Narrative:

I was the pilot flying (sic; sitting right seat); ferrying the aircraft. An uneventful climb up to FL430 (flight planned altitude) was performed using the normal climb profile (climb at mach = 0.62) and autopilot on. When the autopilot transitioned to 'altitude' capture mode at FL430; the pitch of the aircraft repeatedly oscillated between approximately +6 degrees nose up to approximately +1 degrees nose up. The time period from top-of-pitch to the next top-of-pitch cycle was approximately 8 seconds. The altitude was varying by approximately +/- 40 ft from the level-off altitude during these cycles. The indicated airspeed was varying by a few knots. I do not recall seeing the airspeed trend line indicating significant airspeed acceleration or deceleration. The indicated airspeed seemed much more stable than the oscillations observed with the pitch attitude. I recall trying to detect if the airspeed was decaying; indicative of being on the back side of the power curve. I did not observe the airspeed trending toward a significant decay; nor did I detect any airframe indications of buffet. Just to make certain I consulted the laminated card to reaffirm that we were not close to the high-speed or low-speed buffet margins. The wings remained in level flight (no bank); and the air was smooth and VMC. (I do not recall the outside air temperature. Our bags were loaded in accordance with with the 'standard' loading diagram as per our fom and the dispatch release.) this is the highest I have ever flown an encore. I have never received training in the encore in the simulator or in the aircraft to know if this is a 'normal' aircraft reaction at high altitude; or if this was 'normal' autopilot reaction at the very high altitude and that the oscillations would eventually damped out. I have not been trained in the encore regarding the proper manual control techniques at such a high altitude/airspeed to avoid a pilot induced oscillations. I recalled from several fatal accidents at high altitude that when pilots disconnected the autopilot at high altitudes their inputs worsened the oscillations which subsequently caused fatal injuries. Thus I deliberately left the autopilot on and did not disconnect the autopilot because of my concern for causing pilot induced oscillations. The oscillations continued; at approximately the same frequency and amplitudes. While I was trying to figure out what to do regarding the pitch oscillations; the 'heading' miscompare light illuminated. While I focused on aircraft control the pilot not flying (PIC sitting left seat) consulted dispatch via air phone regarding the 'heading' issue. We were instructed to divert. Upon receiving a lower altitude clearance from ATC I changed the mode control from 'altitude' to 'V/south' and began a descent. As we descended below FL410 the pitch oscillations seemed to dampen to normal. Last tour I attended simulator recurrent training. We received the 'high altitude' power point presentation; during which we saw the same slides regarding coffin corner; high speed buffet; low speed buffet; swept wing stall characteristics; etc. However; all of the graphs were for the citation X. None of the graphs contained performance and handling charts pertaining to the straight-winged encore. Nor did any of the graphs discuss unique handling characteristics of the encore in high altitude flight; differences in stall recovery at high altitude versus lower altitudes; proper recovery from oscillations in the encore at high altitude/high-airspeed; etc. I asked the instructor if these pitch oscillations are normal aircraft reactions for the encore at higher altitude; whether this was 'normal' or 'abnormal' autopilot behavior in the encore; and if 'abnormal' what was the proper technique/procedure to correct the oscillations in the encore. He did not know. I have looked through the aircraft's afm and aom and cannot find any information pertaining to these. If this is the 'normal' behavior of the aircraft this high; I never want to fly that high again. 1) I have no idea if this was 'normal' or 'abnormal' aircraft behavior at high altitude in the encore. If this was an 'abnormal' behavior; then I don't know if this is due to insufficient certification testing and/or an inoperative autopilot. If this is 'normal' or acceptable aircraft behavior; I've never been trained in this. The flight safety instructors had no idea either. So obviously my training and their training on high-altitude encore procedures has some deficiencies. 2) I have no idea on the proper method for handling such an oscillation at high altitude in the encore. Neither did the flight safety instructors.

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Original NASA ASRS Text

Title: A CE560 level at FL430 developed pitch oscillations from +6 to +1 degrees in an 8 second period. Unable to determine a cause; the crew descended and the oscillations dampened below FL410.

Narrative: I was the pilot flying (SIC; sitting right seat); ferrying the aircraft. An uneventful climb up to FL430 (flight planned altitude) was performed using the normal climb profile (climb at Mach = 0.62) and autopilot ON. When the autopilot transitioned to 'ALT' capture mode at FL430; the pitch of the aircraft repeatedly oscillated between approximately +6 degrees nose up to approximately +1 degrees nose up. The time period from top-of-pitch to the next top-of-pitch cycle was approximately 8 seconds. The altitude was varying by approximately +/- 40 FT from the level-off altitude during these cycles. The indicated airspeed was varying by a few knots. I do not recall seeing the airspeed trend line indicating significant airspeed acceleration or deceleration. The indicated airspeed seemed much more stable than the oscillations observed with the pitch attitude. I recall trying to detect if the airspeed was decaying; indicative of being on the back side of the power curve. I did not observe the airspeed trending toward a significant decay; nor did I detect any airframe indications of buffet. Just to make certain I consulted the laminated card to reaffirm that we were not close to the high-speed or low-speed buffet margins. The wings remained in level flight (no bank); and the air was smooth and VMC. (I do not recall the outside air temperature. Our bags were loaded IAW with the 'standard' loading diagram as per our FOM and the dispatch release.) This is the highest I have ever flown an Encore. I have never received training in the Encore in the simulator or in the aircraft to know if this is a 'normal' aircraft reaction at high altitude; or if this was 'normal' autopilot reaction at the very high altitude and that the oscillations would eventually damped out. I have not been trained in the Encore regarding the proper manual control techniques at such a high altitude/airspeed to avoid a pilot induced oscillations. I recalled from several fatal accidents at high altitude that when pilots disconnected the autopilot at high altitudes their inputs worsened the oscillations which subsequently caused fatal injuries. Thus I deliberately left the autopilot on and did not disconnect the autopilot because of my concern for causing pilot induced oscillations. The oscillations continued; at approximately the same frequency and amplitudes. While I was trying to figure out what to do regarding the pitch oscillations; the 'HDG' miscompare light illuminated. While I focused on aircraft control the pilot not flying (PIC sitting left seat) consulted Dispatch via air phone regarding the 'HDG' issue. We were instructed to divert. Upon receiving a lower altitude clearance from ATC I changed the mode control from 'ALT' to 'V/S' and began a descent. As we descended below FL410 the pitch oscillations seemed to dampen to normal. Last tour I attended simulator recurrent training. We received the 'high altitude' power point presentation; during which we saw the same slides regarding coffin corner; high speed buffet; low speed buffet; swept wing stall characteristics; etc. However; all of the graphs were for the Citation X. None of the graphs contained performance and handling charts pertaining to the straight-winged Encore. Nor did any of the graphs discuss unique handling characteristics of the Encore in high altitude flight; differences in stall recovery at high altitude versus lower altitudes; proper recovery from oscillations in the Encore at high altitude/high-airspeed; etc. I asked the instructor if these pitch oscillations are normal aircraft reactions for the Encore at higher altitude; whether this was 'normal' or 'abnormal' autopilot behavior in the Encore; and if 'abnormal' what was the proper technique/procedure to correct the oscillations in the Encore. He did not know. I have looked through the aircraft's AFM and AOM and cannot find any information pertaining to these. If this is the 'normal' behavior of the aircraft this high; I never want to fly that high again. 1) I have no idea if this was 'normal' or 'abnormal' aircraft behavior at high altitude in the Encore. If this was an 'abnormal' behavior; then I don't know if this is due to insufficient certification testing and/or an inoperative autopilot. If this is 'normal' or acceptable aircraft behavior; I've never been trained in this. The Flight Safety instructors had no idea either. So obviously my training and their training on high-altitude Encore procedures has some deficiencies. 2) I have no idea on the proper method for handling such an oscillation at high altitude in the Encore. Neither did the Flight Safety instructors.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.