I have read many times that straight down thru a deers back is not acceptable with a bow, why not?
If you miss the spine why wouldn't the broadhead find vitals?The heart and lungs are still in there, why wouldn't an arrow passing thru a lung from top to bottom be as effective as an aroow passing thru the sides?

IMO, aiming for the spine is not an ethical shot. If the shot is straight down, you will have to go through an awful lot of bone to hit the heart. Too many variables there for my taste (deflection off bone for one). As far as lungs, you will only take one out. Finding a single lunged deer is one of the most difficult tracking tasks that there is. A lot has been posted on that one here on the forum.

Again, this is a low percentage shot IMO.

Where as a double lunged deer should be a 100% recovery.

Not that it wouldn't kill the deer but it just leaves more room for problems. Deer have been known to live with one lung and even if they don't, they can go a long way on one lung and the greater distancea deer goes from the point of impact the lesser the chances of recovery. Anyhow if you are facing a straight down shot, then it's hard for me to believe that the deer didn't offer a broadside shot before it got under the stand and even harder for me to believe that it won't offer a broadside leaving from beneath the stand.

BobCo mentioned the crux of the matter : Low percentage.

Shooting straight down a difficult shot to make. That lowers your percentage off of the bat. Then add the fact that your 9 or 10 inch kill zone shrinks to about 4 or 5 inches, that lowers your percentage by another 50%.

IF you hit the center of the spine, yes you will probably drop the deer right there, but the odds are against you. Miss the spine either side and you've probably got a single lung and no heart hit at best . You may also catch a rib at the terminus where it's strongest rather than in it's middle where it's weakest.

In the spirit of full disclosure, I myself would be tempted to take such a shot, but then I'd lean out and look down and probably get a bit of vertigo and have to forget it

also, a complete pass through is not likely, therefor there is no bloodtrail whatsoever with the entrance wound on top of the deer

It may be a smaller kill zone but chances are if it is staight down then the deer is only 5-7 yards away which if you have been practicing like most ethical hunters do,you should not have a problem putting a lethal shot from that distance. Just my opinion.

In my earlier years, I tookthe straight down shot (e.g. <10 ft from the tree) several times and (fortunately) had never lost a deer doing it. (I'd rig my waist belt so I could lean out over the stand, and had well practiced the shot). Toughest(and last) time occured~1988 when I missed the spine and heart on a forkie, and tallow plugged the exit hole. I found (stumbled upon) the buck~200yds away some two hours later. I am a believer that straight downis not a good percentage shot, and now wait for the broadside or quarteting away. Ditto the above posts on single lung hits. If the good angle doesn'thappen, there is always another day...
-fsh

Several years ago I took this shot on a deer I rattled in. He came in fast and stopped under my tree. He was headed in a direction that would not allow for a shot after passing. The arrow entered beside the spine and did not exit. As the deer ran off I was kicking myself because of all the arrow sticking out. I assumed it had not penetrated far enough. After an hour I tried to trail but there was no blood. Apparently the arrow did its job and cut up the lungs as he ran off and eventually worked its way down thru the brisket. The last 10yds was a stream of blood. Fortunately he didn't go far and I was able to recover him by simply following the general direction he went. If I had it to do over I would not take the shot as at the time I remember how sick I felt thinking I had wounded the animal and would not be able to recover it.

I have taken two deer in this situation, one asrecent as 2 weeks ago. Both resulted in very quick kills. The doe I took 2 weeks ago only made it about 70 yards before piling up. She came on a string straight to my stand, i was drawn on her at 25 yards out but never offered a shot. She passed directly underneath me and was about to enter a large pine thicket and i knew i wouldn't have a shot then. I bent at the waist and placed my pin just to the side of the center of the back right behind the shoulder. The arrow entered exactly where i was aiming (shot straight through the backstrap) and exited the deer's brisket, taking out the heart, and stuck in the ground. She ran off about 70 yds and stopped and i could see her tail wagging rapidly and i knew i had her and seconds later she crashed and never moved again. The arrow broke off, most of the arrow was left in the deer, except the broadhead. Came back to where the shot happened and found the broadhead and about 6" of arrow still stuck in the ground.

The other deer was a small buck several years ago with virtually identical results.

I agree that it isn't the highest percentage shot available, but under the right situation i'd take it again.

There are groups of nerves under the spine as well as a major arteries. One way to look at it is if you get a good hit then there will be no tracking, it will drop the deer off it's feet,you may have to put another arrow in it's vitals, but more than likely it will bleed out rather quick with the correct shot to the spine.