the 10th of March this year, a Boeing 737 plane crashed in Ethiopia, killing all 157 on board.
Investigators have released their early findings into another accident involving a 737 operated by Indonesia’s Lion Air, in October last year, and they suggest the cause was “an erroneous angle-of-attack sensor”.
An angle-of-attack sensor is usually positioned to the side of the plane’s nose and detects oncoming air flow, to which it adjusts and sends back data to the plane’s computer system.
You could maybe imagine the sensor system as though it were a spirit level similar to the ones used in the construction trades – the bubble in the spirit level will maintain its own horizontal alignment no matter what the angle of the instrument.
However, if the spirit level – in this case, the sensor – gets stuck for some reason and fails to account for the angle of the plane, then the autopilot system will inevitably receive incorrect information and, therefore, make inappropriate or even dangerous maneuvers.
Boeing has grounded all 737 Max jetliners for now, and in a statement, the company says: “The preliminary report contains flight data recorder information indicating the airplane had an erroneous angle-of-attack sensor input that activated the Maneuvering Characteristics Augmentation System function during the flight, as it had during the Lion Air 610 flight.”
The company adds that it will release an updated version of the MCAS software, and issue new training and education guidelines for pilots.
“The update adds additional layers of protection and will prevent erroneous data from causing MCAS activation,” says Boeing. “Flight crews will always have the ability to override MCAS and manually control the airplane.”
Boeing says MCAS will also now use an additional sensor – already installed – so they effectively cross-check each other’s data, and, as the New York Times reports, if they disagree by a certain amount, the MCAS would not be triggered.
Extra sensory perception
Clearly, if the pilot has put the plane in autopilot mode and that autopilot system relies on data from sensors, then the sensors have to be functioning correctly.
If an angle-of-attack sensor is telling the autopilot system that the plane needs to nosedive, then it will nosedive. How the software update can solve this is difficult to imagine – without the use of additional sensors which can validate or reject the data coming from the angle-of-attack sensor.
On the face of it, it seems like a simple problem to solve – the plane simply needs a gyroscopic sensor which can give it precise data about its angle and orientation.
Gyroscopes are, of course, used in planes, and probably always have been. It would be difficult to understand why they wouldn’t be.
However, apparently, according to an article on Electronics 360, gyroscopes are mostly used for a variety of other things, such as direction or heading – on a 2D plane, so to speak – and to control turn indicators, also a 2D plane issue.
Gyroscopes may be used to acquire aircraft orientation data, but they – and other instruments – are gradually being replaced by what are called “attitude heading and referencing systems”, according to Electronics 360.
Moreover, mechanical gyroscopes appear to have been all but phased out in modern aircraft.
The solution proposed by Carlos Varela, associate professor of computer science at Rensselaer Polytechnic Institute, is to install aircraft with more sensors.
In an article for The Conversation, entitled “Too many airplane systems rely on too few sensors”, Varela, a former pilot himself, says he, too, was once in a situation where an airspeed sensor was generating data which suggested the plane was flying so slow that the plane would not stay in the air – and a nosedive would correct the issue.
“Had I trusted my airspeed sensor, I would have pushed the plane’s nose down in an attempt to regain speed, and possibly put too much strain on the aircraft’s frame, or gotten dangerously close to the ground,” writes Varela.
Varela says he and his team have developed a computer system that looks at information from “many sensors, comparing their readings to each other and to the relevant mathematical formulas”, perhaps underlining Boeing’s initial solution to use data from two sensors instead of just one.
Varela goes on to conclude: “As flight becomes more automated and increasingly reliant on sensors, it is imperative that flight systems cross-check data from different sensor types, to safeguard against otherwise potentially fatal sensor faults.”
Interestingly, a typical new car has approximately 200 sensors installed, whereas most airliners are said to have less than 100. However, this is an unscientific, impressionistic estimate, and should not be taken as definite, especially since Airbus claims to put “10,000 sensors in every wing” of an A380 superjumbo.
Adding to the autopilot’s workload
According to The Air Current, Boeing engineers managed to achieve 14 percent greater fuel efficiency in the 737 by moving the engine slightly further forward and extending the nose landing gear by a few inches.
However, these changes look to have caused the plane to make “an upward pitching movement” which then prompted the development of MCAS, which was “designed to address this”. Essentially, MCAS would compensate for the upward pitching movement by triggering autopilot to bring the nose down without human pilot input.
So, while the angle-of-attack sensor is being pointed at as a possible cause of the problem, there appear to be other issues to resolve with MCAS, which possibly explains Boeing’s software update.
But it’s still the angle-of-attack sensor which feeds the MCAS with the data about the orientation of the plane, so it will remain the center of attention for now.
As The Air Current puts it, MCAS activates when the sensed angle of attack exceeds a threshold based on airspeed and altitude.
The ‘angle-of-attack sensor’
The angle-of-attack sensors of the Boeing plane operated by Lion Air in Ethiopia were produced by Rosemount Aerospace, according to Twin Cities.
The website also reports that other problems with such sensors – though not necessarily from the same company – were found on Lufthansa flights in 2014.
Those planes were manufactured by Airbus, and aviation authorities in Europe and the US subsequently ordered the replacement of angle-of-attack sensors on many Airbus models.
The FAA said at the time that sensors from two companies – Rosemount, which is owned by United Technologies, and Sextant / Thomson – “appear to have a greater susceptibility to adverse environmental conditions” than sensors made by a third company, although the third company appears not to have been named.
Other suppliers of angle-of-attack sensors include:
- Garmin
- Honeywell International
- Rockwell Collins
- Thales
- Transdigm
- AmeTek
- Dynon Avionics
This list is republished from a report on the global angle-of-attack sensors market, by Technavio, which says the sector is competitive and innovative.
And perhaps the most important development in the market appears to be the installation of a greater number of sensors into commercial aircraft, which would necessitate something called “sensor fusion”, where data from a number of different sensors are collected and analyzed by an increasingly centralized system.
“The introduction of sensor fusion technology is one of the key trends that will gain traction in the commercial aircraft angle-of-attack sensors market in the coming years,” says the report.
“In the sensor fusion technology, multiple sensors are integrated with the microprocessor or microcontroller to handle several features of control systems at the interface.”
Main image: Sensors for aerospace applications, courtesy of TE Connectivity
