AIRCRAFT ACCIDENT BRIEF (RE PAYNE STEWART)
by National Transportation Safety Board
Accident No.: DCA00MA005
Operator or Flight Number: Sunjet Aviation
Aircraft and Registration: Learjet Model 35, N47BA
Location: Aberdeen, South Dakota
Date: October 25, 1999
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On October 25, 1999, about 1213 central daylight time (CDT), a Learjet Model 35, N47BA, operated by Sunjet Aviation, Inc., of Sanford, Florida, crashed near Aberdeen, South Dakota. The airplane departed Orlando, Florida, for Dallas, Texas, about 0920 eastern daylight time (EDT). Radio contact with the flight was lost north of Gainesville, Florida, after air traffic control (ATC) cleared the airplane to flight level (FL) 390. The airplane was intercepted by several U.S. Air Force (USAF) and Air National Guard (ANG) aircraft as it proceeded northwestbound. The military pilots in a position to observe the accident airplane at close range stated (in interviews or via radio transmissions) that the forward windshields of the Learjet seemed to be frosted or covered with condensation. The military pilots could not see into the cabin. They did not observe any structural anomaly or other unusual condition. The military pilots observed the airplane depart controlled flight and spiral to the ground, impacting an open field. All occupants on board the airplane (the captain, first officer, and four passengers) were killed, and the airplane was destroyed.
HISTORY OF FLIGHT
On October 25, 1999, the flight crew was scheduled to begin a 2-day trip sequence consisting of five flights. The flights on the first day were to be from Orlando Sanford International Airport (SFB), Sanford, Florida, to Orlando International Airport (MCO), Orlando, Florida; from MCO to Dallas-Love Field Airport (DAL), Dallas, Texas; and from DAL to William P. Hobby Airport, Houston, Texas.
The first flight of the day, a visual flight rules positioning flight operating under 14 Code of Federal Regulations (CFR) Part 91, was scheduled to depart SFB about 0800 EDT bound for MCO, which is approximately 15 nautical miles (nm) away. According to the Sunjet Aviation customer service representative on duty at SFB on the day of the accident, the captain reported for duty at SFB about 0630 EDT, and the first officer arrived about 0645 EDT. She stated that both pilots were in a good mood and appeared to be in good health.
A Sunjet Aviation line service technician stated that the captain asked him to pull the airplane out of the hangar, fuel it to 5,300 pounds fuel weight, connect a ground power unit to the airplane, and put a snack basket and cooler1 on the airplane. The first officer arrived at the airplane just before the fueling process started and stayed in the cockpit while the airplane was being fueled. The first officer then went inside the terminal building while the captain performed the preflight inspection of the airplane.
About 0725 EDT, an instrument flight rules flight plan was filed with the St. Petersburg Automated Flight Service Station for the second flight of the day, MCO to DAL, which would operate under 14 CFR Part 135. The flight plan indicated that N47BA was scheduled to depart MCO about 0900 EDT; follow a route over Cross City, Florida, to 32 degrees, 51 minutes north and 96 degrees, 51 minutes west; and proceed directly to DAL. The requested altitude was 39,000 feet.2 The flight plan also indicated that there would be five persons on board (two pilots and three passengers) and 4 hours and 45 minutes of fuel.
According to a witness, the accident airplane departed SFB about 0754 EDT. The flight arrived at MCO about 0810 EDT. An Aircraft Service International Group employee at MCO stated that after the airplane arrived, the captain told him that they were picking up passengers and did not require additional fuel. According to this witness, the passengers arrived about 30 minutes later and boarded the airplane. The Sunjet Aviation director of operations indicated that an additional passenger who was not on the original charter flight request boarded the accident airplane at MCO. Several bags were placed on board the airplane, including what the Aircraft Service International Group employee described as a big golf bag weighing about 30 pounds.
According to ATC radio transmissions, the flight departed MCO about 0919 EDT bound for DAL. At 0921:46 EDT, the flight contacted the Jacksonville Air Route Traffic Control Center (ARTCC) and reported climbing through an altitude of 9,500 feet to 14,000 feet.3
At 0921:51 EDT, the controller instructed N47BA to climb and maintain FL 260. N47BA acknowledged the clearance by stating, "two six zero bravo alpha." At 0923:16 EDT, the controller cleared N47BA direct to Cross City and then direct to DAL. N47BA acknowledged the clearance. At 0926:48 EDT, N47BA was issued instructions to change radio frequency and contact another Jacksonville ARTCC controller. N47BA acknowledged the frequency change.
