Dead Reckoning. Sounds like a very apt name for the latest scary movie, but as an aviation term you may be disappointed to know that it is not as exciting as it sounds. I have absolutely no idea why the word ‘dead’ is used – but I can take a guess, and say that it is used as a metaphor in the English language for a horizontal, straight and finite position (similar to a corpse). This makes it sound very accurate (maybe it was in the early days of navigation), but add to it the word ‘reckoning’, and we now have something closer to a calculated guess. Regardless, you may find it useful to know what it is for an exam, or just to be educated in all things involving our wonderful flying machines.
This is a method of navigation in which the direction and speed of movement of an aircraft are determined by computing the effect of wind on the aircraft. Essentially, you would use this method to determine your next position, and how long it will take to get there, based on the factors that have affected your current position and time. Yes, yes, I know that modern navigation equipment is by far clever enough to do this, but quite often, dead reckoning is programmed into their micro-chipped brains. So, in order to prevent the dumbing of the human race, let’s see if we can do this ourselves:
The dead reckoning or ‘DR’ position is an expected position of where we will be, once we have calculated the results of flying on a particular heading, with a certain TAS, and the effects of the wind direction and speed. It can never be seen as a set ‘fix’, because nature often has it’s own ideas, and any of these fore-mentioned factors can change in flight. Most commonly, DR is used for three purposes:
- To approximate present position.
- To calculate a new heading to steer to get to a new position.
- To calculate the ETA to the new position.
Below are a few examples of some calculations that may be useful:
Crosswind / Headwind Component
Useful to know which way the wind is causing us to drift, particularly if we are looking at maintaining a heading, or coming in to land on a runway.
Example:
For a landing on runway 23 (227° magnetic) surface W/V reported by the ATIS is 180/30kt. VAR is 13°E. Calculate the cross wind component?
Remember if a wind direction is spoken (ATIS/Tower) then it will be magnetic, and so is the runway. Determine the difference between the wind direction and and the runway direction (use three digits for this!).
For cross wind: 227 -180 = 47 Next, SIN 47 x Windspeed (30kts) = 22kts
For head wind: 227 -180 = 47 Next, COS 47 x Windspeed (30kts) = 20kts
Alternatively, you can use a CX2/3 calculator > ACT leg > X/H Wind entering wind direction, speed and runway (using 3 digits)
Track (T) and Groundspeed (GS)
For this, I find the CX2 quickest, and I will show you a little trick ;).
Example:
Given: TAS = 480 kt, HDG (°T) = 040°, W/V = 090/60kt. Calculate the Track (°T) and GS?
Using the CX2, ACT leg > Unknown wind, enter the information like this:
THdg: 40
GS: Enter the wind velocity 60
TCrs: Enter the wind direction 90
TAS: 480
At the bottom, your answer will read:
Wind Direction: 034 (This is your track!)
Wind Speed: 445kt (This is your GS!)
Drift Angle and Groundspeed (GS)
Calculating the drift angle is important, so that we know how far off our track the wind will push us, and the ground speed will tell us how quickly we will be blown off course if we don’t make corrections.
Example:
Given: TAS = 235 kt, HDG (T) = 076° W/V = 040/40kt. Calculate the drift angle and GS?
Using the CX2, ACT leg > Unknown wind, enter the information like this:
THdg: 76
GS: Enter the wind velocity 40
TCrs: Enter the wind direction 40
TAS: 235
At the bottom, your answer will read:
Wind Direction: 83 (This is your track!)
Wind Speed: 204kt (This is your GS!)
To get the drift we need to find the difference between the track (read from the wind direction on the CX2), and the true heading, so we need one more step:
Drift = Track (CX2 W Dir) – True Heading
= 83 – 76
= 7 degrees RIGHT (Our track is greater than our true heading, so this means that the wind is coming from the right – we need to angle the aircraft into the wind in order to maintain our heading, so the drift correction angle is R)
If Drift = Thdg – Track (CX2 W Dir) , then the drift angle will be less, as the track is less than our THdg, and therefore LEFT.
Time, Distance, Speed
Resolving for time, distance or speed requires two of the three elements, and this simple formula:
Speed = Distance/Time
Which can be manipulated as follows:
Distance = Speed x Time
Time = Distance / Speed
Example:
Given: GS = 105 kt. Distance from A to B = 103 NM. What is the time from A to B?
Time = Distance / Speed
Measure your distance on the map in NM, in the case that you do not know your GS, take the TAS + tailwind or TAS – Headwind to get your GS.
Time = 103 / 105
= 0.980 (To get this into HR: MIN x60)
= 58’51”
While these are just a few examples of what you can calculate. However, especially if you are flying VFR there is something to be said for plain, simple pilotage – that is, art of knowing where you, are by reading a map and comparing it with the surrounding terrain and landmarks. So whether you like to calculate, or follow your GPS, remember to keep your eyes looking out of the window… Lots to appreciate out there!
