During the en route descent phase of flight, an additional benefit of flight management systems is that the FMS provides fuel saving idle thrust descent to your destination airport. This allows an uninterrupted profile descent from level cruising altitude to an appropriate minimum IFR altitude (MIA), except where level flight is required for speed adjustment. Controllers anticipate and plan that you may level off at 10,000 feet MSL on descent to comply with the Part 91 indicated airspeed limit of 250 knots. Leveling off at any other time on descent may seriously affect air traffic handling by ATC. It is imperative that you make every effort to fulfill ATC expected actions on descent to aid in safely handling and expediting air traffic.

ATC issues speed adjustments if you are being radar controlled to achieve or maintain required or desired spacing. They express speed adjustments in terms of knots based on indicated airspeed in 10 knot increments except that at or above FL 240 speeds may be expressed in terms of Mach numbers in 0.01 increments. The use of Mach numbers by ATC is restricted to turbojets. If complying with speed adjustments, pilots are expected to maintain that speed within plus or minus 10 knots or 0.02 Mach.

Speed and altitude restrictions in clearances are subject to misinterpretation, as evidenced in this case where a corporate flight crew treated instructions in a published procedure as a clearance. “…We were at FL 310 and had already programmed the ‘expect-crossing altitude’ 3-23 of 17,000 feet at the VOR. When the altitude alerter sounded, I advised Center that we were leaving FL 310. ATC acknowledged with a ‘Roger.’ At FL 270, Center quizzed us about our descent. I told the controller we were descending so as to cross the VOR at 17,000 feet. ATC advised us that we did not have clearance to descend. What we thought was a clearance was in fact an ‘expect’ clearance. We are both experienced pilots…which just means that experience is no substitute for a direct question to Center when you are in doubt about a clearance. Also, the term ‘Roger’ only means that ATC received the transmission, not that they understood the transmission. The AIM indicates that ‘expect’ altitudes are published for planning purposes. ‘Expect’ altitudes are not considered crossing restrictions until verbally issued by ATC.”