Damage to automatic transmission when parked on steep hill

So I read that parking on a steep hill (without use of parking brake) can damage automatic transmissions, but what kind of damage exactly?

In theory, you could damage the parking pawl. To prevent this, put the parking brake on before putting the car in park.

In reality, I suspect that you would be very hard-pressed to find anyone who had this happen to them.

As beowulff says, parking on a steep hill with an automatic puts a lot of weight on the parking pawl, which is designed to prevent the driveshaft from rolling when parked but is not really meant to be used as a parking brake. The pawl is just a small pin that sticks into the output shaft of the transmission to immobilize it but is not designed to take a lot of stress. The parking brake OTOH actually locks up the wheels so they can’t turn. The recommended procedure is to put the parking brake on before shifting into park, so the vehicle’s weight is born by the parking brake and not the pawl.

As a matter of habit, I always use the parking brake whether on a hill or not. I always put the parking brake on before shifting to P, and take it off after shifting out of P.

^^that’s what I do, as well. And always turn my wheels toward the curb.

What might cause real damage is on some automatic CVT transmissions parked on hills is when you start again, trying to go uphill, the CVT drive ‘belt’ may slip trying to deliver the torque needed to move the car from a stop and also uphill. That would cause a grove in the pulley which would make it far easier to slip in the future upon starting from a stop under loads. Once in a while this start position is OK, but all it takes is once to cause that to happen. As the CVT fluid ages the belt is more likely to slip as friction modifiers lose their ‘stickyness’.

Isn’t it enough to put the parking brake on before putting your foot off the brake pedal?

Why would it slip under this circumstance, as opposed to pushing the accelerator just as hard when departing from a dead stop on level ground?

Yes, and that’s how I do it:

  1. brake to a stop.
  2. shift to park, keeping foot on main brake.
  3. apply parking brake.
  4. release main brake.

The order of steps 2 and 3 doesn’t matter.

I had a Pontiac Catalina years ago with a broken pawl in the transmission. A couple times when I forgot the set the parking brake I found the car had moved on it’s own. Worst one was the time the car rolled backwards into a sign pole. The rear bumper got caught on a very large bolt holding the pole to a cement riser. Bent the crap out of the rear bumper getting it off.

Does the broken pawl have any effect on the car’s driving on a flat surface, or is it only something that comes into play when the car is on a slope angle?

Depending on where it ends up, it might grind up something in the transmission (unlikely).
Other than that, no, it’s only used to lock the driveshaft in Park.