How Much Runway Does It Take to Land a 747?
Hey Mom,
How much runway does it take to land a 747?
Thanks,
Someone Who Wants You To Do My Homework For Me
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hey, Do your Own Homework, Kid,
I’ll show you how!
The answer to this one is that it depends on a lot of things, and pilot skill is just the very tip of that particular ice burg. When discussing the performance of any aircraft, we need to take multiple factors into consideration so we’re comparing apples to apples and not Beechcraft to Boeings. Aircraft weight, wind and runway conditions, air density, pilot technique, and even just which model B 747 we’re talking about here will make a difference. Before attempting a takeoff or landing in any airplane, the regs, specifically FAR 91.103, require us to know how much runway we’ll need under the expected conditions. Let’s start with the metaphorical heavy lifting and discuss the ins and outs of density altitude.
Density altitude is pressure altitude corrected for nonstandard temperature. What, that didn’t clarify the whole mess? Let’s talk pressure altitude. All other things being equal, the atmosphere becomes less dense with an increase in altitude. The higher up we go, the less air there is above us weighing us down. The less dense the air, the less your airplane has to work with, as in less airflow over the wings to produce lift and less air in each cubic inch of your engine to produce power with. This sounds bad, but we do have one thing working in our favor. Keep in mind that less dense air will also mean less air friction so we can expect to cruise a little faster when we do get up there. So, remember that “all other things being equal” part? We’re going to need to correct for nonstandard pressure. On days when the pressure is higher or lower than standard, the air will be more dense or less dense, respectively, than it otherwise would be. A good rule of thumb is 1000 feet per 1 inch of mercury. If 29.92 inches is standard but the pressure happens to be 30.35 inches, we need to correct by .43 inches, or 430 feet. Since pressure is higher, the air will be as dense as it would be 430 feet lower than we actually are. If our airport happens to be 1000 feet above mean sea level, we’re still at 1000 feet MSL but our pressure altitude will be 570 feet. As far as the airplane can tell, the air is as dense as it would be at 570 feet so it will perform as if it is at that altitude. So, we’ve got pressure altitude. That’s only part of the story. Temperature also affects air density. As it gets hotter, the air becomes less dense, and you got it, performance suffers. As it gets colder, the air becomes more dense, and our density altitude decreases proportionally. Humidity also rears it’s ugly head. The more water vapor, the fewer air molecules in that proverbial square inch of air. Though colloquially, we say that the air feels heavy with humidity, it’s actually less dense when it’s waterlogged. Density altitude, then, is the altitude the airplane “thinks” it’s at, regardless of it’s actual true altitude. On a day with low density altitude (relatively dense air) the airplane will perform better. This includes a shorter takeoff and landing roll. Aha.
Still with me? I promise, it gets easier from here. Let’s consider weight. It’s a simple matter of momentum. The more that puppy weighs, the longer it’ll take to slow it down. Load that plane lightly and you can greatly cut down on the distance necessary for landing. This is a major factor when you’re crunching numbers for an airplane with the payload capacity of a 747. A headwind can also greatly decrease your distance; every bit helps. A dry, level runway is much better for braking than a wet, snowy, or icy one and this, too, will be reflected in the calculations. Short field landing technique will affect your results, as will any modifications made to increase your aiplane’s STOL (short takeoff and landing) capabilities.
So now that you know why the numbers vary, lets take a look at answering your question more directly. Here we have the performance charts for the Boeing 747-400, the company’s biggest selling 747. All you americans, take note that distances are in meters and weights are in kilograms unless otherwise noted. As you can see, the distance required for landing can vary considerably based on current conditions. Anecdotally, the pilots who fly them say that without a full load, landing with less than 6ooo feet is not unreasonable. These videos show a landing at St Maarten, right over the beach, on runway 9 with a length of 7, 154 feet. The first video shows the view from the ground, the second is from the cockpit.
Enjoy, and don’t try this at home, kids, unless you hold a B747 type rating and have done the math!
Mom
[youtube=http://www.youtube.com/watch?v=zAfQwDizpRo]
[youtube=http://www.youtube.com/watch?v=ksmDuXO_k6E]
Leave a Reply