In June 2007, a Cessna Citation 550 impacted Lake Michigan shortly after take-off. The two pilots and four passengers were killed, and the airplane was destroyed. At the time of the accident flight, marginal visual meteorological conditions prevailed at the surface, and instrument meteorological conditions prevailed aloft.
The pilots taxied to the departure runway. According to the cockpit voice recorder (CVR) transcript, they discussed some tasks that were on preflight checklists but they did not perform a formal challenge-response checklist procedure as they were leaving.
Soon after takeoff, the CVR recorded the captain stating, “Lights off, yaw damper on.” The first officer then radioed the controllers to advise them that the pilots were starting the airplane’s turn from the runway heading to the assigned heading of 050°. The captain asked, “Why am I fighting the controls here?” The captain again stated, “I’m fighting the controls…” and then he confirmed with the first officer that the landing gear was up and instructed him to retract the flaps.
A few seconds later, the captain made his third mention of an unspecified control problem, stating, “what the [expletive]’s going on? I’m fighting the controls.” The first officer then asked, “How’s your trim set? Is that the way you want it?” The captain began to describe the control problem more specifically, stating, “I’m fighting the controls. It wants to turn hard left,” and the first officer again asked, “How’s your trim down here?” The captain’s response, which began, “trim has nothing…” was interrupted when the first officer erroneously responded to an ATC transmission intended for another airplane. After this, the captain stated, “… something is wrong with the trim…the rudder trim…” The captain questioned the altitude clearance, the first officer stated they were cleared to 3,000 feet msl, and the captain stated, “all right, something is wrong with our rudders. And I don’t know what.” The first officer asked the captain what he wanted to do; then he asked, “How’s that; any better?” The CVR recorded the captain stating, “huh, no, we got a trim problem…[sound of grunt]…tell ‘em we got to come back and land.” Soon the CVR recorded the captain saying, “She’s rolling on me. Help me, help me,” and the first officer responded, “I am.” The captain asked the first officer to pull the autopilot circuit breakers, and the first officer responded, “Where is it?” The captain then said, “tell ‘em we got a control problem,” and the first officer advised the controllers that they had “a control problem, we’ve got to come back in.” The first officer used an incorrect airplane identification number on this transmission, and the captain promptly corrected him. The first officer transmitted, “Milwaukee, we got…a runaway trim, we got an emergency.”
According to the CVR transcript, the first officer then asked, “what circuit breakers?” The CVR recorded the captain telling the first officer, “answer her…” in response to an ATC query, and “keep turning.” The captain then advised the MKE departure controller, “We’re declaring an emergency, yes.”
The first officer repeated that they were “coming back to Milwaukee,” and the captain added, “landing any runway at…Milwaukee. Guide us in please.” The captain transmitted to MKE, “I don’t know what’s wrong,” and then he stated to the first officer, “I don’t know what’s wrong…I see the airport.” The captain then stated, “You hold it, I’m gonna try to pull circuit breakers…” He then stated, “…we’re not…holding it.” The first officer stated, “I’m pulling,” The CVR recorded the captain stating, “awww [expletive]…” The CVR recording ended.
The fragmented airplane wreckage was recovered from Lake Michigan during multiple dives. Physical evidence indicated that the airplane impacted the lake at a speed of about 243 knots, in a steep (about 42°) nose-down, left-wing-low attitude. During post-accident interviews, several pilots who had flown with the captain indicated that although he was a capable pilot, the captain lacked in-depth airplane systems knowledge and did not always adhere to company procedures or comply with regulations. Several former company pilots commented that the captain seemed to focus on business issues rather than on flight operations and safety. The NTSB conducted post-accident interviews with several pilots who had flown with the first officer. One of these pilots characterized the first officer as “a nice guy who had no idea how the airplane operated” and added that he would act without thinking. These interviews revealed that the first officer would get overloaded easily and had difficulty flying a stabilized approach without coaching.
According to post-accident calculations, the airplane’s takeoff weight and the calculated center of gravity for the accident flight were within the required limits.
Flight Control Trim Systems
The accident airplane had separate trim control systems for elevator, aileron, and rudder. Each system included a manual hand-wheel in the cockpit with control cables attached to trim tabs at the related flight control surfaces.
Pitch Trim System Information
The pitch trim system is equipped with an electric servomotor and offers pilots the option of adjusting the elevator trim setting manually or electrically. Either pilot could input electrical pitch trim commands through a toggle switch on his control yoke. After engagement of the autopilot, the autopilot computer could also make pitch trim inputs through a relay to relieve the load on the electric autopilot servomotor. Once engaged, the autopilot would disconnect if either pilot used their electric pitch trim switch.
The Citation 550 airplane flight manual describes an in-depth taxi checklist of the elevator trim system. The CVR recorded no evidence that the pilots performed these system checks during the accident airplane’s ground operations.
