On June 18, 2002, about 1330 Alaska daylight time, a passenger in a Piper PA-31-350 twin-engine airplane, N828KT, became incapacitated during a sightseeing flight over Denali National Park, about 50 miles north-northwest of Talkeetna, Alaska. The airplane was being operated by K2 Aviation, Talkeetna, Alaska, under Title 14, CFR Part 135, as a sightseeing flight in visual flight conditions at 21,000 feet, when the incident occurred. The airline transport certificated pilot and the seven other passengers were not injured. An IFR flight plan was filed, and company VFR flight following procedures were in effect. The flight originated at the Talkeetna Airport about 1300.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on August 1, 2002, an airworthiness inspector with the Federal Aviation Administration (FAA), Anchorage Flight Standards District Office (FSDO), reported that he received a telephone call from the son of a passenger on the flight. The son related that he and his mother were on the sightseeing flight over the Mt. McKinley area. About 30 minutes after departure, while the airplane was flying at 21,000 feet, the mother of the caller became unresponsive. All passengers were utilizing supplemental oxygen delivered from the airplane's oxygen system. The pilot was informed of the situation, and he immediately returned to Talkeetna. The female passenger was met by emergency medical personnel, and was transported to a hospital in Palmer, Alaska.

During a telephone conversation with the NTSB IIC on August 19, 2002, the son of the passenger indicated that the passengers received a briefing from the pilot. The oxygen system was explained, including the need for all passengers to plug their oxygen mask into the airplane oxygen port, and to verify oxygen was flowing into each oxygen mask by the confirmation of a visible green band contained in the oxygen mask tubing. After takeoff, the flight proceeded over numerous glaciers. The pilot then had everyone put on their oxygen mask. Shortly after putting on her mask, the passenger was observed by her son to take some type of medication pill from her purse. The son inquired if the passenger was feeling O.K., and she responded, "yes." The son checked his mother's oxygen mask and it appeared to fit around her nose. The flight proceeded toward the summit of Mt. McKinley. The passenger later recalled seeing climbers on the mountain. She was initially taking pictures, but handed her camera to her son, saying she wanted to lay back and relax. The son inquired whether his mother was feeling sick, but she responded with a thumbs-up gesture and said she was O.K. She laid her head back in the seat and closed her eyes. The son again checked her oxygen mask. About five minutes later, the passenger was unresponsive to her son's questions. He estimated that his mother's incapacitation was about 30 minutes after takeoff, and about 10 minutes after beginning the use of oxygen.

The son of the passenger reported that the pilot was informed of the incapacitation and he began a descent toward Talkeetna. The son said he held his mother's head to maintain an open airway, and noticed that her breathing was labored. Her oxygen mask had visible fogging during her breathing cycles. During the flight to Talkeetna, the passenger remained seated in an upright position and unconscious. At one point, the son yelled his mother's name, which elicited a fluttering of her eyelids. Nine minutes later, the flight landed at Talkeetna. The passenger was transported to a hospital in Palmer, Alaska. During the ambulance ride while lying down, the son reported that his mother became more verbally responsive by answering obliquely to some questions, and ignoring others. She exhibited twitching of her right hand.

After admission to the hospital in Palmer, the passenger was placed on a ventilator for a period of time, and according to her son, his mother initially had a diagnosis of cerebral and pulmonary edema. On June 30, 2002, the passenger was transported to a hospital in Springfield, Illinois, then to a rehabilitation hospital, also in Springfield, and ultimately discharged for in-home rehabilitation on August 18, 2002. The son of the passenger reported that his mother has a diagnosis of an anoxic brain injury, and that she sustained cognitive deficits and physical limitations.

During a telephone conversation with the NTSB IIC, on August 21, 2002, the pilot indicated that the McKinley sightseeing flight usually lasts one hour and 15 minutes. An initial climb is made to about 8,000 feet msl. About 35 to 45 minutes later, a climb is made to 11,000 feet msl where everyone plugs in their oxygen mask to the airplane system, and everyone puts on their mask. The pilot said he verifies oxygen flow by observing a green flow indicator in each passenger's oxygen mask tubing and then climbs the airplane to 21,000 feet for a view of the summit of Mt. McKinley. He banks the airplane left and right so the passengers can get photographs. He then descends toward Talkeetna. The pilot said he was using a Scott brand oxygen mask with a bag reservoir and microphone, at a flow rate of 2.0 liters per minute. The passengers were using a disposable oxygen mask without a reservoir bag, at a flow rate of 1.5 liters per minute.

In the Pilot/Operator Accident/Incident Report (NTSB form 6120.1) submitted by the pilot, the pilot indicated he was informed of the passenger's incapacitation by the passenger's son about 60 minutes into the flight. The son also told the pilot that his mother had taken some type of pill medication. The medication was not identified. The pilot made radio contact with Talkeetna and emergency medical personnel met the flight on the ground. The pilot reported that the passenger was a large, elderly female, who according to her son, had a pre-existing respiratory and heart condition, and had undergone recent heart surgery.

