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Socata TBM-700N C-FBKK

Accident Case Study

There is a common theme to these smaller aircraft slow onset hypoxia incidents. These aircraft usually do not have cockpit voice or flight data recorders, their oxygen systems are less than fool proof, and their pilots do not have a lot of high altitude experience. While they are rarely fully investigated, Transport Canada makes a pretty good effort here, but still come up empty. Regardless, there are many lessons to be learned:

  • Fuselage pressure leaks should be addressed immediately and repaired.
  • Oxygen supply and delivery equipment must be pre-flighted.
  • During every climb, part of your 10,000 foot check must be a check of cabin altitude. Most airplanes have a maximum cabin altitude between 6,000 and 8,000 feet. You certainly should not be above that at 10,000 feet aircraft altitude. You should know your normal cabin climb rate, typically around 300 fpm. Even if it takes you ten minutes to get to 10,000 feet aircraft altitude, you should not see more than 3,000 feet cabin altitude at that point. In any case, if it isn't where it should be, level off and investigate.
  • When troubleshooting a pressurization problem, donning oxygen quickly will not only improve your mental capabilities, it can keep you in the game if things go south quickly or insidiously.
  • When in doubt, descend.

 

Accident Report

  • Date: 8 October 2012
  • Time: 1219 EDT
  • Type: Socata TBM 700N
  • Operator: Privately owned
  • Registration: C-FBKK
  • Fatalities: 1 of 1 crew
  • Aircraft Fate: Destroyed
  • Phase: En route
  • Airports: (Departure) Ottawa/Carp Airport (CYRP), Ontario
  • Airports: (Destination) Goderich (CYGD), Ontario

Narrative

[Aviation Investigation Report A12O0170, pg. 2]

  • On the day of the occurrence, the aircraft departed Goderich, Ontario (CYGD), at approximately 1030 1 with 3 people on board. The aircraft owner was piloting the aircraft from the left cockpit seat, the occurrence pilot was in the right seat, and the owner’s spouse was seated in the cabin. This flight was completed under instrument flight rules (IFR) at flight level 2 250 (FL250) and landed at Carp, Ontario (CYRP), at approximately 1130. The engine was shut down and everyone onboard deplaned. After approximately 15 minutes, the occurrence pilot reboarded the aircraft and closed the main door. The engine was started and the aircraft was taxied for departure. At 1200 the aircraft was taxied into position on Runway 28, with the pilot in the right-hand seat, and departed without event.
  • At 1204 while climbing through 7000 feet above sea level (asl), the pilot contacted Ottawa Terminal air traffic control (ATC) and received an IFR clearance to CYGD. The pilot requested a destination change to Wiarton (CYVV) and was cleared as requested via radar vectors to climb to FL230. Prior to departure, the pilot had decided to change the destination in order to pick up an aircraft manual left in Wiarton, and elected to make this change once airborne to save time.
  • At 1207, Ottawa Terminal instructed C-FBKK, now climbing through 11 500 feet asl, to contact Montreal Centre ATC.
  • At 1208, Montreal Centre cleared C-FBKK, now climbing through 14 000 feet asl, direct to CYVV and instructed the pilot to contact Toronto Centre ATC.
  • At 1211, Toronto Centre cleared C-FBKK to FL240 and a minute later cleared the aircraft to FL260. The pilot read back the clearance to FL260 at 1212:56 while climbing through FL219. This was the last recorded transmission by the pilot.
  • Between 1208 and 1216, the aircraft’s horizontal track varied slightly to the left and right of the direct track to CYVV. Previous flights with the autopilot engaged showed little or no deviation from the direct track.
  • At 1216, the aircraft climbed through FL260 and a minute later, upon reaching FL275, it entered a right turn, which rapidly developed into a spiral dive. During the dive the aircraft reached vertical velocities in excess of 25,000 feet per minute (fpm), and was recorded on radar as leveling off for approximately 6 seconds at 8000 feet asl before reentering the spiral dive. The last recorded radar return at 1218:50 shows the aircraft at 4900 feet asl with a vertical speed of 13 200 fpm.
  • The aircraft was seen and heard by observers below the last recorded altitude executing various unusual manoeuvers described as rapid climbs and descents, loops, steep banks, and inverted flight. The noise was described as rapid and variable high-pitched revving of the engine. The aircraft struck a forest area at an elevation of approximately 1085 feet asl.

