Chapter

8

Long Range Flight and Polar Navigation

Navigation System Degradation . . . . . . . . . . . .	. .	. .	. .	. .	. .		.113
Course and INS Cross-checking							113
Unable to Continue in Accordance with ATC Clearance . . . .	. .	. .	. .	. .	.	.	113
Polar Navigation . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	113
Grid Navigation . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.114
Minimum Time Routes . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	114
Questions . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.115
Answers.							116

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8		Long Range Flight and Polar Navigation

Navigation Polar and Flight Range Long 8

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Long Range Flight and Polar Navigation		8

Navigation System Degradation

Modern navigation systems based on INS and radio navigation aids (including GPS) consisting of either 2 or 3 set systems, include comparator and/or warning devices. Pure equipment failure would be indicated by the illumination of a warning light.

Course and INS Cross-checking

In an emergency course and INS can be cross-checked:

•In a 3 set system, the output of each system should be compared (a voting system) from which inaccuracy in any one system should be quickly detected.

•In a 2 set system, the failure of one system would not be readily detected unless the system captions malfunction codes in which case interpretation of the code should reveal which unit is faulty. If it is possible to obtain a fix (from the weather radar or bearings from ADF and NDB, or a fix from a VOR beacon), comparing with the system positions should reveal the inaccurate system.

If uncertainty still exists more basic methods include contacting another aeroplane in the vicinity and cross-checking spot winds, ground speed and drift. As a last resort, comparison of the outputs from the nav systems could be compared with the flight plan data for wind velocity at the DR position of the aeroplane.

Unable to Continue in Accordance with ATC Clearance

If an aeroplane is unable to continue the flight as per the ATC clearance, a revised clearance shall, whenever possible, be obtained prior to initiating any action. This shall also apply to aircraft which are unable to maintain the specified navigation accuracy. The revised clearance shall be obtained by RTF distress or urgency traffic as appropriate. The subsequent ATC action shall be based on the intention of the pilot and the overall air traffic situation.

Polar Navigation

Polar tracks defined as North/South routes involve navigation at high latitudes (above 65°N). In these areas, the lack of ground radio aids, high rates of change of magnetic variation and steep magnetic dip angles, make conventional airways navigation difficult if not impossible. For the reasons stated, magnetic compasses become unreliable and reference to magnetic north is impractical. In this situation, navigation is achieved by reference to a grid navigation process or reliance on inertial systems and satellite based global positioning (GPS).

In areas where the rate of change of magnetic variation becomes excessive (in close proximity to the North Magnetic Pole), VOR beacons are orientated to true north to assist grid navigation. VORs in the Canadian Northern Control Area are oriented to true north. Where the primary heading information is derived from an INS system, care must be taken to monitor the system (by reference to any other aid or method) for degradation or failure.

It is important to remember that there are a number of different ways in which the autopilot can become unobtrusively disconnected from the steering mode, therefore regular checks of correct engagement are to be made. Where possible it is recommended that when coupled to the autopilot the navigation system should display position co-ordinates throughout the flight; these are then plotted 10 minutes after each waypoint. The navigation system not being

Long Range Flight and Polar Navigation 8

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8		Long Range Flight and Polar Navigation

Navigation Polar and Flight Range Long 8

used to steer the aircraft should display cross-track distance being displayed on the HSI where feasible.

Training and drills should not be conducted to the extent that the flight crew is distracted such that the navigation system is mishandled. If at any point during the flight the autopilot is disconnected (e.g. because of turbulence) care must be taken when the navigation steering is re-engaged to ensure that the correct procedure is followed. Where the system sets specific limits for automatic capture, the cross-track indications should be monitored to ensure proper re-capture of the programmed flight path/profile. Where low angles of bank are used (10° for passenger comfort) it is essential to be particularly alert to possible insidious departures from cleared track.

Other factors which make polar navigation difficult are limited communications with that which is available being mainly restricted to HF, lack of en route alternate aerodromes and high rates of gyro correction (for earth rate and transport wander).

Grid Navigation

Grid navigation in conjunction with a directional gyro can be used in polar areas to resolve polar navigation problems. The procedures for the use of polar stereographic charts and grid coordinates are covered fully in the navigation general syllabus with reference to the associated notes. Similarly, the use and effect of INS and gyro systems is covered in the instrument syllabus including the definition and calculation of transport precession, earth rate precession and convergence factor. However it should be noted that grid navigation, gyroscopic transport precession, earth rate precession and convergence factor are examinable in the OP syllabus.

Minimum Time Routes

A minimum time route is defined as the track flown between two points which results in the shortest time adhering to all ATC and airspace restrictions. Geographically, the shortest distance between any two points is the minor arc of the great circle joining both those points. In reality, airspace restrictions (danger/restricted/prohibited areas), airway routings, and wind and meteorological considerations may make another longer track a quicker option.

Historically, minimum time routes have been ‘manually’ calculated by taking 3, 4 or 5 alternative track options from a point and taking wind into account, determining the route that achieves the greatest track distance in a given time period (usually 1 hour). The process is then repeated for the point at the end of that route and repeated as many times as is required to arrive at the destination. This is a time consuming and laborious process. Modern computers are far better at doing this than man, and today all minimum time routes are computer generated (and far more accurate in the prediction).

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