Guys,

Lately I've been doing a lot traveling via air and everytime the plane lands, it lands from a different direction. I've always wondered about this. I guess I'm curious as I look out the small window to see if I can recognize the major freeways and soon enough, the plane overshoots the airport and comes and makes a U-turn.

Any pilots out there who can explain this? I heard a few things about wind direction, etc, but I'm not sure.

-Vu

I'm not a pilot but I know aircraft usually land and take-off into the wind and of course the wind often changes from day-to-day. A lot of airports also have noise-abaitment rules so they have to change runways and different times of day and week-to-week.

__________________
1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys

The airplane is kept aloft based upon airspeed. So, if the airplane heads into the wind, the ground speed of the airplane is reduced by the speed of the wind. The airplane is then moving slower relative to the ground, but is moving at the same airspeed.

So, there is a definite preference to land into the wind.

Most airports have flight patterns that has the airplane do a downwind leg, followed by a 90 degree turn to "base leg" followed by another 90 degree turn to "final leg" and the runway approach.

However, there are noted exceptions and the three airports around NYC certainly have their share of them.

All the answers are correct. We land and takeoff into the wind so that our ground speed is the slowest possible ( accelerate/abort energies and landing/braking energies need to be the lowest possible). There are a few exeptions. Some tail wind on takeoff and landing is acceptable IF it has been calculated in the takeoff data.
We also have limitations on crosswind and total steady state wind speed that each aircraft manufacture determines. In the certification of the aircraft the manufacture determines the maximum headwind, crosswind and tailwind the aircraft can handle.
The maximum headwind may simply be based on safety. The MD80 I fly has a maximum headwind of 50Kts. Above this wind I can't land or takeoff. Not because of flight control limitations but because iti starts to become "dangerous". 50kt winds at the surface will probably produce severe turbulance in the lower level of the atmosphere. So its not the speed of the wind. Its the turbulence.
For a crosswind my aircraft has a 30Kt "demonstrated" croswind. They design the aircraft with a rudder for 3 reasons. 1, you need a control on the yaw axis if you want complete control of the aircraft. 2, in a multi engine aircraft, you need the rudder to counteract the yaw tendency if you loose an engine. and 3, during a crosswind landing ( the wind is perpendicular to the runway) you use cross controls to align the aircraft with the runway. So at touchdown the aircraft is not sliding over the ground and touches down in a crab. This puts large loads on the aircraft and is just uncomfortable for the passenger. Now this "demonstrated croswind" is actually demonstrated by the test pilots for the manufacture during the aircraft certification. This is the highest crosswind they were able to show the FAA in a crosswind landing. Above this value you are a test pilot. It can be done but it has not been proven to the FAA and if anything happens, your in trouble.
I had to do this in the C141 aircraft I flew in the military. We had a demonstrated crosswind limit of 32kts. We were landing at Lajes in the Azores. The winds were 50kts perpendicular to the runway. BUT we had nowhere else to go. Santa Maria, the only other airport in the Azores had the same wind and we did not have the fuel to go all the way to Portugal, the nearest land. So we landed, actually wreslted the aircraft to the ground. One of the limitations to the demonstrated crosswind is the rudders abiltiy to rotate the nose to align with the runway. On my aircraft winds above 30 kts. I will have full rudder in and the aircraft will not be completely aligned. We run out of control input.

Now in the New York airport area the flight patterns for all three major airports (LGA, EWR, JFK) is complicated. When the runways are changed at one airport it effects which runways can be used at the other airports due to the arriving and departing flight paths.

One other item. Runways are built based on the wind patterns in the area. If the wind comes out of the north 50% of the time and then the south another 20%. Then a north south runway is what is built so that the aircraft are aligned with the wind. However at times geographical factors may be greater. If you don't have a long section of land available then you do with what you have.

Sorry for the long winded answer.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

on the subject of aircraft.... (slight hi-jacking of thread, excuse the pun) y'all might enjoy this picture I took last Friday.

http://www.jetphotos.net/viewphoto.php?id=439038

__________________
1993 320TE M104
---------------------------------------------------
past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
----------------------------------------------
"music and women I cannot but give way to, whatever my business" -Pepys

going into heathrow right?
hope it was close to the airport

__________________
1984 300D Turbo - 231k....totalled 11/30/07 RIP

That's a great shot!!