At 0927:10 EDT, N47BA called the Jacksonville ARTCC controller and stated that the flight was climbing through an altitude of FL 230. At 0927:13 EDT, the controller instructed N47BA to climb and maintain FL 390. At 0927:18 EDT, N47BA acknowledged the clearance by stating, "three nine zero bravo alpha." This was the last known radio transmission from the airplane.4 The sound of the cabin altitude aural warning5 was not heard on the ATC recording of this transmission.6
At 0933:38 EDT (6 minutes and 20 seconds after N47BA acknowledged the previous clearance), the controller instructed N47BA to change radio frequencies and contact another Jacksonville ARTCC controller. The controller received no response from N47BA. The controller called the flight five more times over the next 4 1/2 minutes but received no response.
About 0952 CDT,7 a USAF F-16 test pilot from the 40th Flight Test Squadron at Eglin Air Force Base (AFB), Florida, was vectored to within 8 nm of N47BA.8 About 0954 CDT, at a range of 2,000 feet from the accident airplane and an altitude of about 46,400 feet,9 the test pilot made two radio calls to N47BA but did not receive a response. About 1000 CDT, the test pilot began a visual inspection of N47BA. There was no visible damage to the airplane, and he did not see ice accumulation on the exterior of the airplane. Both engines were running, and the rotating beacon was on. He stated that he could not see inside the passenger section of the airplane because the windows seemed to be dark. Further, he stated that the entire right cockpit windshield was opaque, as if condensation or ice covered the inside. He also indicated that the left cockpit windshield was opaque, although several sections of the center of the windshield seemed to be only thinly covered by condensation or ice; a small rectangular section of the windshield was clear, with only a small section of the glare shield visible through this area. He did not see any flight control movement. About 1012 CDT, he concluded his inspection of N47BA and proceeded to Scott AFB, Illinois.
About 1113 CDT, two Oklahoma ANG F-16s with the identification "TULSA 13 flight" were vectored to intercept the accident airplane by the Minneapolis ARTCC. The TULSA 13 lead pilot reported to the Minneapolis ARTCC controller that he could not see any movement in the cockpit. About 1125 CDT, the TULSA 13 lead pilot reported that the windshield was dark and that he could not tell if the windshield was iced.
About 1133 CDT, a TULSA 13 airplane maneuvered in front of the accident airplane, and the pilot reported, "we're not seeing anything inside, could be just a dark cockpit though...he is not reacting, moving or anything like that he should be able to have seen us by now."
About 1138 CDT, the TULSA 13 lead pilot stated, "my wingman is going to make a final pass and then we are going to head back to the [midair refueling] tanker." The TULSA 13 wingman reported, "we did not get close enough to see any icing on the window due to our configuration...we did get up behind him but did not see anything." About 1139 CDT, TULSA 13 flight departed for the tanker.
About 1150 CDT, two North Dakota ANG F-16s with the identification "NODAK 32 flight" were vectored to intercept N47BA. (TULSA 13 flight had returned from refueling, and both TULSA 13 and NODAK 32 flights maneuvered in close proximity to N47BA.) About 1157 CDT, the TULSA 13 lead pilot reported, "we've got two visuals on it. It's looking like the cockpit window is iced over and there's no displacement in any of the control surfaces as far as the ailerons or trims." About 1201 CDT, TULSA 13 flight returned to the tanker again.
At 1210:41 CDT, the sound of an engine winding down, followed by sounds similar to a stickshaker and an autopilot disconnect, can be heard on N47BA's cockpit voice recorder (CVR), which recorded the final 30 minutes of cruise flight. The CVR also captured the continuous activation of the cabin altitude aural warning, which ceased at 1212:26 CDT. At 1211:01 CDT, ATC radar indicated that N47BA began a right turn and descent. One NODAK 32 airplane remained to the west, while one TULSA 13 airplane broke away from the tanker and followed N47BA down. At 1211:26 CDT, the NODAK 32 lead pilot reported, "the target is descending and he is doing multiple aileron rolls, looks like he's out of control...in a severe descent, request an emergency descent to follow target." The TULSA 13 pilot reported, "It's soon to impact the ground he is in a descending spiral."
Both flight crewmembers were certificated under Sunjet Aviation, Inc., and Federal Aviation Administration (FAA) certification requirements. A review of FAA records indicated that the flight crewmembers had no records of airplane accidents, incidents, or enforcement actions. In addition, both flight crewmembers held valid Florida driver's licenses with no history of accidents or violations during the 3 years before the accident. A search of the National Driver Registry found no history of driver's license revocation or suspension for either flight crewmember.
The captain, age 42 at the time of the accident, held an airline transport pilot certificate (certificate no. 389484668, issued September 21, 1999) with the ratings and limitations of airplane multiengine land, commercial privileges for airplane single-engine land, and type ratings for Boeing 707, Boeing 720, and Learjet. His most recent FAA first-class medical certificate was issued on June 16, 1999, with no limitations.
