Federal Register / Vol. 61, No. 17 / Thursday, January 25, 1996 / Proposed Rules 2163

PROCEDURES FOR EXITING THE

FREEZING RAIN/FREEZING DRIZZLE

ENVIRONMENT:

These procedures are applicable to all

flight phases from takeoff to landing. Monitor

the outside air temperature. While severe

icing may form at temperatures as cold as

¥18 degrees Celsius, increased vigilance is

warranted at temperatures around freezing

with visible moisture present. If the visual

cues specified in the AFM for identifying

possible freezing rain or freezing drizzle

conditions are observed, accomplish the

following:

• Exit the freezing rain or freezing drizzle

severe icing conditions immediately to avoid

extended exposure to flight conditions

outside of those for which the airplane has

been certificated for operation. Asking for

priority to leave the area is fully justified

under these conditions.

• Avoid abrupt and excessive

maneuvering that may exacerbate control

difficulties.

• Do not engage the autopilot. The

autopilot may mask unusual control system

forces.

• If the autopilot is engaged, hold the

control wheel firmly and disengage the

autopilot.

• If an unusual roll response or

uncommanded control movement is

observed, reduce the angle-of-attack by

increasing airspeed or rolling wings level (if

in a turn), and apply additional power, if

needed.

• Avoid extending flaps during extended

operation in icing conditions. Operation with

flaps extended can result in a reduced wing

angle-of- attack, with ice forming on the

upper surface further aft on the wing than

normal, possibly aft of the protected area.

• Report these weather conditions to Air

Traffic Control.’’

(b) An alternative method of compliance or

adjustment of the compliance time that

provides an acceptable level of safety may be

used if approved by the Manager,

Standardization Branch, ANM–113, FAA,

Transport Airplane Directorate. Operators

shall submit their requests through an

appropriate FAA Principal Operations

Inspector, who may add comments and then

send it to the Manager, Standardization

Branch, ANM–113.

Note 3: Information concerning the

existence of approved alternative methods of

compliance with this AD, if any, may be

obtained from the Standardization Branch,

ANM–113.

(c) Special flight permits may be issued in

accordance with sections 21.197 and 21.199

of the Federal Aviation Regulations (14 CFR

21.197 and 21.199) to operate the airplane to

a location where the requirements of this AD

can be accomplished.

Issued in Renton, Washington, on January

19, 1996.

Darrell M. Pederson,

Acting Manager, Transport Airplane

Directorate, Aircraft Certification Service.

[FR Doc. 96–1172 Filed 1–24–96; 8:45 am]

BILLING CODE 4910–13–U

14 CFR Part 39

[Docket No. 96–NM–19–AD]

Airworthiness Directives; Empresa

Brasileira de Aeronautica, S.A.

(EMBRAER) Model EMB–120 Series

Airplanes

AGENCY: Federal Aviation

Administration, DOT.

ACTION: Notice of proposed rulemaking

(NPRM).

SUMMARY: This document proposes the

adoption of a new airworthiness

directive (AD) that is applicable to all

EMBRAER Model EMB–120 series

airplanes. This proposal would require

revising the Airplane Flight Manual

(AFM) to specify procedures that would

prohibit flight in freezing rain or

freezing drizzle conditions (as

determined by certain visual cues), limit

or prohibit the use of various flight

control devices, and provide the flight

crew with recognition cues for, and

procedures for exiting from, severe icing

conditions. This proposal is prompted

by results of a review of the

requirements for certification of the

airplane in icing conditions, new

information on the icing environment,

and icing data provided currently to the

flight crews. The actions specified by

the proposed AD are intended to

minimize the potential hazards

associated with operating the airplane

in freezing rain or freezing drizzle

conditions by providing more clearly

defined procedures and limitations

associated with such conditions.

DATES: Comments must be received by

March 7, 1996.

ADDRESSES: Submit comments in

triplicate to the Federal Aviation

Administration (FAA), Transport

Airplane Directorate, ANM–103,

Attention: Rules Docket No. 96–NM–

19–AD, 1601 Lind Avenue, SW.,

Renton, Washington 98055–4056.

Comments may be inspected at this

location between 9:00 a.m. and 3:00

p.m., Monday through Friday, except

Federal holidays.

