Into The Wind

Question:  I recently traveled on American Airlines' Chicago - New Delhi non-stop flight. It was a full 777 (heavy with all the fuel required for the 8,000 mile trip, I assume). While waiting to take off at O' Hare, the pilot announced that we would be waiting for about 45 minutes because we needed to take off from the opposite end of the runway and into the wind because we were heavy. I had thought that all planes take off (and land) into the wind to reduce relative ground speed. The closest I could come to guessing was that we were taking off from a different, perhaps longer, runway and hence disrupting the normal arrival/departure procedures and therefore had to wait. Could you please throw some light on what the departure procedure is for a heavy flight at ORD and other airports?

Vijay Bettada, Brighton, MA.

Answer:  Airplanes should take off into the wind, but your term "relative groundspeed" isn't quite correct.  In a calm wind, if our takeoff speed is, say, 140 knots, then at 140 knots indicated airspeed we "rotate" and the plane becomes airborne.

Say, though, there is a 20 knot headwind.  Sitting on the runway, our airspeed indicator would indicate 20 knots (we can't necessarily read a speed that low, however).  At the plane's speed of 120 knots, the 20 knots of wind added to that gives us an indicated airspeed of 140 knots and we can fly. 

The plane doesn't care if the plane is creating the speed by its movement, or if wind is creating part of it.  We need less runway to become airborne as it takes less time to attain flying speed.  After all, we have a head start of 20 knots before we even move.

Sometimes, if we're not too heavy, we can tolerate a small tailwind, but if in fact we do have a tailwind we'll use a little more runway attaining flying speed.  This is often no problem.  However, when we’re very heavy we must have no tailwind at all.  Chicago and other airports, but especially the really busy ones, don’t necessarily change runway configurations unless they really have to, when the tailwind "component" exceeds 10 knots—the limit for every airplane I know.  Changing runways is cumbersome and causes some delays while airplanes are reshuffled.

The wind component is the effective headwind or tailwind if the wind is coming at us at an angle.  The only time a headwind or tailwind   component is exactly the same as the wind is when it's coming directly toward our nose (headwind) or our tail (tailwind).  If it's coming at any angle whatsoever, the wind component drops. 

If there is a calm wind (no wind), or if the wind is 90 degrees to the runway, then there is a zero wind component.  And you can see that if we had, say, a direct crosswind of 10 knots, the wind could swing all the way around 360 degrees and at no time would we have to use a different runway.  The exception is when we're very heavy and can't accept any tailwind component at all.  The minute the wind direction is even slightly past that 90 degree point, it's a tailwind and we'll have to use a different runway if we're very heavy.

Sometimes, especially at very busy airports like Chicago O'Hare, instead of changing which runways are in use whenever there is a wind shift but the winds aren't too strong, they'll keep their current configuration but let the ones that need a particular runway use it to take off into the wind, and this was the case for your plane from what you described.

Your plane needed to take off into the wind due to its restrictive weight, but had wait for a break in the other takeoffs going the opposite way on the same runway, which caused the delay.  I’ve had exactly the situation you’ve described happen to me more than once when I was flying the B-777 from Chicago to Osaka.  

Here's something interesting:  Our maximum allowable takeoff weight, if it's less than the maximum allowable structural weight, may actually be increased if there is a headwind.  Our "numbers" that tell us how heavy we can be at a certain temperature, airport elevation, etc.  But there may be an allowance of so much weight per knot of wind, allowing us to take off just a little bit heavier on the same runway.   This might come into play on a hot day (the air is less dense) when we have a weight restriction.  I just looked up a sample in my Flight Manual, and saw I could increase my takeoff weight just over 800 lbs per knot of headwind at a particular runway under specific conditions. 

This could make a difference in getting those last few passengers on board, or that last bit of cargo.  Even though these numbers are figured while we're still at the gate, they're considered reliable enough for this purpose.  If the weather and wind were to change drastically on the way out to the runway, the pilot would have to consider it and decide whether anything needed to be off-loaded.  I've never actually heard of this happening, however.

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