Resolved Different Wind Speed readings?

[Ardy 0]

Hi there,

 

I have recently noticed that when I'm flying my 737-800, the wind speed reading I get from the HSI is different than the wind speed reading from the PROG Page on the FMC.  

Why is this? And which one should I use?

 

Thanks,

 

      Ardy.

[Ardy 0]

One more thing. Are the wind speed readings that ATC give in IAS or in TAS.

 

Thanks

 

[allardjd 1,853]

One more thing. Are the wind speed readings that ATC give in IAS or in TAS.

 

 

IAS and TAS relate to the velocity of an aircraft relative to the air-mass it is travelling in.  IAS is pretty much density relevant and though it is very important with respect to stall speeds and other V-speeds, it can be thorny for navigation purposes unless corrected; TAS is corrected for density effects, including altitude and temperature.   Wind speed is relative to the Earth, so if you are looking for an aircraft analogy, it's more akin to GS than anything else, but not normally characterized as anything but wind speed.  It's the velocity of the air mass with respect to the Earth.

[brett 2,309]

The wind speed given by ATC is at ground level and is calculated by readings taken over a given period of time and averaged out as are the gusts too. Not sure about the difference in the 737 readings, hopefully one of our commercial airliner flyers will pop in for the answer to that one.

[allardjd 1,853]

ATC terminal wind reports include gust information where there is enough wind gust activity present to warrant it.  

I looked around and found some - the current KHDC (Hammond, Indiana) METAR says...

KHDC 060055Z AUTO 11022G34KT 7SM -TSRA SCT013 SCT026 BKN045 27/21 A3005 RMK AO2 LTG NOOP ALQS

...indicating winds of 22 knots at 110 degrees, gusts to 34 knots.

The -TRSA indicates Thunderstorms and light rain. The presence of thunderstorms explains why there might be gusty conditions.

For enroute forecasts, winds aloft data is included. This is a line of winds aloft data for KMSP, Minneapolis-St. Paul, Minnesota

MSP 1826 2227+20 2224+14 2521+07 2220-07 2521-17 272732 284143 305552

It's forecast at 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000 and 39,000 MSL

In this case the 3,000 foot wind is forecast to be 180 degrees at 26 knots. The 6,000 foot wind is forecast to be 220 degrees at 27 knots and the air temperature is forecast to be +20 degrees Celsius. The 30,000 foot wind is forecast to be 280 degrees at 41 knots with the air temperature forecast to be -43 degrees Celsius.

Wind speeds are all relative to the Earth, of course.

John

[stu7708 244]

Hi there,

 

I have recently noticed that when I'm flying my 737-800, the wind speed reading I get from the HSI is different than the wind speed reading from the PROG Page on the FMC.  

Why is this? And which one should I use?

 

Thanks,

 

      Ardy.

 

It's been a while since I took the NGX up for a spin, but I don't recall that the wind data in the FMC is populated automatically. I think you need to update those manually. Not 100% sure though, so don't take this as a certain answer..

[Bruce (a.k.a. brian747) 142]

Hi Ardy!

 

Really good, if a trifle fuzzy, questions there — although I somehow suspect that they're related. I'm afraid this is likely to be a long answer which tries to cover a lot of ground on the basis of not a lot to go on, but I'll do my best....

 

> "I have recently noticed that when I'm flying my 737-800," 

 

OK, sorry to interrupt so soon but it would have been really helpful to know which one, in order to target my reply more precisely.  Are you in the FSX default? The iFly? The PMDG NGX? A.N. Other? They will each model things differently, so the answer to your question could be affected by the source of your aircraft model, as well as giving me a clue about how experienced you are so that I don't bore you with stuff you already know....       

 

> "...the wind speed reading I get from the HSI...

 

< Er... <*cough*>  When you say the "HSI", I am guessing that you are probably referring to the large screen just to the right of the Primary Flight Display? (In Boeing terminology this is normally referred to as the Navigation Display (ND), perhaps to distinguish it from the compact display format HSI that is only displayed following the failure of either an inboard or outboard Display Unit — although since that has no wind indication I assume you can't mean that one. I'm also going to assume that you don't mean the wind speed and direction indications on the HUD, since you don't mention it).

