Figure: Falcon DA-900B SX-ECH in Faro, Portugal, from Wikimedia Commons.
Our airplanes are becoming pretty complex and when part of that complexity includes the flight controls, pilots should exercise an extra measure of caution when dealing with malfunctions. This situation was made worse because the flight manual procedures for diagnosing a "Pitch Feel" light did not give the pilot's an adequate way to diagnose the position of a harmless sounding device called an "Arthur Q unit." This unit determines how pilot inputs are translated to the controls and the pilots were unable to determine its failure mode.
Two months prior to this mishap another DA-900B experienced extreme pitch oscillations when a pilot manually applied elevator inputs without disengaging the autopilot. One month prior to this mishap Dassault Aviation issued Service Newsflash No. 52 reporting two events which resulted in abrupt pitch movements and cautioning pilots to avoid manual inputs with the autopilot engaged.
It is unclear why the autopilot disengaged when it did but it may be that it was caused by a pilot input. Whether or not that was the case, the pilot induced oscillations which followed sent seven passengers and everything not secured in the cabin to the ceiling and to the floor again ten times, killing the passengers not wearing seat belts.
It is clear the pitch control system on the DA-900B is a bit different than many "conventional" aircraft and bears careful study by its pilots and care when dealing with malfunctions. Part of good situational awareness is knowing what your airplane's critical, fault intolerant systems are. When presented with a flight control problem, it is often wise to slow the aircraft down to a maneuvering speed and get it on the ground.
What follows are quotes from the sources listed below, as well as my comments in blue.
- Date: 14 September 1999
- Time: 2135 UTC
- Type: Falcon DA-900B
- Operator: Olympic Airways
- Registration: SX-ECH
- Fatalities: 0 of 3 crew, 7 of 10 passengers
- Aircraft Fate: Repaired
- Phase: En route
- Departure: Athens, Greece
- Destination: Bucharest, Romania
Figure: Pitch oscillations, from Romania Accident Report, Page 54.
[Romania Accident Report, ¶1.1]
- During climb ,after the flaps and slats were retracted, the flight crew noticed, on the warning panel, the "PITCH FEEL" light, was illuminated. The PIC disengaged the autopilot, checked the forces on the control column and re-engaged the autopilot. The "PITCH FEEL" warning light, remained continuously ON, during cruise and descent until SLATS were extended.
- The final cruise level was FL 400, as requested by the crew and cleared by ATC, and was reached 27 minutes after take-off. Based upon crew declaration, during cruise flight, the crew noticed a roll mistrim warning, which disappeared after adequate compensation on ailerons trim, performed by the PIC.
- After 47 minutes from take-off, a normal descent to FL 150 was initiated, with the A/P engaged in vertical speed (V/S) mode. During descent the Indicated Air Speed (IAS) increased from 240 Kts to 332 Kts.
- At 12 minutes from Top of Descent (TOD), approaching FL 150, the F/O had a request for a further descent. Just before FL 150 the ATC recleared OAL3838 to continue descent to FL 50, while PIC briefed F/A about ETA. One second later A/P disengaged and thereafter the aircraft was manually flown by the PIC.
- Between FL 150 and FL 140, for approximately 24 seconds, the aircraft experienced 10 oscillations in pitch axis which exceeded the limit manoeuvring load factor. Maximum recorded values were: +4.7 g and -3.26 g. During the event the thrust power was reduced.
- At about FL 130, after aircraft recovery from the encountered oscillations, F/O declared an EMERGENCY, saying: "We are in emergency sir, request vector to final approach. We have problems with the controls".
- The aircraft was manually controlled by the PIC for 1 minute and 36 seconds, thereafter A/P was re-engaged for approximately 4 minutes, from FL130 to 2500 feet, as recorded by the DFDR.
- At the request of the flight crew, radar vectoring was provided by the ATC, and a VISUAL approach was performed on RWY 08R.
[Romania Accident Report, page 9.]
