- Comair EMB-120 near Monroe, MI
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Comair EMB-120 near Monroe, MI
Photo of EMB-120 in Flight
Photo Copyright Del Laughery - Used with permission
An Embraer EMB-120RT, US registration number N265CA, was manufactured by Embraer in Brazil in December 1991, and delivered to Comair. On February 20, 1992, the airplane received a U.S. Standard Certificate of Airworthiness from the FAA and was put into service as part of Comair's fleet.
On January 9, 1997, Flight 3272, operated by Comair Airlines under the provisions of Title 14 Code of Federal Regulations (CFR) Part 135, departed as a scheduled, domestic passenger flight from the Cincinnati/Northern Kentucky International Airport (CVG) in Covington, Kentucky to Detroit Metropolitan/Wayne County Airport (DTW) in Detroit, Michigan. The flight departed CVG about 1508 local time, with two flight crew members, one flight attendant, and 26 passengers on board.
Instrument meteorological conditions (IMC) prevailed at the time of the accident, and the flight was operating on an instrument flight rules (IFR) flight plan. The IFR flight plan indicated that Flight 3272's final cruise altitude would have been flight level (FL) 190; however, the pilots requested and received clearance to climb to FL 210 to avoid turbulence at the lower altitude.
History of Flight
Following a departure that was delayed due to servicing and de-icing requirements, Comair Flight 3272 was uneventful until arrival in the Detroit terminal area. The flight crew obtained Detroit's automatic terminal information service (ATIS) information that indicated visibility of one mile in light snow and included the remarks, "braking action advisories in effect" and "local ground de-ice procedure in effect."
Detroit approach control requested Flight 3272 to reduce airspeed to 190 knots, descend, and maintain an altitude of 7,000 feet. During the descent, passing through about 8,600 feet, the first officer called for the descent checklist that included an ice protection prompt (to be accomplished before the airplane entered icing conditions). Comair's EMB-120 flight standards manual (FSM) states that "icing conditions exist when the outside air temperature (OAT) is +5 degrees Celsius or below and visible moisture in any form is present (such as clouds, rain, snow, sleet, ice crystals, or fog with visibility of one mile or less). Neither pilot specifically called for the approach checklist. The approach briefing is the first item on the approach checklist. The last two items of the approach checklist; notify flight attendants, and set flaps to the approach setting, were typically accomplished later during the approach when the airplane was closer to its destination airport. Cockpit Voice Recorder (CVR), Flight Data Recorder (FDR) information, and physical evidence indicated that the flaps were in the retracted position when the accident occurred.
Photo of Captain's view of left wing and deicing boot
Following level-off at 7000 feet, the flight crew received instructions to further reduce speed to 170 knots and descend to 6000 feet. The final approach controller informed the flight that they would be receiving vectors for traffic spacing, and then instructed the flight to descend to 4,000 feet. This was followed shortly by instructions to turn to a heading of 180 degrees, and further reduce airspeed to 150 knots. Final instructions to the flight were to turn further left to a heading of 090 degrees. Following this final instruction, the control wheel position (CWP), roll attitude, and magnetic heading indicated that the airplane began a left turn and was in a shallow but steepening left bank at 4,000 feet. The autopilot mode changed from "Altitude Pre Select (Arm)" to "Altitude Hold" mode. As bank angle increased through 23 degrees, the CWP moved to the right, indicating the autopilot's attempt to correct the increasing roll; however, the bank angle continued to increase. At about this same time, engine torque values began to increase from flight idle, and the FDR recorded split engine torque values, with higher torque values recorded for the right engine than the left. This split continued until the autopilot disengaged. The airplane's left roll attitude increased beyond 45 degrees, airspeed decayed to 146 knots, as the stick shaker (stall warning) activated, and the autopilot disconnected. Following autopilot disconnect, roll attitude rapidly increased to 140 degrees and the airplane pitched down to -17 degrees. Airplane pitch continued to decrease until reaching -50 degrees, and left roll increased to more than 140 degrees. (Click here to view NTSB Comair Re-creation Animation)
The airplane struck the ground in a steep nose-down attitude in a level field in a rural area about 19 nm southwest of DTW. Fragmented airplane wreckage was found in and around three impact craters, with airplane debris located up to 340 feet from the largest impact crater. The complete NTSB report on this accident is available here: (NTSB Report)
Photo at crash site - NTSB Docket Photo
Ice Protection System
The EMB-120 was certified for operation in icing conditions as specified in Appendix C to 14 CFR part 25, specifically, 14 CFR 25.1419. During certification testing in both natural icing conditions, and with artificial ice shapes installed on the airplane, the airplane was demonstrated to be in compliance with the applicable regulations relative to operations in icing conditions.
