Pilotage and dead reckoning

From FlightGear wiki
Revision as of 05:28, 11 October 2011 by Johan G (Talk | contribs) (Since I will retire From the FlightGear community, I could at least finish this part.)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

Pilotage and dead reckoning is the fine art of using mainly a chart, a clock, a compass to get from point A to point B.

Pilotage is the art of knowing where you are by reading a map and comparing it with the surrounding terrain and landmarks, while dead reckoning is the art knowing where you currently are by using a compass, your ground speed, a clock and an initial known position.

If you are flying very low or even in bad weather you might be very dependent on your dead reckoning, due to limitations in how much of the surrounding terrain you can see. In essence:

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


Pilotage or map reading is, in fair weather and at a bit higher altitude, nearly independent of dead reckoning. The two main uses for the map 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 turn radius of several miles at high speed, which significantly shifts its course to the next checkpoint.

In a later stage after the calculations are done ticks are added 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 also eases catching up any timing errors.

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 bad habit of trying to figure out where you currently are. This will result in that you mentally might be a few miles behind the aircraft, 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 also studied the map while planning you might remember what places that will be more critical to check the timing against.

(Grabbing map, looking at clock, then 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
FlightGear's terrain and real maps

The larger landforms usually conforms well with the map. 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.

Dead reckoning

Using dead reckoning is more or less the act of following, and updating, a preplanned "time table", often called a (VFR) 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 checkpoint 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. Some of the numbers could also be drawn onto the map.

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 do have, the true air speed or TAS, as there is a varying difference between them, mainly dependent on 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 coming 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 will have to keep to get to the next checkpoint isn't the same as the direction from the last checkpoint to the next, 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.