- The impact of the unfasten passengers with cabin ceiling and aircraft furniture, due to accelerations occurring during the pitch oscillations caused fatal injuries to 7 passengers, serious injuries to 1 crew member and 1 passenger and minor injuries to 2 passengers.
[Romania Accident Report, ¶1.6.2.]
- The primary flight control system is fully power-assisted non-reversible system. The control inputs originate from the cockpit where a system of rigid rods and bellcranks link the control wheels and rudder pedals to the servo-actuators of the rudder, the elevators and the ailerons. Twenty-three turquoise-coloured rods (22 for aircraft 88 onwards) in the fuselage are made of carbon. In case of a total hydraulic failure, control surfaces can be operated manually within restricted flight envelope. In this case the servo-actuators transmit control movements mechanically to the control surfaces.
- The aileron, elevator and rudder controls incorporate the following:
- a system of springs forming an artificial feel unit (AFU),
- an automatic spring load adjusting system ("ARTHUR" Q unit), for increasing the force required to operate the aileron control (according to the speed) and the elevator control (according to the position of the horizontal stabiliser).
Figure: Falcon 900B Elevator-control system, from Flight Safety Foundation, February 2001, Figure 1.
- The pitch [control forces] and roll control forces may be higher or lower than normal, depending on whether the Arthur unit has failed in the high-[speed] or low-speed position.
- Light forces: avoid large displacements and rapid movements of the control surfaces, to avoid inducing high load factors.
- High forces: use normal [trim system] or emergency trim system and execute an approach.
- The PF disengaged the autopilot and perceived that control feel was "normal."
- A perception error was made in determining the Arthur unit position, and this had important influence on the encountered airplane upset.
Figure: Vertical accelerations, from Romania Accident Report, Annex 6, page 1.
[Romania Accident Report, ¶3.1]
- During ground test, the PITCH FEEL failure could not be identified systematically.
- The offset between the HS potentiometers and the Arthur Q potentiometer did not identify the PITCH FEEL malfunction without any doubt.
The Arthur Q unit is an automatic spring load adjustment system for increasing the load force required to operate the aileron and elevator controls. With the unit failed, the aircraft is restricted to 260 KIAS or M 0.76.
- The defective potentiometers were found at the manufacturer's facility and explained the Arthur Q unit failure.
- The crew disengaged the auto-pilot and checked the efforts on the control column to determine an actual failure.
- The PIC could not identify and evaluate the failure. The PITCH FEEL indication was not considered as real malfunction.
The Arthur Q unit failed in the "low speed" position, but there is no cut and dried way for the pilot to know this. The pilot should have reduced speed.
- The A/P design logic allowed an unfavourable STAB trim position, when manually overridden.
- During the established time of the accident the A/P was disengaged, most probably, due to a servo motion monitor trip and then the aircraft was manually operated.
- Initiation of the oscillations was caused by the discontinuity of efforts on the control column when the active channel of the A/P, (which had been overridden) disengaged.
- The speed (332 Kts) and the Arthur unit failure facilitated Pilot Induced Oscillations.
- The aircraft entered 10 pitch oscillations which exceeded the load factor manoeuvring limit, for a period of 24 seconds after A/P disengaged. Maximum recorded values of vertical acceleration were positive 4.7 g and negative 3.26 g.
- The first pilot input on the control column, after A/P disengaged, was large and rapid for an Arthur unit failed in "low-speed" mode.
Figure: Cabin phot, from Romania Accident Report, Photo 2.
- All the dead passengers had the seat belts not fastened during the accident. The flight crew has followed the Company Operational Procedures regarding "fasten SEAT- BELTS" policy.
Final Report Nr. 711 on the Accident of the Falcon 900B registered SX-ECH, 14 September 1999, Romania Ministry of Transport, Civil Aviation Inspectorate, 01.08.2000
"Inadequate Response to Flight-control Problem, Misuse of Autopilot Cited in Falcon 900B Upset," Flight Safety Foundation, Vol 58 No. 2, February 2001
Wikimedia Commons, Public Domain Artwork