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Page 1: VOR Navigation

VOR Navigation

AST 241Chapter 2

Page 2: VOR Navigation

History

• VOR’s derived from the old 4-course radio range from the late 1920’s and 1930’s

• Gained widespread use for navigation in the 1950’s

• Made instrument navigation commonplace• Remain the basis for most of the world’s air

navigation systems- and will be for 5-10 yrs.

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Principles of operation

• VOR’s are commonly described as broadcasting 360 separate radials emanating from the station in all directions.

• How do they really work?• VOR’s broadcast 2 signals- the reference

(or 360-N) signal and the rotating signal.

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Principles of Operation

• If the two signals are in phase the aircraft is on the 360 “radial”, if the receiver detects these signals are out of phase by ¼ then the aircraft is located on the 90 “radial” from the station and so on.

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Principles of operation

• Remember the VOR “Omni” head in the aircraft only tells the pilot one thing which is?

• Where the aircraft is located with respect to the selected radial- IF interpreted correctly.– No aircraft heading information (unless HIS)– No distance information

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Principles of operation

• What is the most important initial action when using a VOR for air navigation?

• Tune and IDENTIFY the Station

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VOR information

• How is a VOR used to Determine which radial an aircraft is located on?

• How is a VOR used to determine a heading to get to a particular station?

• How Can the VOR receiver(s) be used to locate your relative position if lost?

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VOR information

• Most pilots are taught to use the VOR as a “command” instrument- “Which way do I fly?”

• The VOR receiver was originally designed as a SHI- Station Heading Indicator.

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SHI Steps:

• Tune and identify the station• Put the desired radial at the top of the indicator

and interpret where you are in relation to that selected course.

• Use the CDI (course deviation indicator) and the TO/FROM flag to divide the VOR into quadrants.

• The midpoint of the quadrant containing the CDI and the TO/FROM flag will give a 45 degree INBOUND intercept for the selected course.

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SHI steps ctn.

• The midpoint of the quadrant with the CDI but opposite the TO/FROM flag will give a 45 degree OUTBOUND intercept

• REMEMBER- if after that you plan to track TO the station to orient the OMNI Head so that your aircraft heading and VOR indication are the same to avoid reverse sensing- Have a TO indication if going TO the station.

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Tracking

• All previous principles work well in a no-wind situation

• Wind complicates the process in 3 ways:• The greater the distance from the station the

slower the needle reacts (Fig. 2-11)• The stronger the cross-wind the greater the

correction should be• The faster the aircraft the less the correction

should be- less “relative” effect on the aircraft

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Tracking

• 2 Basic means of establishing track:• Bracketing and Estimating• Bracketing- logical trial & Error• The speed of needle drift is an indirect

indication of the crosswind strength

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Tracking

• The 30 degree rule for tracking:• Make a 30 degree initial turn toward the needle

(assuming correct orientation)• When the needle centers- remove ½ of the

correction- 7-8 degrees.• Watch needle• If it drifts back in the direction of the original

drift- add the 7-8 removed degrees back in and go 7-8 in the other direction

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Tracking

• If the needle drifts the other way take 8 degrees out.

• Go through this iteration again using 3-4 degrees and then again with 1-2 degrees until a workable heading is found

• *Then the wind will change!

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Estimating

• Estimating is essentially the same as tracking except you start with a pre-calculated value based on known wind information.

• When using this method begin with 5 degree changes instead of the 15 degrees used when winds are not known.- this is generally a quicker method.

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VOR testing

• How often do VOR receivers have to be tested for tolerance for VFR flight?

• How about for IFR flight?• Every 30 days- with a logbook entry give

date, time/place, name & bearing error.• Many airports have VOT facilties• What are they and how do you know if they

have one?

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Testing

• What is the allowable error if using a VOT?• +/- 4 degrees• What are the acceptable VOR indications

when using a VOT?• 180-TO and 360- From• Some airports have certified VOR

checkpoints on the field- refer the the AFD.

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Testing

• When using checkpoints what are the allowable tolerances?

• Ground +/- 4 degrees• Flight +/- 6 degrees• You are allowed to make your own checkpoints• If tested against each other 2 receivers must be

within ______ degrees?• 4 degrees

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Testing

• It is recommended that VOR’s be periodically calibrated as they may indicate correctly close to the station yet be out of tolerance when at a greater distance.

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VOR range

• The VOR transmission is limited to line of sight and can be disrupted by terrain- to avoid this stay on published airways or refer to the AFD

• Remember the VOR accuracy is limited to 1 degree which may add up to a 28 mile discrepancy at 200 miles if the VOR is at the 4 degree max. tolerance.

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VOR range

• Terminal VOR (T)- from 1,000 ft. to 12,000 ft. out to 25 NM

• Low Alt. (L)- from 1,000 ft. to 18,000 ft out to 40 NM

• High Alt.(H)- from 1,000 ft. – 14,500 out to 40 NM, from 14,500ft. – 60,000 ft. out to 100 NM and from 18,000 ft. - 45,000 ft. out to 130 NM.

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The End