According to his resume, the captain served as a copilot and a standardization/evaluation copilot on a USAF KC-135A from 1981 to 1984. His resume also indicated that he was an emergency procedures evaluator (simulator) during this period. From 1984 to 1988, the captain served as an aircraft commander on a USAF E-3A. From 1988 to 1993, the captain served as a classroom and in-flight instructor pilot on a Maine ANG KC-135E. During his USAF and ANG career, the captain accumulated 3,953 hours flying KC-135 and E-3A airplanes and achieved the rank of Major. For approximately the next 6 years, the captain worked in a nonflying capacity.
The captain was hired by Sunjet Aviation on September 21, 1999.10 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.
Family and coworkers indicated that the captain was in excellent health. He was a nonsmoker who did not take medications or consume alcohol. The captain lived in the Orlando, Florida, area. During the 3 days before the accident, the captain's family reported that he participated in routine activities around the house. They further reported that on the night before the accident, the captain went to bed about 2200 EDT and, on the day of the accident, left the house between about 0530 and 0600 EDT.
The First Officer
The first officer, age 27 at the time of the accident, held a commercial pilot certificate (certificate no. 595521666, issued April 15, 1999) with the ratings and limitations of airplane multiengine land, airplane single-engine land, instrument airplane, and type ratings for Learjet and Cessna Citation 500. The first officer was also certified as a flight instructor. Her most recent FAA first-class medical certificate was issued on October 1, 1999, with the limitation that she must wear corrective lenses.
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.
The first officer's friends reported that she was in good health. Friends further described the first officer as a nonsmoker who did not use caffeine and did not take other medications. The first officer lived in the Orlando, Florida, area. During the 3 days before the accident, she visited with friends in the Daytona Beach and Orlando areas. According to her friends, two nights before the accident, the first officer went to bed about 0100 EDT and awoke about 0900 EDT and, on the night before the accident, went to bed about 2200 EDT and awoke about 0545 EDT.
According to FAA records, the accident airplane, a Gates Learjet Model 35, serial number (S/N) 060, was manufactured in 1976 and had been maintained and operated by Sunjet Aviation since January 1999. The airplane's titleholder as of October 25, 1999, was Jet Shares One, Inc., and the previous titleholder was McMillin Aircraft, Inc. The airplane had a total of eight passenger seats. Two AlliedSignal (Honeywell) Model TFE731-2-2B turbofan engines powered the airplane.
Oxygen System Procedures
According to the Limitations section of the Learjet Model 35/36 Aircraft Flight Manual (AFM), flight crew and passenger oxygen masks are not approved for use above 40,000 feet cabin altitude.11 A "warning" in this section states that "passenger masks are intended for use during an emergency descent to an altitude not requiring supplemental oxygen." The manual also indicates that "passenger masks will not provide sufficient oxygen for prolonged operation above 34,000 feet cabin altitude. Prolonged operation above 25,000 feet cabin altitude with passengers on board is not recommended." In addition, the manual indicates that, above FL 250, in aircraft with ZMR-series oxygen masks, one flight crewmember must wear the oxygen mask around the neck; in aircraft with 6600214-series oxygen masks, the masks must be in the quick donning position.12 Further, the manual indicates that, above FL 410, the pilot, copilot, and passengers must wear oxygen masks. The maximum operating altitude for the airplane is 45,000 feet.
Normal Procedures Checklist
The Learjet Model 35/36 AFM states the following:
Normal preflight procedures (all checklist line items) must be accomplished prior to takeoff at the original departure point of a flight. At each intermediate stop of flight where both engines are shutdown, the Through-Flight Checklist may be used for preflight provided certain criteria are met during a stop.
Procedures on the checklist marked with the symbol (t) denote Through-Flight checklist items. The following items pertaining to the oxygen system are listed in the exterior preflight procedure: "Oxygen Bottle Supply Valve (if applicable)-Open (On)" and "Oxygen Discharge Disc (if applicable)-Condition." (Neither item is marked with the symbol t.) According to the FAA principal operations inspector (POI) assigned to the Sunjet Aviation certificate, the labeling on the oxygen bottle13 supply valve is misleading; the word "OFF" is visible when the valve is open. In addition, according to the Sunjet Aviation chief pilot, during the exterior preflight procedure, it would be possible to confuse the ON/OFF status of the oxygen system because of misleading markings. He stated that he reviewed that issue with the accident captain during training. He further stated that Sunjet Aviation pilots never turn the oxygen system off; this issue is emphasized during preflight training, and it is not company procedure to disconnect the flight crew oxygen masks. In addition, briefing the passengers (which includes oxygen system operation) is a required item in the Cabin Preflight section of the Learjet Model 35/36 AFM.