[Aviation Investigation Report A12O0170, pg. 3]

  • The weather conditions at the time and location of the accident were not considered a contributing factor. The sky was clear, there was no precipitation, and visibility was over 15 statute miles. There were no reports of icing or turbulence from other aircraft in the area.
  • The pilot held a valid airline transport pilot licence, which included a current instrument rating and a type rating on the TBM 700, and had over 19 200 hours of flight experience and approximately 700 hours on type. The pilot had completed initial training and type rating in 2001, and had taken 8 recurrent courses since that year.
  • The pilot had a valid category 1 medical certificate, having successfully completed an examination on 05 September 2012, 33 days before the accident. The examination included an electrocardiogram, which assists the examiner in assessing the condition of the patient’s heart. The pilot was 74 years old and was on medication for high blood pressure and high cholesterol. The Civil Aviation Medical Examiner and the Regional Aviation Medical Officer were aware of the conditions and associated medications, and decided these did not preclude the issuance of a category 1 medical certificate.
  • Data recorded on the Civil Aviation Medical Examination Report shows that, at the time of the examination, the pilot had a body mass index (BMI) of 33.1. A person with a BMI above 30 is considered obese.
  • Under the report section entitled “Cardio Vascular (CV) Risk Factors”, the examiner had checked Hypertension (High Blood Pressure), Serum Lipids (High Cholesterol) and Obesity.
  • The TBM 700N is pressurized by bleed air taken from the compressor section of the engine, and the pressurization is controlled by an onboard computer called the Global Air System Controller (GASC).
  • The output of compressed air from the engine is controlled by a flow control/shut-off valve (FCSOV). The FCSOV is a butterfly-type valve located in the engine compartment forward of the firewall. It is an electro pneumatic regulating and shut-off valve commanded by a torque motor, which is part of the FCSOV assembly. The valve actuates by using engine bleed air pressure. The valve is closed by an internal spring when the torque motor is de-energized or when there is no engine bleed air supply. Electrical current for the torque motor is controlled by the GASC, which receives signals from the bleed differential pressure sensor and the cabin pressure sensor. The valve is normally open during flight. The FCSOV will close if any of the following abnormal conditions occur: an over-temperature condition is sensed at the air inlet or outlet; bleed pressure is lost; electrical power is lost; or the guarded bleed switch on the environmental control system panel is set to OFF. When the FCSOV is closed, an amber BLEED OFF CAS message is displayed on the MFD.
  • The pressurization system is computer-controlled, but there is a cabin pressurization control panel (CPCP) located on the instrument panel in front of the right seat occupant’s left knee. On this panel there is a cabin altitude selector, which is set by the pilot to assist the GASC in smoothly altering the cabin altitude during climb and descent. There is an instruction in the PIM directing pilots to set the selector to cruise altitude +1000 feet while climbing and to the destination airfield elevation while descending.
  • The pilot is warned of the condition of the pressurization system in flight by aural and visual alerts. The upper left quadrant of the MFD displays cabin altitude, cabin differential pressure, cabin altitude rate of change, selected altitude, and oxygen cylinder pressure. If cabin altitude exceeds 10 000 feet, the displayed cabin altitude blinks red, a red CABIN ALTITUDE CAS message appears in the lower left quadrant of the MFD, the red master caution light on the upper left glare shield starts blinking, and a repetitive aural alert (a chime) sounds in the headsets. The aural alert and blinking light continue until acknowledged by pressing the master caution pushbutton indicator.
  • In the event of pressurization loss there are emergency oxygen masks for the rear passengers, and quick-donning emergency oxygen masks for the occupants of the cockpit. If the pilot is wearing a mask and needs to make a radio broadcast, a mask radio transfer switch on the far left side of the instrument panel must be set to ON to allow the push-to-talk (PTT) button to activate the mask microphone.
  • The oxygen cylinder was recovered; it had exploded. The neck-mounted isolation valve was recovered and was found in the OFF position. There were no impact marks or material transfer on the isolation valve to suggest the valve had moved during the accident sequence. However, the possibility that the valve had moved to the OFF position during the impact sequence could not be ruled out.
  • The cabin altitude selector was recovered and determined to be set at an aircraft altitude of 18 300 feet or cabin altitude of 2500 feet.
  • The FCSOV was recovered and found to have been in the closed position upon impact.
  • The engine and propeller were recovered and it was determined that the engine was producing power when the aircraft hit the ground.

