Long Range Flight and Polar Navigation — Page 120, Lesson 184

I want to walk you through a topic that really tests a pilot's planning and systems knowledge: long-range flight and polar navigation. Let's start with some practical handling points. When you're using the autopilot to steer the aircraft in polar regions, the system should display cross-track distance on the HSI — that's the Horizontal Situation Indicator — wherever that's feasible. Cross-track distance tells you how far left or right you've drifted from your intended path. Now, a critical caution: training and drills should not be conducted to the extent that the flight crew gets so distracted that the navigation system is mishandled. In other words, don't let practice exercises take your attention away from actually flying the navigation correctly. If at any point during the flight the autopilot is disconnected — for example, because of turbulence — you must take care when re-engaging the navigation steering. You need to ensure the correct procedure is followed. Where the system sets specific limits for automatic capture, you should monitor the cross-track indications to make sure the aircraft properly recaptures the programmed flight path and flight profile. Here's a subtle but important point: where low angles of bank are used — typically 10° for passenger comfort — it is essential to be particularly alert to possible insidious departures from cleared track. "Insidious" means the aircraft can drift off track gradually without you noticing, because the bank angle is so shallow that the turn rate is slow. You have to stay vigilant. Now, what makes polar navigation difficult beyond the handling? Three main factors. First, limited communications — what's available is mainly restricted to HF, High Frequency radio. Second, a lack of en-route alternate aerodromes — there simply aren't many airports to divert to if something goes wrong. Third, high rates of gyro correction — your gyroscopic instruments need to be corrected for earth rate and transport wander, and those correction rates are high in polar regions. That brings us to Grid Navigation. Grid navigation, used in conjunction with a directional gyro, can be employed in polar areas to resolve these polar navigation problems. The procedures for using 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 — Inertial Navigation Systems — and gyro systems is covered in the instrument syllabus, including the definition and calculation of transport precession, earth rate precession, and convergence factor. But I want you to note this carefully: grid navigation, gyroscopic transport precession, earth rate precession, and convergence factor are all examinable in the Operational Procedures syllabus — the OP syllabus. So even though the detailed maths may be in other books, you need to know these concepts for this exam. Let's move to Minimum Time Routes. A minimum time route is defined as the track flown between two points which results in the shortest time, while 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. That's the great circle route. But in reality, airspace restrictions — danger areas, restricted areas, prohibited areas — along with airway routings, and wind and meteorological considerations, may make another longer track a quicker option. So the shortest distance isn't always the fastest time. Historically, minimum time routes were manually calculated. You'd take 3, 4, or 5 alternative track options from a point, take wind into account, and determine the route that achieves the greatest track distance in a given time period — usually 1 hour. Then you'd repeat that process for the point at the end of that route, and repeat as many times as needed to arrive at the destination. That's a time-consuming and laborious process. Modern computers are far better at doing this than a human, and today all minimum time routes are computer-generated — and far more accurate in the prediction. So you won't be doing manual calculations, but you need to understand the principle behind what the computer is doing.