433,
I come across these clearances alot when approaching SPL from the south. We never have doubt about the meaning of the clearance. Start decent to FL.... with the given rate of descent.
Approaching SPL form the south it seems to us most of the times it is issued to meet some restiction at brussels. As it is always 2000 ft/ min to FL 250 Then switch to Brussels. There runs a tape (it seems) that says descent 220 switch Adam. If 06 or 01 in use your about 2000-3000 ft below profile. For other runways it has you easily about 6000-8000 below profile.
A lot of pilots in my company usually let the airspeed build up with 2000 ft/min to intercept profile sooner (ever noticed?). Say from normal M.79-300kts to .82-330 kts which gives you a less steep descent path (with given rate) and you intercept the descent profile sooner.
The instructions which say reach FL ... in ... minutes are much more confusing. Do I need to start the descent now? Some people start a shallow descent till a point where they can do a idle descent to comply with the clearance etc....
Depending on aircraft type most aircraft fly Mach numbers till about FL 260. Above that you easily reach 4000 ft/min descent rate as airspeed increases (speed of sound increases whilst descending). The aircraft is in effect continiously pitching over the keep the same mach number (idle descent). Below the switch it keeps the same indicated airspeed and rate of descent falls back to 2500 ft/min (depending aircraft type) (and gradually reduces as idle power from engines is increasing with lower altitudes). When reducing speed to say 250 kts it will not be much more than 1300 ft/min.
The controllers are not always aware of the above effects.
An exceptions seems to be the london controllers which are extremely good in planning the descent for you as they are constant aware of the distance to run versus altitude and speed you fly.
Care to comment on the restrictions mentioned above with Brussels?