TBM Loss Puts Renewed Focus on Hypoxia

 - November 1, 2014, 2:35 AM
The TBM900 that crashed on a Rochester, N.Y., to Naples, Fla., flight was the aircraft on display at Sun ’n’ Fun in April.

Two months after the September 5 crash off the coast of Jamaica of TBM N900KN, with two people on board, depressurization and the danger of hypoxia have drawn renewed attention, even though–and perhaps because–the pilot’s actions after he reported an unspecified problem to ATC appear inconsistent with recommended procedures for loss of pressurization.

N900KN was a new TBM900, the updated model of the Daher-Socata turboprop single introduced earlier this year. Owner-pilot Larry Glazer, chairman of the TBM Owners & Pilots Association (TOPA), and his wife, Jane, also qualified as PIC on the airplane, received the keys to this, their third, TBM at the U.S. TBM900 rollout ceremony at Florida’s Fantasy of Flight in March. (No type rating is required for the TBM900, but pilots must have a high-altitude signoff to fly above 25,000 feet. Transition and high-altitude training are included in the purchase price of TBMs.)

On the day of the accident the couple was en route at FL280 from Rochester, N.Y., to Naples, Fla., when Mr. Glazer requested a descent to “about” 18,000 feet and reported “an indication that is not correct in the airplane.” Shortly after he acted on a descent clearance to 25,000 feet and a 30-degree heading change for traffic avoidance, ATC lost contact with the aircraft. Cable news channels followed the unfolding tragedy and Norad scrambled two successive pairs of fighters (F-15s and F-16s respectively) to intercept the TBM, which continued flying south over Cuba and into the Caribbean until it apparently ran out of fuel.

Norad shrouds details of its intercepts and observations to avoid compromising its “tactics, techniques and procedures,” said Norad and U.S. Northcom spokeswoman Maj. Beth Smith, who was on duty during the intercepts. In both encounters crews reported that their efforts to communicate with the TBM pilots were unsuccessful and its windows were “frosted over,” hampering visual inspection, Smith said. The first interceptors reported observing the pilot slumped over the controls; no observation was reported of an oxygen mask (which is large and has distinctive orange tabs on either side), but Smith noted “it’s very difficult to see things” in such conditions. Smith could not confirm published reports that Mr. Glazer was seen breathing.

According to data cited in the NTSB’s preliminary report, the airplane entered a high rate of descent from FL250, its last radar return recorded over open water at about 10,000 feet at 18.3547N, 76.44049W, some 12 miles off Jamaica’s northeast coast. Search-and-rescue operations conducted by the Jamaican Defense Authority and United States Coast Guard observed an oil slick and small pieces of debris scattered over one-quarter mile near the last radar target. Seabed depths in the area range from about 6,000 to 12,000 feet.

Cabin Pressurization System

The TBM900 has redundant independent systems that monitor cabin pressurization. If the cabin altitude rises above 10,000 feet (plus or minus 500 feet), a chiming aural alert sounds and a flashing red Cabin Altitude or Cabin Diff Press (differential pressure) alert appears on the MFD, and the PFD’s cabin-altitude field flashes red. The POH calls for immediately donning the oxygen mask and, if necessary, descending in the event of any such alerts. No alert is audible on Glazer’s transmissions with ATC, but it can be silenced by pressing the Master Warning button, as is common in FAR Part 23 turbine aircraft.

The TBM’s oxygen system is turned on from outside the aircraft before flight, and the preflight includes a half-dozen steps to ensure the system is operational. Annunciations appear on the G1000’s display panels if the oxygen system isn’t properly configured. The drop-down Zodiac oxygen masks are designed to be donned within five seconds, with proper training and practice.

The TBM900’s pressurization system is identical to that of its predecessor TBM850, except that the TBM900 incorporates an automated control managed by the Garmin G1000 avionics suite, adjusting the cabin altitude without pilot input based on GPS positioning and flight plan/altitude preselect data. The TBM had been flying at FL280 for about an hour before the pilot reported a problem, an indication that the pressurization system was working properly and the cabin was maintaining pressurization during the first portion of the flight.

