Landing
According to 14 CFR part 91, no pilot may land when the
flight visibility is less than the visibility prescribed in the
standard IAP being used. ATC provides the pilot with the
current visibility reports appropriate to the runway in use.
This may be in the form of prevailing visibility, runway visual
value (RVV), or runway visual range (RVR). However, only
the pilot can determine if the flight visibility meets the landing
requirements indicated on the approach chart. If the flight
visibility meets the minimum prescribed for the approach,
then the approach may be continued to a landing. If the flight
visibility is less than that prescribed for the approach, then
the pilot must execute a missed approach regardless of the
reported visibility.
The landing minimums published on IAP charts are based on
full operation of all components and visual aids associated
with the instrument approach chart being used. Higher
minimums are required with inoperative components or
visual aids. For example, if the ALSF-1 approach lighting
system were inoperative, the visibility minimums for an ILS
would need to be increased by one-quarter mile. If more
than one component is inoperative, each minimum is raised
to the highest minimum required by any single component
that is inoperative. ILS glideslope inoperative minimums
are published on instrument approach charts as localizer
minimums. Consult the “Inoperative Components or Visual
Aids Table” (printed on the inside front cover of each
TPP) for a complete description of the effect of inoperative
components on approach minimums.
Instrument Weather Flying
Flying Experience
The more experience a pilot has in VFR and IFR flight,
the more proficient a pilot becomes. VFR experience can
be gained by flying in terminal areas with high traffic
activity. This type of flying forces the pilot to polish the
skill of dividing his or her attention between aircraft control,
navigation, communications, and other flight deck duties.
IFR experience can be gained through night flying which
also promotes both instrument proficiency and confidence.
The progression from flying at night under clear, moonlit
conditions to flying at night without moonlight, natural
horizon, or familiar landmarks teaches a pilot to trust the
aircraft instruments with minimal dependence upon what
can be seen outside the aircraft. It is a pilot’s decision to
proceed with an IFR flight or to wait for more acceptable
weather conditions.
Recency of Experience
Currency as an instrument pilot is an equally important
consideration. No person may act as pilot in command of an
aircraft under IFR or in weather conditions less than VFR
minimums unless he or she has met the requirements of Part
91. Remember, these are minimum requirements.
Airborne Equipment and Ground Facilities
Regulations specify minimum equipment for filing an IFR
flight plan. It is the pilot’s responsibility to determine the
adequacy of the aircraft and navigation/communication
(NAV/COM) equipment for the proposed IFR flight.
Performance limitations, accessories, and general condition
of the equipment are directly related to the weather, route,
altitude, and ground facilities pertinent to the flight, as well
as to the flight deck workload.
Weather Conditions
In addition to the weather conditions that might affect a
VFR flight, an IFR pilot must consider the effects of other
weather phenomena (e.g., thunderstorms, turbulence, icing,
and visibility).
Turbulence
Inflight turbulence can range from occasional light bumps to
extreme airspeed and altitude variations that make aircraft
control difficult. To reduce the risk factors associated with
turbulence, pilots must learn methods of avoidance, as well as
piloting techniques for dealing with an inadvertent encounter.
Turbulence avoidance begins with a thorough preflight
weather briefing. Many reports and forecasts are available to
assist the pilot in determining areas of potential turbulence.
These include the Severe Weather Warning (WW), SIGMET
(WS), Convective SIGMET (WST), AIRMET (WA), Severe
Weather Outlook (AC), Center Weather Advisory (CWA),
Area Forecast (FA), and Pilot Reports (UA or PIREPs). Since
thunderstorms are always indicative of turbulence, areas
of known and forecast thunderstorm activity is always of
interest to the pilot. In addition, clear air turbulence (CAT)
associated with jet streams, strong winds over rough terrain,
and fast moving cold fronts are good indicators of turbulence.
Pilots should be alert while in flight for the signposts of
turbulence. For example, clouds with vertical development
such as cumulus, towering cumulus, and cumulonimbus are
indicators of atmospheric instability and possible turbulence.
Standing lenticular clouds lack vertical development but
indicate strong mountain wave turbulence. While en route,
pilots can monitor hazardous inflight weather advisory
service (HIWAS) broadcast for updated weather advisories,
or contact the nearest FSS or En Route Flight Advisory
Service (EFAS) for the latest turbulence-related PIREPs.
Figure 10-14. Maintaining an instrument scan in severe turbulence can be difficult.
To avoid turbulence associated with strong thunderstorms,
circumnavigate cells by at least 20 miles. Turbulence may
also be present in the clear air above a thunderstorm. To
avoid this, fly at least 1,000 feet above the top for every 10
knots of wind at that level, or fly around the storm. Finally,
do not underestimate the turbulence beneath a thunderstorm.
Never attempt to fly under a thunderstorm. The possible
results of turbulence and wind shear under the storm could
be disastrous.
