Ring Laser Gyro, Inertial Navigation System and Air Data Instrument Definitions

Question 1

Describe the principle of operation of a gyro stable platform?

Answer 1

• The platform houses the accelerometers and so isolates them from vibration, turbulence, and attitude changes.
• Remote vertical gyros keep this platform aligned in azimuth and with true north during the aircraft’s travels, thus maintaining the accelerometers in a fixed orientation throughout the flight.
• Alignment occurs during the initial synchronising procedure and is known as “gyro compassing” and must be complete before any flight plan can be entered.

Question 2

How is the stable platform held in alignment with the global (earth) frame?

Answer 2

Achieved by mounting the platform using gimbals (frames) which allow the platform freedom in all three axes. (FIG 20-1).

Question 3

What is the purpose of an accelerometer?

Answer 3

• Provide displacements of the aircraft in all those planes by measuring accelerations experienced by the aircraft both positive and negative.

Question 4

How many accelerometers does the Gyro Stable Platform house? What is the minimum needed and what are their movements?

Answer 4

• Three
• Minimum of two –> one for north-south movement and another for east-west movement and possibly one for vertical movement.
• The vertical movement can be eliminated with altitude data being gained from the Air Data Computer.

Question 5

What is the basic function of the gyro stable platform?

Answer 5

To maintain the accelerometers level regardless of the aircraft’s attitude.

Question 6

What is the product of the first integration of the N-S accelerometer in an INS?

Answer 6

Velocity along the local meridian.

Question 7

What is the product of the second integration of an accelerometer output?

Answer 7

Distance.

Question 8

What is the INS? How does it achieve this?

Answer 8

• The INS is a self-contained long range navigation system that provides a continuous and accurate fix of aircraft position.
• It achieves this by employing GREAT CIRCLE NAVIGATION to provide very accurate steering commands to the AFDS to steer the aircraft via a series of waypoints to its destination, it superseded the hyperbolic systems.

Question 9

Describe the inputs and outputs of the INS?

Answer 9

• This system, through its own computer, is coupled directly with the AFDS computer to steer and navigate the aircraft.
• It also utilises the CDU of the FMS, receives inputs from the ADC and displays information on the EFIS.

Question 10

What is the definition of CAS and EAS?

Answer 10

• Calibrated Airspeed - IAS corrected for position and instrument errors
• Equivalent Airspeed - CAS corrected for compressibility error.

Question 11

What is the definition of mach number?

Answer 11

Is the ratio of the speed of an aircraft to the local speed of sound in the same atmospheric conditions.

Question 12

What is the definition of the local speed of sound?

Answer 12

The speed of sound at a given altitude.

Question 13

What is the definition of VMO and MCRIT?

Answer 13

• Maximum operating speed, same as VNE on the ASI and NZ.

- Critical Mach number. The free-stream Mach number when a local Mach number reaches Mach 1.

Question 14

Explain the principle of operation of a ring laser gyro

Answer 14

• 2 coherent lasers are transmitted through a bore in a triangular/square block in different directions
• All corners have a mirror (three mirrors) with one mirror being semi-transparent which allows both lasers to enter to sense phase.
• Coherent light has sine wave properties.
• The rotation of the block changes path and wavelength so the frequency can be measured.
• When angular acceleration occurs, a phase difference in the light can be detected.
• This change in phase and therefore frequency can be computed to detect angular acceleration over a measured time.