737NG ND Wind Vector

[QAVION]

Not sure if this has been asked before, but....

The Boeing Maintenance Manual says that the Wind speed/angle arrow on the ND is not available until 100Kts TAS. Does this mean there exists the possibility that it may appear on the ground (assuming the local winds are strong enough)? Some have said it only appears in the air.

If it does appear on the ground, I can only see it providing wind data dead ahead (i.e. along the axis of the aircraft).

I realize that this is probably not what pilots are looking at during their takeoff run, but....

Thanks for any feedback.

Regards.
Q.

[avioniker]

It won't appear on the ground because the ADIRU's require aircraft movement to be able to measure the effect of wind on the track of the aircraft. That's where the arrow and wind data come from.
The software inhibits the display under a certain airspeed.

[QAVION]

...on another forum, I've just been told that the wind vector is available after landing until a certain wheelspeed (providing wind data for go-around). Would this simply be hysteresis or perhaps something held in memory?

Thanks.
Regards.
Q.

[Notso Fantastic]

The wind vector cannot give you any sensible information once the wheels are on the ground. It gets its information from:
Which way are you heading and what airspeed
IRS- what has been the change of position (track and groundspeed)
From those it can work out what wind has affected you. Once the wheels are on the ground, it cannot now resolve a wind as the aeroplane is not tracking due to the wind. The only way you could have a wind readout would be a weathervane on top of the aeroplane with a subtraction for groundspeed!

[QAVION]

"The wind vector cannot give you any sensible information once the wheels are on the ground."

Would it be able to give you any data at all (useful, sensible or otherwise)... say a value of wind along the runway? I was never any good at vectors, but wouldn't airspeed minus groundspeed give you a (resolved) wind component along the runway?

I have the somewhat vague feeling I've read/heard about an aircraft with a ground wind display, but I don't know where or when ...making it hard to let me put this one to rest :bored:

Thanks.

Cheers.
Q.

[avioniker]

I think you may be missing the point in the posts above.
The plane doesn't measure the air movement or wind.
The Inertial systems measure the relative movement of the plane along a track and compare that with the commanded course, heading, and attitude the plane maintains plus the airspeed as computed inertially vs. the air data measured airspeed and then presents the difference to the pilot as a wind vector.
If you're doing 100kts CAS with a course of 90deg. and the inertial figures you're doing 50kts along a 90deg track then you have a wind of 90deg at 50kts. (and that's a bit simplified but we don't need to be getting into TAS, GS, IAS, and CAS now do we?)
Or do I have it backward again???

I'm so confused..........:confused:

[Notso Fantastic]

You cannot have a ground wind display unless you have a windvane sticking above the aeroplane (and then you'd need to allow for the groundspeed)! I am not sure when the wind vector becomes 'live', but whilst the wheels are tracking the ground, there is no way the system can detect any sideways drift to assess the wind. I'm never looking down there and neither should anybody else on take off or landing! You always have a simple groundspeed display. If there was a 50 kt wind across the runway, the FMS would have no way of knowing until the wheels lifted off and the aeroplane started being blown sideways.

[QAVION]

"I think you may be missing the point in the posts above.
The plane doesn't measure the air movement or wind."

Er... yes, no... maybe :O

I've been told that TAS and the wind vector cannot be computed without air data coming from the ADC's (or at least the ADC side of the ADIRU's), according to my other Boeing training manuals. Some (kind of) measurement of air movement must be "in the mix" somewhere... No?

The inertial side of the ADIRU is measuring the forward motion of the aircraft (track and groundspeed). TAS, I presume, is being measured above 100Kts by the ADC side of the ADIRU's(?). TAS, on Boeings at least, is simply(!) pitot static data put through the blender (compensated, mixed with TAT, etc)

"If you're doing 100kts CAS with a course of 90deg. and the inertial figures you're doing 50kts along a 90deg track then you have a wind of 90deg at 50kts. (and that's a bit simplified but we don't need to be getting into TAS, GS, IAS, and CAS now do we?)"

That's exactly what I'm trying to say (in a roundabout way). Why not paint a 50Kt vector at 90degrees (dead ahead) on the ground on the ND above 100kts? (Or if the angle of the wind is, say, 30 degrees (relative) to the aircraft track, paint a wind vector at 90 degrees, but 43kts (50kts x cos30deg?) in strength.

I very much understand that pitot sensing errors are going to arise in the TAS calculation if the wind is blowing at an extreme angle... just as it would with the IAS/CAS. I also realise that the aircraft cannot predict (on the ground) the effect a crosswind would have on the aircraft in the air. I simply want to understand what is stopping the aircraft computing the strength of the wind (dead ahead) and displaying that data on the ground. Although, now that I think about it, if this was displayed on a HUD in front of the pilot, I have to ask would this data be any use in visually displaying a rapid change from a headwind to a tailwind, as might be experienced in a microburst? Anyway, I'll leave that for another day ;)

Thanks for all your efforts. :ok:

Cheers.
Q.