INS - Dgca question bank for pilots

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#1. INS errors are classified as “Bounded errors” and “Unbounded errors”.

An “Unbounded error” is an error that increases with time, an example being the distance gone error due to a ground speed error.

An “Unbounded error” is an error that increases with time, an example being an increasing ground speed error due to the platform not being levelled correctly.

A “Bounded error” is an error that is subject to sudden unpredictable random changes. Most notable during pitching manoeuvres and when raising or lowering flap and U/C.

A “Bounded error” is an error that is “tied” to the real wander rates of the gyros on the platform.

ERRORS OF INS

BOUNDED ERRORS
a) Platform tilt due to initial misalignment.
b) Inaccurate measurement of acceleration by accelerometers.
c) Integrator errors in the first stage of integration.

UNBOUNDED ERRORS
Unbounded errors – are either cumulative track errors or distance errors:
a) Initial azimuth misalignment of the platform.
b) Wander of the azimuth gyro.

Errors which give rise to cumulative errors in the recording of distance run:
a) Wander in the levelling gyros. This causes a Schuler oscillation of the platform, but the mean recorded value of distance run is increasingly divergent from the true distance run.
b) Integrator errors in the second stage of integration.

INHERENT ERRORS

The irregular shape and composition of the earth, the movement of the earth through space, and other factors provide further possible sources of error.
Such errors vary from system to system depending upon the balance achieved between accuracy on one hand and simplicity of design, reliability, ease of construction, and cost of production.

#2. Two checks that can be carried out to check that two selected sequential waypoints have been entered correctly are:

select DSR.TK/STS and check that the status is less than 4; select DIS/TIME and check that the time agrees with the flight plan time.

select DIS/TIME and check that the distance agrees with the distance on the flight plan; then check that the time agrees with the flight plan time for the leg

select DIS/TIME and check that the distance agrees with the distance on the flight plan; select DSR.TK/STS and check that the track agrees with the flight plan track for the leg.

select DIS/TIME and check that the distance agrees with the distance on the flight plan; select HDG/DA and check that the heading agrees with the flight plan heading for the leg.

During a flight, the FMS data is cross-checked with the paper flight plan to ensure accuracy. Key parameters like track (TRK), time, fuel, wind, waypoints, and altitude are verified. Discrepancies in route, ETA, fuel consumption, or performance are identified and corrected to maintain safety and efficiency.

second check is to call up the initial great circle track (TK/GS) and distances (DIS/TIME)
between consecutive waypoints, and to compare these values against those shown on the
flight log/flight progress log/flight plan.

Distance and Time
The distance to go from the aircraft’s present
position direct to the next selected waypoint
is shown to the nearest nautical mile in the LH
window.
The lapsed time from the aircraft’s present
position to the next waypoint is shown to the
nearest tenth of a minute in the RH window.
The distance to go here is shown as 140 NM

Desired Track and Status
The desired track (assuming that the aircraft
is on the direct great circle track between the
two selected waypoints) is shown in degrees
true to the nearest tenth of a degree in the
LH window

#3. In an INS the E/W accelerations are converted into an E/W speed (kt) at the first stage of integration and into E/W distance gone (nm) at the second stage of integration. This gives:

departure which is multiplied by Cosine of the present latitude of obtain d'long (min) which is used to automatically up-date the present longitude.

d'long (min) which is used to automatically up-date the present longitude.

departure which is multiplied by Secant of the present latitude to obtain d'long (min) which is used to automatically up-date the present longitude.

departure which is multiplied by Sine of the present latitude to obtain d'long (min) which is used to automatically up-date the present longitude.

2ND STAGE

#4. At the second stage of integration E/W speed is converted into E/W distance gone. To convert this departure into change of longitude is has to:

be divided by Secant of the latitude.

be multiplied by Secant of the latitude

be divided by Tangent of the latitude.

be multiplied by Cosine of the latitude.

