Five Things I learned from flying the Airbus A320
In the early hours of March 14, 2021, I found myself sitting in the left seat of an Airbus A320 Level D Simulator lining up a 180,000lb mammoth onto Runway 04L at JFK international airport. Level D Simulators are ultra-realistic six degrees of motion full flight simulators and typically the only initial training airline pilots get before flying paying customers for the first time. The story of how I ended up in command of a $100 million aircraft is an exhilarating one but not as impressive as some of the pictures I took during this adventure
The A320 is one of the first highly augmented non-developmental control law I have flown in a full flight simulator. It is an absolute breeze to fly (at least in normal law) and requires minimal compensation to meet adequate performance in high gain tasks. Here I summarize the top 5 things I learned about flying the A320 during the 3 magical hours I spent in the “box”:
1. The magic of C*: Neutrally stable first-order Flight Path response throughout the envelope
The Airbus A320, like many of its Fly-By-Wire (FBW) contemporaries, effectively leverages feedback loops to augment and shape aircraft response to stick inputs. In a non-augmented conventional aircraft, the longitudinal stick directly moves the elevators to create a pitch acceleration and sustain a pitch rate. The Pitch Rate is approximately linearly proportional to the elevator deflection. The Pilot then uses a lead-lag approach to close the loop on Pitch (or Flight Path Angle which is proportional but lags by the incidence lag). This is not the case on the Airbus A-320. On the A-320, the stick deflection is not necessarily proportional to the elevator but the elevator deflection is a function of the command augmentation system applied in the flight computers. The response type of the augmented control law on the A320 is called the C*.
Through handling qualities experiments, engineers at Boeing hypothesized that pilots are more concerned with the aircraft’s pitch response at low speeds and normal acceleration response at high speeds. At the same time, engineers were experimenting with C Law augmentation where pilot control displacement was proportional to a normal acceleration or “g” command. They added pitch rate to the C Law and the C* law was born. In C*, Pitch rate feedback is added to the “g” command from the longitudinal stick. At low speeds, where attitude changes are more perceptible cues, stick displacement is proportional to pitch rate. At higher speeds, where load factor is a more perceptible cue, the stick displacement is proportional to normal acceleration, and pitch rate is used only for loop stabilization. Both responses are blended linearly. On the A-320, this crossover speed is around 210kts.
On the A-320, the perfect harmony of pitch rate and load factor allows the pilot to make smooth first order flight path changes at high speed with minimal compensation while maintaining bandwidth in pitch for terminal high gain tasks. C* reduces pilot workload and frees up more time for the pilot to exercise monitoring functions and, of course, play candy crush
2. Autothrust (A/THR) Speed Mode: Uncoupling Speed and Flight Path Angle
Every general aviation weekend warrior pilot is familiar with the pitch power coupling and the inherent speed stability of a longitudinally stable aircraft. The Airbus A320 reduces compensation required in the longitudinal axis with desired velocity vector changes by providing the autothrust speed mode. When A/THR is set and the thrust lever is in the second detent, thrust demand is computed directly by the FADEC and the Flight Computers for the speed setpoint and flight path angle. The thrust level angle is not related to the thrust as the FADEC is controlling thrust to match the speed setpoint. To fly a given speed, set a speed setpoint, enable A/THR and then control the climb and descent of your flight path with the longitudinal stick input. A/THR makes instrument approaches way too easy.
3. Airspeed Invariant Roll Rate: Neutrally stable first-order Roll response throughout the envelope (< 33-degree bank)
The lateral axis is typically neutrally stable or close to neutrally stable in most conventional aircraft, at least for a certain range of bank angles. The dynamics of the aircraft, though, require the pilot to perform a careful choreography of longitudinal, lateral, and directional inputs to turn an airplane. The A320 eliminates the need for any pilot compensation in a turn by damping adverse yaw and increasing elevator deflection to provide extra lift for centripetal force. The A320 actually goes a step further and uses roll rate feedback to account for the inherent roll damping of the aircraft. Compensating for roll damping allows the A320 to provide a roll rate response that is proportional to the stick deflection and has the same roll mode time constant at all airspeeds! Thus, a 0.5-inch deflection of the lateral stick will produce the same roll rate at all airspeeds. Additionally, due to the augmented neutral stability, the aircraft has a bank angle hold response type once the stick is released and maintains the bank angle where the stick was released. Turning flight is no longer a complex choreography of rudder pedal and longitudinal and lateral stick inputs. A pulse input of the lateral stick is all that is required to establish a coordinated and level turning flight.