 

> "...is different than the wind speed reading from the PROG Page on the FMC."

 

Right. Fair enough. But, er, different in what way, I wonder? Evidently I'm going to have to guess.   

 

> "Why is this? And which one should I use?"

 

OK, let's start with a quick summary of the basics (just skip this bit if you've heard it before).   

 

All the big Boeings have dual (e.g. the 737) or triple (e.g. the 747) FMCs and dual IRSs. To decode that a little, let me define a few terms. I'll talk about the 747-400 since I know that pretty well, so just scale it down a little here and there for the 737 — but the principles are the same.   

 

In the 744, the Integrated Display System (IDS) comprises six Integrated Display Units (IDUs) (driving the two PFDs, the two NDs, and the upper & lower EICAS displays). The IDS is interfaced with three EFIS/EICAS interface units (EIUs), each of which receives the same data from a multitude of aircraft systems, and sends that data to selected IDUs. Notice "the same data" — that's important.

 

However, weather radar and terrain images are not routed through the EIUs but get sent directly to the IDUs. Data from navigation radios such as VOR, DME, ADF, ILS, and markers are also routed to the IDUs but via three air data computers (ADCs) which process inputs from various pitot-static probes, temperature probes, and AoA sensors. It's the ADCs that provide us with the airspeeds, barometric altitudes, altitude rates, and other air data.

 

But there's more.

 

The inertial reference system (IRS) comprises three inertial reference units (IRUs). Each IRU contains laser gyros and accelerometers to determine aircraft position, attitude, velocities and many other parameters. For the computation of the vertical speed, each IRU uses its internal velocity detection for short-term updates, and the barometric altitude rate (air data) for long-term updates. To get the barometric altitude rate, each IRU is linked with a certain ADC, and that linked ADC supplies the IRU also with TAS data which the IRU applies in its wind calculations. (So perhaps you can now see why I wearied you with all the above stuff).

 

On those systems, the wind ("At last!", I hear you cry) is an output of the IRSs' calculations of the difference between inertial velocity and air velocity, (and nothing to do with the FMCs, which is why you'll get wind displayed whether or not the FMCs are running).

 

Therefore (to summarise all that) in spite of the fact that the IDS integrates all that data and processes it in quite complex ways, nonetheless comparison processes go on to ensure that a single composite answer comes out of the tin brains regardless. So if you're seeing (as I'm sure you are) a difference in the displayed values, then there must be another reason for the difference.

 

However, since you haven't given me any pics or examples to go on, this is going to be a bit speculative. But I'm wondering whether maybe there's a sort of connection with your supplementary question —

 

> "Are the wind speed readings that ATC give in IAS or in TAS."

 

There are three sets of things that many simmers get confused about (if you're not one of them, please forgive me, I'm just making a general point, here).

 

The first is with regard to IAS vs TAS (without even mentioning SAT and TAT); and the second is true vs. magnetic, and the third is track vs. heading. I'm wondering if we're in that sort of territory?

 

The difference you say you noticed was that "the HSI is different than the wind speed reading from the PROG Page on the FMC". In other words, the winds shown in the upper left corner of the ND in some unexplained way didn't match the ones on the PROGRESS pages shown in the CDU. You didn't tell us what the difference that you observed actually was, but I have a sneaky suspicion that what you're seeing could perhaps be the difference between true and magnetic? 

 

My reason for wondering (on the basis of very little evidence, admittedly) is this. If we take the figures that you see in the CDU first, no matter whether you see the wind direction in the PROGRESS pages (1 and 2), the RTE DATA page, the CRZ page, or the DES FORECASTS page, the values you will read off are all True, as opposed to magnetic. They would normally all agree with each other, because they all emanate from the same source, as discussed above. BUT (there's always a 'but',  <sigh> ) you need to bear in mind that it's also possible in what is laughingly known as the Real World™ that the values shown on the Captain's CDU might not perfectly match the values seen on the FO's CDU, simply owing to a mismatch between the display computers, if they're not perfectly in sync. So IF your model of the 737 incorporates such subtle differences, that might perhaps be a factor.

