User:Johan G/Introduction to Low level VFR navigation

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WORK IN PROGRESS

Low level navigation is the fine art of getting from point A to point B at the precise time, at low altitude, possibly hoping to be unseen, using mainly a clock, a compass, a chart and the combined skills of dead reckoning and pilotage.

To actually get to point B on time requires some planning and knowledge about the aircraft, terrain and weather, and also requires the ability to adjust the plan. To get it exactly right is very rewarding as it can be a bit of a challenge if e.g. the weather shifts and your wind calculations goes out the window.

Dead reckoning and pilotage

Dead reckoning is the art knowing where you currently are by using a compass, your ground speed, a clock and an initial known position, while pilotage is the art of knowing where you are by reading a map and comparing it with the surrounding terrain and landmarks. If you are flying very low or even in bad weather you might be very dependent on your dead reckoning.

  • Lower altitude: More dead reckoning required, less pilotage possible
  • Higher altitude: Less dead reckoning required, more pilotage possible.


Dead reckoning

Using dead reckoning is more or less the act of following, and updating, a preplanned "time table", often called a navigation log or nav log. The difference between the nav log and your local bus' time table is that the nav log, apart from times and places, in this context often called checkpoints or waypoints, also include the heading, altitude and airspeed you need to have to get to the next place on time.

The navigation log

The navigation log, together with the map, is the principal tool in getting from point A to point B on time. Most of the calculations done while planning, often done on scrap paper, goes right into the nav log.

There's a few thing that complicates the calculations that gives the numbers in the nav log. To counter all these things a lot of calculations are needed. Often, at least at student pilot level, these are done with an E6-B or CR flight computer.

  • Firstly, all maps show true north, while navigation almost exclusively is done by magnetic north. The winds are also reported in true north.
  • Secondly, the air speed shown by your airspeed indicator, the indicated airspeed or IAS, is not the same as the speed you actually have, the true air speed or IAS, as the difference between them varies mainly with altitude.
  • Thirdly, unless you are flying in still air the TAS is actually not the speed you have over the ground, the ground speed or GS. If you have a head wind coming towards you your GS will be slower than your TAS, and the opposite if you have a tail wind from behind.
  • Fourthly, if you have the wind from the side, you will drift off from your track. To stay on track you will have to head into the wind. This results in that the (magnetic) heading, or MH, you have to keep to get to the next checkpoint isn't the same as the direction to it, the (magnetic) course or MC. Also since the wind is pushing from the side it will change your GS, though not as much as a head or tail wind.


Pilotage

Pilotage or map reading is, in fair weather and at a bit higher altitude, nearly independent of dead reckoning. It's two main uses is to catch any sings if drift before you have drifted to far off track and to accurately find and turn at the checkpoints. In order to effectively use the map it is usually prepared with checkpoints, tracks, minute marks and various annotations. This can be done in many ways from very simple to all the way to extremely detailed.

One very important thing to keep in mind is that you might not have all that much time in the cockpit to read the map, i.e. the things drawn on the map must be unambiguous and be possible to read on a glance.

Map preparations

Usually, the first step in planning is part of the map preparation. That is finding suitable checkpoints, marking them and plotting tracks between the checkpoints. The tracks might involve turn arcs before, or more preferably, after the checkpoints, shifting the track to the side altering it's course. The size of the turn arcs, if the are drawn at all, are depending on the map scale, the turn angle, and the performance of the aircraft. While a double decker may turn on a coin a jet fighter might have a mile turn radius at high speed, which significantly shifts its course to the next checkpoint.

In a later stage after your calculations are done ticks are to the tracks with a few minutes distance between them annotated with either the total number of minutes from the start, minutes from the last checkpoint or the time. These are called minute marks and significantly eases finding out where you are depending on the time and catching up any errors in the air speed.

Other annotations can include the new course, heading, altitude or air speed, the required fuel to get back, to the next checkpoint or to a divert, alternative or emergence airfield, radio frequencies etc.

Map reading

While this is not the best place to go into map legends, there is a few things that are important to mention.

Clock, chart, terrain

While reading the map it is easy to get into the habit of trying to figure out where you currently are. This will result in that you mentally will be a few miles behind the plane, which can be hazardous in some circumstances. It is more efficient to plan ahead, i.e. what will you pass some time ahead according to the minute marks. This way you wont have to "thumb" the map, constantly reading it, but can read the map more efficiently. If you have studied the map while planning you will know what places that will be more critical to check the timing against.

(Grabbing map) "I should pass a road at an angle in 1 minute and thirty seconds... There's the road, ten seconds late". (Putting map back)

In essence:

  1. Clock
  2. Chart
  3. Terrain
Real maps and FlightGear's terrain

The road networks in FlightGear are often detailed enough that one can use them as checkpoints and navigate between them. Discerning between what is a road, a railway or a (small) river can be a bit of a problem. Cities are not always in the same shapes as on the map. etc.

The tools used

  • Scrap paper
  • VFR log
  • Flight computer (E6-B, CR) or similar tool
  • Straight edge and compass/ruler/plotter/combat plotter
  • Map(s), NOTAMS etc.

Planning

General work flow

  • Two ways
    • Fixed TAS
    • Fixed GS
  • Drawing route
  • Calculating timings
  • Minute marks
  • Fuel calculations
  • Diverts

Execution

  • Thumb rules

Competitions in real life

  • Precision navigation
  • Military precision navigation

More reading etc.

  • Wikipedia
  • FAA NACO
  • Chief of Naval Air Training Academy (CNATRA)