EFIS wind arrow behaviour during takeoff

[Tor]

6 flights and I forgot to look for the wind arrow each time. I guess the whole event of a takeoff is so pre-programmed into the brain that it's difficult to focus on something different.

John H Watson:
A 737 may be a bad example. From what I hear, some even crab during taxy )))

They all do. Hence:

Tor:
..If not if must be the flex that is inherent in the torque links..

[John H Watson]

..If not if must be the flex that is inherent in the torque links..

Or lack of flex, hence the shimmy dampers.

Interesting reading....

http://www.airliners.net/aviation-forums/tech_ops/read.main/76457/

Regards
JHW

[Tor]

The way I have understood:

The Torque Link (or Scissor Link) holds together the top and lover portion of the Oleo leg (shock absorber) and prevents the wheel from rotating/torque freely. Due to the play in the Torque Link, the wheels may however, rotate a little bit (causing the crabbed taxing).

The Shimmy Damper is installed to dampen vibration caused by this play.

So I would say the flex is in the Torque Links and the flex is dampened by the Shimmy Damper.

But since it seems that pilots are always only told "half the truth" I stand by to be corrected.  

[John H Watson]

The Torque Link (or Scissor Link) holds together the top and lover portion of the Oleo leg (shock absorber)

Freudian slip?  :mrgreen:

[effte]

However, almost all aircraft have torque links and only the 737 exhibits this behaviour?

Way I heard it, the shimmy damper was added to (and you won't believe this!) solve a shimmy problem on later models of the 737. The installation of the damper however gave some flexibility to the system as a whole and et voila! The crabbing taxi saw the light of day.

[Hardy Heinlin]

I just learned that on the Airbus after VR during rotation the computed headwind is about 10 knots too slow, i.e. 5 kt headwind is indicated as 5kt tailwind, and zero wind as 10 kt tailwind.

Is this effect aircraft specfic? Is this an air flow problem of the pitot tube geometry?


|-|

[Zinger]

Due to system design characteristics, the wind arrow is irrelevant below flight speed. For B744 which is succeptible to taxi under wet and strong crosswind conditions, the pilot must rely on aerodrome wind information. On the takeoff roll above V1, he gets crosswind cues from the amount of pedal and aileron cross inputs required to maintain wings level and runway centerline. The wind arrow is also not reliable for indication of imminent windshear right after liftoff.
B744 manuals I have do not address crosswind takeoff technique, as opposed to B777 manuals. Boeing design uses 30 knot crosswind limit, 25 for autoland, which is more than adequate for most flights.

[Lasse]

I had a look at the wind arrow yesterday when taking off, and its not pointing right into the nose...
We had 10 kts in the side so for our speeds V2 155 KIAS it showed 1-2 deg off and poped up during takeoff run when the GS and TAS had a diff of 6 kts as it would in the air...

Hope it helped?

/L

[Hardy Heinlin]

Thank you very much, Lasse.

Perhaps on the ground the wind arrow is displayed not only if TAS is higher than 100, but also if the groundspeed-to-TAS difference is greater than 5 knots or so.


Cheers,

|–|ardy

[Lasse]

Hi again

I tried to look it up in the B744 FCOM, but its not written there. (Well at least not in the version I have)...
For the  B737-NG its a little better discriped... The wind arrow will show above TAS 101 Kts and wind magnitude of 6 and will be removed when it falls below 4.
In plan mode it will only show direction and speed.

However did a takeoff more since yesterday and it seams like the angle of the wind arrow is based on those small yaws we have during takeoff run due the crosswind. So the wind arrow is 1-4 degrees off RWY track...

/L

[Tor]

I flew mostly classic in the last week and when NG there was no crosswind. But yesterday I finally had an NG take off with decent crosswind. And I remembered to have a look as well.

I do confirm everything Lasse says.

I can add the following. I noticed that there may be a difference of more than 6 kts, still with no arrow. So the TAS may increase and the GS increasing slower and then catching up. None of them increase smoothly, it's like: increase fast, then stop or slowly then increase again. Goes for both TAS/GS and as said they don't necessarily follow each other linearly. So the difference may be 2 kts one second and 8-10 kts the next. With no arrow indication. Guess there is a criteria in the logic that it has to be more that 6 kts for a certain time. So the arrow doesn't come on and off.

With constantly more than 6 kts difference, the arrow will be displayed and as Lasse said it will indicate what ever drift there may be as you go along the runway. I have to retract my statement that the aircraft goes straight. There is indeed a crab as you go along, even with steady pedal, maybe 2-3 degs. So I think is not just the yaw inputs.
But as mentioned there is a slip in the main gear on the 737 shimmy damper, so i think this is about it (hence the wheels not being dragged sideways pr. previous discussion). Someone with more knowledge about 747 may comment if the 747 main gear also has this play.

When the arrow is displayed in crosswind it will remain at the 2-3 degs during the takeoff run (varying with rudder input as Lasse says), and when coming airborne it will slowly turn to the direction of the actual wind, much slower than the nose swings over. This takes 3 to 5 seconds I would say. So the computers doesn't calculate this instantly when the wheels come off the ground.

[Hardy Heinlin]

Thank you, gentlemen, I'll try to incorporate all these effects in my model as a basis and keep the values in the code flexible for future adjustments ...


Best regards

|-|ardy