 

However, the information displayed on the ND is a little different.

 

AFAIK the ND wind direction given in numerals is always relative to True north, whereas the wind direction arrow is relative to magnetic north. (I think that's true of Airbus as well — Jess?). Could that be the difference you're seeing? In the area where you noticed this, was the magnetic variation significant, or just a degree or two? The wording in the Boeing FCOM for the 747 is — "The wind bearing/speed and direction arrow: this indicates wind bearing, wind speed in knots, and wind direction with respect to display orientation and heading/track reference."

 

Which raises another question, of course... Are you OK with heading vs. track? **

 

Moving on to your supplementary question:

 

> "Are the wind speed readings that ATC give in IAS or in TAS."

 

Well since the wind speeds (and gust speeds) are being measured at ground level at the airfield by equipment that is concreted solidly onto terra firma, AirSpeed (either I or T) doesn't come into the equation, you just get a speed and a direction. However — is the direction you get True, or magnetic?

 

If by any chance there is an element of the true vs. magnetic thing going on here, then there's a wonderful old rule of thumb which quickly helps us to sort out which is which — 

 

If you read it, it's true. If you hear it, it's magnetic.

 

(I don't have any tattoos, but if I ever thought of getting one I might get that great aphorism written along my forearm, for easy reference). In other words, if you see the figures on the CDU or the ND, or on a chart, they are giving you True; if you hear them from ATIS or ATC (or deduce them from the wind arrow on the ND) then they're magnetic. All charts and text sources (METAR, TAF, winds aloft, surface analysis charts, whatever) use True north as the reference, whereas ATIS / AWOS / ASOS or any information a controller gives you over the radio is magnetic.

 

(Although, come to think of it, there's one... well sort of exception to that rule which you might wish to bear in mind. In the U.S they have Flight Service Stations (FSS) which provide information and services to pilots before, during, and after flights — but unlike ATC they're not responsible for giving you instructions or clearances or providing separation. In other words you hear them only because they are reading to you, but the information they're giving you will be the winds referenced to True north, since they're just reading to you from charts or textual information and hence it's the same as if you were reading it yourself). Certainly in Europe, the part of the world that I'm most familiar with, wind information for landings and take-offs given by the tower is always given in degrees magnetic. Mind you, having said that, I have to admit that the magnetic variation for many parts of Europe isn't exactly enormous....

 

So my apologies for having to be a bit vague; but to be fair you didn't give me a lot of information to go on.    

 

Let me know if any of the stuff I've suggested has been of any help. Or if not, please help me out and be a little more specific?   

 

Cheers,

 

bruce

a.k.a. brian747

 

 

** Later edit: I have just added a document to the Commercial Airliners library which illustrates the difference between Track up and Heading up presentations on a Boeing ND. You can find it in the library, which is here: http://forum.mutleyshangar.com/index.php/files/category/15-commercial-sim-pilots-centre/?sort_order=ASC&sort_key=file_name&num=15

 

B.

 

 

[Ardy 0]

Hi Bruce,

 

Thanks for your reply.

 

I'm talking about the PMDG NGX and sorry about the wrong definition. I just couldn't think of the name so I put the closest thing in.

 

The speed is the same on both the ND and the FMC PROG page but the heading is off by 2 degrees. So now I understand that one of the wind speeds is in True North and the other is in Magnetic.

 

Thanks for your other answers as well.

 

Ardy

 

 

[Bruce (a.k.a. brian747) 142]

Hi Ardy!

 

    That's great — I'm happy you got it sorted out. I'll mark the topic as Resolved, if that's OK with you.

 

Many thanks for letting us know!   

 

Cheers,

 

bruce

a.k.a. brian747

 

[albertz 2]

Wind from a tower is magnetic. Wind from a forecast is true. ATIS, taf, metal all true. Exceptions are in extreme latitudes where winds from the tower will be true. Your wind difference is exactly as you surmise.

cheers

Matt