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NTSB Focus Area –Inflight Icing
Dr. Earl F. WeenerBoard Member
International Winter Operations Conference
October 5-6, 2011Montreal, Quebec, Canada
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The NTSB is an independent U.S. Federal agency charged with determining the probable cause(s) of transportation accidents, making
recommendations to prevent their recurrence, conducting special studies and investigations,
and coordinating resources to assist victims and their families after an accident.
NTSB Mission
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NTSB view of Airframe Icing
• Majority of icing conditions encountered are not a problem for certificated aircraft -we deal with the uncommon occurrences
• NTSB recommendations on aircraft icing date back to 1970
• Airframe icing has been on the NTSB’s Most Wanted List of safety improvements from 1997 to 2011
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Accidents: October 31, 1994Roselawn, IN, American Eagle ATR-72
• 68 fatalities
• Involved Supercooled Large Droplets
• Created ridge of ice aft of deice boots
• Caused ailerons to deflect, resulting in loss of control
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• Accretions can cause stall or control anomalies at higher airspeed than normally expected
• Ice can accrete aft of ice protection system
• Sometimes difficult to see or detect
• Pilots may not detect an unsafe condition
Why is SLD important to consider?
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Effect of SLD on ice accretion
Appendix C
SLD
Wing leading edge cross section
Direction of flight
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Lewis Research CenterIcing Branch
Resultant Ice Shapes
Front Edge of Wing
Rough Ice
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Lewis Research CenterIcing Branch
Resultant Ice Shapes
Front Edge of Wing
Rough Ice
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Part 25 Appendix CContinuous Maximum Icing
15µ 40µ
MVD
97-99%
SLD
LWC
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Lift
Coe
ffici
ent -
CL
Alpha (deg)
Stic
k Sh
aker
Stic
k P
ushe
r
Buf
fet
Stal
l Bre
ak
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Lift
Coe
ffici
ent -
CL
Alpha (deg)
Stic
k Sh
aker
Stic
k P
ushe
r
Buf
fet?
Stal
l Bre
ak Buf
fet
Stal
l Bre
ak
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NTSB Recommendations: Roselawn
Summary
• Icing conditions beyond current Part 23 & 25 certification requirements exist
• Continued flight in some icing conditions not possible
• Part 23 & 25 Appendix C certification requirements inadequate for SLD
• Training for Part 121 and 135 needed
• Research into SLD and development of detection and protection needed
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Accidents: January 9, 1997Monroe, MI – Comair Embraer EMB - 120
• 39 fatalities• Operated in icing conditions near lower end of operating airspeed
envelope (flaps retracted)• Issues: use of ice protection, airspeed and configuration information,
stall warning/protection system capabilities, use of autopilot, aircraft icing certification
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Accident Sequence• Approach to DTW with autopilot engaged
• Ice accreted for 4 ½ to 5 minutes (some SLD)
• De-ice boots not operated during approach
• Airplane rolled to 45 deg left roll despite autopilot right roll input
• Rapid roll left when autopilot disconnected due to stick shaker
• Presence of an estimated ¼ to 1/2 of an inch of ice created a rolling moment the A/P could not counteract.
