[NTSB Aircraft Accident Brief, AAB-00/01, page 5]
- The captain was hired by Sunjet Aviation on September 21, 1999. According to Sunjet Aviation records, the captain had accumulated a total of 4,280 hours of flight time (including his military flight time). He had flown a total of 60 hours with Sunjet Aviation, 38 hours as a Learjet pilot-in-command and 22 hours as a Learjet second-in-command. The captain had flown 35 and 6 hours in the last 30 and 7 days (respectively) and 0 hours in the last 24 hours before the accident. Sunjet Aviation records indicate that the captain received his initial Learjet 35 type rating and completed the airman competency/proficiency check for the Learjet 35 on September 21, 1999. According to Sunjet Aviation employees, the captain was an excellent pilot who transitioned into the Learjet without difficulty. They also indicated that he was knowledgeable about the airplane and that he was a confident pilot with good situational awareness.
- The first officer was hired by Sunjet Aviation on February 24, 1999. According to Sunjet Aviation records, the first officer had accumulated a total of 1,751 hours of flight time, 1,300 of which were as a pilot-in-command. She had flown a total of 251 hours with Sunjet Aviation as a second-in-command, 99 hours of which were as a Learjet second-in-command. The first officer had flown 35 and 6 hours in the last 30 and 7 days (respectively) and 0 hours in the last 24 hours before the accident. Sunjet Aviation records indicate that the first officer received her initial Learjet 35 type rating, completed her initial Learjet 35 second-in-command check ride, and completed the airman competency/proficiency check for the Learjet 35 on April 15, 1999. Pilots who had flown with the first officer before she was hired by Sunjet Aviation indicated that she was a knowledgeable pilot with good aircraft handling skills; one pilot stated that she was a serious pilot who had a “meticulous” style in the cockpit and was not someone who abbreviated procedures or neglected checklists. Sunjet Aviation pilots indicated that she was a confident pilot with excellent radio communication skills.
[NTSB Aircraft Accident Brief, AAB-00/01, page 27]
- The flight crew’s last communication with air traffic control (ATC) was at 0927:18 eastern daylight time, when the first officer acknowledged an ATC clearance to flight level (FL) 390 and the airplane was climbing through 23,200 feet. Her speech was normal, her phraseology was accurate and appropriate, and Safety Board testing indicated that she was not using an oxygen mask microphone for this transmission or those that she had made earlier. The flight crew’s failure to respond to repeated ATC radio inquiries beginning at 0933:38, when the airplane was climbing through about 36,400 feet, was the first indication of a problem on board the accident flight. As the flight continued, it deviated from its assigned course and failed to level at its assigned altitude (FL 390).
- The continuous sounding of the cabin altitude aural warning during the final 30 minutes of cruise flight (the only portion recorded by the CVR) indicates that the airplane and its occupants experienced a loss of cabin pressurization some time earlier in the flight. Further, although the severity of the impact precluded extensive analysis, there was no evidence suggesting any alternative reason for incapacitation.
- If the pilots had received supplemental oxygen from the airplane’s emergency oxygen system, they likely would have properly responded to the depressurization by descending the airplane to a safe altitude. Therefore, it appears that the partial pressure of oxygen in the cabin after the depressurization was insufficient for the flight crew to maintain consciousness and that the flight crewmembers did not receive any, or adequate, supplemental oxygen.
- Post accident examination of the left and right bleed air shutoff/regulator valves (modulation valves) indicated that they were near their fully closed positions. Because the modulation valves are spring loaded to the open position and require bleed air to close, the nearly closed position of both valves at impact is consistent with a normal and adequate supply of engine bleed air from one or both engines. Further, these nearly closed valve positions indicate that there was a low demand for bleed air by the airplane’s air conditioning and anti-icing systems and that both BLEED AIR switches, which were not recovered, would have had to have been selected to the ON position. The nearly closed valve positions also indicate that the airplane’s pneumatic system was intact, and, therefore, normal system pressure was being supplied to the air conditioning system flow control valve.
This is faulty logic. If the aircraft was normally pressurized, these valves would indeed be nearly closed. But the aircraft was not normally pressurized and the fact these valves were nearly closed is a major clue. Either not enough air was going in or too much was going out. The airplane had a history of air inflow problems.
- The flow control valve regulates the flow rate of conditioned bleed air entering the cabin for pressurization and heating. If there is no inlet bleed air, the valve main spring will close the flow control valve completely. Although, as previously discussed, bleed air was available to open the flow control valve, the condition of the flow control valve indicated that it was in its fully closed position at impact. The valve requires several seconds to move from its fully open to fully closed position in normal operation, indicating that the valve was in its closed position before impact. This closed valve would have prevented bleed air from entering the cabin, thereby preventing normal pressurization.
- Closure of the flow control valve on a Learjet Model 35 and the resulting loss of bleed air supply to the cabin would cause the airplane to quickly lose cabin pressure (depressurize) at a rate dependent upon the cabin leakage rate. Computer simulations by Honeywell indicated that if a loss of normal bleed air supply to the cabin occurred at flight altitudes above 25,000 feet, the cabin altitude could ascend to 10,000 feet in about 30 seconds and reach 25,000 feet in about 2 1/2 minutes.
