EXIT SAFETY

Spotting, Winds, and Separation

John KallendDepartment of Mechanical, Materials and Aerospace

Engineering

Illinois Institute of Technology

2004 Spring Skydiving Expo

ISSUES

Large jump planes with many groups

Multiple planes on jump run

Mixed disciplines with different fall rates

High exit altitudes (and higher winds, windshears)

GPS spotting

High performance canopies

RESOURCES

http://www.iit.edu/~kallend for download of the 2-d Freefall Separation program, relevant links and a copy of these slides.

AVOIDING THE BAD STUFF

Out-landings in snake filled swamps

Freefall collisions

Canopy collisions

Each DZ has unique circumstances.

There is no single solution that fits all DZs on all occasions.

THE MESSAGE WILL BE:

Check the spot and look for traffic below.

The “45 degree method” doesn’t work.

Belly fliers should go first.

Groundspeed methods are OK in most circumstances; learn the exception.

There’s no hurry

SPOTTING

Most DZs now use GPS for spotting.

It works very well, BUT

EACH SKYDIVER IS RESPONSIBLE

FOR HIS/HER OWN SAFETY.

Don’t be a “wind dummy”.

Look out the door!

Check that no planes etc. are below (GPS doesn’t see these). Other pilots are under no obligation to avoid DZs.

Do a visual check on groundspeed.

Be sure you can get back.

LZ

SPOTTING: NO WIND

On a no-wind day, canopies opening within the shaded area make it back to the LZ. Outside this area will land off.

The boundaries of the shaded area extend about a mile beyond the LZ (depends on opening altitude). OK

OK

OffOff

Off

OK

OK

OK

LZ

SPOTTING: STEADY WIND

On a windy day, the area from which a canopy can reach the LZ is shifted upwind, but its size stays the about the same.

If making low hop’n’pop from a helicopter or balloon, you want to exit over this area.

OK

OK

OffOff

Off

Wind

OK

OK

LZ

SPOTTING: STEADY WINDFrom altitude, freefall drift will displace the exit area from which you can make it back to the LZ. You will need to exit inside the red line to make it back. The exact displacement depends on jumprun airspeed, fall rate, upper winds, and direction of jump run. The SIZE of the safe area to exit is unchanged

Wind

Drift

SPOTTING: GREEN LIGHT

Consider an area for a safe exit and a satisfactory spot that is 2.5 miles across.

With 100kt true airspeed and 40kt uppers, jumprun will take:

Downwind, 56 seconds

Upwind, 2 mins 10 sec.

(no need to rush). That’s 11 2-ways with 13 second separation)

Crosswind, 1 min 22 sec.

wind

SPOTTING

LZ

X marks the spot. First group out cannot wait for the perfect spot, or the last out will be

HOSED.WIND

GETTING BACK FROM A BAD SPOT

Learn the “accuracy trick”

Get “small” to reduce drag.

If upwind of the DZ, use rear risers or brakes to flatten the glide.

If downwind of the DZ, rear risers may still help in light winds. Otherwise use full flight. Front risers only help in very strong headwinds.

GETTING BACK FROM A BAD SPOT

Forward speed depends mostly on wing loading. Risers and brakes mostly affect descent rate.

Have a safe landing spot selected while still above 1000’agl

It is better to walk a mile than to be carried a few yards.

Separation(Collisions are No Fun)

“Now lets talk about separation from other jumpers. First of all, anyone who counts on vertical separation for safety is out of touch with reality. I see people in freefall at 1,500 feet and lower routinely, so just because someone plans to open at 2,500 doesn't mean you should bet your life on it. Everyone needs to open in their own column of air. Horizontal separation is the only guarantee of security.”

Bryan Burke

Skydive Arizona

SEPARATION

HOW MUCH SEPARATION IS NEEDED?

A modern canopy flies at around 30mph = 44ft/sec

(some are notorious for off-heading openings)

It takes about 3 seconds to recognize a

collision hazard and take action

Two canopies on a head on course cover

around 300 feet in this time

Let’s take 300 feet as the absolute minimum acceptable separation.

300 150

300 ft separation means each jumper has a “column of air” 150ft in radius, with himself or herself at the center. These columns of air should not overlap.

