Learning From a Close Call that Could Have Been Avoided
On April 10, 2019 an American Airlines A321T registered as N114NN was substantially damaged following a wingtip strike during takeoff from New York City’s JFK airport. Though no one was injured, the five year-old aircraft would ultimately be deemed a total loss after the struck wing was found permanently deformed.
In the aftermath, the crew – both highly experienced with a combined total of almost 5000 hours in the Airbus A320 series – reported surprise at the sudden left roll during rotation. Initial speculation focused on the Airbus’s fly-by-wire (FBW) flight controls, but the recently released NTSB final report would ultimately reveal a much more simple cause. The event is a model study on human factors.
Fly-by-Wire Primer
In an Airbus A321, the primary flight controls are managed by a series of seven redundant flight control computers, all operating under multiple sets of “laws” depending on the phase of flight. The FBW system takes pilot or autopilot requests – such as moving a side stick aft to climb – and produces appropriate flight control responses to achieve the end result. Though the ailerons and elevators feature no physical connections to their side stick controllers, the rudder pedals are mechanically linked with the rudder itself. While the architecture may be complicated, the end result for pilots is identical to a traditional airplane, with full control authority combined with built in protections.
Crosswind Takeoff That Should Have Been Fairly Routine
During the takeoff roll, a 15 knot crosswind from the right of the aircraft was present. Like any airplane, this would require the flying pilot (the Captain, on this leg) to utilize left rudder to track straight down the runway. Airbus specifically cautions against using any unnecessary aileron into the wind. During the takeoff, the rudder pedal input varied from neutral to about half left input, but as the aircraft reached its 156 knot rotation speed the Captain increased rudder input to its maximum left deflection. As the aircraft rotated the Captain input side stick commands for roll, first right, then left, then right again. The right wing left the ground first, and both pilots applied full right side stick leading to an aural “dual input” alert. The left wingtip struck the ground before the rudder was alternated to the right then released to neutral as the crew stabilized the aircraft and climbed away. They returned to JFK about 30 minutes later, with the First Officer flying the wounded Airbus.
The Power of Certification
Like all Transport Category aircraft, the A321 is certified to be fully controllable with an engine inoperative and the remaining engine at maximum thrust. Doing this requires a powerful rudder, with a large surface area and substantial deflection. Comparatively, the ailerons are small surfaces with relatively little power of their own, assisted by roll control spoilers on top of the wing.
Looking back to Private Pilot aircraft training, we all learned that providing a yaw input with the rudder will cause one wing to ‘lead’ the other, producing a rolling moment alongside the yaw. In the case of the accident Airbus, the large amount of rudder deflection alone may have been managed on the ground, but as the aircraft rotated and lift increased, it allowed the right wing to leave the runway first and ultimately caused the left wingtip to strike the runway surface.
Human Factors Were At Play
As mentioned above, the crew was very well qualified in the Airbus series. However, we are able to see a breakdown in basic airmanship with the over-controlling of the rudder followed by wild side stick inputs including full nose up as well as full roll deflection alternating between left and right.
Finally, during the initial roll to the left both pilots input full right sidestick, leading to the “dual input” aural alert. While it is understandable that the pilots would provide a quick resolution to an unexpected roll, there is no situation that calls for two pilots to be flying at once. The Airbus flight control logic uses a sum of the inputs in a “dual input” situation, so if the First Officer was providing the proper full right aileron and the Captain applied full left, the airplane would sum those as zero deflection, allowing the rudder to continue the yawing and rolling unabated.
It was dark at the time of the accident, so it’s unlikely the First Officer could see the rudder pedals near his feet. However, the Airbus has a takeover button on both side sticks, allowing a pilot to take control at any time. While it’s easy to say with 20/20 hindsight, standard operating procedure in this case would be for the pilot monitoring to state “I have control,” press their takeover button for the side stick, and manually take rudder and thrust control.
The Big Picture: It Took Time to Fully Understand What Happened
In the aftermath of the accident, American Airlines did a data study of 13 months of company A321 takeoffs (totaling 270,000 flights). In these flights, no other departure exhibited the same amount of rudder deflection or duration of deflection during takeoff rotation.
Though the Airbus is deservedly celebrated for its flight control architecture and the protections it’s fly-by-wire system provides, this accident unfortunately came down to a simple bit of aerodynamics, certification standards, and human factors.
In the end, a few seconds of over-controlling on a relatively light wind day ultimately led to the loss of a nearly hundred million dollar airplane. Fortunately, the human cost was zero and the interaction between pilot and aircraft is a situation we can all learn from.