This information may be examined at

the FAA, Transport Airplane

Directorate, 1601 Lind Avenue, SW.,

Renton, Washington; or at the FAA,

Small Airplane Directorate, Atlanta

Aircraft Certification Office, Campus

Building, 1701 Columbia Avenue, suite

2–160, College Park, Georgia.

FOR FURTHER INFORMATION CONTACT: John

W. McGraw, Aerospace Engineer, Flight

Test/Systems Branch, ACE–116A, FAA,

Small Airplane Directorate, Atlanta

Aircraft Certification Office, Campus

Building, 1701 Columbia Avenue, suite

2–160, College Park, Georgia 30337–

2748; telephone (404) 305–7336; fax

(404) 305–7348.

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to

participate in the making of the

proposed rule by submitting such

written data, views, or arguments as

they may desire. Communications shall

identify the Rules Docket number and

be submitted in triplicate to the address

specified above. All communications

received on or before the closing date

for comments, specified above, will be

considered before taking action on the

proposed rule. The proposals contained

in this notice may be changed in light

of the comments received.

Comments are specifically invited on

the overall regulatory, economic,

environmental, and energy aspects of

the proposed rule. All comments

submitted will be available, both before

and after the closing date for comments,

in the Rules Docket for examination by

interested persons. A report

summarizing each FAA-public contact

concerned with the substance of this

proposal will be filed in the Rules

Docket.

Commenters wishing the FAA to

acknowledge receipt of their comments

submitted in response to this notice

must submit a self-addressed, stamped

postcard on which the following

statement is made: ‘‘Comments to

Docket Number 96–NM–19–AD.’’ The

postcard will be date stamped and

returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this

NPRM by submitting a request to the

FAA, Transport Airplane Directorate,

ANM-103, Attention: Rules Docket No.

96–NM–19–AD, 1601 Lind Avenue,

SW., Renton, Washington 98055–4056.

Discussion

In October 1994, a transport category

airplane was involved in an accident in

which severe icing conditions (believed

to be composed of freezing drizzle size

droplets) were reported in the area.

Although the National Transportation

Safety Board (NTSB) has not yet made

a finding of probable cause of the

accident, loss of control of the airplane

may have occurred because ice

accretion on the upper surface of the

wing aft of the area protected by the ice

protection system caused airflow

separation, which resulted in the

ailerons being forced to a right-wingdown control position. There also is

concern that the autopilot, which was

engaged, may have masked the unusual
2164 Federal Register / Vol. 61, No. 17 / Thursday, January 25, 1996 / Proposed Rules

control forces generated by the ice

accumulation. These conditions, if not

corrected, could result in a roll upset

from which the flight crew may be

unable to recover.

The atmospheric conditions (freezing

drizzle) that may have contributed to

the accident are outside the icing

envelope specified in Appendix C of

part 25 of the Federal Aviation

Regulations (14 CFR part 25) for

certification of the airplane. Freezing

rain is an atmospheric condition that

also is outside the icing envelope. Such

icing conditions are not defined in

Appendix C, and the FAA has not

required that airplanes be shown to be

capable of operating safely in those

icing conditions.

The FAA finds that flight crews are

not currently provided with adequate

information necessary to determine

when the airplane is operating in icing

conditions for which the airplane is not

certificated or what action to take when

such conditions are encountered.

Therefore, the FAA has determined that

flight crews must be provided with such

information and must be made aware of

certain visual cues that may indicate the

airplane is operating in atmospheric

conditions that are outside the icing

envelope.

Since such information is not

available to flight crews, and no

airplane is certificated for operation in

freezing drizzle conditions, the FAA

finds that the potentially unsafe

condition (described previously as

control difficulties following operation

of the airplane in icing conditions

outside of the icing envelope) is not

limited to airplanes having the same

type design as that of the accident

airplane.

The FAA recognizes that the flight

crew of any airplane that is certificated

for flight in icing conditions may not

have adequate information concerning

flight in icing conditions outside the

icing envelope. However, the FAA finds

that the specified unsafe condition must

be addressed as a higher priority on

airplanes that are turbopropellerpowered and have unpowered control

systems. Many of these airplanes carry

passengers in regularly scheduled

revenue service in the United States.

Since turbopropeller-powered airplanes

are more likely to operate at low

altitudes and to make more frequent

landings, they are more likely to

encounter icing conditions that are

outside the icing envelope.

