Line of Flight Explained
The topic of “Line of Flight” seems to be a mysterious, yet cool term that is often misused and/or misunderstood. As a freefly load organizer and instructor, I’ve realized the lack of knowledge about this subject so I figured we can take a moment and break it down:
Jump Run – the direction of flight and configuration of the plane while jumpers are exiting
Line of Flight – The 3-dimensional profile of Jump Run
The Line of Flight is essentially the same “line” as Jump Run, however in skydiving, the Line of Flight is discussed in terms of three-dimensional space.
Next, where Jump Run begins (or the point where the first group exits) is known as “Down the Line of Flight” and where Jump Run ends (towards the last group exiting), is called, “Up the Line of Flight.”
According to these illustrations, note the compass rose and which direction the plane is flying. You can determine that the plane is flying from the South, towards the North. This establishes Jump Run and Line of Flight.
So, what makes this “Line of Flight” important? To avoid collisions!!
Potential Collision Hazards
- Freefall Drifting (outside the given exit separation and given column of air)
- Break-Off & Opening
- Canopy Opening and the First 10-15 seconds
On every jump, in any axis, we all experience freefall and canopy drift. (Reference http://www.melissaairheart.com/winds-aloft/) Therefore, pre-planning the spot, Jump Run, Exit Order (reference http://www.melissaairheart.com/exit-order-of-business/), and Exit Separation (reference http://www.melissaairheart.com/exit-separation-time-really-matters/) turn out to be important elements of safety for Line of Flight.
Taking into consideration the day’s Jump Run, the Exit Order for the load and Exit Separation for the day’s conditions, each group (assuming they are a traditional RW, Freefly, student or Tandem group) is given a “Column of Air” for freefall. If a group is moving towards the boundaries of their given column, there now exists a potential for a collision.
How does one get towards the boundaries of the column if they exited in the right Exit Order and given the appropriate Exit Separation?
Example 1: New Freeflier
Freefly speeds are increased from 120mph to roughly 150mph. Typically, new sit flyers have a tendency to lean forward which causes a dramatic backslide, which can cover a great distance. If that jumper is facing Up or Down the Line of Flight, they are increasing their chances of converging with another group.
A solution is to have newer freefliers identify themselves in the loading area, and let others know they’ll be taking the Line of Flight into consideration. Then make sure to face perpendicular to the Line of Flight during freefall.
Example 2: Break-Off
To avoid collision on break off, it is suggested to track perpendicular to the Line of Flight.
Let’s say there is one 4-way RW group (no video), and three 2-way freefly groups exiting from a caravan – given Exit Separation 6 seconds, Jump Run South to North, and each group exited appropriately. To assure avoiding running into groups, the 2-way freefliers are able to track perpendicular to the Line of Flight, allowing more separation between themselves and the other groups.
However, in a 4-way or larger, inevitably, part of the group may track Up and Down the Line of Flight. There are 3-options to this variable:
1: The 2-jumpers tracking Up or Down the Line of Flight may reduce their tracking speed so as not exit their Column boundaries, yet still gaining an appropriate distance; and the 2-jumpers tracking off the Line of Flight do a max track to assist in maximizing group separation
2: The 4-way could adjust their break-off and off-set their trajectory by at least 45° so as to break-off, off the Line of Flight
3: The group exiting after a group of 4 (or more), leave a little extra time before exiting to account for enlarging the Column of airspace for the previous group’s need space for break off
[Larger groups will absolutely need more time between groups to account for a larger distance covered on their break-off.]
Note: Angled, tracking and wingsuit groups are exceptions to the “Column of Air” example as they fly in a broader airspace and need special consideration for their flight paths. This requires communication and awareness from the entire load, including the pilot.
Why is Line of Flight important for canopy?
Example 1: Canopy’s Flight Path
The canopy’s forward movement after opening still increases the distance towards the boundaries of the prior or previous group’s Column of Air.
Therefore, after ensuring a functioning canopy, it’s important to fly OFF the Line of Flight for approximately 10-15 seconds after opening. In theory, you should be able to look Up the Line of Flight and see the group after you breaking off or just opening; and look Down the Line of Flight and identify the group Down the Line of Flight under canopy and slightly below (depending on opening altitudes).
Example 2: Landing Area and Opening Point
This will vary depending on Jump Run’s direction, surface winds, freefall drift, etc. However, if the landing Target Area is under Group 3 shown in the next Illustration, then Group 1 and 2 will have to fly Up the Line of Flight. If you find yourself in Group 1 or 2’s situation, fly off the Line of Flight and identify the groups that exited after you before you fly to the the Holding Area and Landing Pattern.
Try this at home:
1. Figure out Line of Flight (or Jump Run) for the day’s conditions and identify landmarks for specific directions
2. Make sure you note if your group is drifting up or down the Line of Flight; then assure you track accordingly
3. After deployment and opening checks, fly your canopy off the Line of Flight (if safe to do so) for 10-15 seconds.
4. Identify a safe flight path to Holding Area and Landing Pattern
Another great resource is USPA’s Power Point presentation on “Canopy Collisions” found here http://www.uspa.org/USPAMembers/Downloads/tabid/84/Default.aspx (scroll towards bottom, under “Miscellaneous” topics.