The Before Starting Engines checklist requires that the oxygen system must be checked and set as follows:
a. PASS MASK Valve - AUTO.
b. PASS OXY Valve - NORM.
c. OXYGEN PRESSURE Gauge - Check. (t)
d. Crew Masks:
(1) Check oxygen flow available. Select 100% oxygen.
Warning system checks are also included in the Before Starting Engines checklist. The Cabin Altitude Warning Check checklist includes the following:
(5) TEST Selector Switch - Rotate to CABIN ALT, then depress and hold TEST button. Cabin altitude warning horn shall sound.
(6) HORN SILENCE Switch - Momentarily engage. Cabin altitude warning shall cease.
(7) TEST Button - Release.
The Before Starting Engines checklist calls for the pressurization controls to be checked and set as follows:
(4) L [left] and R [right] BLEED AIR Switches - Check, On.
(5) CABIN AIR Switch - OFF.
(6) PRESSURIZATION AUTO-MAN Switch - AUTO.
(7) AIRCRAFT ALT [altitude] Selector Knob - Rotate to cruise altitude. (t)
(8) Cabin RATE Selector - Position as desired.
(9) IN NORMAL-OUT DEFOG Knob - Push in.
The Taxi and Before Takeoff checklist includes the following:
18. Pressurization System - Set. (t)
19. CABIN AIR Switch - NORM. (t)
The After Takeoff checklist includes the following:
6. Pressurization System - Set.
a. Cabin Altitude and Cabin Climb Indicators - Monitor.
b. Cabin RATE Selector - As desired.
The Climb checklist indicates that the following check should be made when climbing through 18,000 feet:
Crew Masks - Positioned to quick donning position at or before FL 250.
Federal Aviation Regulations Part 135: Oxygen-Use Rules
Title 14 CFR 135.89, "Pilot Requirements: Use of Oxygen," states the following:
(a) Unpressurized aircraft. Each pilot of an unpressurized aircraft shall use oxygen continuously when flying-
(1) At altitudes above 10,000 feet through 12,000 feet MSL [mean sea level] for that part of the flight at those altitudes that is of more than 30 minutes duration; and
(2) Above 12,000 feet MSL.
(a) Pressurized aircraft.
(1) Whenever a pressurized aircraft is operated with the cabin pressure altitude more than 10,000 feet MSL, each pilot shall comply with paragraph (a) of this section.
(2) Whenever a pressurized aircraft is operated at altitudes above 25,000 feet through 35,000 feet MSL, unless each pilot has an approved quick donning type oxygen mask -
(i) At least one pilot at the controls shall wear, secured and sealed, an oxygen mask that either supplies oxygen at all times or automatically supplies oxygen whenever the cabin pressure altitude exceeds 12,000 feet MSL; and
(ii) During that flight, each other pilot on flight deck duty shall have an oxygen mask, connected to an oxygen supply, located so as to allow immediate placing of the mask on the pilot's face sealed and secured for use.
(1) Whenever a pressurized aircraft is operated at altitudes above 35,000 feet MSL, at least one pilot at the controls shall wear, secured and sealed, an oxygen mask required by paragraph (b)(2)(i) of this section.
(2) If one pilot leaves a pilot duty station of an aircraft when operating at altitudes above 25,000 feet MSL, the remaining pilot at the controls shall put on and use an approved oxygen mask until the other pilot returns to the pilot duty station of the aircraft.
Abnormal Procedures Checklist
According to the Learjet Model 35/36 AFM Abnormal Procedures checklist, the following must be accomplished for a pressurization loss at altitude:
Up to 10,000 (± 500) Feet Cabin Altitude
1. Oxygen Masks - Don.
2. Engine RPM - Maintain.
3. IN NORMAL OUT DEFOG Knob - Push in.
4. WSHLD [windshield] HEAT Switch - AUTO.
5. CABIN AIR Switch - OFF.
6. AUTO MAN Switch - MAN.
7. UP DN [up down] Manual Control (red) - As required to maintain satisfactory pressurization.
At 10,000 (± 500) Feet Cabin Altitude
1. Cabin altitude aural warning horn will sound.
NOTE: At 10,000 (± 500) Feet Cabin Altitude, control pressure to the outflow valve is trapped. This deactivates the Automatic Mode and stops cabin altitude from rising higher if the failure is in the automatic control system.
