Reduce Dangers to Aircraft Flying in Icing Conditions

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Reduce Dangers to Aircraft Flying in Icing Conditions

Objectives

  • Use current research on freezing rain and large water droplets to revise the way aircraft are designed and approved for flight in icing conditions.
  • Apply revised icing requirements to currently certificated aircraft.
  • Require that airplanes with pneumatic deice boots activate the boots as soon as the airplane enters icing conditions.

Importance
The 1994 in-flight icing encounter and subsequent loss of control and crash of a commuter airliner in Roselawn, Indiana, which took 68 lives, prompted the Safety Board to examine the issue of airframe structural icing and conclude that the icing certification process has been inadequate because the process has not required manufacturers to demonstrate the airplane’s flight handling and stall characteristics under a realistic range of adverse ice accretion/flight-handling conditions.  The Federal Aviation Administration (FAA) has not adopted a systematic and proactive approach to the certification and operational issues of turbine-engine-driven, transport-category airplane icing.

The consequences of operating an airplane in icing conditions without first having thoroughly demonstrated adequate handling/controllability characteristics in those conditions are sufficiently severe that they warrant a thorough certification test program, including the application of revised standards to airplanes currently certificated for flight in icing conditions.

A widespread and firmly held, but incorrect, belief in the aviation community is that the activation of deice boots should be delayed until the buildup of 1/4 to 1/2 inch of ice to prevent “ice bridging.”  As a result, in many cases, flight crews have not activated the deice boots until after the buildup of dangerous levels of icing on the aircraft.

Summary of Action

Revise Icing Certification Criteria and Testing
As a result of the Roselawn, Indiana, accident, the Safety Board called on the FAA to revise (1) the icing criteria and icing testing requirements necessary for an airplane design to be approved within the United States and (2) the operational requirements that specify under what icing conditions it is permissible to operate an aircraft.  More than 10 years ago, this work was referred to an Aviation Rulemaking Advisory Committee (ARAC), which has recommended to the FAA changes to the design requirements for new airplanes to evaluate performance and handling characteristics in icing conditions.  In March 2002, 6 years after it started this work, the ARAC approved a concept to revise the icing criteria in the design requirements for new airplanes.  In March 2006, the ARAC completed its final report on recommended rulemaking and advisory material related to supercooled large drop (SLD) conditions and ice crystal/mixed-phase conditions.  The report included recommendations for defining an SLD environment and ice crystal/mixed-phase conditions.  Included in the report are recommendations addressing 14 Code of Federal Regulations (CFR) Part 25 aircraft performance and handling qualities, engine installation effects, ice protection system requirements, and engine requirements.  Currently, the FAA has completed a preliminary economic analysis of the ARAC proposal and is evaluating the report.  The ARAC also completed much of the work required for a 14 CFR Part 23 SLD rule.  The FAA has initiated a study to compile data for the economic analysis

Currently, there are four rulemaking activities concerning icing:

  • A revision to Part 121, applicable to airplanes with takeoff weights less than 60,000pounds, which addresses when to activate the ice protection system and when the flight crew should exit icing conditions.  The ARAC recommended that the FAA issue a Part 121 rule to require less subjective means of determining when the flightcrew should exit icing conditions.  The FAA completed a preliminary economic analysis of the ARAC proposal and is evaluating the results.
  • A revision to Part 25that addresses when to activate the ice protection system.
  • In March 2006, the ARAC developed and forwarded to the FAA recommended Part 25 rules that include requirements to demonstrate that an airplane can safely operate in certain SLD conditions for an unrestricted time or can detect SLD and enable the flight crew to exit icing conditions.  The ARAC will also make recommendations concerning mixed-phase icing rulemaking.  The FAA performed a rough economic analysis of the ARAC proposal and is evaluating the results of this analysis.
  • The FAA plans to develop similar Part 23rules after completion of the Part 25rulemaking.  The ARAC completed much of the work required to revise Part 23 in a similar manner, and the FAA has initiated a study to compile data for an economic analysis relevant to Part 23. 

During 2007, the FAA took two significant actions to improve flight safety in icing conditions.  On October 9, 2007, the FAA published the final rule concerning revisions to Part 25 for evaluating airplane performance and handling characteristics in icing conditions.    In addition, on April 26, 2007, the FAA published a notice of proposed rulemaking (NPRM) concerning timely activation of the airframe ice protection system (IPS) that would require airplane manufacturers to provide a means to detect ice and ensure that the IPS is activated.  The comment period for this NPRM closed in July 2007, but the FAA has not yet issued the final rule. 

The Safety Board learned in 2007 of FAA activities in response to icing recommendations issued as a result of the February 16, 2005, crash of a Cessna Citation 560 during approach to landing in icing conditions at Pueblo, Colorado.  This accident occurred in SLD conditions, and FAA and Cessna flight testing in response to the investigation used procedures and tests suggested by the ARAC to analyze airplane handling characteristics in SLD conditions.  This suggests that the FAA may be near developing and issuing regulations concerning SLD.  However, the FAA has not provided any projected dates for development and issuance of an NPRM and final rule. 

The pace of the FAA’s activities in response to all of these recommendations remains unacceptably slow, despite some encouraging action during 2007.