Additionally, the flight crew of an

airplane having an unpowered roll

control system must rely solely on

physical strength to counteract roll

control anomalies, whereas a roll

control anomaly that occurs on an

airplane having a powered roll control

system need not be offset directly by the

flight crew.

Subsequent to the accident, the FAA,

in conjunction with certain foreign

airworthiness authorities and airplane

manufacturers, conducted reviews of

certain transport and small category

airplanes to determine if any airplanes

might experience control difficulty

should a ridge of ice form aft of the

deicing boots and forward of the

ailerons. The review focused on

turbopropeller-powered airplanes

having unpowered roll control systems,

since those airplanes are similar in

design to the accident airplane and

because they are frequently exposed to

icing conditions.

During the reviews of these airplanes,

an artificial ice shape was used in the

demonstration of roll control

characteristics. This ice shape was

chosen as representative of a shape that

might form if an airplane were operated

in freezing drizzle. Results of these

reviews revealed that certain airplanes

demonstrated acceptable roll control

forces. However, the dynamics of ice

accretion in freezing drizzle are not well

understood, and the FAA recognizes

that such airplanes could develop ice

shapes other than those tested during

the review. Upon further review, the

FAA may consider additional

rulemaking.

Following examination of all relevant

information, the FAA has determined

that certain limitations and procedures

should be included in the FAAapproved Airplane Flight Manual

(AFM) for the affected airplanes, as

follows:

• EMBRAER Model EMB-120 series

airplanes must be prohibited from flight

in freezing rain or freezing drizzle

conditions (as determined by certain

visual cues); and

• Flight crews must be provided with

information that would minimize the

potential hazards associated with

operating the airplane in freezing rain or

freezing drizzle conditions.

The FAA has determined that such

limitations and procedures currently are

not defined adequately in the AFM for

these airplanes.

This airplane model is manufactured

in Brazil and is type certificated for

operation in the United States under the

provisions of section 21.29 of the

Federal Aviation Regulations (14 CFR

21.29) and the applicable bilateral

airworthiness agreement.

Explanation of the Provisions of the

Proposed AD

Since an unsafe condition has been

identified that is likely to exist or

develop on other airplanes of the same

type design, the proposed AD would

require revising the Limitations Section

of the AFM to specify procedures that

would:

• Prohibit flight in freezing rain or

freezing drizzle conditions (as

determined by certain visual cues);

• Prohibit use of the autopilot when

ice is formed aft of the protected

surfaces of the wing, or when an

unusual lateral trim condition exists;

• Restrict use of flaps in icing

conditions; and

• Require that all icing detection

lights be operative prior to flight into

icing conditions at night.

The prohibition on flight in freezing

rain or freezing drizzle is not intended

to prohibit purely inadvertent

encounters with the specified

atmospheric conditions. However, pilots

should make all reasonable efforts to

avoid such encounters and must

immediately exit the conditions if they

are encountered.

This proposed AD also would require

revising the Normal Procedures Section

of the AFM to specify procedures that

would:

• Limit the use of the flaps and

prohibit the use of the autopilot when

ice is observed forming aft of the

protected surfaces of the wing, or if

unusual lateral trim requirements or

autopilot trim warnings are

encountered; and

• Provide the flight crew with

recognition cues for, and procedures for

exiting from, severe icing conditions.

Cost Impact

The FAA estimates that 227 airplanes

of U.S. registry would be affected by this

proposed AD, that it would take

approximately 1 work hour per airplane

to accomplish the proposed actions, and

that the average labor rate is $60 per

work hour. Based on these figures, the

cost impact of the proposed AD on U.S.

operators is estimated to be $13,620, or

$60 per airplane.

The cost impact figure discussed

above is based on assumptions that no

operator has yet accomplished any of

the proposed requirements of this AD

action, and that no operator would

accomplish those actions in the future if

this AD were not adopted.

In addition, the FAA recognizes that

this proposed AD may impose

operational costs. However, those costs

are incalculable because the frequency

of occurrence of the specified
Federal Register / Vol. 61, No. 17 / Thursday, January 25, 1996 / Proposed Rules 2165

conditions and the associated additional

flight time are indeterminable.

Nevertheless, because of the severity of

the unsafe condition addressed, the

FAA has determined that continued

operational safety necessitates the

imposition of these costs.

