Aeronautical Information Manual (AIM), page 237

Index   236 -- Page 237 -- 238


command must notify ATC as soon as possible and
obtain an amended clearance. In an emergency
situation which does not result in a deviation from the
rules prescribed in 14 CFR Part 91 but which requires
ATC to give priority to an aircraft, the pilot of such
aircraft must, when requested by ATC, make a report
within 48 hours of such emergency situation to the
manager of that ATC facility.
g. The guiding principle is that the last ATC
clearance has precedence over the previous ATC
clearance. When the route or altitude in a previously
issued clearance is amended, the controller will
restate applicable altitude restrictions. If altitude to
maintain is changed or restated, whether prior to
departure or while airborne, and previously issued
altitude restrictions are omitted, those altitude
restrictions are canceled, including departure proce-
dures and STAR altitude restrictions.
1. A departure flight receives a clearance to destination
airport to maintain FL 290. The clearance incorporates a
DP which has certain altitude crossing restrictions. Shortly
after takeoff, the flight receives a new clearance changing
the maintaining FL from 290 to 250. If the altitude
restrictions are still applicable, the controller restates

2. A departing aircraft is cleared to cross Fluky
Intersection at or above 3,000 feet, Gordonville VOR at or
above 12,000 feet, maintain FL 200. Shortly after
departure, the altitude to be maintained is changed to

FL 240. If the altitude restrictions are still applicable, the
controller issues an amended clearance as follows: "cross
Fluky Intersection at or above three thousand, cross
Gordonville V-O-R at or above one two thousand,
maintain Flight Level two four zero."

3. An arriving aircraft is cleared to the destination airport
via V45 Delta VOR direct; the aircraft is cleared to cross
Delta VOR at 10,000 feet, and then to maintain 6,000 feet.
Prior to Delta VOR, the controller issues an amended
clearance as follows: "turn right heading one eight zero
for vector to runway three six I-L-S approach, maintain
six thousand."
Because the altitude restriction "cross Delta V-O-R at
10,000 feet" was omitted from the amended clearance, it is
no longer in effect.

h. Pilots of turbojet aircraft equipped with
afterburner engines should advise ATC prior to
takeoff if they intend to use afterburning during their
climb to the en route altitude. Often, the controller

may be able to plan traffic to accommodate a high
performance climb and allow the aircraft to climb to
the planned altitude without restriction.
i. If an "expedite" climb or descent clearance is
issued by ATC, and the altitude to maintain is
subsequently changed or restated without an expedite
instruction, the expedite instruction is canceled.
Expedite climb/descent normally indicates to the
pilot that the approximate best rate of climb/descent
should be used without requiring an exceptional
change in aircraft handling characteristics. Normally
controllers will inform pilots of the reason for an
instruction to expedite.

4-4-11. IFR Separation Standards
a. ATC effects separation of aircraft vertically by
assigning different altitudes; longitudinally by
providing an interval expressed in time or distance
between aircraft on the same, converging, or crossing
courses, and laterally by assigning different flight
b. Separation will be provided between all aircraft
operating on IFR flight plans except during that part
of the flight (outside Class B airspace or a TRSA)
being conducted on a VFR-on-top/VFR conditions

clearance. Under these conditions, ATC may issue
traffic advisories, but it is the sole responsibility of the
pilot to be vigilant so as to see and avoid other aircraft.

c. When radar is employed in the separation of
aircraft at the same altitude, a minimum of 3 miles
separation is provided between aircraft operating
within 40 miles of the radar antenna site, and 5 miles
between aircraft operating beyond 40 miles from the
antenna site. These minima may be increased or
decreased in certain specific situations.

Certain separation standards are increased in the terminal
environment when CENRAP is being utilized.

4-4-12. Speed Adjustments
a. ATC will issue speed adjustments to pilots of
radar-controlled aircraft to achieve or maintain
required or desire spacing.
b. ATC will express all speed adjustments in

terms of knots based on indicated airspeed (IAS) in
5 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 is
restricted to turbojet aircraft with Mach meters.

ATC Clearances and Aircraft Separation 4-4-7

Page 237 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   236 -- Page 237 -- 238