ADF Intercepting a Track To or From NDB
To navigate with the help of ADF and NDB:
- Visualise your position.
- Intercept the desired course.
- Maintain the course to or from the station.
The first step is to visualise your position. Once you have visualised the aircraft’s position, you will be able to intercept the desired course, which in this case is 035° inbound.
The second step is to make any turn necessary to the heading that gives you a suitable intercept. Observe the instrument readings during the turn.
Inbound To the NDB Beacon
Now look at the corresponding plan view.
The heading of 090° gives you an intercept angle of 55°. The desired QDM is 035° the aircraft will be on track when the RBI indicates a relative bearing of 305° as shown in the diagram below.
When the needle is reaching the desired relative bearing, in this case 305° start your turn towards the station and your aircraft will be on the desired inbound track. When compared with the instrument indications.
Outbound From the NDB Beacon
To intercept a track outbound in nil wind conditions, follow the same procedures. First of all visualise your position.
The relative bearing of 100° combined with the magnetic heading of 125° indicates that you are North and East of the NDB.
The desired track is 050° outbound. Our intercept angle is 75°.
When the relative bearing is 105° we will have reached our outbound course.
At a relative bearing of 105° start turning left 75° to intercept the outbound course.
Heading 050° (only in still air, otherwise drift will have to be applied) with relative bearing 180°, now you are on course.
In order to intercept a specific course:
- First you have to know your position relative to the desired course.
- Then you establish a suitable interception angle.
Consider the following situation – to help you to visualise the situation draw a plan and the instrument indications.
- The aircraft is on a heading of 340°.
- The relative bearing to the NDB is 080°.
- The required course is 090º inbound.
By maintaining a heading of 340°, the aircraft will eventually intercept the 090 course. This would be a rather untidy intercept because a turn of 110° would be required when the aircraft is on track.
A more efficient intercept can be achieved by turning onto an initial heading of 360°, for a 90° intercept. A heading of 030º will lead to a 60° intercept with the required inbound course.
Since the aircraft is on QDR 240, a heading of 060º would turn the aircraft directly towards the station, and the QDM 090 will not be intercepted.
Once you have turned to a correct intercept heading, the rule is a simple one. When the angle formed by the aircraft’s heading and the desired course is the same as the angle between the zero mark at the top of the indicator and the pointer, then the aircraft is on the desired course (QDR or QDM).
If you are intercepting OUTBOUND, the aircraft is on the desired course when the intercept angle is the same as the angle between the zero mark at the top of the indicator and the TAIL of the needle.
know the interception angle. For instance, with a heading of 220° and a clearance to intercept QDM 180, the intercept angle is 40°.
When the needle is 40° to the left of zero, the track has been intercepted.
The aircraft heading is 265° and the RBI indicates 005°. You are required to join QDM 240 at an intercept angle of 60°.
The first step is always to visualise your position:
- What is the QDR?
- Which way do you have to turn to make the intercept, left or right?
The course is to the right of the aircraft, so a right turn has to be made for the interception.
Which heading will you need in order to intercept the QDM 240 with an intercept angle of 60º?
To intercept QDM 240 at 60°, the aircraft should be turned to a heading of 300º.
Maintain a heading of 300º, and observe the RBI needle.
Since this is a 60º intercept, wait until the pointer falls to 60º left of the zero indication on RBI. To mentally superimpose the RBI needle on the directional gyro is always a good crosscheck of your calculations.
The aircraft should be turned a few degrees before the desired QDM is reached. Observe the instruments and initiate the turn a few degrees before reaching QDM 240.The RMI eliminates the need to do any mental calculation. It always displays the QDM under the pointer and the QDR under the tail.
The procedure of intercepting QDRs and QDMs is made a lot easier if you maintain a mental picture of where the aircraft is and where you want it to be.
With no crosswind, a direct inbound course is achieved by:
- Heading the aircraft directly at the NDB.