Regulatory Impact

The regulations proposed herein

would not have substantial direct effects

on the States, on the relationship

between the national government and

the States, or on the distribution of

power and responsibilities among the

various levels of government. Therefore,

in accordance with Executive Order

12612, it is determined that this

proposal would not have sufficient

federalism implications to warrant the

preparation of a Federalism Assessment.

For the reasons discussed above, I

certify that this proposed regulation (1)

is not a ‘‘significant regulatory action’’

under Executive Order 12866; (2) is not

a ‘‘significant rule’’ under the DOT

Regulatory Policies and Procedures (44

FR 11034, February 26, 1979); and (3) if

promulgated, will not have a significant

economic impact, positive or negative,

on a substantial number of small entities

under the criteria of the Regulatory

Flexibility Act. A copy of the draft

regulatory evaluation prepared for this

action is contained in the Rules Docket.

A copy of it may be obtained by

contacting the Rules Docket at the

location provided under the caption

ADDRESSES.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation

safety, Safety.

The Proposed Amendment

Accordingly, pursuant to the

authority delegated to me by the

Administrator, the Federal Aviation

Administration proposes to amend part

39 of the Federal Aviation Regulations

(14 CFR part 39) as follows:

PART 39—AIRWORTHINESS

DIRECTIVES

1. The authority citation for part 39

continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701.

39.13 [Amended]

2. Section 39.13 is amended by

adding the following new airworthiness

directive:

Embraer: Docket 96–NM–19–AD.

Applicability: All Model EMB–120 series

airplanes, certificated in any category.

Note 1: This AD applies to each airplane

identified in the preceding applicability

provision, regardless of whether it has been

modified, altered, or repaired in the area

subject to the requirements of this AD. For

airplanes that have been modified, altered, or

repaired so that the performance of the

requirements of this AD is affected, the

owner/operator must use the authority

provided in paragraph (b) of this AD to

request approval from the FAA. This

approval may address either no action, if the

current configuration eliminates the unsafe

condition; or different actions necessary to

address the unsafe condition described in

this AD. Such a request should include an

assessment of the effect of the changed

configuration on the unsafe condition

addressed by this AD. In no case does the

presence of any modification, alteration, or

repair remove any airplane from the

applicability of this AD.

Compliance: Required as indicated, unless

accomplished previously.

To minimize the potential hazards

associated with operating the airplane in

freezing rain or freezing drizzle icing

conditions by providing more clearly defined

procedures and limitations associated with

such conditions, accomplish the following:

(a) Within 30 days after the effective date

of this AD, accomplish the requirements of

paragraphs (a)(1) and (a)(2) of this AD.

Note 2: Operators must initiate action to

notify and ensure that flight crewmembers

are apprised of this change.

(1) Revise the FAA-approved Airplane

Flight Manual (AFM) by incorporating the

following into the Limitations Section of the

AFM. This may be accomplished by inserting

a copy of this AD in the AFM.

‘‘• Flight in meteorological conditions

described as freezing rain or freezing drizzle,

as determined by the following visual cues,

is prohibited:

—Unusually extensive ice accreted on the

airframe in areas not normally observed to

collect ice.

—Accumulation of ice on the upper surface

(for low-wing airplanes) or lower surface

(for high-wing airplanes) of the wing aft of

the protected area.

—Accumulation of ice on the propeller

spinner farther back than normally

observed.

If the airplane encounters conditions that

are determined to contain freezing rain or

freezing drizzle, the pilot must immediately

exit the freezing rain or freezing drizzle

conditions by changing altitude or course.

Note: The prohibition on flight in freezing

rain or freezing drizzle is not intended to

prohibit purely inadvertent encounters with

the specified meteorological conditions.

However, pilots should make all reasonable

efforts to avoid such encounters and must

immediately exit the conditions if they are

encountered.

• Use of the autopilot is prohibited when

any ice is observed forming aft of the

protected surfaces of the wing, or when

unusual lateral trim requirements or

autopilot trim warnings are encountered.

Note: The autopilot may mask tactile cues

that indicate adverse changes in handling

characteristics. Therefore, the pilot should

consider not using the autopilot when any

ice is visible on the airplane.

• In icing conditions, use of flaps is

restricted to takeoff, approach, and landing

only. When the flaps have been extended for

approach or landing, they may not be

retracted unless the upper surface of the wing

aft of the protected area is clear of ice, or

unless flap retraction is essential for goaround.