The “Abnormal Procedures” section of the flight manual included the following instructions for a jammed elevator trim condition during a takeoff or go-around maneuver: Reduce power as necessary to maintain 120 knots indicated airspeed or less. Do not change flap position. Do not retract landing gear. Land as soon as practical.
The accident airplane was equipped with a Honeywell SPZ-500 integrated autopilot system, which used a computer to collect information from various sensors and control inputs and then to position electric-clutched servomotors at each of the primary flight controls. The rudder servomotor and an internally segregated portion of the autopilot computer also functioned as the yaw damper, providing a limited amount of rudder authority to the rudder servomotor to offset minor yaw motions.
The Citation 550 autopilot control panel is located between the pilots’ seats, aft of the seats’ forward edges. The two buttons that are used to either fully or partially engage the autopilot system (the autopilot and yaw damper buttons, respectively) are located on the aft portion of this control panel, are directly adjacent to each other, and are identical except for their left/right orientation and the labeling text beneath them.
Wiring Service Bulletin
As a result of previous reports of failures of the wiring within Cessna Citations control column shafts, Cessna issued a service bulletin in 1992, advising Citation operators to replace the flat ribbon cable installed inside the control column shafts (which were found folded and damaged in the accident airplane control column) with a rounded type of sheathed wire bundle that would fit better and be better protected within the shaft. This service bulletin had not been accomplished on the accident airplane.
Accident Flight Sequence
According to the CVR recording, the captain had recognized a flight control problem almost immediately after takeoff as he began to turn right from the runway heading. He repeatedly stated that he was “fighting the controls” and confirmed with the first officer that the landing gear had been retracted.
The captain allowed the airplane to accelerate and climb after takeoff consistent with the airplane’s assigned departure heading and altitude while he and the first officer tried to troubleshoot the control anomaly. Comments recorded by the CVR indicated that the pilots did not consult an emergency or abnormal procedure checklist. Further, CVR evidence indicated that, on at least one occasion, it is likely that the first officer made an unrequested trim input.
Inadvertent Autopilot Engagement Scenario
The NTSB considered the possibility that the autopilot was inadvertently activated instead of, or at the same time as, the yaw damper, initiating the accident sequence. Because the captain would normally have hand-flown the initial portion of a flight, he would not have expected the autopilot to be engaged. Additionally, because he did not request trim adjustments, the captain would not have anticipated having to counter trim-related forces while haphazardly troubleshooting the problem.
According to the NTSB’s performance study, the lateral and directional handling problems that would result from inadvertent autopilot activation and mistrim under these circumstances would result in control forces that were light at first and larger as the airplane accelerated and turned further from the runway heading. The effects of the autopilot turning the airplane toward the runway’s heading and the forces resulting from the mistrimmed condition would have demanded strenuous inputs from both pilots to maintain control.
The performance study results were consistent with many aspects of this interpretation of the accident sequence. For example, the captain first complained of a control problem just moments after the airplane would have begun to turn from the initial autopilot engagement heading in accordance with the departure clearance, when an inadvertently activated autopilot would have been trying to return the airplane to the initial autopilot engagement heading.
The final loss of control began just after the captain transferred the controls to the first officer to look for the autopilot circuit breaker. The performance study showed that the transfer of control occurred when the airplane was heading back towards MKE and passed through and moved away from the autopilot engagement heading, which, according to this scenario, would have resulted in dynamically changing forces on the control wheel as the autopilot reversed the direction of its input in an attempt to return to the runway heading.
Other evidence was inconsistent with this scenario, however. For example, examination of the recovered auto-pilot servomotor clutches and the unstretched filaments in the cockpit autopilot-engaged light bulb (which was not illuminated at impact) suggested that, although the autopilot did have power at impact, it was likely not engaged. Some stretching was observed in the yaw-damper-engaged bulb filaments. If the autopilot had been engaged, similar stretching should have been observed in the autopilot-engaged bulb filaments.
Runaway Pitch Trim Scenario
Because of evidence of short circuits in the first officer’s control column wiring that could have affected the airplane’s pitch trim, the NTSB also considered the possibility that a runaway pitch trim resulted in uncommanded nose-down inputs, initiating the accident sequence. This scenario interprets the captain’s reactions as consistent with an increasing control force and efforts to decrease that force. These efforts would have been complicated by the first officer’s trim inputs. Actuation of the accident airplane’s electric pitch trim required completion of circuits providing power to the motor and to a ground path. Wreckage examination did reveal physical evidence that, sometime before impact, at least two short circuits occurred in a bundle of wires in the first officer’s control yoke/column. Investigators also found evidence of heat damage consistent with the short circuits and chafing on some of the wires in this area.
