AIRCRAFT SPEED AND ALTITUDE
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’
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.”