User:Johan G/Introduction to Low level VFR navigation
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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 rewarding as it can be a bit of a challenge if e.g. the weather shifts and your wind calculations goes out the window.
Pilotage and dead reckoning
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.
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.
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)
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.
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, 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.
The tools used
- Scrap paper
- Navigation log
- Flight computer (E6-B, CR) or similar tool
- Straight edge and compass/ruler/plotter/combat plotter
- Chart(s), NOTAMS etc.
General work flow
There are mainly two ways to do the planning, in relation to various speeds. One way is to use a fixed ground speed GS, which would make the prepared chart usable in other wind conditions. The other way is to use a fixed true air speed TAS, maybe optimised for range or speed. The downside of using a fixed TAS is that different winds will alter the timing calculations, i.e. you will arive at another time and the minute marks have to be redrawn. In essence:
- Fixed ground speed GS: The same timing and reusable minute marks, even if wind changes.
- Fixed true air speed TAS: Possibility to optimise the TAS for long range or high speed.
If you would go for the fixed ground speed GS, there's a small thing that might come in handy while preparing the map and while flying: Using a ground speed GS dividable by 60.
- Drawing the route
- Finding suitable checkpoints and marking them
- Draw turn arcs
- Draw tracks
- Finding the numbers
- True and magnetic course
- Minimum safe altitude (for emergencies)
- Course, air speed and timing calculations
- True course TC to magnetic course MC
- Ground speed GS to indicated air speed IAS or true air speed TAS to indicated air speed IAS
- Timing at the checkpoints and entire flight
- Adding minute marks to the chart
- Fuel calculations, for each leg and in total
- To diverts, and alternatives (for longer flights)
- Emergency diverts along the route (for longer flights)
- Two alternative airports (for longer flights)
- Possibly preparing an "abbreviated" chart, a strip chart
- Last minute wind calculations
- Finding the wind numbers
- True wind to magnetic wind
- Wind correction angle WCA
- New magnetic heading MH
- New true air speed TAS or new ground speed GS
- Prior to flight: Folding the chart
Drawing the route
Finding suitable checkpoints and marking them
I recommend spacing the check points about 5 minutes apart, but not closer than 2 minutes. The reason to not have them closer than 2 minutes is that you need time if you have to adjust engine power, air speed or trim the plane. The check points don't have to be turn points, and they actually doesn't have to be on the track. When turning you naturally have to do that over a checkpoint that is on the track.
Good check points when flying in FlightGear tend to be where roads, rail roads, and/or rivers intersect. Also airfields, modelled landmarks, and distinct terrain features like hills and parts of lakes might be good check points.
When marking the check points, do so by drawing a pencil figure around them making sure you do not obstruct the feature itself or any useful information on the map such as altitudes or radio frequencies. Commonly a large circle is used, but do as you may.
Drawing turn arcs
After each checkpoint after which you turn, you might need to draw an arc. The reason for this is that if you are doing a turn at high speed, your turn radius will be large enough to put you off track if you draw the track directly from the checkpoint.
If you are flying a high performance jet, possibly an attack or fighter jet, all below is irrelevant if you have a combat plotter or something similar, since it literally is a template intended for map preparation, having turn arcs for various air speeds and map scales ready. A combat plotter, Combat mission plotter PLU-6/C, suitable for being printed on a transparency have been uploaded to the wiki.
You could possibly make your own plotter, complete with turn arcs, in e.g. Inkscape, even if you do not fly a high performance jet.
If you are doing a procedural turn, a turn that in which a full circle will take either 2 minutes or 4 minutes, the diameter of the turn can be calculated using this formula:
- ((GS / 60) * n) / pi, where n is either 2 or 4 minutes and pi is 3.14.
What you are doing is that you find the circumference of the turn through (GS / 60) * n, a one minute turn would be GS / 60, and then get the diameter by dividing the result by pi.
Coming in handy while flying, as a rule of thumb, the bank angle in a 2 minute procedural turn is about 15% of the air speed. To get the bank angle divide the air speed by 10 and add half of that. E.g. for an air speed of 300 kts the bank angle is about 30 + 15 = 45 degrees. Or use this formula:
- BA = AS * 0.15, where BA is the bank angle and AS is the air speed.
To summarise this in a table, for the same ground speeds GS mentioned above:
|NM dia. (2 min proc. turn)||0.6||1.3||1.9||2.6||3.2||3.8||4.4||5.1||5.7|
|Aprox. BA (2 min proc. turn)||9||18||27||36||45||54||63||72||81|
|NM dia. (4 min proc. turn)||1.3||2.6||3.8||5.1||6.4||7.6||8.9||10.2||11.5||12.7||14.0||15.3|
Just like when marking the checkpoints, don't draw the track over important information like altitudes and frequencies. It might be a good idea to draw the track in pencil at first. Draw the track between the next turn point and tangential to the turn arc.
If you are going for the fixed ground speed GS method you can add the minute marks now. If you went for an air speed dividable by 60 you can find the distances in the table below. The recommended way to get the distances on the map is to use the ticks along the meridians (north-south lines) closest to the current leg. To add minute marks to the turn arcs, you add them for every 180 or 90 degrees (360 degrees / 2 minutes or 360 degrees / 4 minutes).
Finding the numbers
Now lets find the numbers that you will need in the calculations that will follow. While we wont use the minimum safe altitude in the calculations, it will be esier to find in this step.
True and magnetic course
We will need both of them. The map and the weather are from true north, while the navigation is done from magnetic north.
The distance is needed for timing calculations. If you have gone for the fixed ground speed GS method, or even better, have a combat plotter or similar the timing calculations are already done.
The altitude is needed for the air speed calulations.
Minimum safe altitude
The minimum safe altitude MSA might be good to have in case you have e.g. engine trouble or get lost.
- Thumb rules
- Precision navigation
- Military precision navigation
More reading etc.
- Chief of Naval Air Training (CNATRA)
- FAA NACO
- Perry-Castañeda Library at Library of Texas University
- Nought Feet – A Film on Low Flying Navigation Technique. Published by YouTube. – Part one of two, 10 min, WWII era instruction film produced for the Royal Air Force
- Nought Feet – A Film on Low Flying Navigation Technique. Published by YouTube. – Part two of two, 6 min, WWII era instruction film produced for the Royal Air Force
- Low Level Air Navigation. Published by Texas Archive of the Moving Image (TAMI). – 23 min training film produced for the US Navy. (Seems it was used as recent as 2012 (!). )