There are always exceptions to the norm and many variables. Therefore, maintain awareness and use your best judgement in each situation.
Note: If this does not make sense, please consult an instructor at your Drop Zone for further explanation. This is not meant to be a sole training tool for skydiving or parachute flying. Full instructional methods will be provided at your skydiving school.
Drawings are not to scale
Nastyn8: You are incorrect in your dismissal of using ground speed for horizontal separation. In your example of an 80 mph airspeed into an 80mph headwind, a second group exiting the plane would indeed drift the same path as the previous group (assuming constant winds and skydivers). The only separation between groups would then be based on the path the first group took after deploying their parachutes.
For your second example using the boat and dropping sticks into the water, X seconds after dropping the second stick into the water it will be in the same place that the first stick was X seconds after it was dropped into the water. The only reason the sticks do not hit each other is because the first stick continues to move.
A different example in the boat with no "ground" speed would be dropping a metal sphere into the water such that it lands on the bottom of the river and then dropping a second similar object into the water. Assuming constant movement of the water, the second object will hit the first. If the boat had some forward upstream speed relative to the "ground", then the second object would land upstream of the first with the separation distance as a product of the forward speed and the time between dropping the two objects.
In reply to: "If you are in the boat, and drop one stick into the stream a few seconds after another, would the second stick land on top of the first?"
If the 1st stick suddenly "stopped", and began "holding" (relatively) in place (think of the canopy opening) - - - Then YES! ...Yes, it would!!
This is great, except for the reference to the widely accepted and yet completely false theory that exit separation should be a function of ground speed. An example to illustrate why it's false: imagine an 80 mile per hour headwind on the aircraft (uniform to the ground - keep it simple for the sake of illustration), if the aircraft has an AIRSPEED of 80 miles per hour, this would mean the aircraft is not moving relative to the ground. According to the ground speed theory, the second group could never go as they would supposedly end up right on top of the other group. That is a false conclusion, as exit speed relative to the air mass is what determines horizontal separation, and has nothing to do with ground speed (wind shear could be a factor, but that is not typically factored into the fallacious ground speed theory). You can also think of the boat with a motor pushing against the stream, with no speed relative to the ground. If you are in the boat, and drop one stick into the stream a few seconds after another, would the second stick land on top of the first?
clsalo: In my example, a second group leaving a few seconds later (assuming same body positions etc...) would follow a similar looking path, but WOULD NOT end up opening in the same place - their path in the sky is translated over by aircraft speed*exit separation. I probably should have used a leaf instead of a stick to illustrate what I was talking about with the stream analogy. I'm attempting to show the folly of ignoring motion relative to the air mass. The sphere or massive object sticking to bottom of the stream is not representative of a skydiver in an air mass, as we can all agree that the skydiver will "drift" with the air mass (eventually reaching essentially zero speed relative to the air mass in a neutral body position, regardless of whether the wind is 1mph or 1000 mph relative to the ground.
Scrumpot: Yes, of course. And what does that have to do with ground speed? It's all a function of speed relative to the AIRMASS, not the ground. The reference frame for this discussion on exit separation is the air that we are playing in - the ground does not influence where we end up relative to each other (other than our landing pattern obviously). Think of it this way: during your skydive, does the ground exert any force on you other than one acting straight down? How then can it physically influence the horizontal separation between you and another group?
NWPoul: 1) As you mentioned, the airmass movement at different altitudes is what actually determines what is going on - but it still doesn't matter what the ground is doing relative to the aircraft in determining minimum exit separation. 2) There would of course be nothing wrong with giving yourself more time if you can afford it - my problem is with people that think the MINIMUM safe exit separation changes based solely on ground speed, which is an incorrect and dangerous assumption (the MAXIMUM separation should logically change with ground speed as you are implying)
The ground speed should be taken for exit delay coz:
1 If you have strong upper wind than you probably have significant wind shift at some altitude
2 If you have strong upper wind and therefore low ground speed you can afford more time for more separation (and safety) and still be in a good spot
Interesting theory about the stick / leaf floating down the stream although I would have thought that if there was "GRAVITY" pulling from the river bank it would pull the item along a similar course ....
jamesfreefall: Yes, the leaves will follow a similar path along the bank, but they will maintain the separation they had due to the time between when they were released, thus they don't ever occupy the same "piece" of water (the "piece" of water that the other leaf is in is already downstream). Think of watching all of this happen while you are floating down the river in a life jacket with your favorite beverage. If you passed the boat from which the leaves are dropped at the same time that the first leaf was dropped, the first leaf would remain beside you, and the next leaf dropped would remain behind you as it floated down the river, never ending up in the same "piece" of water that you and the first leaf are in (unless you started swimming up-river, analogous to tracking directly up jump run or flying your canopy directly up jump run = bad idea as Melissa points out in the article).
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