- Maintaining the ADF needle on the nose of the aircraft.
If there is no drift then the aircraft will home to the NDB.
Any crosswind will cause the aircraft to be blown off track. In the cockpit, the ADF needle indicates this as it starts to move away from the top of the indicator.
To fly a straight course to the station is called TRACKING. To track to the station, a wind correction angle (WCA) has to be established which compensates for the drift caused by the crosswind. If the exact W/V is not known, then use an estimated WCA obtained from the available information (forecasts, pilots’ reports, etc.). Remember that the higher the crosswind the greater the WCA and, for the same crosswind, slower aircraft will need to establish a greater WCA than faster aircraft.
You are established on a course with a wind correction angle to compensate for drift. Observe the instruments and look at where the crosswind is coming from and what affect it is having on the aircraft.
If the ADF needle indicates a constant relative bearing while you are maintaining a constant magnetic heading, your wind correction angle is correct and the aircraft is tracking directly to or from the station. A wind correction angle that does not compensate for the present wind will cause the aircraft to drift off course and the ADF needle to show a gradually changing relative bearing.
If the head of the ADF needle moves to the right, it indicates that a turn to the right has to be made to maintain the course to the NDB and, conversely, if the head of the needle moves to the left, a left turn has to be made.
How large each correcting turn should be depends upon the deviation from the course. A simple method is to double the angle of bearing change. Observe that if the aircraft has deviated 10° to the left, the needle will have moved 10° to the right.
To double the angle of bearing change simply means that you alter your heading 20 degrees to the right.
Having regained the course, turn left by only half the correcting turn of 20°. That is to say, turn left 10° to maintain the track. This WCA should provide reasonable tracking.
In real life the perfect track is difficult to achieve and the pilot will make a number of minor corrections to the heading. This technique is known as bracketing the track.
The ADF needle will become more and more sensitive as the NDB station is approached. Minor displacements to the left or right of the track will cause larger and larger changes in the relative bearings and the QDM. When passing overhead the NDB, the ADF needle will oscillate then move toward the bottom of the dial and settle down. When close to the NDB do not change heading. Maintain the heading until accurate readings are obtained on the out bound flight.
To facilitate the QDR calculations when tracking outbound, you should remember that the QDR is equal to the Magnetic Heading plus or minus the deflection of the tail of the needle. Suppose that the desired course outbound from an NDB is QDR 040 and the pilot estimates a WCA of 10° to the right to counteract the wind from the right.
To fix the QDR 040 in a no wind condition is achieved by flying heading 040°. Since we have a right crosswind that requires a 10° WCA, the heading in this case is 050°.
VOR Station Passage
To plan an intercept and follow a specific radial, first determine your position in relation to the desired track. If you are tracking a radial outbound, set the CDI to the desired radial. CDI deflection will now tell you which way to turn in order to make an intercept. The intercept angle will depend on different factors. If ATC wants you to join the new track as soon as possible, you can make an initial intercept of up to 90° and, when the CDI starts to move, you start leading the turn to establish on the new radial. If you are close to the VOR station, the needle will move quite fast. Conversely, if you are far from the station the needle will move more slowly. Aircraft speed will also affect the needle movement.
If there are no restrictions regarding the intercept, an intercept angle of 30° or 45° is normally a good alternative. This will be taught during your practical training.
If we are to intercept a radial and track it inbound, the procedure will be as above, except that we set the reciprocal of the radial, which is the inbound course. The procedure is otherwise the same. When the selected course has been intercepted, the procedure is the same asdescribed earlier.
If you are tracking TO a VOR station and you are to continue on the same course after you have passed the station, when close to the station, needle-movement becomes very erratic. The TO/FROM flags will flicker during the passage of the station and the warning flag (NAV/OFF) will appear momentarily. This is because of the cone of confusion that is directly overhead the VOR.