VIEW FROM ABOVE

SEPARATION WITHIN THE GROUP

TRACKINGA survival skill that is fun.

A breakoff track should be flat and fast.

Tony Hathaway

To obtain 300’ separation between jumpers in a 4-way, need to track 212 ft from the center so the individuals’ columns of air don’t overlap.

3 seconds after opening there could be canopies anywhere in an area 724 ft in diameter. Each group needs its own column of air.

The corresponding area for an 8-way is more than 1,000 ft. across. These numbers are minima.

724ft

How Far to Track, and What Size is the Group’s Air Column?Group Size Tracking Distance (ft) Radius of Group’s Space (ft)

1 0 150

2 150 300

3 173 323

4 212 362

6 300 450

*7 345 (300) 495 (450)

*8 391 (345) 541 (495)

*10 485 (438) 635 (588)

Note: this table gives the absolute minimum distance to track and the absolute minimum radius of the group’s space for 300’ separation, assuming efficient tracking and equally spaced tracks. In reality, more space will be needed.

* For groups larger than a 6-way, the smaller values in parentheses are for when one jumper deploys in place and the others track.

AND

Make sure the first wave doesn’t stop too soon.

A staged breakoff gives better separation for any group of 12 or more skydivers.

BUT

The “first wave” needs to go 300’ (or more) farther than the next wave.

SO

Make sure the second wave waits long enough before turning and tracking.

SEPARATION BETWEEN GROUPS

Centers of groups should be separated by > sum of their radii to minimize chance of collisions between groups.

Group 1 Group 2

R1 + R2

R1 R2

Example: Group 1 is 8-way, radius 541; group 2 is 4-way, radius 362. Separation should be > 541 + 362 (= 903ft).

Since these are absolute minima, you should space more conservatively. Plan on, say, 1,200ft to allow for long tracks, sliding, etc.

How much spacing is needed?Group Sizes

1 2 4 6 8 10

1 6-800 800 1000 1000 1000 1000

2 1000 1000 1000 1100 1100

4 1000 1200 1200 1300

6 1200 1350 1500

8 1450 1500

10 1600

HOW DO WE ACHIEVE THIS?

People do the strangest things

(like tracking up the line of flight)

SO

We won’t try to predict what they will do

Computer model will use virtual “Spaceballs” with fall rate adjusted for the discipline, to remove human factor. Freefly spaceball falls 11,000ft in 50 seconds.

RW spaceball takes 65 seconds to fall the same vertical distance. Spaceballs define the “perfect”

trajectory, no backsliding or tracking.

ACHIEVING SEPARATION

1. Watch angle from vertical to previous group.

2. Look down and watch until 1,200 ft have passed. (Skratch’s method).

3. Count to 5, then jump (Otter covers about 750 ft in this time on jump run at 13,000ft, no wind)

4. Wait for time taken to cover 1,200ft across ground (groundspeed method)

Question: Do these methods work?

First we’ll take a look at the “angle” method.

“Wait until the group in front of you makes an angle of 45 degrees behind

the plane, then exit.”

45o ?

1. Can you judge 45 degrees ?

2. If you can, does the method work anyway ?

Computer model Freefall does the math.

Uh-Oh

Conclusion 1

The angle made between the vertical and the previous group varies very little after the first two or three seconds. It does not depend on wind speed. It rarely reaches 45 degrees.

There is no physical or mathematical basis for this method. DON’T USE IT.

Video by Bill von Novak

From 13,000 feet, a 2.75 degree error in judging the vertical leads to a 1,200 foot

error on the ground!

How well can you judge the vertical from a moving

aircraft?

2. Look at the ground.

Ask the pilot the groundspeed and you do the math. 1 knot is roughly 1.5 ft/second, or 100kt = 150ft/sec

Example: groundspeed = 100ft/sec and you want 800ft separation. Then you wait 800/100= 8 seconds between exits. Does this work?