It's difficult to judge the altitude of that aircraft from the photo. Would you say 300 ft.?

One of the scariest things I witnessed was a DC-8 (IIRC) landing in St. Lucia.

Same deal, one runway and the need to use the controls to hold the aircraft parallel to the runway in a crosswind.

However, this fellow chose to use a crabbed approach for whatever reason and he held it all the way to touchdown. The aircraft instantly twists and the tires align themselves with the direction of flight (down the runway). It had to be just brutal on the gear, and, I'm sure the passengers didn't appreciate it one bit.

We find 2 kinds of ex military pilots in the airline industry. And this is not always a hard and fast rule. Those who flew fighters and those who flew tranport aircraft or large bombers. Fighters are designed to land in a crab. Their gear is designed for the sideload and it works well for them. They also don't use an extensive flare technique. Large aircraft pilots are taught wing low landing technique. You approach in a crab. At the point that you are comfortable ( and this varies by pilot from 30-200ft above the runway) you apply the rudder to align the aircraft with the runway. To counteract the aircrafts tendancy to drift off the runway you apply, or drop the upwind wing slightly. This cross control manuever will allow the crab to be eliminated. With high winds you may touchdown on the upwind main wheel first ant then the downwind wheel will touchdown with spoiler deployment. All aircraft up to the An225 should be flown this way.
A few, and I mean very few pilots think the aircraft should be landed in a crab. I personally dont' like it. And I dont' think the aircraft likes it either.
( the C-5 and the B52 had crasswind landing gear, you rotate the gear to align them with the runway while the body is still crabbed. I understand this was removed inthe C-5B model.

I know that civilian trained pilots are taught this techique from day one. So it is natural for them.
An interesting sidenote. The newer Airbus aircraft that use computer controled flight controls are landed differently. The side stick controller when applied gives a "certain role RATE". Not a control input. Boeings give you a actual aileron movement depending on the amount of control input. When we apply the cross control techniques we input the controls and then maintain the input. But the airbus is giving you a role rate. Watch a A320 the next time it lands in a cross wind. They have to pump the control stick, in aileron movement, to create the crosswind technique.

Heres a few "non standerd landings"

http://www.airliners.net/open.file?id=229317&WxsIERv=Obrvat%20747-2W6OZ&WdsYXMg=Nve%20Puvan&QtODMg=Ubat%20Xbat%20-%20Xnv%20Gnx%20Vagreangvbany%20%28UXT%20%2F%20IUUU%29%20%28pybfrq%29&ERDLTkt=Puvan%20-%20Ubat%20Xbat&ktODMp=1993&BP=0&WNEb25u=Qnely%20Punczna&xsIERvdWdsY=O-2450&MgTUQtODMgKE=Lrg%20nabgure%20aba-fgnaqneq%20ynaqvat%20ng%20Xnv%20Gnx.%20Guvf%20jnf%20gur%20svefg%20cvpgher%20V%20rire%20tbg%20choyvfu rq.&YXMgTUQtODMgKERD=19469&NEb25uZWxs=2002-04-14%2000%3A00%3A00&ODJ9dvCE=&O89Dcjdg=&static=yes&sok=JURER%20%20%28nvepensg_trarevp%20YVXR%20%27Obrvat%20747%25%27%29%20NAQ%20%28cynpr%20%3D%20%27Uba t%20Xbat%20-%20Xnv%20Gnx%20Vagreangvbany%20%28UXT%20%2F%20IUUU%29%20%28pybfrq%29%27%29%20NAQ%20%28ZNGPU%20%28nve pensg%2Cnveyvar%2Ccynpr%2Ccubgb_qngr%2Cpbhagel%2Cerznex%2Ccubgbtencure%2Crznvy%2Clrne%2Cert%2Cnvepen sg_trarevp%2Cpa%2Cpbqr%29%20NTNVAFG%20%28%27%2B%22ynaqvat%22%27%20VA%20OBBYRNA%20ZBQR%29%29%20%20beq re%20ol%20cubgb_vq%20QRFP&photo_nr=38&prev_id=247814&next_id=223246