Diagram of EMB-120 Ice protected areas
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Photo of an example of inflated deicing boots
Photo of EMB-120 cockpit
Photo copyright Tim Samples - used with permission
The EMB-120 ice protection system includes electrical anti-icing for the windshields, pitot/static tubes, static ports, and the angle of attack, sideslip, and total air temperature sensors. The propeller blades are electrically deiced, and the leading edges of the wings, horizontal and vertical stabilizers, the engine inlets, and the engine bypass ducts are protected by pneumatic deicing boots.
According to their manufacturer, the pneumatic boot system protecting the wings and tail surfaces operates on engine bleed air and is intended to remove ice that forms on the protected surfaces by inflating the rubber boots. Engine bleed air operating through flow control valves causes the boots to cyclically inflate and deflate. When the boots inflate, accumulated ice is cracked and removed by the airstream. The wing boots are divided into three spanwise segments, with tubes in each segment oriented span-wise along the leading edges.
As configured at the time of the accident, the system was controlled by a timer select switch and a cycle switch, both located in the flight deck. The cycle switch was a two-position switch selectable for heavy or light icing conditions.
Photo of Ice/rain Protection System
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When operating, if a "heavy" cycle is selected, the boots are cyclically inflated for six seconds, and deflated for 54 seconds. If "light" is selected, the deflation time is increased to 174 seconds between inflation cycles. Inflation is symmetric, beginning with the most outboard wing segments, moving to the middle and inboard boots, then sequencing through the other protected areas of the airplane before beginning the sequence again for as long as the system is turned on. A system description excerpted from the Embraer Maintenance Manual, including a system schematic, is available at the following link: (Embraer AMM)
The stall warning system was not designed to communicate with the anti-ice system, and there was no stall warning bias (earlier stall warning) following an icing encounter or during deice system operations, nor was one required by regulation. Some airplanes incorporate a bias in the stall warning system such that if the ice protection system is activated any time during the flight, the bias is invoked and stall warning is shifted to a higher speed. This results in an earlier stall warning based on and proportional to the increase in stall speed associated with ice accumulation in order to compensate for the stall speed increase.
Previous Events and Manufacturer's Follow-up Actions
Photo of EMB-120 Wing with flaps extended
Photo copyright Cameron Bowerman - used with permission
Prior to the accident there were six icing related incidents between June 1989 and April 1995. All six events involved uncommanded speed reductions, some to stall warning, and roll upsets. The associated investigations concluded that all six events were the result of ice accumulations on the wing. In some of the events, the flight crew had observed ice accretions on the wings, but did not believe that accumulation was sufficient to activate the ice protection system. In other events the flight crew failed to recognize that they were in icing conditions and, therefore, did not activate the ice protection systems. The six events are detailed in the Precursors section of this summary.
Following the October 31, 1994 crash of an ATR-72 in icing conditions, near Roselawn, Indiana, the FAA conducted a review of in-service incidents related to upsets in icing conditions. The review encompassed about 50 events, including the six previously identified EMB-120 events. Following this review on November 7, 1995, the FAA held a meeting attended by the Brazilian Centro Técnico Aerospacial (CTA), the Airline Pilots Association (ALPA), Embraer, the NTSB, and EMB-120 operators, including Comair. At this meeting the results of controllability testing accomplished on the EMB-120 in supercooled large droplet (SLD) icing conditions were discussed. The FAA also made a presentation summarizing the six previous in-service events and discussed with the attending parties possible remedial actions that could be taken to mitigate operational issues when flying in icing conditions, and the characteristics of the airplane when ice is accumulated on the airframe without the activation of the de-ice system.
