# Avionics Questions and Answers – Control Laws in FBW

This set of Avionics Multiple Choice Questions & Answers (MCQs) focuses on “Control Laws in FBW”.

1. What are control laws?
a) Algorithms relating pilots stick and control surface
b) Laws that govern aerodynamics forces
c) Laws that govern flight motion
d) Laws used to predict range and endurance

Explanation: The term ‘control laws’ is used to define the algorithms relating the control surface demand to the pilot’s stick command and the various motion sensor signals and the aircraft height, speed and Mach number.

2. What is θi in pitch rate command law?
a) Tailplane demand angle
b) Pilot input angle
c) Gain in loop
d) Pitch rate gearing

Explanation: Pitch rate command law is given by
ηD = K(θi − Gqq)
where ηD is the tailplane demand angle; θi is the pilot’s input command; q is the pitch rate; K is the forward loop gain, and Gq is the pitch rate gearing.

3. What is false true with respect to K in the pitch rate command law?
a) Value of K is infinite
b) Value of K is constant with height and altitude
c) Value of K changes with height and altitude
d) Value of k is constant with altitude and airspeed

Explanation: As the altitude and airspeed are varied, the magnitude of hinge moment in the control surface varies. This raises the need to vary gain K with change in flight conditions.

4. Derivative of a control system provides _________
b) Gain proportional to error
c) Reduced overshoot
d) Infinite gain

Explanation: Derivative of error control provides a phase advance characteristic to
compensate for the lags in the system, for instance, actuator response, and hence improve the loop stability. This increases the damping of the aircraft response and reduces the overshoot to a minimum when responding to an input or disturbance.

5. Integral of a control system provides __________
b) Gain proportional to error
c) Reduced overshoot
d) infinite gain

Explanation: Integral of error control eliminates steady-state errors and reduces the following lag. The integral of error term increases the loop gain at low frequencies up to theoretically infinite gain at dc so that there are zero steady-state errors due to out of trim external moments or forces acting on the aircraft.

6. Phase advance control is also known as __________
a) Proportional error controller
b) Derivative error controller
c) Integral error controller
d) One plus derivative error controller

Explanation: A phase advance controller is usually known as phase advance controller with a transfer function.
The control term is filtered to limit the increase in gain at high frequencies and smooth the differentiation process which amplifies any noise present in the error signal.

7. What is the MTBF of a solid state rate gyro?
a) 1,000 to 5,000 hrs
b) 5,000 to 10,000 hrs
c) 10,000 to 50,000 hrs
d) 50,000 to 100,000 hrs

Explanation: Solid state rate gyros are very reliable sensors with an MTBF in the region of 50,000 to 100,000 hours – ‘fit and forget’ devices. Also, they are mounted internally and suffer less damage from the environment.

8. What are the shortcomings of using an angle of incidence indicator for feedback sensor?
a) Damage from bird strikes and local airflow disturbance
b) Damage from ice formation and local airflow disturbance
c) Ground and impact damage from landing
d) Works only in subsonic ranges

Explanation: Although the angle of incidence indicator can provide motion feedback sensor information the major shortcomings of using them are damage from bird strike as they are mounted external to an aircraft and effect of the local airflow around the fuselage cause variations in measured values.

9. What is the angle of incidence limiting?
a) Warning indication to the pilot during high angles of incidence
b) The minimum angle of attack that can be measured
c) Keeping angle of incidence under safety limits whilst maneuvering
d) Reducing the error that occurs in an angle of incidence indicator

Explanation: The incidence terms from the airstream sensors can be blended with the pitch rate error terms and the ‘gearings’ (or gain coefficients) of these terms adjusted as higher angles of attack are approached. This enables the angle of incidence to be kept within the safe limits whilst maneuvering at high angles of incidence, i.e., the angle of incidence limiting.

10. The aircraft is a rigid body.
a) True
b) False

Explanation: The aircraft structure is flexible and has a variety of flexural and torsional modes. The frequency of this structural mode is typically between 8 and 15 Hz for a fighter/strike aircraft and 2 to 4 Hz for a large transport aircraft.

11. FCS sensors which sense aircraft motion also sense the structural deflections.
a) True
b) False

Explanation: The FCS sensors which sense the aircraft motion also sense the structural deflections and hence couple the structure modes into the control loops. The location of the motion sensors with respect to the nodes and anti-nodes of these structural modes is important.

12. What happens to the phase lag in an actuator as the frequency is increased?
a) Increases
b) Decreases
c) Constant
d) Cannot be determined

Explanation: The response of the actuators at low frequencies is basically that of a low pass (or first-order) filter, but as the frequency increases the lags in the first stage actuation system become dominant. The output/input ratio falls at an increasing rate and the phase lag rapidly increases.

13. What can occur when the control valve travel limits are reached under conditions of large amplitude demands?
a) Stall
b) Structural damage
c) Rate limiting
d) Rate delimiting

Explanation: Rate limiting can occur when the control valve travel limits are reached under conditions of large amplitude demands. The behaviour under rate limiting conditions is non-linear and the onset and the effects are dependent on the amplitude and frequency of the input demand.

14. What is PIO?
a) Pitot indicator
b) Pilot induced oscillations
c) Pressure indicator observed
d) Primary Input/Output

Explanation: Pilot Induced Oscillations are sustained or uncontrollable oscillations resulting from the efforts of the pilot to control the aircraft. They can have catastrophic results in extreme cases.

15. Flight control systems comes under the category of _______
a) SISO
b) SIMO
c) MIMO
d) MISO

Explanation: Flight control systems come into the category of multi-input/multi-output (MIMO) closed-loop control systems as control is exerted about three axes and there are six degrees of freedom. The classic control theory approach is very suitable for single input/single output (SISO) closed-loop control systems and some single input/multi-output (SIMO) systems.

Sanfoundry Global Education & Learning Series – Avionics.

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