• All icing detection lights must be

operative prior to flight into icing conditions

at night. [NOTE: This supersedes any relief

provided by the Master Minimum Equipment

List (MMEL).]’’

(2) Revise the FAA-approved AFM by

incorporating the following into the Normal

Procedures Section of the AFM. This may be

accomplished by inserting a copy of this AD

in the AFM.

‘‘WARNING

‘‘If ice is observed forming aft of the

protected surfaces of the wing, or if unusual

lateral trim requirements or autopilot trim

warnings are encountered:

• If the flaps are extended, do not retract

them until the airframe is clear of ice.

• The flight crew should reduce the angleof-attack by increasing speed as much as the

airplane configuration and weather allow,

without exceeding design maneuvering

speed.

• If the autopilot is engaged, hold the

control wheel firmly and disengage the

autopilot. Do not re-engage the autopilot

until the airframe is clear of ice.

• Exit the icing area immediately by

changing altitude or course.

• Report these weather conditions to Air

Traffic Control.

CAUTION

Severe icing comprises environmental

conditions outside of those for which the

airplane is certificated. Flight in freezing

rain, freezing drizzle, or mixed icing

conditions (supercooled liquid water and ice

crystals) may result in extreme ice build-up

on protected surfaces exceeding the

capability of the ice protection system, or

may result in ice forming aft of the protected

surfaces. This ice may not be shed using the

ice protection systems, and may seriously

degrade the performance and controllability

of the airplane.

THE FOLLOWING SHALL BE USED TO

IDENTIFY FREEZING RAIN/FREEZING

DRIZZLE ICING CONDITIONS:

• Unusually extensive ice accreted on the

airframe in areas not normally observed to

collect ice.

• Accumulation of ice on the upper

surface (for low-wing airplanes) or lower

surface (for high-wing airplanes) of the wing

aft of the protected area.

• Accumulation of ice on the propeller

spinner farther back than normally observed.

THE FOLLOWING MAY BE USED TO

IDENTIFY POSSIBLE FREEZING RAIN/

FREEZING DRIZZLE CONDITIONS:

• Visible rain at temperatures below +5

degrees Celsius [outside air temperature

(OAT)].

• Droplets that splash or splatter on impact

at temperatures below +5 degrees Celsius

OAT.
2166 Federal Register / Vol. 61, No. 17 / Thursday, January 25, 1996 / Proposed Rules

PROCEDURES FOR EXITING THE

FREEZING RAIN/FREEZING DRIZZLE

ENVIRONMENT:

These procedures are applicable to all

flight phases from takeoff to landing. Monitor

the outside air temperature. While severe

icing may form at temperatures as cold as

¥18 degrees Celsius, increased vigilance is

warranted at temperatures around freezing

with visible moisture present. If the visual

cues specified in the AFM for identifying

possible freezing rain or freezing drizzle

conditions are observed, accomplish the

following:

• Exit the freezing rain or freezing drizzle

severe icing conditions immediately to avoid

extended exposure to flight conditions

outside of those for which the airplane has

been certificated for operation. Asking for

priority to leave the area is fully justified

under these conditions.

• Avoid abrupt and excessive

maneuvering that may exacerbate control

difficulties.

• Do not engage the autopilot. The

autopilot may mask unusual control system

forces.

• If the autopilot is engaged, hold the

control wheel firmly and disengage the

autopilot.

• If an unusual roll response or

uncommanded control movement is

observed, reduce the angle-of-attack by

increasing airspeed or rolling wings level (if

in a turn), and apply additional power, if

needed.

• Avoid extending flaps during extended

operation in icing conditions. Operation with

flaps extended can result in a reduced wing

angle-of-attack, with ice forming on the

upper surface further aft on the wing than

normal, possibly aft of the protected area.

• Report these weather conditions to Air

Traffic Control.’’

(b) An alternative method of compliance or

adjustment of the compliance time that

provides an acceptable level of safety may be

used if approved by the Manager, Atlanta

Aircraft Certification Office (ACO), FAA,

Small Airplane Directorate. Operators shall

submit their requests through an appropriate

FAA Principal Operations Inspector, who

may add comments and then send it to the

Manager, Atlanta ACO.

Note 3: Information concerning the

existence of approved alternative methods of

compliance with this AD, if any, may be

obtained from the Atlanta ACO.