Photo of crash site (left)
Photo of Wreckage of Comair Flight 3272 - NTSB docket photo (right)
Following the FAA meeting, Embraer, on November 15, 1995, held a Flight Crew Awareness seminar for all EMB-120 operators. The six previous in-service events were discussed in detail. The stated purpose of the seminar was to discuss operation of the EMB-120 in icing conditions, to generate recommendations for a flight crew awareness program, and to discuss the operational documentation (flight manual and flight crew operations manual) regarding operations in icing conditions.
The seminar resulted in a number of recommendations. Among those were recommendations to modify the FAA-approved Airplane Flight Manual (AFM) to emphasize recommend operational speed increases in icing conditions, distribution of an Operational Bulletin detailing specific modes of operation in icing conditions, including autopilot usage, operational speeds, proper use of ice protection systems, stall warning, and refined definitions of, and means to identify, icing conditions and formation of ice.
In December 1995, Comair issued an interoffice memo to all EMB-120 flight crew members discussing the major factors that had resulted in the controllability problems experienced by other operators in the six in-service events. The memo cited the primary causes as lack of airspeed control, use of the autopilot in a vertical mode that resulted in airspeed deviations, and failure to recognize ice accumulations and use the ice protection system. The memo further issued instructions that in icing conditions, autopilot use should be restricted to a single (Indicated Airspeed) mode (IAS mode), which would initiate a descent if proper airspeed could not be maintained, and would maintain proper stall margins. Operational speeds were increased in icing conditions to a minimum airspeed of 160 knots, and 170 knots when airframe ice was present. It also recommended close monitoring of airspeed, especially in turns.
Photo of Left engine and propeller of Comair Flight 3272- NTSB docket photo
In April 1996, Embraer published Operations Bulletin 120-002/96, which provided results of SLD testing and a general discussion of EMB-120 operations in icing conditions. The bulletin was distributed to Embraer operators and additionally to various FAA certification and aircraft evaluation groups, and many FAA flight standards offices. Guidance in the bulletin regarding ice accretions formed in non-SLD conditions without activation of the de-ice system, described a drag increase associated with ice accretion that required an increase in angle of attack to maintain altitude. The increase in angle of attack in that condition could potentially lead to a stall and uncommanded roll excursions. Stall would occur at a "speed somewhat above" a normal stall speed.
Finally, on April 23, 1996, Embraer issued AFM revision 43, which revised icing procedures to activate the de-icing system at the first signs of ice formation. Based on a concern related to ice bridging that might be caused if the de-icing system was activated too early in icing conditions, Comair decided not to incorporate the contents of AFM revision 43 into their operations manual or training procedures.
Icing Related Procedures
Photo of Right engine and propeller of Comair Flight 3272 - NTSB docket photo
Prior to these in-service incidents, the FAA approved Airplane Flight Manual (prior to revision 43), produced by Embraer, and Comair's Flight Standards Manual (FSM - the Comair equivalent of the AFM) allowed by 14 CFR 121.133 and .141, containing icing procedures that advised delaying actuation of deicing boots until a specific ice thickness (¼ to ½ inch) had accumulated on the wing. The Comair FSM recommended an ice accumulation of approximately ½ inch prior to activation of the deicing boots. It also added that when it was difficult to see the wing leading edge or it was night, that a speed decay of 10 to 15 knots was a "good indicator of ice accumulation." (Comair FSM) Embraer's Revision 43 to the EMB-120 AFM changed this guidance, and recommended activation of wing and tail deicing boots "at the first sign of ice formation." As stated previously, Comair elected not to incorporate the AFM recommended procedural changes into their operational procedures.
