I have heard that pilots learn in simulators to fly also commercial planes without working engines, so that they glide. And still I have the impression, the chances that a plane could float from 11000 meters height are nearly 0. Could someone explain, if a commercial plain CAN BE THEORETICALLY (Physics) and PRACTICALLY landed from 11km with all failed engines? How important is the altitude from which all engines fail?
All standard fixed wing aircraft are able to glide. The key measure of performance in gliding is the glide ratio which is related to the lift-to-drag ratio of the aircraft. The glide ratio of a clean A320 is 17:1 which means it can travel 17 units of distance forwards for every 1 unit of distance downward at best glide speed. Using your figure of 11,000 meters away and multiplying by 17 results in a maximum range of 187km . In practice, the aircraft would need to be higher than this (or land closer) to accomodate turns and other factors, but if the A320 was at 1000 meters AGL or higher I would expect it to have no problem landing at an airfield or airport 11km away. Note that aircraft routinely glide every time you fly even though you may be unaware of it. When a commercial flight begins its descent often power will be brought back to idle, so the aircraft is gliding at that point.
There have been several instances of large commercial aircraft landing fully unpowered. One well known example is the case of the "Gimli" glider in which a Boeing 767 ran out of fuel and glided to landing at a nearby airbase.
EDIT: It has been brought to my attention that the OP is asking about a starting altitude, not how far away from the airfield. As I imply in my answer what matters is the ratio of altitude and distance, not altitude alone.
On Airliners.net a participant named Longhauler posted the following in 2005
If you wish to maintain the highest altitude, you maintain your best lift drag ratio. For the A320 that's about 220 knots. You will descend about 1500 ft/minute. So from 35000 ft, it will take you roughly 25 minutes to reach the ground. You will travel roughly 90 nm.
To travel the furthest distance, you descend at M0.74/270 knots. Again, from 35000 ft, you will travel roughly 95-100 nm.
To cover a distance the quickest you descend at M0.80/330, but you be gliding like a simonized crowbar. (namely dropping like a stone). You will travel only about 65 miles, but you'll get there quicker!
As someone said above, you have to remember, pretty well all descents are done with the thrust at idle.
Another consideration, is that without engines, your cabin will depressurize. Slowly on newer aircraft, increasing with the age of the aircraft. It is a balance with finding a place to land within your gliding range, but also looking at your cabin altitude rising.
11000 meters is 5.94 nautical miles - so that's easy from cruising altitude.