The longitudinal channel of the fly-by-wire system has three operational modes:
• TAKEOFF-LANDING mode: In this mode, the stabilizer deflects in accordance with control
stick inputs and rate of pitch.
• FLIGHT mode: In this mode, the stabilizer deflects in accordance with the control stick
inputs, rate of pitch, and g force.
• DIRECT CONTROL EMERGENCY mode: in this mode, the stabilizer deflects in accordance
with the control stick inputs only.
The TAKEOFF-LANDING and FLIGHT modes of the fly-by-wire system are switched automatically in
accordance with landing gear position.
The DIRECT CONTROL mode is switched on by pressing the [S] key if the fly-by-wire system fails.
Piloting in this mode requires special care; the flight is characterized by:
• Insufficient pitching stability of the aircraft
• Low aerodynamic damping
• Increased longitudinal stick sensitivity
When in this mode, compensate for the aircraft’s tendency to change pitch by using short,
preemptive stick inputs. Refrain from abrupt, large stick inputs. Angle of attack in this mode should
not exceed 10 degrees (the AoA/G limiter does not restrict the angle in this mode); perform turns
with a bank no more than 45 degrees.
It is in this mode that the “Pougachev’s Cobra” aerobatic maneuver is performed. The mode is
switched off by pressing the [S] key again. The on/off control setting is seen on the left cockpit
console.
The fly-by-wire system is a trajectory control system. In other words, if the control stick remains in
the same position, the system will hold the set flight path. This is why the reduction of airspeed (for
example) and resulting decreased lift and increasing angle of attack, will lead to the aircraft
attempting to hold the initial flight path and prevent the aircraft from descending. This will result in
the airspeed stability degradation up to neutral stability.
The longitudinal control system includes an airspeed trimming law that generates a signal
proportional to the ram air. When the indicated airspeed is increased, the FCS causes the stabilizers
deflection (up to 5 degrees) to nose up. When decelerating, it causes the nose to come down. This
imitates the airspeed stability of the aircraft, which is neutral in the presence of a g loading feedback
signal. The airspeed stability imitation allows the pilot to use the control stick like he or she would
with a stable aircraft.
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