Have you ever wondered what would happen if you were a passenger on a long flight over the Pacific and something terrible happened? I know I have, and like most of us, we probably envision treading water for a long time, or sitting in a life raft because the water is too cold for survival. (In the North Pacific in the winter, the water is survivable for about four minutes without protection or a lifeboat.) Fortunately for today’s passengers and crew, this possibility is quite remote, thanks to many modern conveniences like excellent communication equipment, well designed and highly reliable aircraft engines, and safety procedures that are well thought out.

I can think of three major emergencies that we as pilots can plan and prepare for, that if the safety procedures are followed, are completely survivable, even when there are thousands of miles of ocean below the airplane.

These are: Fire, Engine Failure, and Cabin Depressurization.

There are many other things that could go wrong, but they are not as severe as these three biggies. Bigger than these, like the wings falling off, either usually don’t happen or aren’t survivable anyway, so why plan for or worry about them?

An onboard fire is probably the worst of the big three because it can quickly escalate from harmless to deadly in just a few minutes. Recall all those warnings about tampering with the smoke detectors that you hear prior to departure? Yeah, those are deadly serious — no pun intended. The cargo compartments will seal themselves shut in the event of a cargo compartment fire to starve it of oxygen and fire suppression systems like Halon extinguishers take care of the rest of the fire. The cabin is a bigger problem because unless the fire is in a galley or lavatory trash can, it requires people like flight attendants to fight it. There have been several major fire disasters because the cabin caught on fire, like this Air Canada DC-9 and this South African 747 Combi (A combi 747 is one where the back half of the plane is cargo, and the front half is set up to carry passengers.) I’m sure that the lack of passenger smoking on planes has saved many lives.

But what about the cargo planes, like the one I fly? The cargo section of the main deck of the 747 is cavernous and there is no way to have enough Halon to put out a fire on that type of scale. But surely we have a way to fight it while it’s three a.m. over the Pacific or Himalayas, right? The answer is yes, and it’s actually a better system than in the passenger versions of the 747. Because we don’t have to worry about keeping 400 people alive down below us, we depressurize the entire airplane while wearing oxygen masks, to starve the fire of oxygen. When we get a fire warning, we put on our oxygen masks and raise the cabin altitude to 25,000 feet, where there isn’t enough air to burn an open flame. Once the fire is out, we lower the cabin back down to five or six thousand feet and take off our masks. It’s a simple system that should prove to be very effective, however, I hope I never need to test it.

Along those same lines, another major problem that we can deal with effectively is cabin depressurization. If our plane can’t maintain pressurization, then we can’t maintain consciousness and everyone goes to their final sleep. Contrary to what some believe, aircraft don’t carry large tanks of oxygen to allow people to breathe, they simply pack air into the cabin to provide pressure similar to sea-level pressure. See, the ratio of oxygen in the air, 21% is the same at sea-level as it is at 40,000 feet, there just isn’t enough pressure in the atmosphere to breathe it in at high altitudes. Aircraft don’t have to constantly supply oxygen, it is already there at 21%, they just have to make the pressure available for us to be able to breathe. They do that by taking high pressure air off the engines and sending it into the cabin. The cabin’s “altitude” is controlled by how much of that pressurized air we let out, with a special valve called an outflow valve. As the valve opens, the pressure in the cabin lowers, and as the valve closes, the pressure gets greater inside the cabin. At high altitudes, we need lots of pressure to maintain breathable air, so at cruise, the valve is mostly closed.

When that system fails, or a hole gets blown into the side of the cabin, we lose all our pressure. The masks drop and provide air for a little while, but only for a few minutes at high altitude. We have to descend down to more breathable air, but doing that increases our fuel burn dramatically. On our Cathay 747’s, we first descend to 14,000 feet, until all the oxygen in the masks are depleted, and then we descend to 10,000 feet, where everyone can breath normally. Our fuel burn will be around 50% more down at 10,000 feet, so if we are out over the Pacific, our destination is off the table. We will have to go to an en-route alternate, decided upon by using our point of no return, or PNR, described in the last blog here. Again, we always have to have enough fuel when we depart, to have a depressurization at any point along our route, and still be able to reach a suitable alternate.

And finally, there is the little matter of engine failure. Do any nervous flyers ever worry about that? I didn’t think so :o) No, it seems everyone’s fear is engine failure, especially when traveling over such huge expanses of water. The good thing about the 747 is that it comes with four engines, so there is little chance of major trouble. But even superb aircraft like the 777 that only has two engines doesn’t cause much concern for alarm as engine failure is so remote. These engines are not like a car’s engine. They are millions of dollars worth of precision, and because they rotate, there are very few parts that wear like engine parts wear, and their maintenance is much improved over the maintenance you and I give our cars, for sure!

However, with that said, failures do occur, and when they do, pilots need to be ready for them. Much like the last blog entry described about PNR’s here, we have a three engine PNR, where if one fails prior to a certain point, we know we’ll need to return to a certain airfield, and if it fails beyond that point of no return, we will be able to continue to another airfield. This is because we now may not have the fuel to continue to our destination.

Why? Because we actually burn more fuel with three engines than with four. It sounds counter intuitive, but think of it like this: When we go from four to three engines operating, we now have less thrust. Less thrust means we can’t maintain the same altitude, and have to descend a few thousand feet. This is because the higher we fly, the more thrust it takes to keep all that weight up in the sky and when we lose some of that thrust, we start to slow down. If we lost an engine and didn’t descend, we would slow down beyond the speed at which our wing would be able to maintain lift, and we would stall — where the wing can’t produce enough lift for the weight it is having to carry. The trouble is, when we do descend, we burn more fuel and that is why losing an engine will actually cause fuel troubles. Also, I believe the synergy affect applies here, too. The thrust from four engines is better and more efficient than the increased thrust of three, making up for the lost engine, if the aircraft maintains the same weight and altitude.

So, I write this to give you a little bit of insight into what we plan for in the 747 as we get ready to cross large expanses of inhospitable terrain, either mountains or oceans. When crossing the United States, there are so many airports to duck into if trouble pops up that there isn’t much planning to do. But when there are vast distances between places of refuge, proper planning is essential. Never fear, as even with the three biggest troubles that can occur, there is always enough fuel to get to a suitable landing field, because if there wasn’t, we wouldn’t be taking off in the first place. Cathay doesn’t pay me enough to take that type of risk!

TobyLaura.com