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and several surrounding boundary-wind indicators. With
10-26
Figure 10-17. Glide slope Deviations Due to Wind Shear Encounter.
this system, controllers are alerted of wind discrepancies
(an indicator of wind shear possibility) and provide this
information to pilots. A typical wind shear alert issued to a
pilot would be:
“Runway 27 arrival, wind shear alert, 20 knot loss 3
mile final, threshold wind 200 at 15”
In plain language, the controller is advising aircraft arriving
on runway 27 that at about 3 miles out they can expect a
wind shear condition that will decrease their airspeed by 20
knots and possibly encounter turbulence. Additionally, the
airport surface winds for landing runway 27 are reported as
200° at 15 knots.
Pilots encountering wind shear are encouraged to pass along
pilot reports. Refer to AIM for additional information on
wind shear PIREPs.
VFR-On-Top
Pilots on IFR flight plans operating in VFR weather
conditions may request VFR-on-top in lieu of an assigned
altitude. This permits them to select an altitude or flight level
of their choice (subject to any ATC restrictions).
Pilots desiring to climb through a cloud, haze, smoke, or
other meteorological formation and then either cancel their
IFR flight plan or operate VFR-on-top may request a climb
to VFR-on-top. The ATC authorization will contain a top
report (or a statement that no top report is available) and a
request to report upon reaching VFR-on-top. Additionally,
the ATC authorization may contain a clearance limit, routing,
and an alternative clearance if VFR-on-top is not reached by
a specified altitude.
A pilot on an IFR flight plan, operating in VFR conditions,
may request to climb/descend in VFR conditions. When
operating in VFR conditions with an ATC authorization to
“maintain VFR-on-top/maintain VFR conditions,” pilots on
IFR flight plans must:
1. Fly at the appropriate VFR altitude as prescribed in 14
CFR part 91.
2. Comply with the VFR visibility and distance-fromcloud
criteria in 14 CFR part 91.
3. Comply with instrument flight rules applicable to this
flight (minimum IFR altitudes, position reporting, radio
communications, course to be flown, adherence to ATC
clearance, etc.).
Pilots operating on a VFR-on-top clearance should advise
ATC before any altitude change to ensure the exchange of
accurate traffic information.
ATC authorization to “maintain VFR-on-top” is not intended
to restrict pilots to operating only above an obscuring
meteorological formation (layer). Rather, it permits operation
above, below, between layers, or in areas where there is no
meteorological obstruction. It is imperative pilots understand,
however, that clearance to operate “VFR-on-top/VFR
conditions” does not imply cancellation of the IFR flight
plan.
Pilots operating VFR-on-top/VFR conditions may receive
traffic information from ATC on other pertinent IFR or
VFR aircraft. However, when operating in VFR weather
conditions, it is the pilot’s responsibility to be vigilant to see
and avoid other aircraft.
This clearance must be requested by the pilot on an IFR flight
plan. VFR-on-top is not permitted in certain areas, such as
Class A airspace. Consequently, IFR flights operating VFRon-
top must avoid such airspace.
10-27
VFR Over-The-Top
VFR over-the-top must not be confused with VFR-ontop.
VFR-on-top is an IFR clearance that allows the pilot
to fly VFR altitudes. VFR over-the-top is strictly a VFR
operation in which the pilot maintains VFR cloud clearance
requirements while operating on top of an undercast layer.
This situation might occur when the departure airport and the
destination airport are reporting clear conditions, but a low
overcast layer is present in between. The pilot could conduct
a VFR departure, fly over the top of the undercast in VFR
conditions, then complete a VFR descent and landing at the
destination. VFR cloud clearance requirements would be
maintained at all times, and an IFR clearance would not be
required for any part of the flight.
Conducting an IFR Flight
To illustrate some of the concepts introduced in this chapter,
follow along on a typical IFR flight from the Birmingham
International Airport (BHM), Birmingham, Alabama to
Gulfport-Biloxi International Airport (GPT), Gulfport,
Mississippi. [Figure 10-18] For this trip, a Cessna 182
with a call sign of N1230A will be flown. The aircraft is
equipped with dual navigation and communication radios, a
transponder, and a GPS system approved for IFR en route,
　
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