Principia, Inc.

Pilots of pressurized aircraft like the Piper Meridian practice switching smoothly to emergency oxygen in case cabin pressure is lost.

Captain Daniel (Boone) Gibson and his crew in the 79th Physiological Training Flight at Andrews Air Force Base in Maryland do a bang-up job educating civilian pilots on this topic. Ordinarily, they work with military pilots. But about once a month they open their altitude chamber and expertise up in full day classes that can be arranged through the FAA in Oklahoma City. Included are in-depth briefings on decompression sickness, hypoxia, trapped gasses, and other ill effects of sudden exposure to high altitude, along with proper use of supplementary oxygen. A closely-supervised ninety minute simulated flight in the chamber exposes trainees to conditions identical to what occurs at an altitude of 25,000 feet in actual flight. Each pilot has his or her own unique set of symptoms to contend with. The controlled environment in the chamber gives flight crews a chance to explore their symptoms safely. In flight, early recognition of symptoms can help crews detect problems with cabin pressure or oxygen systems.

The Piper Meridian, PA46-500TP, is certified for flight up to 30,000 feet. Behind the copilot’s seat is a cabinet within reach of the pilot with a mask designed to be put on quickly with one hand – a “quick don”. It is fed from a green 4.25 cubic foot high pressure cylinder designed to withstand 1800 to 2000 lbs/sq. in. The Pilots Operating Handbook (POH) estimates that when fully charged the system has sufficient supply for one individual for 25 minutes at 30,000 feet when used with the mask supplied, a model MC-10 manufactured by In-Eros. A diluter demand regulator included with the mask normally mixes oxygen with cabin air to conserve supply. There is a switch on the mask that can be used to select higher pressure 100 percent oxygen when needed in an emergency. In this position, supply duration is reduced. There is a MIC SELECT switch in the lower left corner of the instrument panel to allow the pilot to select a built-in microphone in the mask. In-Eros also makes a mask that mates with airtight goggles. That is not standard equipment in the Meridian, but it would be a useful upgrade since the cabin might be decompressed to deal with a smoke condition, and that smoke could be so thick or acrid that it would interfer with vision.

The other five seats in the Meridian, including the copilot’s position, are provided with less robust solid state oxygen generators, also referred to as sodium chlorate candles. Beneath the copilot’s seat is a tray with two masks connected to an oxygen generator thru a clear plastic tube and a lanyard. The tray slides out into the aisle between the copilot’s and pilot’s seat. Pulling on either mask and the attached lanyards fires a percussion igniter cap to start a chemical reaction between sodium chlorate salt and iron wool, resulting in the release of heat, oxygen, sodium chloride, and ferrous oxide. Once started, the reaction cannot be stopped. The POH estimates the supply is sufficient for fifteen minutes with the use of the continuous flow masks attached to the generators, with rebreather bags that help conserve supply. In the event of failure or exhaustion of the pilot’s emergency supply, the copilot’s masks can be used by the pilot.

Solid state systems with rebreather bags are less robust because they do not allow for 100 percent oxygen flow, do not provide a tight seal between the mask and the user’s face, do not include a microphone, last about half as long as the high pressure pilot’s supply, cannot be turned off, and pose some fire risk due to the high temperatures generated by the chemical reaction.

At a typical cruise altitude of 25,000 feet in the Meridian, with a maximum cabin pressure differential of 5.5 psi, cabin pressure is the equivalent of about 8000 feet, which means oxygen partial pressure (pO2) is at about 135mm. Flatlanders in reasonably good health are comfortable under those conditions. A rapid decompression to 25,000 feet quickly reduces pO2 to about 50mm. Along with other possible stresses in a decompression like wind noise, fog, sudden temperature drop, expanding intestinal gasses, possible smoke or fire in the cabin, a decompression can be very distressing, especially for untrained passengers.

If it can be accomplished safely, the crew may wish to initiate an emergency descent to at least 10,000 ft, where the p02 is 100mm and most people will again be comfortable. Often announcing that a descent to an altitude where oxygen will not be needed will help reduce passenger stress.

To make a maximum rate descent in the Meridian, the autopilot is turned off, power is reduced to idle, the nose is pitched up to reduce airspeed to 168 KIAS or lower, the landing gear are extended to provide additional drag, and pitch is adjusted to keep airspeed below 168 KIAS in smooth air, or below 143 KIAS in rough air, to avoid overstressing the airframe. In the simulator, 3000 feet per minute descents were accomplished. In any case, the descent should ideally be completed within the 15 minute endurance of the solid state devices on board.

Dr. Wayne Isom, a heart surgeon and a pilot, suggests that flight crews advise passengers to try to remember to breathe slower and deeper than normal to avoid possible hyperventilation when first going on emergency oxygen. What apparently happens in some cases is that oxygen chemoreceptors in the carotid bodies command unnecessarily fast and deep respirations in response to the sudden decrease in oxygen blood concentration following a decompression. This in turn drives blood CO2 levels lower when they are often already at a proper level. Paradoxically, this negative feedback loop can lower blood oxygen concentration.

To check their own oxygen blood concentrations, flight crews could use battery-powered fingertip pulse oximeters such those made by Nonin, Med Choice, or Checkmate to confirm a blood saturation level of 90 percent or better. However, given the increased workload in a pressurization failure, it is usually more advisable to simply slow one’s breathing to prevent hyperventilation and conserve a limited oxygen supply.