When moderate to severe turbulence is encountered, aircraft
control is difficult, and a great deal of concentration is
required to maintain an instrument scan. [Figure 10-14]
Pilots should immediately reduce power and slow the
aircraft to the recommended turbulence penetration speed
as described in the POH/AFM. To minimize the load factor
imposed on the aircraft, the wings should be kept level and the
aircraft’s pitch attitude should be held constant. The aircraft
is allowed to fluctuate up and down because maneuvering
to maintain a constant altitude only increases the stress on
the aircraft. If necessary, the pilot should advise ATC of
the fluctuations and request a block altitude clearance. In
addition, the power should remain constant at a setting that
maintains the recommended turbulence penetration airspeed.
The best source of information on the location and intensity
of turbulence are PIREPs. Therefore, pilots are encouraged to
familiarize themselves with the turbulence reporting criteria
found in the AIM, which also describes the procedure for
volunteering PIREPs relating to turbulence.
Structural Icing
The very nature of flight in instrument meteorological
conditions (IMC) means operating in visible moisture such
as clouds. At the right temperatures, this moisture can
freeze on the aircraft, causing increased weight, degraded
performance, and unpredictable aerodynamic characteristics.
Understanding avoidance and early recognition followed
by prompt action are the keys to avoiding this potentially
hazardous situation.
Structural icing refers to the accumulation of ice on the
exterior of the aircraft and is broken down into three
classifications: rime ice, clear ice, and mixed ice. For ice
to form, there must be moisture present in the air, and the
air must be cooled to a temperature of 0 °C (32 °F) or less.
Aerodynamic cooling can lower the surface temperature of
an airfoil and cause ice to form on the airframe even though
the ambient temperature is slightly above freezing.
Rime ice forms if the droplets are small and freeze
immediately when contacting the aircraft surface. This type
of ice usually forms on areas such as the leading edges of
wings or struts. It has a somewhat rough-looking appearance
and a milky-white color.
Figure 10-14. Temperature ranges for ice formation.
0 °C to –10 °C
–10 °C to –15 °C
–15 °C to –20°C
Clear
Mixed clear and rime
Rime
Figure 10-15. Temperature ranges for ice formation.
Clear ice is usually formed from larger water droplets or
freezing rain that can spread over a surface. This is the most
dangerous type of ice since it is clear, hard to see, and can
change the shape of the airfoil.
Mixed ice is a mixture of clear ice and rime ice. It has the
bad characteristics of both types and can form rapidly. Ice
particles become embedded in clear ice, building a very
rough accumulation. The table in Figure 10-15 lists the
temperatures at which the various types of ice form.
Structural icing is a condition that can only get worse.
Therefore, during an inadvertent icing encounter, it is
important the pilot act to prevent additional ice accumulation.
Regardless of the level of anti-ice or deice protection offered
by the aircraft, the first course of action should be to leave
the area of visible moisture. This might mean descending
to an altitude below the cloud bases, climbing to an altitude
that is above the cloud tops, or turning to a different course.
If this is not possible, then the pilot must move to an altitude
where the temperature is above freezing. Pilots should report
icing conditions to ATC and request new routing or altitude
if icing will be a hazard. Refer to the AIM for information
on reporting icing intensities.
Fog
Instrument pilots must learn to anticipate conditions leading
to the formation of fog and take appropriate action early in
the progress of the flight. Before a flight, close examination
of current and forecast weather should alert the pilot to the
possibility of fog formation. When fog is a consideration,
pilots should plan adequate fuel reserves and alternate landing
sites. En route, the pilot must stay alert for fog formation
through weather updates from EFAS, ATIS, and ASOS/
AWOS sites.
Two conditions lead to the formation of fog. Either the air
is cooled to saturation, or sufficient moisture is added to the
air until saturation occurs. In either case, fog can form when
the temperature/dewpoint spread is 5° or less. Pilots planning
to arrive at their destination near dusk with decreasing
temperatures should be particularly concerned about the
possibility of fog formation.
Volcanic Ash
Volcanic eruptions create volcanic ash clouds containing
an abrasive dust that poses a serious safety threat to flight
operations. Adding to the danger is the fact that these ash
clouds are not easily discernible from ordinary clouds when
encountered at some distance from the volcanic eruption.
When an aircraft enters a volcanic ash cloud, dust particles
and smoke may become evident in the cabin, often along with
the odor of an electrical fire. Inside the volcanic ash cloud,
the aircraft may also experience lightning and St. Elmo’s fire
on the windscreen. The abrasive nature of the volcanic ash
can pit the windscreens, thus reducing or eliminating forward
visibility. The pitot-static system may become clogged,
causing instrument failure. Severe engine damage is probable
in both piston and jet-powered aircraft.
