Aircraft Performance
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II. AIRCRAFT PERFORMANCE This section is intended to provide a review of CMR 3272’s flight profile based upon DFDR information, identify and discuss previous EMB-120 ice induced roll upsets and the similarities between them, and discuss EMB-120 handling qualities through certification requirements and flight test results. Conclusions: . FAR 23 (Air Taxi Operations) contains more conservative roll control stall requirements than FAR 25 (Transport Category Aircraft Operations). Initial high angle-of-attack, dry-air flight testing conducted on the EMB-120 aircraft identified the potential for uncontrollable roll-offs and high roll rates. i . SLD, quarter-round testing conducted in September of 1996 identified potential autopilot Inadequacies, high roll rates and high control column and control wheel forces. A review of the previous EMB-120 ice induced roll upsets by the FAA in 1996 identified significant handling quality issues for the EMB-120 aircraft. A comparison of the CMR 3272 accident profile and the other EMB-120 ice induced roll upsets show signifcant similarities in aircraft handling and performance. A review of both the CMR 3272 and WestAir 7233 aircraft response to engine torque indicates that both aircraft were experiencing high drag at the time of the upsets. The flightcrew of CMR 3272 was conducting their flight in accordance with ATC clearances and company guidance. As the aircraft reached its target airspeed of 150 knots (as directed by ATC), signitIcant engine torque inputs by the flightcrew did not have any acceleration effect on the aircraft’s airspeed. The autopilot was unable to overcome the left roll rate the aircraft was experiencing during the turn to a heading of 090”. As the autopilot disconnected, the aircraft experienced significant roll and pitch excursions. The angle-of-attack where the upset occurred was below the threshold of the Stall Avoidance System certificated and installed on the accident aircraft. The results of Embraer simulator studies in January 1998 showed that the timing of engine torque inputs was critical in avoiding an upset. The results of Embraer simulator studies in January 1998 showed that the use of the autopilot was a critical factor in causing an upset. NOTE: Several of the documents referenced within this section are Proprietary in nature and are paraphrased. The documents have been requested by the Aircraft Performance Group and remain in the possession of the NTSB Aircraft Performance Group Chairman. 7 A. CMR 3272FDR / FLIGHT PROFILE REVIEW Information from the flight data recorder (FDR) and cockpit voice recorder (CVR) indicates that as the airplane was descending from an altitude of 6,000’ to 4,000’ with the autopilot engaged and wing flaps zero, air traffic control (ATC) instructed the Comair 3272 flightcrew to ‘I.__turn right heading one eight zero, reduce speed lo onefive zero”. The FDR shows the aircraft rolled out on the assigned 180” heading as it was descending through 4,100 feet. ATC then instructed the flight to ‘I... turn left heading zero nine zero-plan a vector across the localizer”. Comair 3212 then began the left turn and leveled off at 4,000 feet with the airspeed stabilizing at 150 KIAS per the ATC clearance. Within one second of the airspeed reaching 150 KIAS (-8 seconds prior to the upset) the FDR shows engine torque steadily increasing to 100% at the time of the upset. It can be assumed that, to achieve the left turn to the 090” heading as instructed by ATC, the flightcrew entered 090” in the heading selector of the autopilot. The autopilot then initiated a left wing down (LWD) input to a maximum target of 25”angle of bank. As the bank (roll) angle reached approximately 20” LWD, the FDR shows the c&trol wheel moving in an opposite direction to command right wing down (RWD) in an attempt to control the LWD roll rate. The left bank angle gradually increased beyond the autopilot target of 25 degrees LWD while the pilots began commanding a torque increase of over 90 percent. The RWD autopilot inputs continued to increase. FDR data also shows the autopilot inputting airplane nose-up pitch trim during the turn, although the pitch attitude of the aircraft remained at approximately 3” nose-up while airspeed gradually reduced to 147 KIAS and altitude remained at 4.000’. As the bank angle reached 45” LWD, the autopilot bank angle limit was exceeded and the autopilot disconnected. The stick shaker activated momentarily while both engine torque parameters recorded readings well over 100 percent. Prior to the autopilot disconnect, the control wheel was deflected about 20” to the right (approximately half travel). After the autopilot disconnected, the control wheel abruptly deflected to at least 20” to the left, and the aircraft rolled abruptly from 45” LWD to 140’ LWD in approximately two seconds. At the same time, pitch attitude rapidly