The only official rulebook I found is from 2006. (I assume the rule hasn't been changed.) But, an illegal forward pass is dead ONLY if it hits the ground:
Rule 8 Forward Pass, Backward Pass, Fumble
Section 1 Forward Pass
Article 1
The offensive team may make one forward pass from behind the line during
each play from scrimmage provided the ball does not cross the line and return behind
line prior to the pass.
(a) Any other forward pass by either team is illegal and is a foul by the passing team.
(b) When any illegal pass is caught or intercepted, the ball may be advanced and the
penalty declined.
Penalties:
a) For a forward pass not from scrimmage: Loss of five yards from the spot of
the pass. It is a safety when the spot of the pass is behind the passer’s goal
line.
b) For a second forward pass from behind line, or for a pass that was thrown
after
the ball returned behind the line: Loss of five yards from the previous
spot. Note: See 8-3-1 for intentional grounding.
c) For a forward pass from beyond the line: Loss of down and five yards from
the spot of the pass (combination penalty). See 14-8-2. See S.N. 3 below.
SUPPLEMENTAL NOTES
(1) Eligibility, pass interference, and intentional grounding rules apply to a second pass
from behind the line or a forward pass that was thrown from behind the line after the
ball returned behind the line. On all other illegal passes, eligibility rules do not apply.
(2) Roughing the passer rules apply on all passes (legal or illegal) from behind the line
of scrimmage (12-2-12).
(3) The penalty for a forward pass beyond the line is to be enforced from the spot where
the ball is released when the passer’s entire body and the ball are beyond the line
of scrimmage. This includes either when the passer is airborne or touching the
ground.
(4) When a distance penalty in Penalty c) leaves the ball in advance of the necessary
line, it is first-and-10 for the offensive team.
(5) An intentional fumble forward is a forward pass. See 8-4-2-Exc. 1.
(6) For when any legal or illegal pass becomes incomplete, see 8-1-5.
(7) For team possession during a forward pass (loose ball) or when it ends, see 3-2-3.
A.R. 8.1
Second-and-10 on A40. A forward pass is batted back by a defensive player. The ball
goes back in the air to the quarterback behind his line. He throws it again to his end who
catches it on the B40 and goes for a score.
Ruling:
No score. Second-and-15 on A35.
A.R. 8.2
Second-and-18 on A4. A second forward pass from behind the line is caught by offensive
end A1 in his end zone. He is downed in his end zone.
Ruling:
Safety. Safety kick A20 or second-and-20 on A2.
A.R. 8.3
Second-and-10 on A40. A second forward pass from behind the line is intercepted by
the defensive team at midfield. A defensive player returns it for a touchdown.
Ruling:
Touchdown. Illegal passes may be caught or intercepted.
A.R. 8.4
A punt is caught on the receiving team’s 20-yard line. The player who caught the ball
attempts to throw a backward pass, but the ball goes forward and hits the ground. The
kicking team falls on it.
Ruling:
Illegal forward pass. The ball is dead when it hits the ground. Penalize from the
spot of the pass. B’s ball first-and-10 on B15 (8-1-1-Pen. a).
A.R. 8.5
A forward pass is intercepted by a defensive player in his end zone. While in the end
zone, he attempts to pass backward. The pass goes forward, hits the ground on the
1-yard line and is recovered by the first passing team.
Ruling:
Safety. Forward pass not from scrimmage in the end zone.
Incomplete
Illegal Passes
Intentional
Fumble
Forward
Necessary
Yardage
One Forward
Pass Legal
Illegal Pass
Caught or
Intercepted
OFFICIAL NFL PLAYING RULES 49
B1-Rule Book RULES p1-76.qxd 6/7/2006 10:02 PM Page 49
A.R. 8.6
Third-and-10 on B35. A second forward pass is thrown from behind the line to flanker
A1. Defensive player B1 interferes with A1 on the B20, but A1 catches it anyway and is
downed on the B20.
Ruling:
Double foul. Illegal pass by the offensive team and interference by the defensive
team. Interference rules apply on the second forward pass from behind the line
(14-3-1). A’s ball third-and-10 on B35 (replay).
A.R. 8.7
Third-and-15 on A30. During a forward pass from beyond the line on the A40, offensive
player A1 clips on the A40. The pass is incomplete.
Ruling:
Choice for defensive team. Loss of down and five from the spot of the pass or
loss of 15 from the spot of the pass (unless offensive player fouls behind that spot—
spot of foul). A’s ball fourth-and-10 on A35 or third-and-20 on A25.
A.R. 8.8
Third-and-15 on A30. During a forward pass from beyond the line on the A40, defensive
player B1 clips on the A40. The ball falls incomplete.
Ruling:
Double foul (14-3-1). Replay at the previous spot. A’s ball third-and-15 on A30.