Photo of Left wing leading edge with deicing boot
NTSB docket photo
The specified delay in deicing system activation was based on a long-standing, industry-wide, historical assumption related to the potential for ice bridging on early deice boot designs, and a means by which it could be avoided. The occurrence of ice bridging, where the deicing boots deform but don't break accumulated ice would render the system ineffective and prevent ice removal. It was believed that waiting for an ice accumulation of a sufficient thickness would always result in ice removal and avoid the possibility of bridging. Testing conducted after the accident revealed that there was no evidence of ice bridging on the EMB-120. Further, the testing led to conclusions that the duration of the system cycle times was such that, if bridging was a problem, it might occur during an intercycle interval (from one to three minutes, depending on system setting). There was no in-service data indicating that ice bridging was ever encountered, leading to the eventual conclusion that ice bridging was not a problem for the EMB-120. As a result, in April 1996, Embraer issued AFM Revision 43 which recommended boot activation at the first sign of ice on the aircraft, and after the accident, led to industry wide recommendations that deicing systems be activated immediately upon entering icing conditions.
An animation describing ice bridging is available at the following link: Ice bridging animation
Post Accident Testing
Following the accident, a number of studies were conducted by Embraer, to measure performance and handling effects of both the thrust asymmetry resulting during the accident sequence, and the drag levels associated with various ice accumulations. Further, in separate studies, the NTSB contracted with NASA-Lewis, and the FAA contracted with the University of Illinois, Urbana-Champaign (UIUC) to investigate various icing related characteristics. The results of the noted tests are included in the NTSB accident report for this accident.
Photo of icing tunnel ice accumulation and illustration of ice shape formation
Photo of EMB-120 During icing tanker testing
The NTSB noted that the EMB-120 had exhibited a history of icing-related incidents prior to the Comair accident. Following the noted incidents, Embraer had issued, and the FAA had approved, an AFM revision that changed operational procedures in icing conditions. The AFM revision was distributed to all EMB-120 operators and to the FAA. Comair subsequently elected not to incorporate the revised procedures into their company operations manual, in part due to their earlier issuance of a company memo on December 8, 1995, advising pilots not to operate at airspeeds below 160 knots in icing conditions, and to observe a minimum airspeed of 170 knots when climbing on autopilot or holding in icing conditions. The memo had been issued in response to the previously discussed November 1995 meetings. Other reasons cited by Comair for not adopting the AFM revision were conflicts with Comair's trained procedures and practices, specifically with practices regarding ice bridging. Comair concluded that the revised procedures were potentially unsafe.
Photo of intercycle ice
This December 1995 memo was distributed to all Comair pilots, but was not subsequently incorporated into a flight standards bulletin (FSB) or a revision to Comair flight standards manual until a flight standards bulletin was issued in October 1996. The NTSB noted that the FSB did not repeat the earlier memo's blanket 160 knot minimum airspeed in icing conditions, and used different language and airspeeds than the earlier memo. These factors, coupled with Comair's methods of distribution and failure to incorporate the information as a permanent revision to the FSM, may have resulted in confusion among flight crews regarding appropriate airspeeds in icing.
Ultimately, the NTSB criticized the FAA for not having required adoption of AFM revision 43 into Comair's AFM or incorporation into company procedures. They cited FAA's lack of internal coordination regarding the safety implications of AFM revision 43 as a crucial factor in the chain of events leading to the accident, and recommended development of an organizational structure and communications system to ensure effective continuing airworthiness oversight.
Photo of EMB-120 deicing boots on/off
Upper photo, boots inflated. Lower photo, boots deflated.
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The NTSB concluded that at the time of the accident, FAA personnel, specifically the Comair POI, "lacked information critical to the continued safe operation of the EMB-120 fleet and would have been unable to evaluate the need to incorporate AFM revision 43 or any alternatives proposed by air carriers." The NTSB stated the belief that flight standards personnel at all levels needed to be informed about all manufacturer operations bulletins and AFM revisions, including background and justification for revisions. As a result of the NTSB finding, FAA internal procedures have been revised, and AFM revisions are now coordinated between all pertinent FAA offices.
In November 2006, the DOT/FAA Office of Aviation Research and Development released a report detailing their investigation into the performance of pneumatic deicing boot systems, surface ice detectors, and quantified information regarding intercycle ice.