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ATC Clearances and Aircraft Separation

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.


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no longer in effect.


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.


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.



11. IFR Separation Standards


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



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.


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.



12. Speed Adjustments


ATC will issue speed adjustments to pilots of

radar−controlled aircraft to achieve or maintain

required or desire spacing.


ATC will express all speed adjustments in

terms of knots based on indicated airspeed (IAS) 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 is

restricted to turbojet aircraft with Mach meters.

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