(c) Special flight permits may be issued in

accordance with sections 21.197 and 21.199

of the Federal Aviation Regulations (14 CFR

21.197 and 21.199) to operate the airplane to

a location where the requirements of this AD

can be accomplished.

Issued in Renton, Washington, on January

19, 1996.

Darrell M. Pederson,

Acting Manager, Transport Airplane

Directorate, Aircraft Certification Service.

[FR Doc. 96–1175 Filed 1–24–96; 8:45 am]

BILLING CODE 4910–13–U

14 CFR Part 39

[Docket No. 96–NM–17–AD]

Airworthiness Directives;

Construcciones Aeronauticas, S.A.

(CASA) Model C–212 and CN–235

Series Airplanes

AGENCY: Federal Aviation

Administration, DOT.

ACTION: Notice of proposed rulemaking

(NPRM).

SUMMARY: This document proposes the

adoption of a new airworthiness

directive (AD) that is applicable to all

CASA Model C–212 and CN–235 series

airplanes. This proposal would require

revising the Airplane Flight Manual

(AFM) to specify procedures that would

prohibit flight in freezing rain or

freezing drizzle conditions (as

determined by certain visual cues), limit

or prohibit the use of various flight

control devices, and provide the flight

crew with recognition cues for, and

procedures for exiting from, severe icing

conditions. This proposal is prompted

by results of a review of the

requirements for certification of the

airplane in icing conditions, new

information on the icing environment,

and icing data provided currently to the

flight crews. The actions specified by

the proposed AD are intended to

minimize the potential hazards

associated with operating the airplane

in freezing rain or freezing drizzle

conditions by providing more clearly

defined procedures and limitations

associated with such conditions.

DATES: Comments must be received by

March 7, 1996.

ADDRESSES: Submit comments in

triplicate to the Federal Aviation

Administration (FAA), Transport

Airplane Directorate, ANM–103,

Attention: Rules Docket No. 96–NM–

17–AD, 1601 Lind Avenue, SW.,

Renton, Washington 98055–4056.

Comments may be inspected at this

location between 9:00 a.m. and 3:00

p.m., Monday through Friday, except

Federal holidays.

This information may be examined at

the FAA, Transport Airplane

Directorate, 1601 Lind Avenue, SW.,

Renton, Washington.

FOR FURTHER INFORMATION CONTACT:

Gregory Dunn, Aerospace Engineer,

Standardization Branch, ANM–113,

FAA, Transport Airplane Directorate,

1601 Lind Avenue, SW., Renton,

Washington 98055–4056; telephone

(206) 227–2799; fax (206) 227–1149.

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to

participate in the making of the

proposed rule by submitting such

written data, views, or arguments as

they may desire. Communications shall

identify the Rules Docket number and

be submitted in triplicate to the address

specified above. All communications

received on or before the closing date

for comments, specified above, will be

considered before taking action on the

proposed rule. The proposals contained

in this notice may be changed in light

of the comments received.

Comments are specifically invited on

the overall regulatory, economic,

environmental, and energy aspects of

the proposed rule. All comments

submitted will be available, both before

and after the closing date for comments,

in the Rules Docket for examination by

interested persons. A report

summarizing each FAA-public contact

concerned with the substance of this

proposal will be filed in the Rules

Docket.

Commenters wishing the FAA to

acknowledge receipt of their comments

submitted in response to this notice

must submit a self-addressed, stamped

postcard on which the following

statement is made: ‘‘Comments to

Docket Number 96–NM–17–AD.’’ The

postcard will be date stamped and

returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this

NPRM by submitting a request to the

FAA, Transport Airplane Directorate,

ANM–103, Attention: Rules Docket No.

96–NM–17–AD, 1601 Lind Avenue,

SW., Renton, Washington 98055–4056.

Discussion

In October 1994, a transport category

airplane was involved in an accident in

which severe icing conditions (believed

to be composed of freezing drizzle size

droplets) were reported in the area.

Although the National Transportation

Safety Board (NTSB) has not yet made

a finding of probable cause of the

accident, loss of control of the airplane

may have occurred because ice

accretion on the upper surface of the

wing aft of the area protected by the ice

protection system caused airflow

separation, which resulted in the

ailerons being forced to a right-wingdown control position. There also is

concern that the autopilot, which was

engaged, may have masked the unusual

control forces generated by the ice

accumulation. These conditions, if not

corrected, could result in a roll upset