ADF Flashcards
Limitations
● Navigating by an NDB does have some quirks, and pilots should be aware of them! These include:
○ Night Effect
○ Mountains
○ Shorelines
○ Electrical storms
○ Bank error
Night Effect
● Radio waves are reflected by the ionosphere and they return to the earth 30 to 60 miles from the station.
● As the sun rises or sets, the ionosphere will change its position in terms of height above the earth since sun pushes down on it.
● This can cause the direction of the NDB to appear to change as the reflection angle changes, and it may also cause the ADF pointer to fluctuate.
● This effect is greatest within one hour of sunset or sunrise, and at distances that are greater than 30 miles from the station
● We can minimize Night Effect by averaging the needle’s fluctuation, by flying at a higher altitude, or by selecting a station with a lower frequency
Mountains
● Mountains or cliffs can reflect radio waves, producing a terrain effect.
● Furthermore, some of these slopes may have magnetic deposits that cause indefinite indications.
● Pilots flying near mountains should use only strong stations that give definite directional indications, and should not use stations obstructed by mountains
Electrical Storm
● When an electrical storm (thunderstorm) is nearby, the ADF needle points to the source of lightning rather than to the selected station because the lighting sends out radio waves.
● The pilot should note the flashes and not use the indications caused by them.
● An electrical storm causes the greatest error associated with signal reception of the NDB
Bank Error
● The ADF is also subject to errors when the aircraft is placed in a banked attitude.
● Bank error is present in turns because the loop antenna which rotates to sense the direction of the incoming signal is mounted so that its axis is parallel to the normal axis of the aircraft.
● Bank error is a significant factor during NDB approaches
Advantages
● While the ADF has drawbacks in the situations that were outlined in the last few slides, the system also has some advantages.
● Two of these benefits are:
○ The low cost of installation of the NDB, and
○ Their relatively low degree of maintenance
● They are also able to provide greater range than VOR’s
Tuning
● After tuning the receiver, the pilot must positively identify the station!
● All air facilities have radio beacons that transmit a continuous two-or three-unit identification in Morse code keyed to provide continuous identification except during voice communications.
● If there is not an audible Morse code identification, then the NDB is undergoing maintenance or calibration. It should not be used. BE CAREFUL, as the ADF will may appear to function normally
Bearing Indicator: Pointer
● The bearing indicator displays the bearing to the NDB or station, relative to the nose of the aircraft.
● Let’s say that again, since it is so important:
○ The bearing indicator displays the bearing to the station, RELATIVE to the NOSE of the aircraft.
● This is why we call it the Relative Bearing
● If the pilot is flying directly to the station, the bearing indicator will point to 0°.
● If the pilot is flying away from the station, then the bearing indicator will point to 180°.
Magnetic Bearing
● The angle formed by the intersection of a line drawn from the aircraft to the radio station, and a line drawn from the aircraft to magnetic north
● The pilot calculates the magnetic bearing by adding the relative bearing shown on the indicator to the magnetic heading of the aircraft.
● For example, if the magnetic heading of the aircraft is 40° and the relative bearing 210°, the magnetic bearing to the station is 250°
RB + MH = BTS
BTS +/- 180 = BFS
RB = BTS - MH
Homing
● One of the most common ADF uses is “homing to a station.”
● When using this procedure, the pilot flies to a station by keeping the bearing indicator needle on 0° when using a fixed-card ADF
Winds: Homing to an NDB
● It also shows us that the pilot must constantly change the magnetic heading to hold the aircraft on a relative bearing of 0°.
● This causes the pilot to follow an “arc” to the station.
○ That is to say, if the pilot does not make crosswind corrections, then the aircraft flies a curved path to the station – even though the bearing indicator remains pointed at zero
Tracking to an NDB / Station Passage
● Tracking – unlike homing – allows the pilot to track directly to the station by correcting for winds.
● The major difference is that tracking a VOR radial is a track bar needle, whereas tracking an ADF magnetic bearing requires the pilot to identify it by using both the bearing indicator and the heading indicator.
intercept heading (inbound/out)
Intercept heading (if track is to right) = pre-determined intercept angle + track to be intercepted
- if track is to left subtract
- Turn to track heading when relative bearing is the intercept angle
Shorelines
● Shorelines can refract or bend low frequency radio waves as they pass from the land to water.
● A pilot flying over the water should not use an NDB signal that crosses over the shoreline to the aircraft at an angle of less than 30°.
● The shoreline has little or no effect on radio waves that reach the aircraft at angles that are greater than 30