Analysis

[Aviation Investigation Report A12O0170, pg. 3]

  • The oxygen cylinder valve position is normally checked before the first flight, as is the quantity of oxygen and operation of the oxygen system from the cockpit. If this switch is in the OFF position and the aircraft is flown, a noticeable CAS warning appears on the MFD. It is dangerous to continue to fly above 10 000 feet without emergency oxygen: in the event of depressurization, a pilot would quickly become incapacitated. It could not be determined with any certainty whether the position of the valve changed during the accident sequence.
  • It could not be determined why the pilot decided to occupy the right-hand seat for the return trip given that the pilot was the sole occupant. While the aircraft can easily be operated from either seat, certain switches, such as the mask radio transfer switch, become more difficult to operate, and the master warning and caution lights and MFD CAS messages are no longer directly in the field of view from the right-hand seat.
  • Radar recordings of the horizontal flight path and lack of altitude capture at FL260 suggest that the autopilot was not being used. It would be considered abnormal not to use it, especially during higher workload scenarios such as single-pilot IFR operations. The PIM states that autopilot operation is contingent on a pilot being seated in the left seat. This restriction may have been known to the pilot, which might explain why the autopilot was not used.
  • The cabin altitude selector on the CPCP was found set to an altitude that did not match any settings recommended in the PIM. The setting would not disable the computer-controlled pressurization system, but it would not provide an optimized rate of change of cabin altitude. It was considered possible that this selection was made in error or during incapacitation or rapid descent.
  • The FCSOV was determined to have been in the closed position when the aircraft struck the ground. There are 5 possible reasons for the valve being closed, but only 1, the loss of upstream pressure, could be mostly ruled out due to known engine operation. If the valve were closed during flight, the cabin would stop receiving pressurized air, and eventually its pressure would equalize with the outside air pressure. This loss of pressure would trigger several warnings in the cockpit including a master warning and a constant repetitive aural alert.
  • The rapid development of a spiral dive is difficult to explain without pilot initiation. If the aircraft is climbing with the engine set to climb power and the trim condition set to normal, it does not have a tendency to enter this type of manoeuver unaided. Likewise, it is difficult to account for the dramatic change in the rate of descent during the spiral dive and the manoeuvers observed below radar coverage without control input.
  • The pilot’s age and medical condition were considered as a possible source of incapacitation. A medical incapacitation event is consistent with a loss of control but inconsistent with the determined position of the FCSOV, the rapid entry into the spiral dive, the reduction in the rate of descent, and the observed manoeuvers.
  • An undetermined loss of pressurization is consistent with the FCSOV position and, combined with the unavailability of onboard oxygen, could explain the controlled initiation of an emergency maximum rate of descent shortly followed by incapacitation and loss of control, possibly due to hypoxia. However, the reduction in the rate of descent, observed manoeuvers, and other unusual factors present too many uncertainties to conclude that hypoxia was the cause.

Probable Cause

[Aviation Investigation Report A12O0170, pg. 10]

  • The pilot lost control of the aircraft for undetermined reasons and the aircraft collided with terrain.
  • Operating an aircraft above 13 000 feet asl without an available emergency oxygen supply increases the risk of incapacitation due to hypoxia following depressurization.

See Also:

Hypoxia

Oxygen

Slow Onset Hypoxia

References

Aircraft Crashes Record Office, Bureau of Aircraft Accidents Archives, (B3A), Geneva, Switzerland

Loss of Control and Collision With Terrain, Socata TBM 700N, C-FBKK, Renfrew, Ontario 10 NM SW, 8 October 2012, Aviation Investigation Report A12O0170, Transportation Safety Board of Canada

Revision: 20150712
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