Garmin declined to answer questions about the G1000 and its installation in the TBM900. Those familiar with the installation say an SD card collects engine performance and voltage data, but no other system information is recorded.

The pilot cabin door, a popular option on TBMs since certified in 1999 on the 700B, is standard on the TBM900. The door creates a potential breach point in the pressure vessel, but any leak sufficient to cause an incapacitating depressurization would have created a sound impossible for a pilot to ignore and loud enough to have been audible on radio transmissions, according to Andrew Knott, TOPA’s executive director and former marketing director for Socata.

Depressurization Preparedness

Dr. Paul Buza, founder, president and medical director of the Southern AeroMedical Institute in Melbourne, Fla., is an expert on the effects of slow depressurization and has provided scenario-based high-altitude physiological training to some 2,500 pilots at his facility. (TOPA is among the pilot groups and OEMs for whom the institute offers sanctioned training.) Buza sees slow depressurization as a much graver danger than the rapid depressurization events for which the military trains pilots in its altitude chambers. “When you have slow depressurization, and the cabin [altitude] is rising at 1,000 feet per minute, and you think you have time to fix it before you put the mask on, you do not have time,” he said.

Among lingering questions: If indeed depressurization contributed to the accident and the aircraft provided the proper alerts, did the pilot’s (presumed) failure to don the mask indicate he was already affected by hypoxia? Dr. Ian Fries, an aeromedical expert who, like the Glazers, recently purchased a TBM900, his third TBM, noted that charts showing the time of useful consciousness are based on studies performed “on young Air Force men who do not represent general aviation pilots.” He also noted that the pilot “will lose judgment” well before losing consciousness. Questions have also been raised about ATC’s response and whether a controller should take more immediate and proactive steps to get an aircraft to a lower altitude in such a situation.

The Garmin 5000 installation in the new Cessna Citation X+ includes an Emergency Descent Mode (EDM) that, in the event of detecting a loss of pressurization, turns the aircraft 90 degrees off airway and descends to a pre-set altitude without pilot intervention. Whether it’s time to bring this capability to a wider range of aircraft that operate in the flight levels is another issue likely to be raised, though not necessarily supported in all quarters.

“The real answer to hypoxia is to get the mask on right away,” said Fries. “An emergency descent creates two emergencies: the one you have and maybe one for ATC. We have a definite solution: get the mask on. Then you don’t have to descend right away.”

On September 24, nearly three weeks after the accident, Daher-Socata issued Service Information 2014-008, “Review of high altitude operations,” covering the TBM’s pressurization and emergency oxygen systems and emphasizing the limited “time of useful consciousness” at high altitudes.

The Jamaican Civil Aviation Authority (JCAA) has formally delegated authority for the accident investigation to the NTSB, and the FAA is assisting the Board “as needed,” according to an FAA spokesperson. Neither U.S. agency is a party to or involved in discussions regarding salvage of the wreckage, and Daher-Socata isn’t aware of any recovery effort, according to Philippe de-Segovia, the company’s director for product marketing. Such a recovery operation would be challenging, costly and carry no assurance of success, according to knowledgeable individuals. In the absence of wreckage the NTSB will “go about our normal process, kicking over every single rock,” said Stephen Stein, the NTSB investigator in charge of the accident investigation.

Stein said the NTSB’s final report on the loss of N900KN will be released about a year after the accident and include a public docket with all the information gathered during the investigation. A brief, with an analytical summary that draws conclusions from the factual report, and likely a causal statement, will be released 60 days later.

France is an accredited representative in the investigation, as the aircraft manufacturer, Daher-Socata, is French, and so is Canada, where Pratt & Whitney Canada, the aircraft’s engine manufacturer, is based. The manufacturers serve as technical representatives to their government’s transportation safety investigative agencies, the Bureau d’Enquêtes et d’Analyses (BEA) and Transportation Safety Board of Canada, respectively. The JCAA is also an Accredited Representative, and other manufacturers and suppliers will participate in the investigation as well, Stein said.