Airplane Information

The incident airplane is non-pressurized, and has a service ceiling of 24,000 feet. The airplane's oxygen system, manufactured by Scott Aviation, consists of a high-pressure oxygen cylinder that supplies oxygen to an oxygen port at each seat. A continuous flow of supplemental oxygen is delivered to an oxygen mask when the airplane's oxygen system is turned on, and a fitting is inserted into an oxygen port. Each oxygen mask is attached at the end of plastic tubing running from a metal fitting to the mask. The flow rate of each mask is determined by the size of an orifice contained in the fitting. Flow of oxygen is visually determined when the fitting is plugged into an oxygen port, and a green band in an in-line flow indicator, becomes visible to the user.

According to the pilot's operating handbook for the incident airplane, the oxygen mask equipment is a Scott Aviation partial rebreather mask equipped with a reservoir bag. The bag acts as a reservoir where oxygen from the airplane's oxygen system mixes with partial exhalations from the passenger. The flow rate of Scott oxygen masks are identified by a stripe on the oxygen fitting. The operating handbook specifies that passenger's oxygen masks be identified with a gold stripe (1.5 liters per minute) or a red stripe (2.0 liters per minute). The pilot's oxygen mask, a partial rebreather oxygen mask equipped with a reservoir bag and a radio microphone, utilizes a flow rate of 2.0 liters per minute.

The FAA inspector indicated that following the call from the passenger's son, he inspected the oxygen system on the incident airplane. He did not discover any deficiencies with the oxygen system hardware. The inspector noted that at each passenger seat position, the operator was not utilizing the Scott oxygen mask with a reservoir bag, but was utilizing a disposable hospital type, Hudson RCI model 1041 medium concentration oxygen mask without a reservoir bag, with a flow rate of 1.5 liters per minute.

Aircraft Certification Information

Certification standards for the use of oxygen are published by the FAA in Title 14, CFR Part 23.1441 through 23.1449.

Paragraph (a)(3) of Federal Aviation Regulation Sec. 23.1443, states, in part: "(3) The minimum mass flow of supplemental oxygen supplied for each user must be at a rate not less than that shown in the following figure for each altitude up to and including the maximum operating altitude of the airplane." A review of the chart, with graph lines added by the NTSB IIC as a visual aid, revealed that at 21,000 feet, the minimum mass flow is about 1.8 liters per minute. A copy of the FAA's chart, with added graph lines, is included in the public docket of this report.

The FAA has a published Technical Standard Order (TSO) C64a, that prescribes the minimum performance standards for continuous flow, passenger oxygen masks. TSO-64a states, in part, that masks must meet the standards set forth in the Society of Automotive Engineers, Inc., (SAE), Aerospace Standard (AS) 8025. In addition, SAE Aerospace Standards for "Continuous Flow Aviation Oxygen Masks for Non-Transport Category Aircraft" are contained in AS 1224. AS 1224, Revision B, includes a description of four types of masks:

a. Open port dilution rebreathing mask - incorporates a rebreather bag into which exhaled gases are mixed with oxygen flowing into the bag. Valving is not present between the mask facepiece and the rebreathing bag.
b. Valved or restrictive phase dilution rebreathing mask - (1) utilizes a rebreather bag into which a constant flow of oxygen is introduced. A check valve or a restrictive sponge between the mask and ambient air, is provided so that ambient air will not be admitted before the rebreather bag has been depleted, or (2) utilizes a rebreathing bag but incorporates a restrictive sponge or other means which admits dilution air when subjected to a significant decrease in intra-mask pressure. Valving is not present between the mask facepiece and the rebreather bag.
c. Valved or restrictive phase dilution reservoir mask - provides the most efficient physiological use of constant flow oxygen and provides a mask utilizing a reservoir bag incorporating a check valve between the mask facepiece and the reservoir bag to prevent introduction of exhaled gases.
d. Open port, or restrictive dilution mask without rebreathing or reservoir bag - this mask is defined as a mask which incorporates dilution ports or restrictive dilution such as by the use of open-cell foam construction or one or more air inlet valves. This mask is not recommended for efficient physiological use of the aircraft oxygen, commensurate with requirements for pilot safety or survival in hypoxic environments at altitude.

The Scott Aviation oxygen mask specified in the operating handbook for the incident airplane is a type (b) mask. The Hudson mask being utilized by the operator at the time of the incident is a type (d) mask.

The mask performance section of AS 1224, 5.1.1 Altitude Limitations, contains recommended altitude limitations for constant flow masks for use in pressurized, or non-pressurized aircraft, whether worn by passengers or crew. The limitations are based on theoretical equipment capabilities and do not necessarily account for variations in the degree of training, physical activity, duration of exposure, general health, and/or altitude tolerance of the user. An open port or restrictive phase dilution mask without a rebreather or reservoir bag (type d) for use in non-pressurized aircraft, has a recommended altitude limitation of 15,000 feet.

