Would it be practical, or even possible, to construct a parachute large enough to stop a jumbo jet from reaching a fatal velocity? I know that most airline jets don’t crash because thier engines fail and they drop out of the sky, but even so a parachute that could do this would be a nice contingency.
My gut reaction is that a metal aircraft is just much much much much too heavy to be even minimally slowed by an enormous decent parachute. As a reference, we can look at the parachute used to slow the lunar lander capsule on its way towards its water-cushioned landing. And that was only three people. Or maybe four. Anyway it was a small fraction of the number of people as compared to the passenger compliment of a jumbo jet. Now let’s add all the baggage & the sheer weight of the aircraft itself. Not gonna happen.
It would be more efficient to equip each passenger with a personal parachute. Now, how do you usher 250 passengers through the emergency exits as the plane nose-dives towards the ground?
Parachutes for airplanes gave you your answer in the first several threads, before it deteriorated into a squabble over airline andauto safety statistics.
I’d say no. Jumbos are pretty damn heavy.
To back that up, let’s break out the back of the envelope. Using the parachute equations on this page, you can make an estimate of the size of the chute needed. A 747 has a max takeoff weight of 396,000 kg. If you want to keep the rate of descent to 5 m/s (which is still pretty damn fast), I calculate that you need a parachute that is 465 m in diameter. That works out to be an area of 170,000 m[sup]2[/sup]. A football field is 5,390 m[sup]2[/sup] (110m x 49m). So the parachute would be as big as 30 football fields. That’s a big parachute. Besides the sheer size, it would also be pretty damn heavy. Ripstop nylon weighs around 42 grams/m[sup]2[/sup]. That’s a very light fabric, used for kites and such. That size parachute would therefore weigh 6,800 kg – about the same as 2 Chevy Suburbans.
You’d have the same problem as with space ships and fuel, wouldn’t you?
You’d need a pretty big chute to hold a jumbo jet to a decent decent rate. And, You have to add the weight of the chute, so you need a bigger chute, which would be even heavier, so you’d need a bigger chute than that…
The question is though: Could the 747 take off with a normal sized runway with the added weight of the chute? Solid fuel rocket boosters might help…
Assuming SmackFu is on the ball with the parachute calculations, the additional weight would not be a major problem, except for long flights, like say, L.A. to Melbourne. I’d guess that 6800kg of fuel would be in the neighborhood of 1100 miles range (figured at 6kg/mi fuel consumption rate of 3mpg, which I’ve seen elsewhere, which is a rough WAG and I don’t want to hear about mixing English and SI measurements).
I know a guy who’s a 747 flight engineer; if accuracy is a real issue in these musings I suppose I could ask him.
Another possible complication on the parachute idea is the structural integrity problems involved in hanging the aircraft from its tail. I don’t think the chute would do much good to anyone except those in the back 2 or 3 rows on the airplane. Weight incurred by the additional strengthening of the airframe to account for this could put the aircraft in danger of a takeoff overshoot.
Oops, I meant 3 gallons per mile, not 3 miles per gallon.
That’s one of the things I mentioned in the other thread. (By the way, I think there’s another thread along the same lines.) If the 'chute is in the tail, the passengers will have tons of aluminum crashing down on them at touchdown.
I was attempting to imply that the rest of the aircraft would break off and fall free, denying any benefit to 95% of the passengers left in free fall, so to speak. Of course, like you said, having the thing telescope into itself on impact would have similarly fatal consequences to that 95% as well as the other 5% when they are jammed into the pile of wreckage and bodies in front of them.
I don’t think there would be a structural failure. By releasing a drogue 'chute first, the airframe could be spared the sudden decelleration of a fully-depolyed 'chute. There is a “total recovery system” for some light aircraft that uses a “stepped” parachute. A limiter ring keeps the 'chute from deploying fully intil a safe speed is reached. The thing to remember is that light aircraft are… well, light! The parachute keeps the aircraft in a somewhat-level attitude and the landing forces are absorbed by the aircraft’s landing gear. (From what I’ve read, the aircraft might be destroyed, but the passengers will be safe.) “Total recovery” 'chutes are often used on ultralight aircraft too, I’ve heard with good results.
In any case, if designers wanted to put a 'chute in the tail, I think they’d design it so it doesn’t break the airframe.
Another thing to consider is this: If the aircraft is in such a state that the only way to save it would be with a total recovery system, it might be moving through the air in such a way that such a system would be unusable. The TWA flight that went down a few years ago, or the Pan Am flight that went down over Scotland would not have been helped by a 'chute. What about the Alaska Airlines plane that went down off of Port Hueneme? Well, a 'chute might have saved it. But how often does a crash like that happen?
(Oh, sewalk, I’m not debating you. We’re on the same side. I just tend to ramble a bit…)
Back to the OP: Yes, it would be possible to build a total recovery system for a 747. Practicality is another matter. The airframe would have to be built to accommodate it (read: more weight) and the system itself would be heavy. You might have a situation where the aircraft could lift itself and the 'chute, but no passengers or fuel. Then there’s the question of whether it would work 100% of the time. If it didn’t, and assuming the aircraft were still capable of carrying passengers, there would be liability issues.
I agree, Johnny. I was going to go into the ultra-light issue, but in the spirit of the OP I thought I’d try to answer within the realm of large commercial aircraft.
One clarification on the weight issue, though: the real issue with modern commercial aircraft in terms of weight is not takeoff distance, but range. A Lockheed C-5 proves that you can get a huge amount of weight airborne in a reasonable distance, but the range of that pig when loaded is abysmal. I hitched a ride on one from Ramstein AB, Germany to Tinker AFB, Oklahoma about 11 years ago. The crew made three inflight tankings to get the thing there in one hop. Keep in mind that fuel received in mid-air gives much more range than fuel lifted off the ground.