9: Navigation

The Flight Computer

VHF Omni-Directional Range (VOR)

Horizontal Situation Indicator (HSI)

The Flight Computer

ASA’s CX-3 is an electronic flight computer and can be used in place of the E6-B. This aviation computer can solve all flight planning problems, as well as perform standard mathematical calculations.

Finding True Course, Time, Rate, Distance, and Fuel

True course is expressed as an angle between the course line and true (geographic) north. Lines of longitude (meridians) designate the direction of north-south at any point on the surface of the Earth and converge at the poles. Because meridians converge toward the poles, true course measurement should be taken at a meridian near the midpoint of the course rather than at the point of departure.

The flight computer can be used to solve problems of time, rate and distance. When two factors are known, the third can be found using the proportion:

Rate (speed) =

Distance

Time

Problem:

Find the time en route and fuel consumption based on the following information:

Wind: 175° at 20 knots

Distance: 135 NM

True course: 075°

True airspeed: 80 knots

Fuel consumption: 105.0 pounds/hour

Solution using the E6-B:

  1. Using the wind face side of the E6-B computer, set the true wind direction (175°) under the true index.
  2. Place a wind dot 20 units (wind speed) directly above the grommet.
  3. Rotate the plotting disk to set the true course (075°) under the true index.
  4. Adjust the sliding grid so that the TAS arc (80 knots) is under the wind dot. Note that the wind dot is 15° right of centerline, so the wind correction angle (WCA) = 15°R.
  5. Read the ground speed under the grommet: 81.0 knots.
    1. Determine the time en route, using the formula:

      Distance

      = Time

      Ground speed

      135 ÷ 81 = 1.67 hour

    2. The E6-B may also be used to find the time en route:
      1. Set the 60 (speed) index under 81 knots (outer scale).
      2. Under 135 NM (outer scale) read the time of 100 minutes or 1 hour 40 minutes (inner scale).
  6. Find the amount of fuel consumed, using the formula:

    Time × fuel consumption rate = total consumed

    1.67 hours × 105.0 pounds/hour = 175.35 pounds

The E6-B may also be used to find the amount of fuel consumed:

  1. Set the 60 (speed) index under 105.0 pounds/hour.
  2. Over 100 minutes or 1 hour 40 minutes (inner scale) read 175 pounds required (outer scale).

Solution using the CX-3:

  1. Select Wind Correction from FLT menu and enter the given information:

    True Airspeed (TAS): 80 KTS

    True Course (TCrs): 0.075°

    Wind Speed (Wspd): 20 KTS

    Wind Direction (WDir): 175°

    Find a ground speed (GS) of 81 knots.

  2. Select Ground Speed from the FLT menu and enter the given information:

    Distance (Dist): 135 NM

    Ground Speed (GS): 81 KTS

    Find a duration of 1 hour, 40 minutes.

  3. Select Fuel from the FLT menu and enter the given information:

    Duration (Dur): 1:40:00 HMS

    Fuel Burn (Rate): 105 US GPH

    Find a total fuel consumed (Wt) of 175 pounds.

Problem:

Based on the following information, determine the approximate time, fuel consumed, compass headings, and distance traveled during the descent to the airport:

Cruising Altitude: 10,500 feet
Airport elevation: 1,700 feet
Descent to: 1,000 feet AGL
Rate of descent: 600 ft/min
Average true airspeed: 135 knots
True course: 263°
Average wind velocity: 330° at 30 knots
Variation: 7° east
Deviation: +3°
Average fuel consumption: 11.5 gal/hr

Solution using the E6-B:

  1. Find the time en route. The time to descend is based on the vertical distance from the cruising altitude of 10,500 feet down to the lower altitude of 2,700 feet MSL (1,000 feet AGL + 1,700 feet MSL):

    10,500

    feet cruising altitude

    –2,700

    feet lower altitude

    7,800

    feet altitude change

    Distance

    = Time

    Rate

    7,800 feet ÷ 600 feet/min = 13 minutes

  2. Find the amount of fuel consumed using the formula:

    Time × fuel consumption rate = fuel consumed

    13 minutes × 11.5 gal/hr

    = 2.5 gallons

    60 min/hr

  3. Use the wind face of the E6-B to find the ground speed (120 knots) and wind correction angle (+ 12°R). Due to wind being present, the TAS is not the ground speed. The speed used to calculate the distance covered must be the ground speed.
  4. Calculate the distance:

    Time × speed = distance

    13 minutes × 120 knots

    = 26 NM

    60 min/hr

  5. Determine the compass heading, by applying the wind correction angle that was found on the wind face of E6-B to the true course, which gives the true heading. The true heading is corrected by variation to give the magnetic heading. Finally, the deviation is used to correct magnetic heading to give the compass heading as follows:

    263°

    TC

    + 12°R

    ±WCA

    275°

    TH

    – 7°E

    ±VAR

    268°

    MH

    + 3°

    ±DEV

    271°

    CH

Solution using the CX-3:

  1. The time to descend is based on the vertical distance from the cruising altitude of 10,500 feet down to the lower altitude of 2,700 feet MSL (1,000 feet AGL + 1,700 feet MSL):

    10,500

    feet cruising altitude

    –2,700

    feet lower altitude

    7,800

    feet altitude change

  2. Determine your time of descent:

    Distance

    = Time

    Rate

    7,800 feet ÷ 600 feet/min = 13 minutes

  3. Select Wind Correction from the FLT menu:

    True Airspeed (TAS): 135 KTS

    True Course (TCrs): 263°

    Wind Speed (WSpd): 30 KTS

    Wind Direction (WDir): 330 KTS

    Find a ground speed (GS) of 120 knots and a true heading (THdg) of 275°.

  4. Subtract the variation of 7° east and add the deviation of 3° to get a compass heading of 271°.
  5. Select Fuel from the FLT menu:

    Duration (Dur): 13 MIN

    Fuel Consumption (Rate): 11.5 US GPH

    Find a volume of total fuel consumed (Vol) of 2.49 U.S. gallons.

  6. Select Ground Speed from the FLT menu:

    Duration (DUR): 13 MIN

    Ground Speed (GS): 120 KTS

    Find a distance (Dist) of 26 NM.

Finding Density Altitude

Density altitude is the altitude in standard air where the density is the same as the existing density. It is affected by the pressure, temperature, and moisture content of the air. Both a decrease in pressure and an increase in temperature decrease the density of the air and increase the density altitude.

To find density altitude, refer to the right-hand window on the computer side of the E6-B.

Problem:

Find the density altitude from the following conditions:

Pressure Altitude: 5,000 feet
True air temperature: + 40°C

Solution using the E6-B:

  1. Refer to the right-hand “Density Altitude” window. Note that the scale above the window is labeled air temperature (°C). The scale inside the window itself is labeled pressure altitude (in thousands of feet). Rotate the disc and place the pressure altitude of 5,000 feet opposite an air temperature of +40°C.
  2. The density altitude shown in the window is 8,800 feet.

Solution using the CX-3:

Select Altitude from the FLT menu:

Pressure Altitude (PAlt): 5,000 FT
Temperature (OAT): 40°C
Find a density altitude (Dalt) of 8,846 feet.

Finding Wind Direction and Velocity

The E6-B can be used to solve for an unknown wind. To determine wind direction and wind speed, the true course, wind correction angle, true airspeed and ground speed are necessary. The WCA may not be given directly in a problem, but can be determined from the true course (TC) and true heading (TH).

Problem:

Determine the wind direction and wind speed under the following conditions:

True course: 095°
True heading: 075°
True airspeed: 90 knots
Ground speed: 77 knots

Solution using the E6-B:

  1. Set the true course (095°) under the true index located at the top of the computer.
  2. Move the sliding grid to place the ground speed (77 knots) under the center grommet.
  3. Determine the wind correction angle. Above the true heading (075°) read the wind correction angle, 20°L. Draw the wind dot over the true airspeed arc (90 knots) and 20° (wind correction angle) to the left.
  4. Finally, rotate the window until the wind dot is lined up directly above the grommet. The wind direction is read under the true index (020°). For convenience, the sliding grid may be moved so that 100 knots is placed under the grommet. The difference between the grommet and the wind dot indicates the wind speed (31 knots).

Solution using the CX-3:

Select Wind Correction from the FLT menu:

Ground Speed (GS): 77 KTS
True Airspeed (TAS): 90 KTS
True Course (TCrs): 95°
True Heading (THdg): 75°

Find a wind direction (WDir) of 019° and wind speed (WSpd) of 31.7 knots.

VHF Omni-Directional Range (VOR)

All VOR stations transmit an identifier. It is a three-letter Morse code signal interrupted only by a voice identifier on some stations, or to allow the controlling flight service station to speak on the frequency. Absence of a VOR identifier indicates maintenance is being performed on the station and the signal may not be reliable.