Company Briefing Information

According to the FAA inspector, the pilot provided the passengers with a verbal briefing about the proper use of the oxygen masks. Additional information about the oxygen masks was contained on a briefing card available at each passenger seat. The briefing did not include any questions about the passenger's health or preexisting medical conditions.

High Altitude Operations in Alaska

In the United States, sightseeing operations to high altitudes in non-pressurized airplanes, where oxygen is required, are only being conducted in Alaska, primarily during the summer. Two operators are conducting these operations from Talkeetna, and market their tours as providing a view of the summit of Mt. McKinley. According to the FAA, during the high altitude sightseeing season, the two operators conducted about 500 flights, carrying about 3,500 to 4,000 passengers.

Anoxia/Hypoxia Physiology

A review of medical literature available on the internet, revealed that hypoxia and anoxia refer to an altered amount of oxygen availability to the brain. The terms are used interchangeably, but hypoxia refers to a decreased amount of oxygen. Anoxia refers to an absence of oxygen. The three principal mechanisms capable of producing brain anoxia are: Decreased oxygen crossing from the lungs to the bloodstream; Decreased circulation of blood to the brain; Insufficient amount or ability of red blood cells to deliver oxygen to the brain.

Exposure to high altitudes without supplemental oxygen can produce hypoxia. An anoxic brain injury may develop from hypoxia, or from other injury mechanisms. The most common anoxic brain injury is stagnant ischemic anoxia, also called hypoxic-ischemic injury. Ischemia refers to a deficiency of blood to a part, due to constriction or obstruction of a blood vessel. Hypoxic-ischemic injury mechanisms can include cardiac arrest, trauma, electrocution, choking, near drowning, drug overdose, atherosclerostic cardiovascular disease, and anesthesia/operative complications. These may produce brain injury symptoms related to cognitive deficits and physical deficits.

Passenger Medical History

A request was made to the passenger's son, and to the passenger's attorney for copies of the passenger's medical history on September 4, 2002. The passenger's medical records were received by the NTSB on March 12, 2003. The records were reviewed by the National Transportation Safety Board's Medical Officer, Washington, D.C.

A review of the passenger's medical records revealed that she had a long history of significant heart disease, with a heart bypass operation in 1994. She had two vein grafts onto a severely diseased right coronary artery, and had some abnormal heart rhythms while hospitalized for the bypass operation.

A review of the passenger's hospitalization records from her postincident hospitalization revealed that her initial assessment included hypoxia, and possible high altitude pulmonary edema and high altitude cerebral edema. Her hospitalization records documented EKG changes consistent with cardiac ischemia (decreased blood flow to the heart), chest X-ray and chest CT scan results consistent with congestive heart failure, and serum Troponin elevation consistent with a heart attack.

The NTSB's Medical Officer's report is included in the public docket of this report.


Oxygen masks utilized by the medical community, by their individual design, vary in their ability to deliver increasing oxygen concentrations to the user. A simple mask, without any reservoir bag, receives oxygen directly to the mask at a specified flow rate, and any exhalations are directed to the atmosphere through holes in the sides of the mask.

Oxygen concentration is increased to the user in a partial rebreather type mask. This type of mask receives oxygen into a reservoir bag, from which the user inhales. Exhalations are partially directed back into the reservoir bag, mixing with a new supply of oxygen, and partially directed to the atmosphere through holes or a vent in the mask.

Maximum oxygen concentration is delivered to the user by a non-rebreather type mask. This type of mask receives oxygen into a reservoir bag, from which the user inhales. Because of a one-way valve, exhalations are blocked from reentering the reservoir bag, and are directed to the atmosphere through flapper valves in the sides of the mask.

The FAA's Civil Aeromedical Institute (CAMI) conducted a study titled, "Performance of Continuous Flow Oxygen Mask Designs Commonly Utilized with Portable Oxygen Bottles on Transport Aircraft." The study, co-authored by CAMI research staff, compared oxygen mask performance at 25,000 feet in an altitude chamber. Three styles of masks were evaluated. 1) Portable oxygen mask with an open port dilution-rebreather bag. 2) A hard shell, phase dilution mask with rebreather bag. 3) A soft shell, constant flow with rebreather bag. Oxygen flow to all masks was 2.0 liter per minute. The study concluded that all three masks, utilizing rebreather bags, provided adequate delivery of supplemental oxygen. A study of an open-port mask, without a rebreather bag, was not conducted.

Following the incident, the operator changed the passenger oxygen masks in the incident airplane to a Scott Aviation partial rebreather type mask, and increased the oxygen flow rate to 3.0 liters per minute.

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