744 GS calculations

[jtr]

Was looking at the prog page the other day, and the figures were something like...

TAS 495 kts
T/Wind 160 kts

and the GS on the ND said
GS 665 kts

All figure for illustrative purposes, but in a nutshell, the GS indicated GREATER than the total of TAS and T/wind by about 10 kts. FWIW wind was coming from our 7 o'clock.

It would seem to do with vector triangles, but be damned if I can figure it out.

[Intruder]

It's not the vector, Victor -- it's some kind of translation error in the software.

I often notice minor discrepancies between CDU (PROG, POS REF) and ND values. There are even discrepancies between the left and right side. Depending on whether or not you have GPS to minimize the errors, you may notice them more if there is a discrepancy between the system feeding the left and right side at the time.

Next time you see something like that, compare the ND values with the "master" CDU (usually left), and see if they're closer.

[jtr]

Thanks, I had thought this may have had an influence, but I was looking at the master FMC, the GS on the other side was 1 kt different, and the A/C was GPS equipped.

10kts seems a bit much for a 1010111001 error

[LOMCEVAK]

It will also depend on whether the displayed tailwind is with respect to heading or track. If it is relative to heading, then if there is also a crosswind component relative to heading, the tailwind relative to track will be greater than that displayed. As you surmised, a vestor triangle issue.

[Intruder]

Regardless, in reality, your groundspeed can never be greater than TAS + wind.

[Ka8 Flyer]

I'm curious, but doesn't the altitude play a role for GS?
Not for the calculation of TAS/IAS etc, but rather due to the fact that if you are high above a surface and flying along a curved surface (ie the earth), then the distance you cover at a high altitude is longer than the distance you would cover on the surface if you were directly over it.
I wonder if the 35000 ft or whatever you were at play a role in this equation?
Also, I wonder if´the IRU's make any consideration of this? I assume an IRS would bascially compute the location "at altitude", rather than the true position you would be at on the surface, but a GPS could calculate your "true" position.
I hope any of this makes sense?
Regards,

Mark

[Intruder]

OK...

The circumfrence of the earth is (in 'round' numbers) about 21,600 NM (360 deg * 60 NM per deg). Divide by 2*PI, and the radius is about 3437.746771 NM.

At 36,000' (6 NM) altitude, the ratio of ground speed at altitude to at the earth's surface, IF it is calcualted on a radial arc, is 1.001454441. So, at 500 KTAS, the difference would be 0.7272205 Knots (approximately). :ok:

That said, I have no clue as to whether such a correction is built into the software. OTOH, distance covered over the ground would be LESS, not MORE, than distance through the air -- you have to fly farther at the higher altitude to get across the same number of latitude lines -- so your "ground speed" at 500 knots True AirSpeed would be 499.2727795 knots (approximately).

:sad:

[The African Dude]

Intruder
Approximately 499.2727795? :D

Ka8 Flyer In real terms, the size of the Earth is so great that when flying at 36000ft you will only cover approximately 37nm further in a full equator orbit than if you were at MSL.

Just poking my nose in. :O

[Ka8 Flyer]

OK thanks for clearing that up for me!
I thought the influence was small, but that it would be so small.. oh well!

So actually the ND was not 10 knots off, but indeed 10.901...?
Regards,

Mark

[Intruder]

No! It was 9.985476713 knots off! Can't you get ANYTHING right? :p

[Intruder]

Ka8:
In real terms, the size of the Earth is so great that when flying at 36000ft you will only cover approximately 37nm further in a full equator orbit than if you were at MSL.
Hmmm... I get approximately 31.4159256 NM further at 36,000'... OTOH, that is about 36.1 statute miles... :8

BTW... Just for exercise some time, try to figure out in your head (no pencil, paper, or notes allowed) the mass of the earth, using commonly-known values and gross approximations.

It took me 2 dark nights' worth of walking home from the bus stop, about a half hour each time. The figure I came up with was right in between 2 [significantly different] values I could find in a web search...

FWIW, I never flew a Ka8, but the Ka6 I flew back in '77 or so was a SWEET machine!

[The African Dude]

Intruder The circumfrence of the earth is (in 'round' numbers) about 21,600 NM (360 deg * 60 NM per deg). Divide by 2*PI, and the radius is about 3437.746771 NM.

Add 6nm (altitude 36000ft) to radius giving 3443.746771 NM. Multiply by 2pi giving circumference of 21637.6991131943440470267223105866 need I go on with the decimal places????

Calculation was based on the figures you provided :ok: How have you managed a different answer?!? "One" would have thought all those bloody decimal places would have IMPROVED accuracy??? :E :E :)

Andy

[Ka8 Flyer]

To Intruder:

Ehm yeah well at least I figured out how to post in this forum ;)

Regarding the Ka8, it's similar to the Ka6 (wood construction) but is for one seater.
I never flew the Ka6 (training was done on the ASK-13 and 21), but the Ka8 has a remarkeable stall speed ;)

Regards,

Mark

[bigmountain]

YOu are right about the vector triangle . Im sure if you can work on your CR 2 computer you will find the answer to your question.

Also the B747 being similiar to the B777 , the display on the ND displays magnetic ( Hdg Tru not selected or not over polar routes) while the prog page 2 shows true. You would also then have a difference because of the wind driection true vs magnetic:ok:

[Intruder]

Calculation was based on the figures you provided How have you managed a different answer?!? "One" would have thought all those bloody decimal places would have IMPROVED accuracy???

It doesn't prevent "calculator error" due to fumble-fingers, though... I got the same number you did, this time... :* :mad: