CHI00FA006

The flight experienced a series of pitch oscillations while on descent to an assigned altitude of 11,000 feet mean sea level(msl). The flight attendant was seriously injured. The Captain reported that when he was at about 11,400 feet msl, it appeared the airplane was going to descend below the assigned altitude. The First Officer, who was the flying pilot in the left seat, reported that he pulled back on the airplane's yoke to initiate the level off without disengaging the autopilot. He reported that when he relaxed the back pressure on the yoke, the airplane pitched nose down violently and a series of three to four pitch oscillations occurred before the airplane was brought under control. The aircraft load factors reached magnitudes between +3.3g and -1.2 g. The flight attendant was not wearing a seat belt at the time of the incident. The inspection of the airplane revealed no pre-existing anomalies. Neither the autopilot's pilot manual nor the airplane's flight manual contained a Warning, Caution, or Note that explicitly stated not to manually override the autopilot. On October 18, 1999, Falcon Aviation issued Newsflash 52 that notified operators not to manually apply control yoke forces in an attempt to control the aircraft when the autopilot is engaged.

History of flight

On October 9, 1999, at 1510 eastern daylight time, a Dassault Aviation, DA-900B, N523AC, operated by the Amway Corporation experienced a series of pitch oscillations while leveling off at 11,000 feet mean sea level (msl) during a descent into Grand Rapids, Michigan. The 14 CFR Part 91 flight had departed the Portland International Airport, Portland, Oregon, at 0900 PDT en route to the Kent County International Airport, Grand Rapids, Michigan. The Captain reported that at approximately 11,400 feet msl, it appeared the airplane was going to descend below the assigned level off altitude of 11,000 feet msl. The First Officer, who was the flying pilot in the left seat, pulled back on the airplane's yoke to initiate the level off without disengaging the autopilot. The First Officer reported that when he relaxed the back stick pressure on the yoke, the airplane pitched nose down violently. A series of three to four pitch oscillations occurred before the airplane was brought under control. The Captain, First Officer, and the two passengers had their seat belts on and were not injured. The Flight Attendant who was in the airplane's galley received serious injuries. The flight continued on to Grand Rapids, Michigan, which was about 32 nautical miles away, and an uneventful landing was made. Visual meteorological conditions prevailed and an IFR flight plan had been filed.

Personnel information

Both the Flying Pilot (FP) and the Non-flying Pilot (NFP) were rated as Captains on the Dassault 900B airplane. The NFP had been designated as the Captain for the entire flight, which included the flight from Grand Rapids, Michigan, to Portland, Oregon, on the previous day. For that leg of the mission, the designated Captain had been in the left seat and was the FP. But on the accident flight, the designated Captain was the NFP in the right seat, and the designated First Officer was the FP in the left seat.

The designated Captain was an airline transport pilot with single and multi-engine land ratings. He held a First Class medical certificate. He had a total of about 10,423 hours of flight time. 9,315 hours were in multi-engine airplanes and about 191 hours in make and model of the accident airplane.

The designated First Officer was an airline transport pilot with single and multi-engine land ratings. He held a First Class medical certificate. He had a total of about 5,838 hours of flight time. 3,625 hours were in multi-engine airplanes and about 993 hours in make and model of the accident airplane.

Aircraft information

The airplane was a three engine Dassault Aviation DA-900B, Falcon 900, serial number 139. The airplane seated 15 and had a maximum gross weight of 46,500 pounds. The engines were TFE-731-5BR Garrett engines which produced 4,750 pounds of thrust. The last continuous airworthiness inspection (OSIP Block 25) was conducted on September 30, 1999. The airplane had flown 9 hours since the last inspection and had a total time of 2,701 hours and 1,087 cycles.

Meteorological information

At 1453, the weather conditions reported at Holland, Michigan, located about 22 nautical miles southeast from the occurrence were: winds 140 degrees at 5 knots, sky broken with 1,900 foot ceiling, visibility 9 miles, temperature 67 degrees Fahrenheit, dew point 60 degrees Fahrenheit, altimeter 30.08.

Inspection of aircraft

An inspection and operational check was made of N523AC's elevator system for any binding or excess movement. No anomalies were found. No anomalies were noted on visual inspection of the ARTHUR Q Unit, control box, and artificial feel unit (AFU). A functional test of the Elevator Arthur Control System was conducted in accordance with the Falcon 900 Maintenance Manual procedure 27-307 utilizing a test box. All tests passed with no anomalies noted.

A continuity check of the horizontal stabilizer potentiometers was conducted in accordance with the test described in the Falcon 900 Service Newsletter No. 16, November 1990. No high discontinuities were found.

Flight recorders

The Cockpit Voice Recorder (CVR) and the Flight Data Recorder (FDR) were sent to the National Transportation Safety Board for examination.

The CVR did not contain any audio relevant to the investigation and a transcript was not prepared. (See Cockpit Voice Recorder Factual Report)

Examination of the FDR data showed the aircraft in a steady descent at 355 KIAS until about 10,900 feet, with the autopilot mode discrete indicating engaged. At that point, the aircraft pitch increased from about 2.75 degrees aircraft nose down (AND) to about 1.5 degrees aircraft nose up (ANU) in just over one second with the autopilot still engaged. The aircraft then immediately pitched over to approximately 4 degrees AND and the autopilot mode discrete changed to NOT engaged. The pitch oscillations subsequently continued between about 3 degrees ANU and 4.5 degrees AND. The aircraft load factor followed the aircraft pitch attitude and reached magnitudes between +3.3g and -1.2 g. (See Systems Group Chairman's Factual Report)

Tests and research

The airplane's two FZ-800 Digital Flight Guidance Computers (DFGC) were sent to Honeywell, Inc. in Phoenix, Arizona, to obtain a readout of the data recorded within the memory of the components. (See Systems Group Chairman's Factual Report)

Additional information

Examination of the autopilot's pilot manual and airplane's flight manual revealed that neither manual contained a Warning, Caution, or Note that explicitly stated not to manually override the autopilot.

On October 18, 1999, Dassault Aviation issued a Service Newsflash 52 for the Falcon 900. The Newsflash contained the following information:

"As we have previously reported, two Falcon 900 aircraft have recently experienced difficulties during descent which resulted in abrupt pitch movements.

Although the root causes of these incidents have not yet been determined and may not be related, the French airworthiness authority has requested that Falcon operators be reminded of the following basic piloting techniques, when flying with the autopilot engaged:

* Do not manually apply control yoke forces in an attempt to control the aircraft.

* If aircraft manual control is required or desired:

* Hold the control yoke and disengage the autopilot using any of the pushbuttons available for disconnection (refer to your model aircraft's documentation for pushbuttons to disconnect).

* Manually control the aircraft as required.

* Trim the aircraft and re-engage autopilot if desired."

Parties to the investigation were the Federal Aviation Administration, Bureau Enquetes Accidents (BEA), Amway Corporation, Dassault Aviation, and Honeywell, Inc.

The aircraft components were released to the Amway Corporation.