On the aircraft I fly, an idle descent in a clean configuration will result in a 3500-3800ft/min descent at M0.81 which is about a 4.0-4.2 degree descent. As constraints are imposed (like a 250kt speed limit), you'll leave the thrust levers at idle and slow from M.81 to 250kts by reducing vertical speed to like 500-1,000ft/min depending on how quickly you want to decelerate. Then resume the descent once you've decelerated.
One place where it might be appropriate to use a steeper descent than normal is going into Santa Barbara. LA Center typically keeps you high to keep you clear of the LA basin arrivals, crossing FIM at FL240, then after crossing FIM at FL240, they clear you to cross KWANG at 9,000ft and hand you off to SoCal approach. Now you have 30 miles to lose 15,000ft. Normally we plan a descent to lose 10,000ft in 30 miles (normal 3-1 rule). Our technique going into Santa Barbara (without having to get descent vectors) is to cross FIM at 24000 as slow as you can go - 210kts, then upon receiving the descent clearance, idle, speed brakes out, and accelerate from 210 to Vmo of 330kts in the descent. It results in about a 6,000-7,500ft/min descent rate, until it's time to decelerate back down to 250kts to go below 10,000ft.
One place you might use a steeper approach is into Aspen. It's still not steep but it is a steeper approach than normal. You're looking at a 1,200-1,500 ft/min descent from the final approach fix to the point where you intercept the VASI for a visual landing onto 15 where you transition to a normal descent rate. A normal 3 degree glideslope angle will result in a 750-800ft/min descent (Aspen does not have a glideslope). For the Aspen approach, we prebrief that until we get within 500ft of the touchdown zone elevation, we ignore the GPWS "sink rate" alert as that is normal on the approach and that we may exceed the 1000ft/min descent rate inside 1000ft which is normally a mandatory go around as the 1000ft/min descent inside 1000ft AGL is a non-stabilized approach. We don't ignore the sinkrate alert, we just respond with "Noted" instead of "Correcting." The only GPWS alert we respond to is the "terrain, terrain, pull up" one.
Good judgement comes from experience. Experience comes from surviving bad judgement.
There's a net trajectory angle, but that depends on the situation... and the limits are Vmo and Mmo, so it's really a curved trajectory (like normal descent too!
Quoting aerotech777 (Thread starter): d) Is it possible to use steep descent in normal flight (no emergency descent)?
e) Is it possible to use steep approach in normal landing?
How steep is steep?
And which method? Drop like a rock (Vmo/MMo, idle, speedbrakes), or leaf-ing it down (full flaps, gear down, idle, flap limit speed)?
Bear in mind, rock method or leaf method, both are uncomfortable for the pax. Rock method has additional risks of overspeeding to the final. Leaf method has a risk of not adding power early enough (otherwise, U'll fall faster when you pull up!). Both, are just "nice to knows", not for normal ops. Besides, you got additional maintenance costs thanks to those "fun" methods!
When losing situational awareness, pray Cumulus Granitus isn't nearby !