July 7, 2012

Background: The Air France Flight 447 crash has become one of the most well studied in recent memory, primarily due to the relentless efforts by the French to retrieve the plane’s black box. A recent report attributed fault to a number of factors, including malfunctioning sensors and pilot error.

Another potential contributing factor was Airbus’ fly-by-wire system which operates in an asynchronous mode, allowing each pilot to independently direct the plane. During the last moments of Flight 447, the plane was reputed to be in dual-input mode, where if one pilot were to push UP on the side-stick and the other DOWN, it would average inputs and in this case, negate them, without either pilot having explicit knowledge of this occurring. In a high-pressure situation like the ones the AF447 pilots were in, these latent differences have the potential to be lethal.

A modern Airbus A380 cockpit with two side-sticks: 

A closer look at the side-stick design: 

Proposed Improvement:

Option A: under the thumb indent, add a small pulse vibrator that emits a specific vibration sequence in the event the other side-stick is pulling in a different direction. The pilot can disregard the input in the event this difference is expected or communicate with the other pilot that the execution plan isn’t being followed.

Option B: is more complex and uses a ‘force feedback’ mechanism that periodically ‘nudges’ the pilot’s side-stick in an opposing direction (nudge back if the stick is being pushed forward). The nudge doesn’t change the intended trajectory, but it does act as a clear signaling mechanism for the pilot. The nudge can keep triggering every 3 seconds initially and then switch to a 5-second interval when it realizes that the pilots are continuing with the dissonant inputs. 

My suggestion will not change how the Airbus fly-by-wire system works other than enhancing awareness of either pilot’s intent to each other in a passive and unambiguous manner. What it attempts to do is emulate the same characteristics of Boeing’s yoke-style navigation without wholesale re-engineering of the Airbus platform. The proposed enhancement assumes that Airbus’ flight computer tracks side-stick positions in real-time and the requisite telemetry is available to interpret and process these positions. 

Note: I am not a pilot, nor a mechanical engineer. But I do write software and it seemed like a system of this kind could be implemented, tested, certified and put to good use by all Airbus aircraft in the future. I might have misunderstood how the fly-by-wire system works or made irrational assumptions, I am all ears to be told otherwise! 

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