One of the short circuits occurred between the pitch trim power supply wire and the adjacent steel surface of the first officer’s control yoke shaft; the recovered wires were chafed around the area where the exposed wire had become welded to the steel of the control shaft. This short circuit exposed the cores of multiple wires, providing a potential source of power to the pitch trim motor. The short circuit would also have caused the pitch trim circuit breaker to trip, which would render the electric pitch trim system unavailable for normal use by the pilots.
Examination of the wreckage also revealed that a contact in one of the two switches in the first officer’s pitch-trim control was bent inward, eliminating the space between it and the center reed. In normal pitch trim operations, the center reed moves outward and contacts the fixed contact to complete the ground path circuit; this condition might have provided the pitch trim motor with the nose-down latent ground path.
Other evidence is inconsistent with this scenario. For example, previous NTSB investigations involving pitch control problems indicate that pilots are typically immediately aware of the nature of pitch-related problems and struggle to keep the airplane upright. Because the captain would have routinely adjusted the pitch trim with each configuration change and in response to any unwanted trim forces experienced during departure, it is likely that he would have recognized a runaway pitch trim situation shortly after it began.
Further, in this scenario, the pilots would have been struggling to keep the airplane from nosing over during the accident flight, yet there was no specific reference to “pitch” or “pull” by either pilot until about five seconds before impact.
Autopilot Panel Design
FAA airworthiness standards require cockpit controls to be located and identified to prevent confusion and inadvertent operation. Additionally, human factors engineering principles indicate that, in a well-designed control system, every control must be detectable and discernable from other controls. The current design configuration of the Citation yaw damper/autopilot control push-button switches appears contrary to this standard. The switches present identical size, texture, mode of operation (on/off pushbutton), and actuation pressure to the pilot. Further, they are located immediately adjacent to each other, outside of the pilots’ normal range of vision, without an intervening divider or partition. In periods of high workload or other distractions, a pilot might be inclined to activate the yaw damper by feel and location; under such circumstances, it would be easy to select the wrong switch.
Aileron Trim Sensitivity and Responsiveness
Post accident interviews indicated that many Citation pilots have been surprised by the disproportionate sensitivity and responsiveness of the Citation aileron trim control system. In addition, numerous Citation pilots (including the owner of the accident airplane) reported problems or even declared an emergency as a result of aileron trim issues. The anecdotal evidence indicates that it is easy to set the Citation aileron trim inappropriately, possibly resulting in a hazardous out-of-trim condition. The disproportionate sensitivity and responsiveness of this system would likely have compounded any aileron trim inputs made by the first officer during the accident flight and exacerbated an already challenging troubleshooting effort.
The accident sequence initiated as a result of a control problem that was related to either an inadvertent autopilot activation or a pitch trim anomaly, the effects of which were compounded by aileron and/or rudder trim inputs; however, it was not possible to determine the exact nature of the initiating event.
Regardless of the initiating event, if the pilots had simply maintained a reduced airspeed while they responded to the situation, the aerodynamic forces on the airplane would not have increased significantly; at reduced airspeeds, the pilots should have been able to maintain control of the airplane long enough to either successfully troubleshoot and resolve the problem or return safely to the airport. The design and location of the yaw damper and autopilot switches on Cessna Citation series airplanes do not adequately protect against inadvertent activation of a system, which could have disastrous consequences.
A rounded type of sheathed wire bundle would fit better and be better protected within the control column shaft than the currently installed flat ribbon cable; replacement of the flat ribbon cable with a rounded type of sheathed wire bundle could result in fewer short circuits and other electrical events.
If circuit breakers that a pilot might need to quickly access during an abnormal or emergency situation were equipped with identification collars, pilots would be able to locate them more readily and pull them more easily during such a situation.
Limiting the deflection of the Cessna Citation’s manually operated aileron trim tab to the deflection certification limit for powered trim tabs and reducing the Citation’s aileron trim sensitivity (the unexpectedly significant aileron trim deflection that results from a relatively small amount of trim knob input) would help pilots avoid sudden and excessive aileron trim deflections.
If Cessna Citation pilots and operators were informed of the potential hazards related to the sensitivity and responsiveness of the airplane’s aileron trim system, they would be better able to avoid problematic aileron trim inputs until a more permanent solution (an aileron trim system retrofit) is in place.
About The Author
For more information, in the U.S.A. (N.T.S.B.) The NTSB is an independent Federal agency charged by Congress with investigating every civil aviation accident in the U.S. and significant accidents in other modes of transportation-railroad, highway, marine and pipeline. WEBSITE = http://www.ntsb.gov/ For more information, in CANADA (T.S.B.) The Transportation Safety Board of Canada (TSB) is an independent agency that advances transportation safety by investigating occurrences in the marine, pipeline, rail and air modes of transportation. WEBSITE = http://www.tsb.gc.ca/eng/View all articles by N.T.S.B..