 

Apply Revised Icing Requirements to Currently Certificated Aircraft

As a result of the Safety Board’s investigation of the in-flight encounter with icing and subsequent uncontrolled collision with terrain of Comair flight 3272, an Embraer 120RT, near Monroe, Michigan, on January 9, 1997, in which all 29 persons onboard the airplane were killed, the Safety Board asked the FAA to review the icing certification of all turbopropeller-driven airplanes currently certificated for operation in icing conditions and to perform additional testing.  On August 16, 2006, the FAA issued Advisory Circular (AC) 20-73A, “Aircraft Ice Protection” which includes certification guidance relative to the effects and criticality of deicing boot intercycle and residual ice accumulations and ice accumulations on unprotected surfaces aft of protected surfaces. 

The icing certification regulations and advisory material developed by the FAA are sufficiently developed to determine whether additional action is required for any airplanes currently certificated and in service.  The FAA has stated that no unsafe conditions exist that warrant actions beyond those that have already been completed or are in the process of being completed.  The Board is concerned that the FAA has reached this conclusion based on a lack of accidents or serious incidents.  During the 1990s, a number of accidents occurred involving airplanes that had passed the certification standards and for which the FAA believed there was no unsafe condition requiring action.  Before another accident or serious incident occurs, the FAA should evaluate all existing turbo propeller–driven airplanes in service using the new information available, such as critical ice shapes and stall warning margins in icing conditions.

Activate Deice Boots as soon as the Airplane Enters Icing Conditions
A widely held belief in the aviation community, among both operators and manufacturers, is that the deice boots should not be activated until the ice buildup is estimated to be between 1/4 and 1/2 inch thick and that early activation of the boots may result in ice bridging on the wing.  However, in AC 25.1419-1A, “Certification of Transport Category Airplanes for Flight in Icing Conditions,” the FAA states that, although ice may not be completely shed by one cycle of the boots, the residual ice will usually be removed by subsequent cycles and does not act as a foundation for a bridge of ice to form.  Further, information gathered at a 1997 Airplane Deice Boot Bridging Workshop, icing tunnel tests, and flight tests revealed that ice bridging did not occur on modern airplanes equipped with deice boots that quickly inflate and deflate.  The icing tunnel tests also revealed that thin (1/4 inch or less), rough ice accumulations on the wing leading edge deice boot surfaces could be as aerodynamically detrimental to an airplane’s performance as larger ice accumulations.

A search of the Safety Board accident database revealed no accidents related to ice bridging.  Conversely, the Board has investigated many icing accidents in which the airplane stalled prematurely and the stall warning system did not activate before the stall because of ice accumulation on the wing leading edges.  Accident investigations, Safety Board accident data, and existing icing information clearly show that delaying the activation of the deice boots can create an unsafe condition.  The Board concludes that ice bridging does not occur on modern airplanes; therefore, it is not a reason for pilots to delay activation of the deice boots.

Action Remaining
Complete efforts to revise icing certification criteria, testing requirements, and restrictions on operations in icing conditions.  Evaluate all aircraft certified for flight in icing conditions using the new criteria and standards.  Require manufacturers and operators of deice boot-equipped airplanes to revise the guidance to emphasize that leading edge deice boots should be activated as soon as the airplane enters icing conditions.

 

Safety Recommendations
A-96-54 (FAA)
Issued August 15, 1996
Added to the Most Wanted List: 1997
Status:  Open—Unacceptable Response
Revise the icing criteria published in 14 [Code of Federal Regulations] CFR  Parts 23 and 25, in light of both recent research into aircraft ice accretion under varying conditions of liquid water content, drop size distribution, and temperature, and recent developments in both the design and use of aircraft.  Also, expand the Appendix C icing certification envelope to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary.  (Source: In-flight Icing Encounter and Loss of Control Simmons Airlines, d.b.a. American Eagle Flight 4184 Avions de Transport Regional (ATR) Model 72-212, N401AM, Roselawn, Indiana, October 31, 1994 [NTSB/AAR-96-01]).

A-96-56 (FAA)
Issued August 15, 1996
Added to the Most Wanted List: 1997
Status:  Open—Unacceptable Response
Revise the icing certification testing regulation to ensure that airplanes are properly tested for all conditions in which they are authorized to operate, or are otherwise shown to be capable of safe flight into such conditions.  If safe operations cannot be demonstrated by the manufacturer, operational limitations should be imposed to prohibit flight in such conditions and flight crews should be provided with the means to positively determine when they are in icing conditions that exceed the limits for aircraft certification.  (Source: In-flight Icing Encounter and Loss of Control Simmons Airlines, d.b.a. American Eagle Flight 4184 Avions de Transport Regional (ATR) Model 72-212, N401AM, Roselawn, Indiana October 31, 1994.  [NTSB/AAR-96-01])

A-07-14 (FAA)
Issued February 27, 2007
Added to the Most Wanted List:  2008

Status:  Open—Acceptable Response
Require manufacturers and operators of pneumatic deice boot-equipped airplanes to revise the guidance contained in their manuals and training programs to emphasize that leading edge deice boots should be activated as soon as the airplane enters icing conditions. (Source: Crash During Approach to Landing, Circuit City Stores, Inc., Cessna Citation 560, N500AT, Pueblo, Colorado, February 16, 2005.  [NTSB/AAR-07-02])

A-07-16 (FAA)
Issued February 27, 2007 (Superseded A-98-100)
Added to the Most Wanted List: 2007

Status:  Open—Unacceptable Response
When the revised icing certification standards and criteria are complete, review the icing certification of pneumatic deice boot-equipped airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. (Source: Crash During Approach to Landing, Circuit City Stores, Inc., Cessna Citation 560, N500AT, Pueblo, Colorado, February 16, 2005. [NTSB/AAR-07-02])

 


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