That is a pretty rapid rate, hard to miss. It is hard to imagine the flow control valve was operating normally and then suddenly closed without getting someone's attention.
- Investigators also considered the possibility that the pilots failed to select the CABIN AIR switch to NORM, which activates the air conditioning system (and pressurizes the airplane), before takeoff. Even though the Taxi and Before Takeoff checklist specifies, in item 19, “CABIN AIR SWITCH - NORM,” the FAA Special Certification Review (SCR) team observed that “there is incentive to leave the pressurization system off during taxi and takeoff in warm weather because inflow air can be hotter than cabin ambient air.” However, without the cabin air conditioning system, the occupants of the airplane likely would have perceived a high cabin climb rate after takeoff, possibly causing discomfort. At about 10,000 feet cabin altitude, the cabin altitude aural warning should have begun to sound, further alerting the flight crew to the lack of pressurization. Although the pilots could have manually silenced the warning, they would have had to repeat this action every 60 seconds. At about 14,000 feet cabin altitude, deployment of the passengers’ oxygen masks would have provided an additional cue that the cabin was not properly pressurized. It is unlikely that the flight crew would have continued to climb despite this clear information that the airplane was unpressurized.
- In summary, as previously discussed, an uncommanded closure of the flow control valve would have been sufficient to depressurize the airplane. However, there was insufficient evidence to determine whether the depressurization was initiated by a loss of bleed air inflow (caused by a malfunction of the flow control valve or by inappropriate or incomplete flight crew action) or by some other event.
It is unclear why the airplane failed to properly pressurize, but the preponderance of evidence points to a lack of inflow air.
Figure: LR-35 Crew Oxygen System Schematic, from NTSB DCA00MA005, Airworthiness 13 Attach 2 - Maintenance Manual Extracts.
[NTSB Aircraft Accident Brief, AAB-00/01, page 32] Following the depressurization, the pilots did not receive supplemental oxygen in sufficient time and/or adequate concentration to avoid hypoxia and incapacitation. The wreckage indicated that the oxygen bottle pressure regulator/shutoff valve was open on the accident flight. Further, although one flight crew mask hose connector was found in the wreckage disconnected from its valve receptacle (the other connector was not recovered), damage to the recovered connector and both receptacles was consistent with both flight crew masks having been connected to the airplane’s oxygen supply lines at the time of impact. In addition, both flight crew mask microphones were found plugged in to their respective crew microphone jacks. Therefore, assuming the oxygen bottle contained an adequate supply of oxygen, supplemental oxygen should have been available to both pilots’ oxygen masks.
There was a lot of speculation in the Learjet world about the position of the oxygen system pressure regulator and shutoff valve, reasoning that many operators shut the valve off between flights to prevent system leakage. The valve was found in the open position and this operator did not, as a practice, close the valve.
[NTSB Aircraft Accident Brief, AAB-00/01, page 31]
- The sequence of maintenance actions from July 22 through October 23, 1999, indicate that there were several pressurization-related discrepancies during this period. Maintenance records indicate that Sunjet Aviation personnel attempted to correct the discrepancies by cleaning the pressurization system outflow valve and performing system ground checks. Work on a staggered engine throttle condition, which resulted in the replacement of the left modulation valve on October 23, 1999, was also related to concerns about the pressurization system (as shown by Sunjet Aviation’s reference to pressurization on the removed modulation valve’s part tag). However, Sunjet Aviation was not able to provide records of pilot-reported discrepancies that led to these maintenance actions.
- The airplane’s maintenance records indicate that the oxygen bottle was last serviced with oxygen (by Sunjet Aviation) on September 3, 1999.
That is nearly two months prior to the mishap flight.
- A Sunjet Aviation official stated to the Safety Board that the accident captain had reported that the oxygen pressure gauge was in the green zone, indicating adequate pressure of 1,550 to 1,850 psi, during preflight checks on the day of the accident.
This appears to me to be fully fabricated, a way of covering up the fact the oxygen had not been serviced in nearly two months. I have never reported that a required item was in the normal zone to any official following my aircraft preflight. I have, many times, reported that the oxygen was insufficient and asked that it be serviced. But why would any pilot report the oxgyen is okay in this situation?
- The Safety Board contacted fixed-based operators (FBO) at 15 known destination airports visited by the accident airplane between September 26 and October 20, 1999, and none had any record of charges for oxygen servicing of the accident airplane. However, the Board cannot exclude the possibility that the airplane was serviced with oxygen after September 3, 1999, at a different airport or at no charge to Sunjet Aviation61 and that no record was made.
- In summary, the Safety Board could not determine the quantity of oxygen that was on board the accident flight.
[NTSB Aircraft Accident Brief, AAB-00/01, page 35]
- Investigations of other accidents in which flight crews attempted to diagnose a pressurization problem or initiate emergency pressurization instead of immediately donning oxygen masks following a cabin altitude alert have revealed that, even with a relatively gradual rate of depressurization, pilots have rapidly lost cognitive or motor abilities to effectively troubleshoot the problem or don their masks shortly thereafter. In this accident, the flight crew’s failure to obtain supplemental oxygen in time to avoid incapacitation could be explained by a delay in donning oxygen masks of only a few seconds in the case of an explosive or rapid decompression or a slightly longer delay in the case of a gradual decompression.