Every effort must be made to avoid volcanic ash. Since
volcanic ash clouds are carried by the wind, pilots should plan
their flights to remain upwind of the ash-producing volcano.
Visual detection and airborne radar are not considered
a reliable means of avoiding volcanic ash clouds. Pilots
witnessing volcanic eruptions or encountering volcanic ash
should immediately pass this information along in the form of
a pilot report. The National Weather Service (NWS) monitors
volcanic eruptions and estimates ash trajectories. This
information is passed along to pilots in the form of SIGMETs.
As for many other hazards to flight, the best source of
volcanic information comes from PIREPs. Pilots who
witness a volcanic eruption or encounter volcanic ash in flight
should immediately inform the nearest agency. Volcanic
Ash Forecast Transport and Dispersion (VAFTAD) charts
are also available; these depict volcanic ash cloud locations
in the atmosphere following an eruption and also forecast
dispersion of the ash concentrations over 6- and 12-hour
time intervals. See AC 00-45, Aviation Weather Services.
Thunderstorms
A thunderstorm packs just about every weather hazard known
to aviation into one vicious bundle. Turbulence, hail, rain,
snow, lightning, sustained updrafts and downdrafts, and
icing conditions are all present in thunderstorms. Do not
take off in the face of an approaching thunderstorm or fly an
aircraft that is not equipped with thunderstorm detection in
clouds or at night in areas of suspected thunderstorm activity.
[Figure 10-16]
There is no useful correlation between the external visual
appearance of thunderstorms and the severity or amount of
turbulence or hail within them. All thunderstorms should be
considered hazardous, and thunderstorms with tops above
35,000 feet should be considered extremely hazardous.
Figure 10-16. A thunderstorm packs just about every weather hazard
known to aviation into one vicious bundle.
Tailwind Shearing to Headwind or Calm Headwind Shearing to Tailwind or Calm
Front Front
OM(outer marker) OM(outer marker)
Figure 10-17. Glideslope deviations due to wind shear encounter.
Weather radar, airborne or ground based, normally reflects
the areas of moderate to heavy precipitation (radar does not
detect turbulence). The frequency and severity of turbulence
generally increases with the radar reflectivity closely
associated with the areas of highest liquid water content of
the storm. A flightpath through an area of strong or very
strong radar echoes separated by 20 to 30 miles or less may
not be considered free of severe turbulence.
The probability of lightning strikes occurring to aircraft is
greatest when operating at altitudes where temperatures are
between –5 ° C and +5 ° C. In addition, an aircraft flying in the
clear air near a thunderstorm is also susceptible to lightning
strikes. Thunderstorm avoidance is always the best policy.
Wind Shear
Wind shear can be defined as a change in wind speed and/or
wind direction in a short distance. It can exist in a horizontal
or vertical direction and occasionally in both. Wind shear can
occur at all levels of the atmosphere but is of greatest concern
during takeoffs and landings. It is typically associated
with thunderstorms and low-level temperature inversions;
however, the jet stream and weather fronts are also sources
of wind shear.
As Figure 10-17 illustrates, while an aircraft is on an
instrument approach, a shear from a tailwind to a headwind
causes the airspeed to increase and the nose to pitch up with
a corresponding balloon above the glidepath. A shear from
a headwind to a tailwind has the opposite effect, and the
aircraft will sink below the glidepath.
A headwind shear followed by a tailwind/downdraft shear is
particularly dangerous because the pilot has reduced power
and lowered the nose in response to the headwind shear. This
leaves the aircraft in a nose-low, power-low configuration
when the tailwind shear occurs, which makes recovery more
difficult, particularly near the ground. This type of wind
shear scenario is likely while making an approach in the
face of an oncoming thunderstorm. Pilots should be alert for
indications of wind shear early in the approach phase and be
ready to initiate a missed approach at the first indication. It
may be impossible to recover from a wind shear encounter
at low altitude.
To inform pilots of hazardous wind shear activity, some
airports have installed a Low-Level Wind Shear Alert
System (LLWAS) consisting of a centerfield wind indicator
and several surrounding boundary-wind indicators. With
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 contains 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-from-
cloud criteria in 14 CFR part 91.
3. Comply with IFR 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 VFR-
on-top must avoid such airspace.
VFR Over-The-Top
VFR over-the-top must not be confused with VFR-on-
top. 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 is flown. The aircraft is equipped with
dual navigation and communication radios, a transponder,
and a GPS system approved for IFR en route, terminal, and
approach operations.
Preflight
The success of the flight depends largely upon the
thoroughness of the preflight planning. The evening before
the flight, pay close attention to the weather forecast and
begin planning the flight.
Figure 10-17. Route Planning.
GREENWOOD
122.1R
KEWANEE
113.8 EWA 8 5
N 32 ° 2 2 . 01' W 8 8 ° 2 7 . 5 0'
Figure 10-18. Route planning.
