How Much Do You Actually Know About Landing An F-18 On An Aircraft Carrier?

You will see the Velocity Vector at -2.7° on the pitch ladder because as the ship moves away from you, you will have to travel a greater horizontal distance over the earth to reach the touchdown point on the flight deck, decreasing your flight path angle relative to the earth.  Since the Velocity Vector shows you your flight path angle relative to the earth, you will see the Velocity Vector at -2.7° on the pitch ladder.The video below shows the same concept calculated for 25 knots of ship's speed.  

There are 12 cells on the IFLOLS, 10 yellow and 2 red. Each yellow cell has an angular coverage of .13°, and each red cell covers approximately .2°. For a 3.5° Basic Angle, the top of the top cell is angled at 4.28° and the bottom of the bottom red cell is angled at 2.58°. 

Because the Velocity Vector shows you your flight path angle relative to the earth, the only factor that affects where you will see the Velocity Vector on the pitch ladder is ship's speed. In this case, ship's speed is only 15 knots, which corresponds to a -3.1° glide path. Even though the 10 knots of natural wind contributes to 25 knots of Wind Over the Deck, it will have no affect on where you will see the Velocity Vector on the pitch ladder.

Your VSI in both cases will be exactly the same because your closure to the ship is the same. In these examples the closure is entirely based on ship's speed OR wind speed. As you combine the two, things start to get more complicated

Your velocity vector shows you your flight path angle relative to the earth, and the aircraft carrier is on the earth.  This video shows you your flight path angle through an air mass (represented by the hot air balloon), caused by wind speed.  The only indication of this shallower angle will be from a decrease in VSI.  You will not see any affect on your Velocity Vector because the wind is not changing your flight path angle relative to the earth. This video shows you your flight path angle relative to the earth caused by ship's movement.  In this case, your VSI will match that in the first video, but because you are also flying a shallower flight path angle relative to the earth, you will also see your Velocity Vector at -2.8° on the pitch ladder.   The fact that the Velocity Vector's placement on the pitch ladder is only affected by ship's movement makes it especially useful for pilots, because the ever changing winds have no affect on what you will see in the HUD.  Even a basic understanding of what angles you will see can significantly increase the safety and consistency of your landings.

Groove distance is based on closure between the aircraft and the ship. The greatest distance occurs with the greatest closure, which happens when aircraft is moving the fastest and the Recovery Headwind is the weakest. Minimum Recovery Headwind for Hornets is 15 knots, and the maximum approach speed in the normal full flaps landing configuration is 150 knots. With 135 knots of closure, an aircraft will cover .7 NM in 18".The table below show all the possible groove distances for the different allowable approach speed and recovery head wind combinations. As you can see the most common groove distance is .4 NM to .6 NM.These distances are important to know when flying a CV-1 approach because if you start flying the ball further away than you are used to from the day pattern, you are more likely to make unnecessarily large glidepath corrections, increasing the probability of an unsafe pass.  Therefore, you should not transition to the ball at night until inside .5 NM.

The IFLOLS is designed so the prescribed Basic Angle emanates from between cells 6 and 7. The datums are also aligned between cells 6 and 7. Like Paddles says, there's no centered ball.

You will see the Velocity Vector at -3.5° because natural winds have no affect on where you will see the Velocity Vector on the pitch ladder. The Velocity Vector shows you only your flight path angle relative to the earth, and in this case, the ship is not moving. Like any normal field landing, if you are able to maintain a centered ball with a 3.5° Basic Angle the Velocity Vector angle will equal the Basic Angle.This makes the Velocity Vector especially useful for pilots because the only variable that will affect where you see the Velocity Vector on the pitch ladder is ship's speed. Notice in this video that even though the flight path angle through the air mass (represented by the hot air balloon) is shallowed out by windspeed, if the aircraft's flight path angle relative to the earth did not equal the Basic Angle (represented by the yellow line emanating from the carrier), the aircraft would not land in the correct spot on the flight deck. 

A -4.64° flight path angle from a Full High Start behind a non-moving ship with a 3.5° Basic Angle will fly you straight into the Round Down.  This combination of conditions gives you the steepest angle you would ever need if you wanted to hit the back of the ship (which you should not want).  Any increase in ship's speed or decrease in Start altitude will only shallow the angle that will put you into the Round Down.The table below shows you the required flight path angles relative to the earth (where you will see the Velocity Vector on the pitch ladder) to maintain any cell at any ship's speed.  As you can see, with a 3.5° Basic Angle, there is almost no circumstance in which you would need to set the Velocity Vector to steeper than -4° or shallower than -2° on the pitch ladder.  Except in the most extreme circumstances, pilots should never allow the Velocity Vector below -4° or above -2° if they see a ball on the lens.