decreasedfrom a 3” nose-up attitude to 50” nose-down attitude. There was only a momentary sound of stick shaker activation at the time of the upset, and there were no aural indications of aircraft buffet prior to the upset. At the time of the upset, the airspeed was approximately 147 knots. After the initial upset, engine torque is immediately reduced to a level consistent with flight idle. During the recovery attempt, the airplane experienced large oscillations in roll attitude and pitch oscillations between 20 degreesnose-down and 80 degrees nose-down until it impacted the ground in a steep nose-down attitude approximately 17 seconds after the initial upset. B. EMB-120 HANDLING OUALITIES 1. Initial Dry-Air/High Angle-of-Attack Flight Testing During the Aircraft Performance Group meeting conducted in Brazil in January of 1998, the group reviewed volumes of documents relating to the handling qualities of the EMB-120 aircraft. The testing addressedin these documents varied from original high angle-of-attack (AOA) dry-air testing 8 to the most recent Supercooled Large Droplet (SLD) testing in 1995/6. From as early as the original high AOA testing, the aircraft appearedto exhibit controllability problems at high AOA’s’. As the aircraft approached approximately 18” AOA, it had a tendency to roll-off rapidly to the left. In some instances, test pilots.described high roll rates and uncontrollable roll-offs. It is necessary to point out that these tests were conducted on a clean aircraft (i.e. no ice contamination). It must also be highlighted that, with ice contamination on the airframe (in this case, specifically the wings), the wing can stall at a significantly lower AOA than the stall AOA for a clean wing. These high roll rates and uncontrollable roll-offs were the reason for the introduction of a stall identification system (stick shaker and stick pusher) on the EMB- 120 aircraft. This stall i identification system is designed to activate at a much lower AOA than those identified during the high AOA, clean aircraft testing. The system is intended to activate well ahead of any roll-off event and provide warning to the flightcrew that the aircraft is approaching an aerodynamic stall. The EMB-120 stick shaker and stick pusher activation AOA’s are 10.5” and 12.2” respectively. Both of these thresholds are based on a dry, uncontaminated wing. There is no allowance or adjustment made for ice contamination of any degree. Embraer conducted quarter round testing on an EMB-120 aircraft in September of 1995 to quantify any aircraft performance and handling qualities problems as a result of an SLD encounter. Embraer arrived at several significant conclusions*. These conclusions bear a strong resemblance to the events surrounding the CMR 3272 upset. Due to the proprietary nature of the document, several significant conclusions are paraphrasedhere: l The EMB-120 autopilot will keep the wings level until the roll servo torque is reached. After that, the torque will remain constant and the bank angle will increase to 45’ where the autopilot will automatically disconnect due to an excessive bank angle attitude. l In the event of asymmetric ice accretions while flying, the airplane will roll fast and high stick forces will be required to recover the wings level attitude. The significance of the above bullet items are their strong resemblance to the facts surrounding the CMR 3272 accident aircraft, considering the fact that CMR 3272 was operating for a short period of time (-30-40 seconds) in a micron range characterized by the weather group as between 30-80 microns. Although the aircraft was in a turn, the autopilot was attempting to correct the left roll rate by inputting right control wheel. When the bank angle reached the autopilot bank angle limit of 45”, & the autopilot disconnected and the aircraft rolled rapidly to the left. We have no sense as to the resulting control forces on the accident aircraft. 1 Excerpts from the original high AOA, dry-air flight test data was requested of Embraer by the NTSB Aircraft Performance Group during the January 1998 meeting in Brazil. This data remains proprietary, but should be in the possession of the Aircraft Performance Group Chairman. 2 Reference Embraer document 120.EV-165, C&trol Demadation Susceptibility Following Operation in Supercooled Large Droplet Icing Environment. This document has been requested of Embraer by the NTSB Aircraft Performance Group Chairman and should be in the possession of the Aircraft Performance Group Chaimun. 9 2. Stall Behavior - Design Considerations Section 6 of Advisory Circular (AC) 25-7, the Flight Test Guide for Transport Category Airplanes, provides guidance material for the testing of an aircraft’s stall characteristics. Paragraph b (3)(ii) of this AC describes one of four acceptable means for the recognition of the stall as “an uncommanded, distinctiveand easily recognizablenose down pitch that cannotbe readily arrested.