H-34 Choctaw/Avionics

SECTION 3. AVIONICS

1 VOR Navigation

1.1 Introduction

Figure 1: VOR Radials

Figure 2: Navigation (NAV) Radios

The S-58 supports VOR (Very High Frequency Omni-Directional Range) navigation. VOR navigation is a ground-based navigation system in which a VOR station transmits a signal that is picked up by a NAV radio (see Figure 2) in the aircraft. The VOR transmits two types of signals, an omni-directional pulse, and a rotating directional pulse. The omni-directional pulse is synchronized to occur when the directional pulse is at magnetic north. By measuring the time between the two pulses, the receiver can determine its relative direction, called a radial, from the VOR station. For SL purposes, a virtual VOR station is "placed" at every airport in the[Shergood Airport Map at the location of the airport marker on the map. See Wikipedia: VOR Omnidirectional range for more details on VOR operation.

1.2 Radials

VOR navigation is based on radials. A radial is line of position from the station location, and radiating out on one of 360 directions. Each of these lines of position is called a radial. The radials correspond to the magnetic direction from the station with radial 0 corresponding to North, radial 90 to East, radial 180 to South and radial 270 to West. Figure 1 shows an example of the radials around a station. The aircraft at position (a) is on the 060 radial, the aircraft at position (b) is on the 090 radial, and the aircraft at position (c) is right of course for an inbound approach on radial 075.

1.3 Nav Radios

Each VOR station has an associated frequency between 108.0 and 117.95 MHz that is used to select it. The frequency for the VOR at a particular airport can be found on the Shergood Airport Map by clicking on the airport symbol to bring up the data box for that airport. The S-58 is equipped with two NAV radios (see Figure 2) on the lower panel that can be used for VOR-based navigation. The top radio is NAV1 and the bottom radio is NAV2. To enter the frequency for a radio, you can touch the frequency adjustment knobs, or use a chat command. For example, to set the frequency for NAV1 to 117.30, you can use the chat command:

nav nav1 117.30

VOR stations have limited range, typically about 12 SL regions, and stations that are greater than 30 sims apart can share the same frequency. For this reason, it is important that you identify the station that you have selected. Press the IDENT button on the NAV radio to identify the station that is selected. This will produce a chat message of the form:

[10:06:15] SA - S58 v1.0 (N61KS): You hear ....  -...  --.  .-. (HBGR)

with the Morse code for the station, and the station identifier.

1.4 CDI

Figure 3: Course Deviation Indicator (CDI)
1: Radial, 2: TO/FROM Indicator, 3: Course Deviation, 4: Relative Course, 5: Flag

The aircraft is equipped with two CDI indicators (see Figure 3) on the pilot's side of the console. Each CDI is linked to one of the NAV radios. The right-most CDI is linked to the #1 NAV radio, and the right-most CDI is linked to the #2 NAV radio. After selecting a frequency for a CDI, the pilot must also select the radial to be used. This is done with the CRS knob on the CDI. You normally set the active radial with a chat command. For example:

nav crs1 60

will select the 060 radial from the VOR station on the #1 CDI. A radial can be selected as either the radial "FROM" the station, or a radial "TO" the station. The TO radials are 180 degrees from the FROM radials. The selected course will be displayed at the top of the indicator.

The CDI indicator includes the following elements:

1. Radial - The radial from or to the VOR the pilot wishes to track.
2. TO/FROM Indicator - Indicates if the pilot is flying TO or FROM the VOR based on the selected radial and the aircraft heading
3. Course Deviation - Vertical needle that swings left or right to indicate the direction the pilot must fly to intercept the radial. Each large dot represents 10 degrees off-course.
4. Relative Course - Heading of the helicopter relative to the selected radial.
5. Flag - Will display OFF, if no valid VOR signal is being received.

1.5 VOR Tracking a Course

Once established on a radial, VOR tracking is flown similarly to GPS tracking with the exception that the CDI shows deflection based on angle from the selected radial, instead of distance from the track. The CDI will be centered when you are on course, and when it is deflected you should turn toward the needle to fly back to the course. Like with the GPS, the CDI indication does not depend on your actual heading.

shows an example of an HSI for an aircraft flying a 047 heading inbound on the 047 radial to a VOR. The selected radial is shown in the "COURSE" window. No range information is available for VORs. You can also see the the needle is centered indicating that you are exactly on the radial. Finally, the triangle above the aircraft symbol indicates that you are flying TO the station since it is in the direction of the selected radial.

shows an example of an HSI for an aircraft flying a 227 radial FROM the VOR on a 227 heading. In this case, the triangle is point away from the selected course indicating that is a FROM course. Note that the 227 FROM radial and the 047 TO radial refer to the same line-of-position, just in different directions.