3. GROUNDSPEED METHOD

Case 1. Groundspeed = 40ktWind at 3000ft = 30ktExit delay = 8 seconds

How Groundspeed Can Mislead

(Unless you deploy on the ground)

Click on the link Freefall Simulation

Case 2: Same groundspeed = 40kt

Wind at 3000ft = 30 kt tailwind

Exit delay same (8 seconds)

Use Freefall Simulation again

Observations:

1. Groundspeed calculation works if winds are in same direction at all altitudes

2. Separation also depends on wind at opening altitude, if in opposite direction to uppers, BEWARE.

CONCLUSION 2It’s not the speed over the ground

that counts.

It’s the speed relative to the air at opening altitude.

Usually groundspeed methods work because the winds at 2,500’ are light

and/or in the same direction as the upper winds.

CONCLUSION 2 (cont.)

To achieve separation between groups that have the same fall rate, methods based on groundspeed work and have margin for error

UNLESS

the lower winds are opposite the uppers,

IF THAT IS THE CASE THEN

extra spacing is needed. ADD the lower wind speed to the jumprun headwind, or SUBTRACT the lower wind speed from the groundspeed.

HOW TO GET WINDS ALOFT

Ask the pilot, or

Call 1-800-WX BRIEF, or

Point your browser at:

aviationweather.gov/products/nws/fdwinds/

TO GET 1000FT OF SEPARATIONJumprun indicated airspeed= 80kt

Headwind(kt)

Delay between exits (sec).

0 6

10 6.5

20 7.5

30 8.5

40 10

50 12

This assumes lower winds are light and/or in same direction as uppers.

Next we’ll look at:

Fall Rate Differences

Forward throw

Freefall drift due to upper winds

For Forward Throw

Click on Freefall.exe

CONCLUSION 3

In no-wind conditions, a freeflier will have a forward throw down the line of flight of about 1,800 ft.

A belly flier will have a forward throw of around 1,200 ft.

CONCLUSION 3 (cont) For any jumprun airspeed, Freefliers will have a forward throw down the line of flight that is greater than that of belly fliers by a distance approximately equal to the distance covered by the jump plane in 4 seconds. For 80kts IAS this is around 600 feet.

What About Winds?

WIND DRIFT

Even in freefall you “blow along” with the winds, which may be quite strong at altitude. Freefliers spend less time in the upper winds.

Compare wind drift for freefliers and bellyfliers: Freefall.exe for winds.

CONCLUSION 4In headwind, freefliers have a steeper trajectory than belly fliers.

For each knot of average upper wind, a belly flier will drift 20 feet farther than a freeflier (so a 40kt average wind will result in 800 feet more drift for the slow faller).

This adds to the forward throw difference if the jumprun is into the wind

CROSSWIND AND DOWNWIND JUMPRUNS

Forward throw is unaffected.

Wind drift effects on separation go away on crosswind jumpruns

Wind drift effects are reversed on downwind jump runs.

CONCLUSION 5

Separation can be achieved with any exit order.

BUT

If the freefliers go first, adjacent groups CONVERGE. This is not fail-safe!

If RW groups go first, groups DIVERGE, a fail-safe situation. It is easier and takes less thought to achieve horizontal separation if freefliers exit after RW groups.

IF FREEFLIERS EXIT BEFORE RW GROUPS

extra spacing is needed to achieve adequate horizontal separation, maybe tens of seconds.

For example, if a freefly 2-way is followed by a large RW group (like a 16-way) that will take a lot of time to climb out, it may

be easy to get the required separation.

But it is not always a bad idea…

IF FREEFLIERS EXIT FIRSTUse your usual method to calculate spacing.

Add 4 seconds to account for extra forward throw.

Add 2 seconds for every 10kt of upper winds

Example: Want 1000ft spacing, no anomalous winds, groundspeed =70kts, uppers=30kts. Normal spacing = 9 seconds.

Now require 9 + 4 + (3 x 2) = 19 seconds between exits.

THE MESSAGE WAS:

Check the spot and look for traffic below.

The “45 degree method” doesn’t work.

Belly fliers should go first.

Groundspeed based methods are OK in most circumstances; learn the exception.

There’s no hurry

Discussions with:

Winsor Naugler III

Skratch Garrison

Tamara Koyn

Tim Wagner

Articles:

Bryan Burke

Bill von Novak

Video: Bill von Novak

ACKNOWLEDGEMENTS