Links:
http://www.ineros.com/
http://en.wikipedia.org/wiki/Chemical_oxygen_generator
http://en.wikipedia.org/wiki/Pulse_oximeter
http://www.portablenebs.com/choiceoximeter.htm
 

At 1405Z on February 2, 2007, a typically cold day in Goose Bay Labrador (CYYR), three pilots departed together in three new Cirrus SR20s carrying registration numbers N901SR, 806SR and 720SR. They filed for Reykjavik, Iceland (BIRK), 1350 nm east, on the first leg of a delivery flight to Phuket, Thailand. Each plane carried a ferry tank in the back seat with 80 US gallons additional fuel. Fritz Schoeder, a Swiss living in Florida, had the delivery contract. He had subcontracted with pilots Michael Bradford, an American, and Siggy Lehr, a German living in Florida.  It had been equally cold on earlier legs from the Cirrus plant at Duluth MN (KDLH), 1300 nm west. For this leg, Fritz Schoeder flew N901SR, while Michael and Siggy piloted the other two. When an enroute weather briefing reported deteriorating conditions at Reykjavik, the flight of three diverted toward Narsarsuaq (BGBW), about 675 nm from Goose Bay.

They were about 100 nm out when Fritz, flying at 13,000 feet, first reported fluctuations in his oil pressure and temperature. Michael, who had been in the lead plane at 9,000 feet, circled back to trail Fritz with Siggy, in case he could not make BGBW, and at 1729Z Fritz advised ATC at Sondrestrom in Greenland, that he might have to declare an emergency. ATC immediately notified Sondrestrom Rescue Coordination Center.  At first Fritz thought it was a problem with the gauges, but at 1749Z, he declaring the emergency.

By early 2007, the SR20 fleet had already accumulated a record of oil breather tube ice blockages on the Continental IO-360-ES engine, which had led Cirrus to issue a winterization kit and a non-mandatory service bulletin #SB 2X-71-10, dated October 8, 2004. The problem was the location of the tube at the front of the engine, directly in the path of cooling air. There has never been a problem reported in the larger Cirrus SR22, which uses the Continental IO-550-N, with the breather line in the back of the engine.

Water vapor produced as a normal byproduct of combustion is intended to vent through the breather tube. But in extremely cold operating conditions the temperature inside the tubes in the SR20 sometimes stayed below freezing long enough to allow the water vapor to turn to ice, occasionally in sufficient quantity to completely block the tube. The tendency is increased at low power settings typically used to extend range on a ferry flight, which resulted in lower overall engine operating temperatures. Denied an outlet, the water vapor can causes a pressure increase over the oil in the pan, and drive it to break out elsewhere in the system, through the filler cap or dipstick tube, for example. The winterization kit restricted the flow of cooling air by reducing the size of the cowl opening and the service bulletin called for insulating the oil breather line.

Many Atlantic ferry pilots were aware of this problem and elected to install either or both the kit and the sleeve. Michael and Siggy had recommended to Fritz that they install the sleeve. Fritz, perhaps concerned about time and money, declined.

In N901SR, the situation quickly deteriorated. Oil appeared on the windscreen, oil pressure began to drop while oil temperature rose. Eventually the engine lost partial power, and the airplane began to descend. Fritz radioed his wingmen that he was heading for Simiutaq (SI), an NDB now about 50 nm northeast of his position at the mouth of the fiord that leads north toward Narsarsuaq, He said he hoped to be able to make an emergency landing near there if unable to make the field. He said to tell his wife he loved her.

There was a solid undercast which he did not break through until 800 feet AGL, by which time the engine had quit and he reported smoke in the cockpit. He could see land, but could also see it was not practical to land there, so he turned back toward the position he had last reported, 60 38N 46 41W, WSW of SI. By this time his altitude was only a few hundred feet AGL. His last radio transmission was about 1810 Zulu, as a AS350 rescue helicopter was departing Qaqortoq (BGJH) just east of SI. A few minutes later, a second helicopter, an S61, departed Narsarsuaq.  Michael and Siggy descended through the clouds and searched for the plane with the helicopters.

Visibility was good, the sea was calm, winds were light, but there were hundreds of small icebergs in the area, making it difficult to pick out a small white airframe. Siggy finally spotted the wreckage sitting nose down in water so clear and calm he could see the wings intact below the surface. The tail had been destroyed and the left door ripped off. Daylight only lasts about seven hours in that area in February, and it was getting too dark so the two SR20s headed for Narsarsuaq, while the helicopters continued the search. They soon found Fritz’s body near the wreckage, in a survival suit and a life preserver. While the helicopter crew was recovering the body, the rocket in the Cirrus airframe fired, spreading the parachute across the water.

An autopsy performed in Greenland concluded Fritz had drowned, though he also had a broken femur. The damage to the airframe in conjunction with the injury may indicate the plane struck one of the small icebergs while ditching. The airplane was not recovered.

Michael is now flying a Lear and Siggy is now an airline pilot. On May 24, 2007, Cirrus made the oil breather tube insulation mandatory in SB 2X-71-10 R1.

Links:

Map of accident area

Service Bulletin

Oil loss report in 2004

Accident report