4. Load Factor Protection: A Control Law for Maximum Performance
It is hard to think of a maneuver representative of the concept of operations for a commercial airliner where a 2.5g pull-up is required. The ones that do come to mind are maneuvers where the 2.5g really really counts. Evasive maneuvers to deconflict from traffic or to avoid an unexpected CFIT event are the primary candidates where aggressive load factor command is not only required, but necessary to ensure the safety of flight. G-load information is not continuously provided in the cockpit and thus airline pilots are not used to controlling this parameter. Inflight experience tells us that in emergencies, the initial reaction of the PF (Pilot Flying) on the controls is hesitant, then aggressive.
With load factor protection on the A320, the pilot may immediately and instinctively pull the sidestick full aft. The aircraft will initially fly a 2.5g pull-up without losing time in hesitation. The control law ensures that the aircraft does not exceed the prescribed load factor limits, thus ensuring protection from structural ramifications of exceeding load factor. If the pilot needs to maintain the pull, the angle of attack will increase and the high Angle of Attack Protection can also kick in and enhance the load factor protection.
5. Artificial Feel Dynamics of the Passive Sidestick
For a general aviation pilot, the initial awkwardness of the augmented response type of the A320 is further exacerbated by the stick. The Airbus A320 replaces the conventional Yoke or a Centerstick with a passive sidestick. Unlike most conventional aircraft, the side stick is not directly connected to control surfaces. The Pilot does not feel the actual untrimmed hinge moment on the ailerons or the elevator. The Stick deflection is measured as a position change or a force change which is measured by sensors like RVDTs, LVDTs, and load cells. This Position or Force value is sent to the Flight Computers which command the surfaces to move to meet the pilot demands.
With fly-by-wire architecture and sidesticks, the force feedback from control surfaces is no longer available, although this is part of the tactile information which in conventional control schemes gives the pilot important information about the flight behavior, surface response, and flight conditions. Instead, artificial feel systems have to be used to provide the pilot with the required control forces. On passive side sticks like the ones on the A320, artificial feel dynamics are provided using springs and dampers. For passive side sticks, this artificial feel, specifically the force/deflection feeling is decoupled from flight processes, autopilot inputs, co-pilot inputs, and flight envelope exceedance. Decoupling the tactile information from the pilot could lead to problems of awareness about the situation of the aircraft. Specifically in a critical situation, with a high workload, this additional force feedback information can contribute to enhancing both situational awareness and pilot’s controllability because force sensation is natural, very fast, and does not influence mental workload. An example would be stalling out at low speeds by continuously increasing back-pressure on the longitudinal stick. The A-320 mitigates the danger of wrong pilot inputs by establishing envelope protection features to ensure safe flight.
From my limited exposure to the A320, I estimate the A320 grip force profile in the longitudinal axis to look similar to the figure below. The gradient is extremely smooth until the soft stop and not overpowering. Past the soft stop, the force gradient is extremely steep. The steep force gradient provides an indirect awareness of control margin to the Pilot in nominal conditions and gives the illusion of tactile feedback that is present in conventional control strategies.
The entry into service of the Airbus A320, the first civil digital fly-by-wire commercial airliner, defined a new standard of fly-by-wire in flight controls and system integration. It augments the dynamic closed-loop response of the aircraft to improve flying qualities and produce a response type that is conducive to the mission and concept of operations for a transport category aircraft. I look forward to getting back in the cockpit and evaluating the flying qualities in more detail. My next trip will include, hopefully, an evaluation of the flight envelope protection features and the overall closed-loop characteristics in degraded laws. I am incredibly thankful to JetBlue, ATOP, Captain Wayne Phillips, and Yellow Jacket Flying Club for this incredible opportunity!