2ND STAGE

#5. The amber ALERT light on an INS control and display unit:

illuminates steadily 2 minutes, in AUTO mode, before reaching the next waypoint.

start flashing 2 minutes before reaching the next waypoint and goes out at 30 seconds to run.

illuminates if power from the aircraft bus bar has been lost and the system is operating on standby battery.

illuminates steadily after passing a waypoint in manual mode, until the next leg is programmed in.

The ALERT annunciator warns the operator that the aircraft is approaching the next waypoint.
In AUTO mode the alert light will come on, steady, 2 minutes to run to the waypoint, and will
extinguish as the track changes overhead the waypoint. In MANUAL mode the alert light will
come on, steady, 2 minutes to run to the waypoint; the light will then flash 30 seconds before
the waypoint, and will continue to flash until the track is changed. The annunciator will not
illuminate below a set speed (typically either 100 kt or 250 kt)

#6.

all the above statements are true.

only (ii), (iii) and (iv) of the above statements are true.

only (i), (ii) and (iii) of the above statements are true.

only (ii) and (iii) of the above statements are true.

at the first stage of integration to convert acceleration, with respect to time,
into speed, (when in NAV mode)

at the second stage of integration to convert speed, with respect to time,
into distance gone, (when in the NAV mode)

#7. The computer of a north referenced Inertial Navigation System (INS) in flight, provides compensation for:

aircraft manoeuvres, real wander, apparent wander, transport wander.

coriolis, real wander, apparent wander, transport wander.

earth rotation, transport wander, coriolis.

transport wander, apparent wander, coriolis, magnetic variation.

AIRCRAFT is operating on an earth which is rotating and an earth which is assumed to be round. In order to keep the accelerometers level with respect to the earth so that they sense acceleration of the aircraft in a horizontal direction only, some compensation
must be made for the earth rotating and the earth being assumed to be round.

Corrections must be made to gyroscopically stabilized platforms to allow for apparent
wander due to earth rotation and aircraft movement over the earth. The required earth rate
compensation is a function of latitude since what is being compensated for is the horizontal
component of the earth rate felt by the gyros, and that varies with latitude. At the equator,
this value is zero degrees per hour and with travel either further north or south, it increases
until it becomes a maximum of +/- 15.04 at the poles.
Transport rate compensation is developed using the velocity signal. The electronics through
which it is sent contain a term proportional to the earth’s radius. So, in reality, the transport
rate signal torquing the gyro is the velocity of the aircraft divided by the earth’s radius.
Both the earth rate and transport rate compensations are compensated by torquing the gyro.
The following diagram should be used to follow the system as explained.

There are a number of compensations generated within the system.
Coriolis and centrifugal effects must be compensated for within the system. Other
compensations are necessary because the earth is not a perfect sphere

• Centrifugal accelerations caused by platform rotation to maintain the local earth vertical.
• Coriolis accelerations caused by the aircraft following a curved path in space when flying
normal earth referenced flights.

#8. The diagram below shows the situation after an aircraft, equipped with INS, has passed over way point 2 and is tracking along the line TK (dashed). Using the information given in the diagram, and the fact that with DA/HDG selected on the control and display unit (CDU) of the INS, the display shows 6L/080, answer the following question ..When DSRTK/STS is selected on the CDU, the left window will show:

074

086

068

080

#9. When XTK/TKE is selected on the CDU, the display will show (to the nearest whole number):

LEFT DISPLAY 5L RIGHT DISPLAY 6R

LEFT DISPLAY 5R. RIGHT DISPLAY 6R

LEFT DISPLAY 5L RIGHT DISPLAY 6L

#10. During initialisation of an INS the aircraft must not be moved until:

The ramp position has been inserted and checked.

The platform is levelled.

The gyros and accelerometers are in the “null” position.

The green “ready NAV” light has been illuminated and the mode selector switch has been set to the “NAV” position

Finish