2. If cabin pressurization cannot be maintained, execute EMERGENCY DESCENT as follows:
a. Oxygen Masks - Don. Select 100% oxygen.
b. Thrust Levers - Idle.
c. Autopilot - Disengage.
d. Spoiler Switch - EXT [extend].
e. LANDING GEAR Switch - DN below [maximum operating limit speed] or [maximum landing gear extended speed] as appropriate for altitude. Keep sideslip angles to a minimum (ball centered) when extending landing gear.
f. Descend at [maximum operating limit speed] or [maximum gear extended speed] as appropriate for altitude. Descent from 45,000 feet to 15,000 feet requires approximately 2 minutes, 45 seconds.
g. Transponder - 7700.
h. Oxygen Mic Switches (Pilot and Copilot - On).
i. Notify ATC.
j. Check and Assist Passengers.
The airplane oxygen system provides emergency oxygen for the flight crew and passengers and consists of a single oxygen bottle, an oxygen bottle pressure regulator/shutoff valve, an oxygen pressure gauge, an overboard discharge relief valve and indicator, flight crew mask quick disconnect valves, flight crew masks, a manual passenger shutoff valve (labeled PASS OXY), an oxygen aneroid valve, an oxygen aneroid bypass shutoff valve (labeled PASS MASK), passenger oxygen actuator lanyard valves, and passenger masks. The quick disconnect valve allows the connection of the flight crew oxygen masks to the oxygen system.
Oxygen Bottle Components and Cockpit Oxygen Pressure Gauge
Oxygen is available to the flight crew at all times when the oxygen bottle pressure regulator/shutoff valve is open. Oxygen is available to the passengers automatically above 14,000 ± 750 feet cabin altitude or manually (at any cabin altitude) by opening the normally closed oxygen aneroid bypass shutoff valve, which is located on the pilot's sidewall.
The oxygen bottle has a storage capacity of 38 cubic feet at 1,800 pounds per square inch (psi). Oxygen pressure for the flight crew and passenger distribution systems is reduced to 70 psi via the oxygen bottle pressure regulator/shutoff valve that is mounted directly on the bottle. The oxygen bottle and attached oxygen bottle pressure regulator/shutoff valve are located in the nose cone of the airplane and are inaccessible to the flight crew during flight.
Flight Crew Masks/Regulators
This airplane was equipped with two different types of flight crew masks: the ZMR-series mask and the 6600214-series mask. Both flight crew masks have mask-mounted regulators manufactured by Puritan-Bennett and are plugged into quick disconnect valves located in the left and right sidewalls of the cockpit. Each mask has a microphone controlled by the OXY-MIC-ON-OFF switch on the crew microphone jack panel near the pilot's and copilot's armrests. The masks are stowed in bracket and strap assemblies located behind the captain's and first officer's seats.
The ZMR-series regulator has two positions (NORMAL and 100%) and provides oxygen diluted with cabin air upon demand when the selector lever (located on the side of regulator) is in the NORMAL position. When the lever is in the 100% position, the regulator provides 100 percent oxygen upon demand, regardless of cabin altitude.
6600214-Series (Rogers) Regulator
The second regulator, a Rogers regulator, part number 112145A, has three positions (NORMAL, 100%, and EMERGENCY) and functions similarly to the ZMR-series regulator in the NORMAL and 100% positions. This regulator design also incorporates a dilution aneroid that will progressively shut off the diluter (cabin) port upon rising cabin altitudes, thereby supplying 100 percent oxygen at cabin altitudes above 33,000 feet. When the selector lever is in the EMERGENCY position, the regulator supplies 100 percent oxygen, regardless of altitude, at a positive pressure of approximately 0.15 psi. This regulator will also automatically supply oxygen under positive pressure (approximately 130 liters per minute at 0.5 psi) at cabin altitudes above 39,000 feet, regardless of the regulator-selected mode.
Passenger Oxygen Distribution System
Manual Passenger Shutoff Valve and Oxygen Aneroid Bypass Shutoff Valve
Normal operation of these valves requires approximately two complete turns of the valve actuator shaft. Normal oxygen system configuration is with the manual passenger shutoff valve open and the oxygen aneroid bypass shutoff valve closed. (Aside from the knobs and the identification labels, the valves are identical.)
Passenger Oxygen Actuator Lanyard Valves
After the passenger oxygen masks fall from their compartments, passengers must pull a lanyard that is attached to the passenger oxygen actuator valve to initiate the flow of oxygen to the masks.
The pneumatic system uses bleed air extracted from the engine compressor sections and includes controls for the regulation and distribution of low-pressure air from the fourth stage axial compressor and high-pressure air from the centrifugal compressor.
Bleed air is provided to a bleed air shutoff/regulator valve (modulation valve) on each engine. When open, these valves regulate the flow of bleed air to a common manifold that supplies the pneumatic systems. This regulated bleed air is used for cabin pressurization and heating, anti-icing systems (for example, the engine nacelles, wing and stabilizer leading edges, and windshield), the pressurization system jet (vacuum) pump, and pressurization of the hydraulic reservoir.