Figure 10-17. Route planning, Gulfport-Biloxi.
SC-4, 16 DEC 2010 to 13 JAN 2011
SC-4, 16 DEC 2010 to 13 JAN 2011
The Weather Channel indicates a large, low-pressure system
has settled in over the Midwest, pulling moisture up from
the Gulf of Mexico and causing low ceilings and visibility
with little chance for improvement over the next couple of
days. To begin planning, gather all the necessary charts and
materials, and verify everything is current. This includes en
route charts, approach charts, DPs, STAR charts, the GPS
database, as well as an A/FD, some navigation logs, and the
aircraft’s POH/AFM. The charts cover both the departure
and arrival airports and any contingency airports that will
be needed if the flight cannot be completed as planned. This
is also a good time for the pilot to consider recent flight
experience, pilot proficiency, fitness, and personal weather
minimums to fly this particular flight.
Check the A/FD to become familiar with the departure and
arrival airport, and check for any preferred routing between
BHM and GPT. Next, review the approach charts and any
DP or STAR that pertains to the flight. Finally, review the
en route charts for potential routing, paying close attention
to the minimum en route and obstacle clearance altitudes.
After this review, select the best option. For this flight, the
Birmingham Three Departure [Figure 10-2] to Brookwood
VORTAC, V 209 to Kewanee VORTAC, direct to Gulfport
using GPS would be a logical route. An altitude of 4,000 feet
meets all the regulatory requirements and falls well within
the performance capabilities of the aircraft.
Next, call 1-800-WX-BRIEF to obtain an outlook-type
weather briefing for the proposed flight. This provides
forecast conditions for departure and arrival airports, as well
as the en route portion of the flight including forecast winds
aloft. This also is a good opportunity to check the available
NOTAMs.
The weather briefer confirms the predictions of the Weather
Channel giving forecast conditions that are at or near
minimum landing minimums at both BHM and GPT for
the proposed departure time. The briefer provides NOTAM
information for GPT indicating that the localizer to runway
32 is scheduled to be out of service and that runway 18/36 is
closed until further notice. Also check for temporary flight
restrictions (TFRs) along the proposed route.
After receiving a weather briefing, continue flight planning
and begin to transfer some preliminary information onto
the navigation log, listing each fix along the route and the
distances, frequencies, and altitudes. Consolidating this
information onto an organized navigation log keeps the
workload to a minimum during the flight.
Next, obtain a standard weather briefing online for the
proposed route. A check of current conditions indicates
low IFR conditions at both the departure airport and the
destination, with visibility of one-quarter mile:
SURFACE WEATHER OBSERVATIONS
METAR KBHM 111155Z VRB04KT ¼ SM FG –RA VV004
06/05 A2994 RMK A02 SLP140
METAR KGPT 111156Z 24003KT ¼ SM FG OVC001 08/07
A2962 RMK A02 SLP033
The small temperature/dewpoint spread is causing the low
visibility and ceilings. Conditions should improve later in
the day as temperatures increase. A check of the terminal
forecast confirms this theory:
TERMINAL FORECASTS
TAF KBHM 111156Z 111212 VRB04KT ¼ SM FG VV004
TEMPO1316 ¾ SM OVC004
FM1600 VRB05KT 2SM BR OVC007 TEMPO 1720 3SM
DZ BKN009
FM2000 22008KT 3SM –RA OVC015 TEMP 2205 3SM
–RA OVC025 FM0500 23013KT P6SM OVC025
FM0800 23013KT P6SM BKN030 PROB40 1012 2SM BR
OVC030
TAF KGPT 111153Z 111212 24004KT ¼ SM FG OVC001
BECMG 1317 3SM BR 0VC004
FM1700 24010KT 4SM –RA OVC006 FM0400 24010 5SM
SCT080 TEMPO 0612 P6SM SKC
In addition to the terminal forecast, the area forecast also
indicates gradual improvement along the route. Since the
terminal forecast only provides information for a 5-mile
radius around a terminal area, checking the area forecast
provides a better understanding of the overall weather picture
along the route, as well as potential hazards:
SYNOPSIS AND VFR CLOUDS/WEATHER FORECASTS
SYNOPSIS… AREA OF LOW PRESSURE CNTD OV AL
RMNG GENLY STNRY BRNGNG MSTR AND WD SPRD
IFR TO E TN. ALF…LOW PRES TROF ACRS CNTR PTN
OF THE DFW FA WILL GDLY MOV EWD DURG PD.
NRN LA, AR, NRN MS
SWLY WND THRUT THE PD. 16Z CIG OVC006. SCT
–SHRA. OTLK… IFR SRN ½ … CIG SCT – BKN015
TOPS TO FL250 SWLY WND THRUT THE PD. 17Z AGL
BKN040. OTLK…MVFR CIG VIS.