All VOR receivers have at least the essential components shown in Figure 9-1. The pilot may select the desired course or radial by turning the omni-bearing selector (OBS). The course deviation indicator (CDI) centers when the aircraft is on the selected radial or its reciprocal. A full-scale deflection of the CDI from the center represents a deviation of approximately 10° to 12°.

Figure 9-1. VOR indicators

The TO/FROM Indicator (ambiguity indicator) shows whether the selected course will take the aircraft TO or FROM the station. A TO indication shows that the OBS selection is on the other side of the VOR station. A FROM indication shows that the OBS selection and the aircraft are on the same side of the VOR station. When an aircraft flies over a VOR, the TO/FROM indicator will reverse, indicating station passage.

The position of the aircraft can always be determined by rotating the OBS until the CDI centers with a FROM indication. The course displayed indicates the radial FROM the station. The VOR indicator displays information as though the aircraft were going in the direction of the course selected. However, actual heading does not influence the display. See Figure 9-2.

Figure 9-2. VOR display

VOR radials, all of which originate at the VOR antenna, diverge as they radiate outward. For example, while the 011° radial and the 012° radial both start at the same point, 1 NM from the antenna, they are 100 feet apart. When they are 2 NM from the antenna, they are 200 feet apart. So at 60 NM, the radials would be 1 NM (6,000 feet) apart. See Figure 9-3.

Figure 9-3. Radial divergence

The VOR indicator uses a series of dots to indicate any deviation from the selected course, with each dot equal to approximately 2° of deviation. Thus, a one-dot deviation at a distance of 30 NM from the station would indicate that the aircraft was 1 NM from the selected radial (200 feet × 30 = 6,000 feet).

To orient where the aircraft is in relation to the VOR, first determine which radial is selected (look at the OBS setting). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator), to find which hemisphere the aircraft is in. Last, determine how far off course the aircraft is from the selected course (look at the CDI needle deflection) to find which quadrant the aircraft is in. Remember that aircraft heading does not affect orientation to the VOR.

VOR accuracy may be checked by means of a VOR Test Facility (VOT), ground or airborne checkpoints, or by checking dual VORs against each other. A VOT location and frequency can be found in the Chart Supplements U.S. and on the Air-to-Ground Communications Panel of the Low Altitude Enroute Chart.

To use the VOT, tune to the appropriate frequency and center the CDI. The omni-bearing selector should read 0° with a FROM indication, or 180° with a TO indication. The allowable error is ±4°. VOR receiver checkpoints are listed in the Chart Supplements U.S. With the appropriate frequency tuned and the OBS set to the published certified radial, the CDI should center with a FROM indication when the aircraft is over the designated checkpoint. Allowable accuracy is ±4° for a ground check, and ±6° for an airborne check. If the aircraft is equipped with dual VORs, they may be checked against each other. The maximum permissible variation when tuned to the same VOR is 4°.

The pilot must log the results of the VOR accuracy test in the aircraft logbook or other record. The log must include the date, place, bearing error if any, and a signature.

Horizontal Situation Indicator (HSI)

The HSI is a combination of two instruments: the heading indicator and the VOR. See Figure 9-4.

Figure 9-4. Horizontal situation indicator (HSI)

The aircraft heading displayed on the rotating azimuth card under the upper lubber line in Figure 9-4 is 330°. The course indicating arrowhead that is shown is set to 300°. The tail of the course indicating arrow indicates the reciprocal, or 120°.

The course deviation bar operates with a VOR/LOC navigation receiver to indicate either left or right deviations from the course that is selected with the course indicating arrow. It moves left or right to indicate deviation from the centerline in the same manner that the angular movement of a conventional VOR/LOC needle indicates deviation from course.

The desired course is selected by rotating the course indicating arrow in relation to the azimuth card by means of the course set knob. This gives the pilot a pictorial presentation. The fixed aircraft symbol and the course deviation bar display the aircraft relative to the selected course as though the pilot was above the aircraft looking down.

The TO/FROM indicator is a triangular-shaped pointer. When this indicator points to the head of the course arrow, it indicates that the course selected, if properly intercepted and flown, will take the aircraft TO the selected facility, and vice versa.

The glide slope deviation pointer indicates the relationship of the aircraft to the glide slope. When the pointer is below the center position, the aircraft is above the glide slope, and an increased rate of descent is required.

To orient where the aircraft is in relation to the facility, first determine which radial is selected (look at the arrowhead). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator) to find which hemisphere the aircraft is in. Then, determine how far from the selected course the aircraft is (look at the deviation bar) to find which quadrant the aircraft is in. Finally, consider the aircraft heading (under the lubber line) to determine the aircraft’s position within the quadrant.

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