Cold case

“It was a close call,” wrote the captain in her report to NASA’s Aviation Safety Reporting System. “If we had not realized what was happening and acted on it, all the occupants of the aircraft would have likely lost consciousness and suffocated.”

What catastrophic event caused one airline captain to have a near-death experience on board an aircraft parked at the gate?It all began when the crew arrived for a scheduled dawn departure. The airplane was already fueled and loaded. The captain noticed the main cargo door and L2 door were still open. She noted on the manifest that their load was primarily food and palletted dry ice.

Pre-dawn, the aircraft was quite cold because the power had been off all night. The auxiliary power unit normally runs the environmental controls, but this plane had been dispatched with the APU placarded inoperative.

The crew called for Air Start carts and shivered through their pre-flight checks. Ground crew connected the air start carts and closed all doors. Immediately, it became difficult for all on board to breathe.

The flight crew managed to start Number One engine. The captain reported that the Central Load Planning Coordinator had barely made it back to the flight deck from the main deck after checking the L1 door.

Mental confusion quickly set in. The F/O just managed to get to “packs on,” while the captain, vision dimming and breath becoming shallow, called for emergency O2 masks on. Several minutes later everyone began to breathe normally and the mental confusion abated.

What had happened to this flight crew?

Hypoxia. But, you ask, weren’t they at the gate and not at altitude?

Many pilots associate hypoxia with altitude sickness, or lack of oxygen due to decreased air density. In reality, hypoxia is caused by lack of oxygen however that might occur.

In this case, the lack of oxygen resulted from the excessive presence of carbon dioxide. Remember the pallets of dry ice, otherwise known as frozen carbon dioxide?

The Dangers of Dry Ice

Dry ice is extremely useful for freezing and keeping things frozen because of its very cold temperature (-109.3F/-78.5C). Also because it gives more than twice the cooling energy per pound of weight than regular water ice, it’s great for use in long-range transport of frozen goods.

Plus, as it doesn’t melt, it doesn’t make a mess, unless you drop it in your favorite party punch. Dry ice sublimates directly from a solid state to a gaseous state, and when mixed with a liquid containing water, it vaporizes the water content. That would be one smoky cloud bubbling off your party punch.

How much is too much dry ice? According to the Flight Operations Manual for the company where the captain worked: “For wide body transport aircraft on the ground for more than one hour, when carrying more than 440 pounds of dry ice in any compartment, one air conditioning pack should be on.”

In their packs-off condition due to an inoperable APU, 440 pounds was too much.

According to the Notice to Crew (NOTOC) that accompanied the crew’s manifest, their flight had more than 10,000 pounds of dry ice on board. Way too much.

When the ground crew inadvertently left the cargo and L2 doors open after the pallets of dry ice had been loaded, they allowed sublimated CO2 gas to enter the flight crew and passenger compartments. When the crew sealed up the plane before engine start, they also sealed in all that extra CO2.

Normally, carbon dioxide is a trace gas in our atmosphere, about .04% of a given volume of air. Increase its presence to 1% and drowsiness will occur. At 5% exposure, you’ll become confused and start to go deaf and blind. Your heart rate and blood pressure will dramatically increase. You’ll also begin suffocating as CO2 intake outpaces oxygen absorption in your lungs. That’s what the flight crew felt just before saving themselves. That’s why the FAA labels dry ice a hazardous material.

So what if you don’t pilot an aircraft boasting a cargo area capable of holding 10,000 pounds of dry ice? What if you’re only toting 100 pounds or so?

One C-208 pilot discovered how dangerous that amount can be. In his NASA report, he stated that the aircraft had been loaded more than an hour prior. Part of that load was 67 kgs. of dry ice (or a little more than 147 pounds).

Due to stiff breezes on the tarmac, his ramp crew had closed and locked all aircraft doors and windows to prevent wind damage. Within two minutes of boarding the aircraft and buttoning it up for departure, the pilot experienced labored breathing and confusion.

A couple of minutes later, he realized sublimating dry ice might be the culprit. He immediately opened all windows and used his oxygen mask after engine start. About four minutes later all symptoms abated and he proceeded with the flight.

The pilot suggested in his conclusion that dry ice should not be stored in the same compartment as humans.

Food and Pets

In fact, there are a number of NASAs submitted by airline flight attendants who have experienced dry ice’s ill effects. Many airlines use dry ice packs to keep food cold.

In one report, a DC-10 flight had to be diverted and five flight attendants treated by paramedics. One passenger was also removed and sent to the hospital by ambulance after suffering a severe asthma attack. Maintenance personnel determined the cause of the flight attendants’ illness to be “a combination of a large amount of dry ice in the lower lobe galley, combined with the low air flow due to an inoperative air pack that had been deferred.”

The reporting pilot suggested that his airline emphasize re-training of ramp, in-flight and maintenance personnel on the importance of proper cabin ventilation when dry ice is on board. He felt the situation severe enough that the company should consider fixing a broken air conditioning pack or even removing the aircraft from service and swapping it out, rather than risking lives.

The issue of CO2 toxicity is grave enough that most airlines prohibit the carriage of live animals and dry ice in the same cargo compartment. A dozen pilots filed NASAs after discovering ramp crews had done just that. Some discoveries were made only after takeoff. Four crews even chose to return to field or divert to an alternate airport and correct the situation.

In one instance, a B-777 flight crew elected to store the animals in the crew relief area on the flight deck rather than risk the animals suffocating.

“After having left the gate, we were notified of having cargo containing dry ice in the aft cargo area, where there were also two live animals. After this notification, we took a lengthy ground delay, waiting on ramp and load control to decide whether to offload either dry ice or animals. The relief pilot suggested placing the animals in the empty crew bunk area. I reminded him that the ops manual restricts using that area for storage and that I would require some type of waiver before I would concur. The relief pilot then asked ramp control to try and contact someone in management who would have the authority to issue such a waiver or direct us to someone who could.

“We eventually reached Capt. Y, domicile chief pilot, on his cell phone. We explained the ops manual restriction and our predicament. He then verbally approved putting the animals in the crew bunk area and added he would furnish written approval if needed when he returned to the flight office. We taxied back to the ramp and had the dog and cat moved.”

I learned a couple of fascinating things about dry ice and the hazards of carbon dioxide poisoning in researching this column. For instance, did you know that dry ice cools the air around it by absorbing the heat from it? That’s called an endothermic process. Did you know that the endothermic process also causes the volume of the CO2 to expand rapidly as the molecules move from a solid to a gas state?

The downside of the endothermic process is that it causes the rapid expansion of the dry ice transitioning from a solid to a gas. If sublimating dry ice is trapped inside a sealed container, it could suddenly, violently exceed the volume of the container. Imagine overinflating a balloon….

A B-767 captain relayed in his NASA a story about a dry ice near-miss. A passenger told the captain he had intended to bring a thermos of dry ice on his trip. The thermos of dry ice had been sealed up and placed in his home refrigerator. While in the refrigerator, enough dry ice sublimated, causing the thermos to explode, blowing the refrigerator door off and causing serious damage to the passenger’s home.

Imagine if that had happened in an overhead bin? If a thermos full of dry ice can blow the door off a fridge and damage someone’s house, imagine what it can do to a small airplane.

Source: http://generalaviationnews.comCold case