Passing over Sandspit en route to Anchorage at 36,000 feet, we are crabbing mightily toward the west against a 160 m.p.h. crosswind to maintain our course. Outside, deep cold and blackness; no moonlight, no aurora borealis, and not much starlight. There must be a thin layer of cirrus clouds over us. The air mass, moving at 160 m.p.h. from the west, is absolutely smooth. That, in itself, is incredible. In fact, it is smooth enough to take an eight second exposure of the ink well outside.We have been on duty eight hours, with about three more to go. The Anchorage weather is not too bad; light snow showers, broken cloud layer at 3,000 feet, 5 miles visibility, 15 m.p.h. winds from the southwest, and 20 degrees Fahrenheit. Although, the forecast is marginally OK, we still have a landing alternate. The fuel tanks were completely full at takeoff, minus 300 pounds taxi fuel. She was a heavy beast... The co-pilot used a sharp pencil to figure the take-off speeds and flap setting.
Far ahead of us... A tiny twinkling light. It is a big freight wagon on the same routing and altitude. We have been watching him for hundreds of miles; our speed must be the same. The air traffic control frequency is eerily quite... Very little traffic up here tonight. The controller will give a new frequency to the freight dogs ahead of us, then when we get to that same point in the black sky, we get the same frequency. It is kind of cool, actually.
The third hour's fuel burn was 5,200 pounds and we moved 502 miles closer to Anchorage. When I compare my numbers to the flight plan's numbers, they are very close. Life is good tonight in the Earth's shadow.
16 comments:
Dave, how do you crab at cruise speed? I thought crabbing required use of the rudder, but I thought the rudder is ineffective past a certain speed.
Crabbing in flight is nothing more than adjusting the course heading to maintain the desired track to your next waypoint or destination...an example would be if you desire to fly a course of say 270 (west) but the wind is coming from a direction of 300 ( Northwest)and you did nothing, you would miss your destination to the south by many miles as the wind blew you off track from right to left. To correct for the wind, you would actually point, or fly the airplane on a heading more into the wind, say a heading of 290 degrees. This would depend on the speed of the cross wind that you were fighting...Hope this is clear as mud....Dispatch bear in Orlando
And the crabbing reduces your speed-on-track, this is why statistically there are more headwind (components) than tailwinds. No free lunch ;-)
anon #1, you might be thinking of slipping, which is a cross control maneuver we do for crosswind landings, i.e., we remove the crab with rudder and aileron so that we can land with the fuselage parallel to the runway. Also, the rudder is effective at speeds above 80 knots or so; more speed yields more effectiveness. In fact, the rudder can tear off the vertical stab, even at low speeds, say around 200 knots. Remember the A300 at JFK, November 2001, I believe. We are very, very careful with the rudder.
Anon #2- correct
Stu - correct
Hi Dave,
You say "The co-pilot used a sharp pencil to figure the take-off speeds and flap setting". Was it a "sharp" pencil or a "SHARP" pencil? I've no heard that term for a few years - brings back memories... Thanks. ISH
About crabbing, just think you are trying swim across a river. If you wanna reach a point on the other bank, you just can't aim this point. You need to swim a little against the current. It's the same for plane. And about rudder, this crab flight is symetric too, you have the ball in the middle.
Anon #1 here again and still a bit confused. I understand chaging the heading to correct for the wind direction, but my question is how do you do that? Are the ailerons constantly in use so that the plane flies with one wing lower than the other? Or does the airplane maintain level wings somehow? Thanks for humoring me by answering these questions.
Anon #1 ...
the river analogy is probably the easier one to understand. Don't think of the wind as "wind" but as an airmass... The wind isn't really "blowing" as much as it's moving as a large mass and everything in it is moving at the same speed. A bit like a leaf in a river.
In order to move in a "straight" line through the moving air mass you have to point the plane a little bit left or right of where you want to go.
The plane isn't "turning" as much as it's direction is balanced against the wind.
When you're flying crabbed into a crosswind the plane is perfectly level and the rudder is nuetral. In fact, the only thing that is different is the direction that the nose is pointing. Pilots actually call this the "Wind Correction Angle" or WCA. That's probably a better description than "crabbing". So rather than constantly turning back onto course as the air mass moves us off course we simply compute an angle left or right of course that will cause the airplane to follow the groundtrack that we want.
Great job there. Couldnt say it better. Hats off.
Dave,
When you are flying along in that inky blackness, do you ever check the chart, see a CTAF airport down there, and use the radio to trigger the lights?
Or is it not possible from FL390? I did it the other night on a long night cross country STS to SMO, turning on the lights at PRB and it was nice to see. Comforting that civilization was down there, listening for me.
Good job with the picture. Where did you leave from to be flying for eight hours?
colin- nope, I do not do that. It probably would work from 39,000 feet.
lawrence- that is duty hours, not flight hours. We can fly eight hours per day; duty fourteen hours.
I'm a long-hooked UK reader of your posts and would like to wish you and yours a very merry Christmas, and a prosperous new year - many more to come in fact... Your posts help us dullards off the ground
I'm familiar with the sight of black black nothing out the window, but I'm too young to have ever seen an inkwell. When I went to school the desks still had little holes in the corners where our inkwells were supposed to sit. I'd never thought until now of what it would be like to look down into a pot of black ink. Funny when a metaphor works backwards.
My explanation of crabbing is that the airplane is going perfectly straight forward through the air, but the air is moving in a different direction over the ground, so the end result is a combination of the two movements. The pilot's job is to choose a direction to fly through the air such that when that movement is added to the movement of the air, the airplane follows a straight line to the destination. The difference between the heading (the way the airplane is pointing) and the track (the way the airplane is moving when wind is taken into account) is the crab angle, or the wind correction angle.
Dave-
Thanks for the great blog. My "self-loading cargo" ramblings for you at blog602.com. Take care and keep up the fine work in the sky and on the screen.
Merry X-mas! Hope Santa brought you everything you asked for.
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