When tracking along an airway between two VORs, the normal procedure is to switch from tracking FROM one VOR to tracking TO the next VOR when midway between the two facilities. Sometimes the changeover point is specified elsewhere on the route segment. This is because of signals being restricted by terrain or by frequency interference. When this is the case, it will be specified on the appropriate instrument chart.
When tracking from one VOR to another, the published radial for the inbound track to one VOR should match the outbound track or radial from the other, but it is not always so. Radials are magnetic tracks from the VOR and the directions of the radials depend on the magnetic variation at the different VORs. The difference between the corresponding radials equals the difference in variation at the position of the two VORs. At high latitudes, convergence of the meridians will also contribute noticeable differences.
Interception of an Inbound Track
Tune and identify the required VOR beacon and orient the aircraft to ascertain the magnetic bearing of the aircraft to the beacon. Then rotate the OBS until the required inbound track is shown in the Bearing Selector Window.
The TO indicator should then be visible (If FROM appears, either the wrong track has been set or the aircraft is on the other side of, or has passed the beacon—see Interception of an Outbound Track).
The aircraft should then be turned in the direction of the Deviation Indicator on to a suitable intercept heading, which is determined from the position of the aircraft as visualised from the information available from the orientation.
Closure of the required track is shown by the Deviation Indicator moving towards the central position, and when this occurs, the aircraft should be turned onto the same heading as the required inbound track. The track should be maintained as described in Homing Directly to the VOR Beacon.
Intercepting a Track Inbound
The procedure for intercepting an outbound track differs from the procedure for intercepting an inbound track only in that FROM rather than TO appears in the TO/FROM Window.
Intercepting a Track Outbound
Orienting the Aircraft to Intercept a Track
If, when orienting the aircraft to intercept an inbound or an outbound track it is found that the aircraft is on a bearing which is 90 degrees or more removed from the required track, then the TO/FROM indications as specified in the paragraphs Interception of an Inbound Track and Interception of an Outbound Track, will not apply until the aircraft is within a sector 90 degrees on either side of the required track.
When holding in the conventional racetrack pattern, it must be appreciated that the outbound leg of the pattern is not via a radial, but parallel to and on the reciprocal track to the inbound leg which is almost invariably on a designated radial.
Therefore on reaching the holding point to commence the pattern, the inbound track is set by the OBS, and this setting is retained while flying in the holding pattern. This setting will provide information to intercept the inbound track when turning inbound at the end of the outbound leg.
Instrument Descent Procedure
After passing over the station, set the OBS to the published outbound track and fly this track outbound, descending if necessary, until the commencement of the procedure turn. During the procedure turn, set the OBS to the published inbound track.
Intercept this track and continue descent to the minimum altitude, maintaining the inbound track by reference to the Deviation Indicator.
If a missed approach is necessary, carry out the missed approach in the manner specified in the missed approach procedure, and if a change of track is prescribed, re-set the OBS to the required track at the point specified for change of track in the procedure.
It is essential to remember the following points concerning the VOR and its operation:
- A radial is a magnetic bearing from the VOR beacon.
- The Deviation Indicator can be centred on either of two bearings by the OBS. These bearings will be 180° apart with the ambiguity automatically resolved by the TO/FROM indicator.
- The VOR system, in practice, should be considered to be accurate only to the order of ± 5°.
- As the VOR propagation is ‘line of sight’, the distance at which a VOR beacon can be received increases with aircraft elevation above the beacon.
- The VOR beacon must be identified aurally before information derived from the beacon is used.
- The Omni Bearing Indicator always indicates magnetic bearings TO or FROM the VOR beacon—never relative bearings.
- Heading of the aircraft at any instant does not affect VOR bearings. The bearing obtained depends on the aircraft location with relation to the VOR beacon.
- Bearing selection by the OBS is by 1 degree intervals. When a bearing is selected, full travel of the Deviation Indicator from one side to the other represents 20° of azimuth, that is 10° either side of the selected bearing.