http://www.airliners.net/open.file?id=047556&WxsIERv=Obrvat%20747-4...&WdsYXMg=Puvan%20Nveyvarf&QtODMg=Ubat%20Xbat%20-%20Xnv%20Gnx%20Vagreangvbany%20%28UXT%20%2F%20IUUU%29%20%28pybfrq%29&ERDLTkt=Puvan%20-%20Ubat%20Xbat&ktODMp=Whar%201997&BP=0&WNEb25u=Fnzhry%20yb&xsIERvdWdsY=&MgTUQtODMgKE=Penml%20ynaqvat%20nf%20lbh%20pna%20frr%20fcbvyref%20naq%20gur%20gver%20genpx%20pyrneyl. %20Shyy%20senzr%20naq%20gnxra%20sebz%20Ornpba%20Uvyy%20jvgu%20800%20zz%20yraf.&YXMgTUQtODMgKERD=817601&NEb25uZWxs=1999-09-19%2000%3A00%3A00&ODJ9dvCE=&O89Dcjdg=&static=yes&sok=JURER%20%20%28nvepensg_trarevp%20YVXR%20%27Obrvat%20747%25%27%29%20NAQ%20%28cynpr%20%3D%20%27Uba t%20Xbat%20-%20Xnv%20Gnx%20Vagreangvbany%20%28UXT%20%2F%20IUUU%29%20%28pybfrq%29%27%29%20NAQ%20%28ZNGPU%20%28nve pensg%2Cnveyvar%2Ccynpr%2Ccubgb_qngr%2Cpbhagel%2Cerznex%2Ccubgbtencure%2Crznvy%2Clrne%2Cert%2Cnvepen sg_trarevp%2Cpa%2Cpbqr%29%20NTNVAFG%20%28%27%2B%22ynaqvat%22%27%20VA%20OBBYRNA%20ZBQR%29%29%20%20beq re%20ol%20cubgb_vq%20QRFP&photo_nr=90&prev_id=047761&next_id=NEXTID

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Dave -- that's funny... we deployed to Lajes for the war, and though I didn't make that trip, I heard all about the winds! All the flight line vehicles have streamers on the antenna to indicate which was the wind is blowing. During the in-briefing, my colleagues were all warned to always park INTO the wind or they'd be sure to lose a door. The first day, a youg crew chief parked with the wind, and the wind ripped the door off its hinges. They quit tethering our tankers overnight; it was damaging the gear too much. They said it was easier to let the plane "walk" a few feet overnight and just reposition it later.

Crazy... that is some serious wind! I remember watching B-52s land in crosswinds; that gear setup they use for that purpose is really neat.

Chris

__________________
1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

Not only is it windy at Lajes, but the wind is often highly variable. The tower would report the wind direction and speed at the approach end, midfield, and departure end of the runway. But if you think Lajes is windy, you probably have not operated out of Adak!!!

Incredible! So if it's this complicated to keep a big plane on course and oriented correctly, could somebody explain it to me again how a bunch of terrorists with no real flight experience, and with navigation transponders switched off, managed to hit three targets so accurately on 9-11? The Pentagon one is especially mind-boggling as it needed such a long run-in at nearly ground level. Who was doing such a good job at compensating for wind direction on that one?

It wasn't windy that morning at the Pentagon..............I was on the roof of my building 5 minutes after that happened that morning 1/2 mile away ( I can see the pentagon from there). and there was no wind.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Well, my take is as follows: the VAST majority of the time pilots get their landings right, so while it's very difficult and nerve-wracking, I'm sure, it's far from impossible. Plus, those 9/11 goofs weren't landing airplanes, they were crashing them, so whether they were "crabbed" approaches or not didn't much matter.

One of the pilots in this thread will chime in and set us straight, hopefully. They're the ones to ask. I can tell you about flap overspeed inspections and servicing landing gear snubbers, rigging flight control cables, etc..., but none of the exciting stuff. 8^)

Chris

__________________
1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

First, they did have real flight experience. Second, transponders are not used to navigate. Transponders allow an aircraft to show up on radar electronically; they are for the benefit of air traffic controllers, not the aircrew. Third, wind compensation during landing is quite different from wind compensation during other phases of flight. During landing, it is important that the longitudinal axis of the aircraft be parallel to the direction of travel (unless the aircraft is equipped with crosswind landing gear.) This is not necessary (or desirable) during other phases of flight.