• Airplane did not recover in the 3000 feet agl available
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ENTERING THE LEFT TURN
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START OF A/P WHEEL TO RIGHT
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AUTOPILOT DISCONNECT
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2 SECONDS AFTER A/P DISCONNECT
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NTSB Recommendations: Monroe
Summary
• Both thin and imperceptible, as well as rough ice can be hazardous
• Boots need to be activated when entering icing conditions
• Minimum maneuvering speeds for all turbine airplanes, all flight conditions
– Effects of various types, amounts & locations of ice accumulations
– Address SLD, rough ice & tailplane icing• Training for minimum maneuvering operations
• Review/revise AFM & OPS Manuals
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NTSB Recommendations: Monroe (Cont’d)
• Further development of effective detection & protection systems necessary
• Cockpit warning systems for stall in icing conditions– Aural warnings– Stick shaker/pusher
• Hand fly in icing conditions with icing protection systems operating
• Review/revise certification testing requirements– Incorporate realistic certification ice shapes
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• Pilot reported using de-icing boots (3 minute activation cycle) ; SAT = -4 Deg C
• Autopilot engaged
• Indications from FDR, CVR Ice Detector was active
• Loss of control in icing conditions
• 8000 ft altitude loss, structural damage to horizontal tail and elevator
• Intercycle Ice case
Incidents: March 19, 2001West Palm Beach, FL – Comair Flight 5054 EMB-120
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18:22:00 18:22:30 18:23:00 18:23:30 18:24:00 18:24:30 18:25:00 18:25:30 18:26:00
0
4
8
12
16
20
18:22:00 18:22:30 18:23:00 18:23:30 18:24:00 18:24:30 18:25:00 18:25:30 18:26:00
0
2
4
6
8
18:22:00 18:22:30 18:23:00 18:23:30 18:24:00 18:24:30 18:25:00 18:25:30 18:26:00
120
160
200
240
Airs
peed
Autopilot Disconnect
FDR Data - Comair 5054 A
OA
NTSB
MasterCaution
Pitc
h Tr
im
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23:23:00 23:24:00 23:25:00 23:26:00 23:27:00 23:28:008000
10000
12000
14000
16000
18000
(FDR SFRN 162952)(FDR SFRN 162472)
Comair 5054 - March 19, 2001FDR/Radar Altitude Comparison During Event
Altit
ude
UTC Time
USAF RADES Radar Altitude FDR Altitude
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2 3 4 5 6 7 8 90.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90NTSB
Calculated Lift Coefficient "Clean" Cl-Alpha relationship
from FDR data earlier in flight
Comair 5054Lift Coefficients Determined from FDR Load Factors
18:15:00 through 18:25:00C
L
Airplane Angle of Attack (Deg)
Mach Number range: 0.22 - 0.28Reynolds number range: 6.4 million - 8.2 million
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View from above Horizontal Stabilizer
Elevator
Stabilizer
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Right Side Stabilizer, Inboard of Elevator
Elevator
Stabilizer
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NTSB Recommendations: West Palm Beach
• No new recommendations on icing issued – nothing new to recommend
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Accidents: February 16, 2005Pueblo, CO – Cessna 560
• 8 fatalities
• Issues: airspeed and deice boot activation
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Altitude Time History
Clear ice mentioned
Stall
De-ice boots operated
Gray ice mentioned
PUBSLD
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Accident Sequence • Airplane slowed below Vapproach
• De-ice boots not operated in second icing layer
• Presence of an estimated 1/6 of an inch or less of ice accreted in SLD conditions caused the airplane to stall prior to stick shaker
• Airplane entered a rapid left roll prior to stall warning
• Airplane did not recover in the 1,500 feet agl available
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Accident Airplane Relevant Speeds
76
81
86
114
Clean Stall Speed
“Ice” Stall Speed
“Ice” Stick Shaker
Approach speed in icing
Accident Stall Speed90
Knots
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NTSB Recommendations: Pueblo
Summary
• In icing conditions – increase speed and operate boots
• Activate boots when entering icing conditions
• Incorporate automatic boot cycling capability
• Review all boot equipped airplanes against revised certification standards when complete
• Specific modifications of Cessna 560 stall warning system regarding both thin & rough ice
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Incidents: January 2, 2006San Luis Obispo, CA - American Eagle Saab 340B
• Slowed on autopilot• Lost 5000 feet altitude• Nearly inverted
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Lift
Coe
ffici
ent -
CL
Alpha (deg)
American Eagle 3008 Clean lift curve
Saab 340Lift Coefficient Extraction
Stall Warning
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NTSB Recommendations: San Luis Obispo• NTSB issued 4 recommendations
– Minimum airspeed– Modify stall protection – Ice detection– Disengage autopilot in icing conditions,
except in periods of high workload
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Accidents: January 27, 2009Lubbock, TX – ATR42-320
• Unstable approach
• Failure to go round
• Airspeed
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NTSB Recommendations: Lubbock• Simulator fidelity for ice accretion should be
consistent with icing accidents and incidents
• Flight crew simulation training for icing should include
– Recognizing changes in flight characteristics as icing develops
– Monitoring and maintaining appropriate icing speeds
– Stalls and approach to stalls with and without ice protection systems
• Performance monitoring system
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Accidents and Incidents Demonstrate:• Icing continues to be a threat to aviation safety
• Airplanes are operating in SLD environments for which they are not certified, particularly in lower layers of the atmosphere
• Rough ice shapes and intercycle ice shapes can cause large aerodynamic penalties, larger than some ice shapes currently used in certification
• Updating simulator models and incorporating previous icing incident/accident data to ensure accuracy can help to mitigate the icing threat
• Consideration by airplane manufacturers of an aircraft performance monitoring system for better flight crew awareness of severe icing encounters (like ATR's)
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