Control of pneumatic bleed air is accomplished with the left and right BLEED AIR switches, which are located on the copilot's lower right switch panel. When the BLEED AIR switch is placed in its OFF position, a shutoff solenoid on the respective modulation valve is energized, and the spring-loaded open modulation valve is closed using bleed air pressure. When the BLEED AIR switch is placed in its ON position, the respective shutoff solenoid is deenergized, causing the modulation valve to regulate a downstream pressure of 27 to 35 psi.
Bleed air check valves, located downstream of each modulation valve, allow flow in one direction and prevent the loss of bleed air during single-engine operation. A bleed air manifold serves as a collection/distribution point for regulated bleed air from either engine. From the manifold, bleed air is distributed to the cabin for pressurization and heating via the flow control valve. The bleed air manifold also supplies the various other pneumatic systems previously identified.
Windshield Anti-Ice (Defog) Shutoff Valve
The windshield anti-ice shutoff valve is used to provide an alternate bleed air source for emergency pressurization when the IN NORMAL/OUT DEFOG knob, which is located below the instrument panel to the left of the pedestal, is pushed in. The shutoff valve is controlled by one of two switches mounted on the anti-ice control panel, which is located on the left side of the instrument panel, by positioning the WSHLD HEAT AUTO/MAN switch to AUTO or by placing the same switch in MAN and using the ON/OFF switch to open and close the shutoff valve.
In the event of a loss of normal pressurization, windshield anti-ice (defog) air can be routed into the cabin as an emergency source of pressurization16 by ensuring that the IN NORMAL/OUT DEFOG knob is in the IN NORMAL position, setting the WSHLD HEAT switch to AUTO, and setting the CABIN AIR switch to OFF (closing the flow control valve). Pressurization will then be maintained automatically. If pressurization is not maintained in the AUTO position, cabin altitude can be maintained by manually controlling the outflow valve using the UP/DN control switch, located on the pressurization module.
Air Conditioning System
The air conditioning system regulates the volumetric flow rate and temperature of bleed air entering the cabin and cockpit areas. Primary system components include system switches, the flow control valve, hot air bypass valve, ram air heat exchanger, and distribution ducts/check valves.
Flow Control Valve
The flow control valve, which is solenoid controlled by the CABIN AIR switch and pneumatically operated, is located in the tail cone and regulates the flow rate of conditioned bleed air entering the cabin for pressurization and heating.
Hot Air Bypass Valve
The position of the hot air bypass valve is controlled by the cabin temperature control system and determines how much bleed air passes through the ram air heat exchanger to control the temperature of bleed air supplied to the cabin.
CABIN AIR Switch
The CABIN AIR switch has three positions (OFF, NORM, and MAX). When the CABIN AIR switch is in the OFF position, the shutoff solenoid is energized, and the flow control valve is closed. When the CABIN AIR switch is in the NORM and MAX positions, the shutoff solenoid is deenergized, and the flow control valve supplies bleed air for cabin pressurization.
Cabin pressurization is provided by conditioned air entering the cabin through the air distribution ducts and is controlled by modulating the amount of air exhausted from the cabin. The major components of the pressurization system include the cabin air exhaust control valve, cabin safety valve, differential pressure relief valves, cabin altitude limiters, pressurization jet pump (a vacuum regulator), and the pressurization module. During flight, the pressurization system is completely independent of the electrical system.
Normal pressurization is controlled with the altitude controller and RATE selector, located on the pressurization module. Before takeoff, the pressurization module AUTO/MAN switch should be set to AUTO, the CABIN AIR switch should be set to NORM, the AIRCRAFT ALT selector knob on the altitude controller should be set to cruise altitude, and the IN NORMAL/OUT DEFOG knob should be pushed in. After takeoff, the RATE selector may be adjusted to obtain the desired rate of cabin pressurization. The rate is monitored by the flight crew with the cabin rate-of-climb indicator, and the cabin altitude is monitored with the cabin altimeter.
Outflow Valve17 and Safety Valve Cabin Altitude Limiters
There are two cabin altitude limiters; one is associated with the outflow valve, while the other is associated with the safety valve. If cabin pressure decreases for any reason (such as a loss of bleed air or a faulty outflow valve), the cabin altitude limiters would command the outflow valve and the safety valve closed at an altitude of 11,000 ± 1,000 feet in an attempt to maintain cabin pressure.
Differential Pressure Relief Valve
There are two differential pressure relief valves; one is installed in the outflow valve control pressure line to limit the normal operating cabin differential pressure to 8.9 psi, while the other is set to limit the maximum cabin differential pressure to 9.2 psi by opening the safety valve.
Cabin Altitude Controller
The cabin altitude controller consists of a selector dial with a window and pointer that displays the cabin altitude setting in relation to aircraft altitude.
