Aeronautical Information Manual (AIM)

AIM

4/3/14

7−1−58

Meteorology

6. The Terminal Weather Information for

Pilots System (TWIP).

(a)

With the increase in the quantity and

quality of terminal weather information available

through TDWR, the next step is to provide this

information directly to pilots rather than relying on

voice communications from ATC. The National

Airspace System has long been in need of a means of

delivering terminal weather information to the

cockpit more efficiently in terms of both speed and

accuracy to enhance pilot awareness of weather

hazards and reduce air traffic controller workload.

With the TWIP capability, terminal weather

information, both alphanumerically and graphically,

is now available directly to the cockpit on a test basis

at 9 locations.

(b)

TWIP products are generated using

weather data from the TDWR or the Integrated

Terminal Weather System (ITWS) testbed. TWIP

products are generated and stored in the form of text

and character graphic messages. Software has been

developed to allow TDWR or ITWS to format the

data and send the TWIP products to a database

resident at Aeronautical Radio, Inc. (ARINC). These

products can then be accessed by pilots using the

ARINC Aircraft Communications Addressing and

Reporting System (ACARS) data link services.

Airline dispatchers can also access this database and

send messages to specific aircraft whenever wind

shear activity begins or ends at an airport.

(c)

TWIP products include descriptions and

character graphics of microburst alerts, wind shear

alerts, significant precipitation, convective activity

within 30 NM surrounding the terminal area, and

expected weather that will impact airport operations.

During inclement weather, i.e., whenever a predeter-

mined level of precipitation or wind shear is detected

within 15 miles of the terminal area, TWIP products

are updated once each minute for text messages and

once every five minutes for character graphic

messages. During good weather (below the predeter-

mined precipitation or wind shear parameters) each

message is updated every 10 minutes. These products

are intended to improve the situational awareness of

the pilot/flight crew, and to aid in flight planning prior

to arriving or departing the terminal area. It is

important to understand that, in the context of TWIP,

the predetermined levels for inclement versus good

weather has nothing to do with the criteria for

VFR/MVFR/IFR/LIFR; it only deals with precipita-

tion, wind shears and microbursts.

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27. PIREPs Relating to Volcanic Ash

Activity

Volcanic eruptions which send ash into the

upper atmosphere occur somewhere around the world

several times each year. Flying into a volcanic ash

cloud can be extremely dangerous. At least two

B747s have lost all power in all four engines after

such an encounter. Regardless of the type aircraft,

some damage is almost certain to ensue after an

encounter with a volcanic ash cloud. Additionally,

studies have shown that volcanic eruptions are the

only significant source of large quantities of sulphur

dioxide (SO

) gas at jet-cruising altitudes. Therefore,

the detection and subsequent reporting of SO

is of

significant importance. Although SO

is colorless, its

presence in the atmosphere should be suspected when

a sulphur-like or rotten egg odor is present throughout

the cabin.

While some volcanoes in the U.S. are

monitored, many in remote areas are not. These

unmonitored volcanoes may erupt without prior

warning to the aviation community. A pilot observing

a volcanic eruption who has not had previous

notification of it may be the only witness to the

eruption. Pilots are strongly encouraged to transmit a

PIREP regarding volcanic eruptions and any

observed volcanic ash clouds or detection of sulphur

dioxide (SO

) gas associated with volcanic activity.

Pilots should submit PIREPs regarding volcanic

activity using the Volcanic Activity Reporting (VAR)

form as illustrated in Appendix 2. If a VAR form is

not immediately available, relay enough information

to identify the position and type of volcanic activity.

Pilots should verbally transmit the data required

in items 1 through 8 of the VAR as soon as possible.

The data required in items 9 through 16 of the VAR

should be relayed after landing if possible.

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28. Thunderstorms

Turbulence, hail, rain, snow, lightning, sus-

tained updrafts and downdrafts, icing conditions−all

are present in thunderstorms. While there is some

evidence that maximum turbulence exists at the

middle level of a thunderstorm, recent studies show

little variation of turbulence intensity with altitude.