The accident site was located at 45 degrees, 25 minutes north latitude and 98 degrees, 45 minutes west longitude and was characterized by a crater that measured 42 feet, 4 inches long (oriented east to west) by 21 feet, 7 inches wide (oriented north to south). The crater measured 8 feet, 6 inches at its deepest point, which was approximately 7 feet south of the crater's northern wall. The local terrain was relatively flat. A marsh was located approximately 80 feet due east of the crater.
The main airframe wreckage was located in or near the impact crater. The majority of the rest of the wreckage was found within an approximately 75-foot radius. Additional wreckage was recovered up to 150 feet away. Almost all of the wreckage found outside of the crater was located east of the crater.
A debris field of smaller wreckage, including instrument panel components, the flight manual, seat cushions, life vests, and personal effects, extended outward from the impact crater in a north-northeasterly direction toward the marsh. The debris field formed a conical shape of approximately 35 degrees.
Oxygen Bottle Components and Cockpit Oxygen Pressure Gauge
The oxygen bottle was dented but not breached during the impact sequence. All oxygen lines to the bottle were separated during the breakup. The oxygen bottle regulator/shutoff valve was found in the open position. The cockpit oxygen pressure gauge viewing glass was broken. The needle remained attached and read just below zero; needle slap marks were found at this reading.
Quick Disconnect Valves
Both flight crew oxygen mask quick disconnect valves were recovered.18 Minor scratches were noted on the face of the pilot's quick disconnect valve and internally near one of the retainer pins. The retainer pins that interlock with the flight crew oxygen mask connector were found intact; one pin was slightly bent.
The inner walls of the copilot's quick disconnect valve showed minor scoring and gouges, which penetrated the nickel plating of the retainer assembly and gouged the (brass) base metal beneath. The internal scoring was predominately noted midway between the two retainer pins and was in line with the bore of the valve. Minor scoring was also noted immediately adjacent to each retainer pin.
Flight Crew Masks/Regulators
Portions of two different flight crew mask regulators were recovered.19 Investigators could not determine the installation location of the flight crew masks at the time of impact because of severe disruption of the cockpit furnishings.
One regulator was identified as a ZMR-series diluter-demand regulator. Because of impact fragmentation, no determination could be made regarding the position of the selector lever from the section of the mask recovered.
6600214-Series (Rogers) Regulator
Approximately 70 percent of the Rogers regulator selector knob, including its pointer, was not recovered. Examination of the disassembled regulator revealed that the regulator was in the EMERGENCY position after impact; rotational scoring was noted external to the regulator assembly and on the lower surfaces of the remaining portion of the selector knob.
Passenger Oxygen Distribution System
Manual Passenger Shutoff Valve and Oxygen Aneroid Bypass Shutoff Valve
The manual passenger shutoff valve and the oxygen aneroid bypass shutoff valve were found intact, but their operating knobs and identification labels were not recovered. Neither valve exhibited additional external damage, and each valve was packed with dirt and debris. One valve was found in the open position, and the other valve was found in the closed position. No determination could be made regarding which valve was the manual passenger shutoff valve and which was the oxygen aneroid bypass shutoff valve because of the missing knobs and identification labels.
Passenger Oxygen Actuator Lanyard Valves
All five passenger oxygen actuator lanyard valves were recovered and examined. No determination could be made as to whether these valve assemblies were pressurized at the time of impact. None of the lanyards remained attached to the oxygen actuator lanyard valve assemblies.
Passenger Oxygen Masks
Several pieces of passenger oxygen masks were recovered. Enough fragments were recovered to compose approximately four separate masks, but none of the pieces composed a single mask. Several miscellaneous sections of passenger oxygen mask hoses were also recovered.
Bleed Air Shutoff/Regulator (Modulation) Valves
The left and right modulation valves were recovered, and both valves were determined to be near their closed positions. Assembly data plates were not found; therefore, no determination could be made regarding their installation on the left or right engine. Because the modulation valves are redundant, the malfunction of one valve would not disable the pneumatic system on the airplane.20
Windshield Anti-Ice (Defog) Shutoff Valve
The motor-operated windshield anti-ice (defog) shutoff valve21 was found in the closed position. Investigators determined the valve position by comparing the valve OPEN/CLOSED indicator flag and actuator arm with that of an exemplar shutoff valve. Neither WSHLD HEAT switch was recovered.
Air Conditioning System
Flow Control Valve
The flow control valve was recovered from the wreckage in its fully closed position. The flow control valve upper actuator housing and servo mechanism were missing. (The valve was in its spring-loaded closed position when the lower actuator housing was destroyed.) The valve main spring was in place and fully relieved because of the missing upper valve housing. The valve was free to operate in its full range of motion.
Hot Air Bypass Valve, Ram Air Check Valve, and a Cabin Air Distribution Check Valve
The hot air bypass valve vane was found intact and in an intermediate position. No obstructions were noted. The ram air check valve was examined and found to operate properly. One of two cabin air distribution check valves was recovered intact and removed from its crushed duct; the valve was complete and was found installed correctly.
Cabin Altitude Controller
The cabin altitude controller display assembly was found crushed; the selector dial and underlying cabin/aircraft altitude dial remained together. The aircraft altitude setting was found set to approximately 36,000 feet.22
Outflow Valve and Safety Valve Cabin Altitude Limiters
The outflow valve cabin altitude limiter, which is located in the pressurization module, was disassembled, and the ball valve and valve stem assembly moved freely after cleaning. No damage was noted to the upper surface of the aneroid bellows (capsule). The safety valve cabin altitude limiter was not recovered.
Differential Pressure Relief Valve
One of two differential pressure relief valves was recovered. The valve was disassembled, and its internal metering valve moved freely. The valve diaphragm was torn around its entire perimeter, and the housing was destroyed; the valve spring was present. Investigators could not determine which relief valve was recovered.
MEDICAL AND PATHOLOGICAL INFORMATION
Tissue specimens from the first officer tested negative for a wide range of drugs, including major drugs of abuse.23 The FAA's Final Forensic Toxicology Fatal Accident Report indicated that 41 mg/dL of ethanol and 1 mg/dL of acetaldehyde were detected in muscle. The report noted that the "ethanol found in this case may potentially be from postmortem ethanol formation and not from the ingestion of ethanol."
No toxicology testing was completed for the captain because of the difficulty of identifying and isolating tissue samples.
TESTS AND OTHER RESEARCH
Learjet Model 35 Flight Test
At the Safety Board's request, Learjet performed a flight test of a Model 35 airplane to capture CVR audio for comparison to the accident CVR tape24 and to validate the performance of the pressurization system during an ascent similar to that of the accident flight. The test of the pressurization system was performed with the air conditioning system selected OFF at takeoff. (The CABIN AIR switch was placed in the OFF position.)
The flight test revealed that during the climb with cabin air secured, the cabin altitude lagged the actual altitude of the airplane by approximately 3,500 feet. The cabin altitude aural warning25 activated at a cabin altitude of 10,000 feet while the airplane was passing through a flight altitude of 13,500 feet.
After the accident, engine teardowns performed by Honeywell, Inc., revealed that the type and degree of damage observed on the right engine was indicative of windmilling engine rotation, but not operation, at the time of impact. The type and degree of damage observed on the left engine was indicative of engine operation at the time of impact. Further inspection revealed that no preaccident condition on either engine would have interfered with normal operation.
Pressurization System Computer Simulations
At the Safety Board's request, Honeywell performed two computer simulations to provide a better understanding of the cabin rate of climb during ascent. The first simulation assumed that the air conditioning system (the CABIN AIR switch) was selected OFF at takeoff, resulting in the loss of bleed air to the cabin; the second simulation assumed that a loss of cabin air occurred at several altitudes at or above 25,000 feet.26
The first simulation predicted that the cabin altitude would lag slightly behind the flight altitude of the airplane as it continued its climb. (The cabin reached an altitude of 10,000 feet as the airplane passed through a flight altitude of 10,600 feet.)
The second simulation considered the loss of cabin air at flight altitudes of 25,000; 30,000; 35,000; and 40,000 feet. The results predicted that the cabin altitude would ascend to 9,500 feet in approximately 24 to 44 seconds, depending on the cabin altitude at the time of inflow valve closure. The simulation further predicted that the cabin altitude would ascend to 25,000 feet in approximately 2 ½ minutes and approach the aircraft flight altitude in 4 to 5 minutes from the initiation of the failure condition.
Oxygen Depletion Calculations
The airplane's maintenance records indicated that the oxygen bottle was last serviced on September 3, 1999. Between September 3 and the date of the accident flight, Sunjet Aviation operated the airplane for about 104.6 flight hours, on 90 flights. The Safety Board was unable to determine exactly how many of these flight hours were above 35,000 feet. ATC voice tapes from one of the flights indicated that the airplane was cleared to FL 370 on one leg. Although no radar data for that flight was available, the Board estimated (using ground speed and distance) that the airplane would have cruised above 35,000 feet for at least 30 to 40 minutes during that round trip itinerary. The captain of that flight told investigators that when the airplane was above 35,000 feet during that flight, he used supplemental oxygen. Board calculations indicated that this reported oxygen usage would have depleted the airplane's oxygen supply by 14 to 25 percent, depending on which mask was used.