G/S rule of thumb

[Harvey SM]

Without using the whizz-wheel, can someone tell me a quick formula of how to work out ground speed, given the true airspeed and wind direction.
cheers

[RVR800]

Use the 15/60 30/60 45/60 rule

e.g. If wind is 45 degrees off then its
3/4 of the wind speed

Speed 20 kts
Wind 345
Hdg 300
Headwind - Minus 15 kts

TAS 95kts - 15 kts headwind = G/S 80 kts

etc..


[This message has been edited by RVR800 (edited 30 April 2001).]

[Tor]

But what has that got to do with G/S?

[RVR800]

Cos wind affects G/S

[Harvey SM]

RVR800

Cheers -
so, what if for example I have the following:

TAS 228kts
wind dir 180
wind speed 30kts
hdg 110

That's 70 degrees off right? And I know it's a head wind, so the ground speed is bound to be less.

Sorry to be a pain
Cheers for your help

[Tor]

Don't mean to be word picking here. But the wind doesn't affect the G/S at all (it's very constant and normally 3 degs http://www.pprune.org/ubb/NonCGI/tongue.gif).

However, it do affects the RoD. The true "G/S rule of thumb" is:

RoD = (GS/2)x10

Or to calculate your optimum altitude:

alt = G/S angle x distance x 100.



[This message has been edited by Tor (edited 30 April 2001).]

[GARDENER]

Tor, attention to detail G/S=groundspeed
GS=glideslope

[Noddy Staltern]

The method described by RV800 is also known as the 'clock face' method and has several applications. Take the angle between heading and wind direction. Imagine this number of degrees as minutes on a clock face. The number of minutes on the clock-face (as a fraction of 60 minutes) represents the proportion of the wind-speed to be used.

Therefore:

15 degrees off = 15/60 = 1/4 of the clock-face
20 degrees off = 20/60 = 1/3 of the clock-face
30 degrees off = 30/60 = 1/2 of the clock-face
40 degrees off = 40/60 = 2/3 of the clock-face
45 degrees off = 45/60 = 3/4 of the clock-face
60 degrees off = 60/60 = 1/1 of the clock-face
Anything greater than 60 degrees is considered as 1/1


FOR CROSSWIND ON TAKEOFF:

Use the fraction above and all of the wind-speed.

eg. The wind is 20 degrees off at 15 knots.
20/60 = 1/3.
1/3 of 15 knots = 5 knots.
Therefore the crosswind at takeoff is 5 knots.

FOR DRIFT WHEN AIRBORNE:

Take the wind-speed and divide by TAS (in miles-per-minute).
This value is the maximum drift.
Use the fraction above with the maximum drift.

eg. The wind is 40 degrees off at 30 knots.
TAS is 120 knots.
Maximum drift = 30 (knots) divided by 2 (miles-per-minute) = 15 degrees
40/60 = 2/3.
2/3 of the maximum drift = 10 degrees.
Therefore the drift experienced is 10 degrees.

FOR HEAD/TAILWIND:

Use all of the windspeed and (90 - wind angle).

eg. The wind is 50 degrees off at 30 knots.
90-50 = 40.
40/60 = 2/3.
2/3 of 30 knots is 20 knots.
Therefore the head/tailwind component is 20 knots


Harvey,

In your example above, the wind is 70 degrees off at 30 knots.
FOR HEAD/TAILWIND:
90-70 = 20
20/60 = 1/3.
1/3 of the wind is 10 knots.
As you rightly say it is a headwind.
G/S =228 - 10 = 218 kts.

The above is commonly known in the RAF as 'dead-reckoning'. It works! Hope it is of some use.


[This message has been edited by Noddy Staltern (edited 30 April 2001).]

[Tor]

Gardener

The G/S and GS are absolutely intentional. As you correctly writes, one of them is Ground Speed and the other is Glide/Slope

[Devils Advocate]

Err, I just take a look at the approach plate upon which it actually has written just what ROD I should be roughly looking at in order to maintain the glide-slope.

Also, it might be fair to say that all the above formula seem to assume that the G/S is always three degrees ( which sometimes it isn't ! ) and that the last thing you need to be doing during an ILS approach for real is mental arithmetic - e.g. a ball-park figure to aim at is all you need (imho).

NB. For those of you flying light aircraft and / or those with a barometric based VSI - you might like to remember that what you're looking at is not an IVSI (where the 'I' means Instantaneous) and that it accordingly suffers from Hysteresis - accordingly, if so, please do keep this instrument in your scan but be aware of its limitations.

Ultimately perhaps the best bit of advice for making an ILS appear easy would be to be fully configured for landing (i.e. gear down, landing flap selected, landing checks completed) just prior to the FAF, i.e. get fully configured in the level segment of the platform height because then pretty much all you then have to do is to reduce the power just a tadge (or depending on a/c type, select landing flap and leave the power where it is) then pitch-over to follow the glide-slope downwards to the DA, and make sure that you are always in trim !

NB. The very last think you want when you're new to the business of flying in crap Wx is to be configuring the a/c (e.g. changes to Power / Attitude / Trim ) during the portion of the flight when you're descending with the ILS - e.g. Airlines don't (shouldn't) do this during Cat III approaches, and neither should you during your approach to your Cat I minima !

So, get configured early..... and remember that, as per the saying, "it ain't no sin to drag it in !"

[You Aint Seen Me. Roit!]

Devils Ad

Are you sure that hysteresis is the problem here.
Surely the main limitation with a basic VSI is lag. Hysteresis may mean that the instrument doesn't return exactly to "zero", but it should have little effect on the general day to day operation.

[Devils Advocate]

The word hystersis is derived from Latin and it means 'to lag behind' - which something which an IVSI does not do (obviously).

[Snap]

Devil

If you cant multiply the figure your DME is giving you by 5 to get the ROD on an ILS approach it would be very sad indeed.

[Devils Advocate]

Snap, I agree, and I too do occasionally have recourse to the old 5x table (typically once every six-months, when I'm in the simulator jumping though the hoops of the LPC / OPC checks) and / or in conjunction with the 3x height your height rule especially during 'visual approaches' with either runway DME distance (or an equivalent point via the FMC's 'fix' page) available.

Now the DME on the RDMI's which I use only give the slant range Nm to the tuned nav aid - and not Groundspeed (that is displayed, incredibly acurately, on our Electronic HSI's).
That said, I suppose that if you're using either a Bendix/King (e.g. KN62A / KN63 / KN64 / KDM706A / KTU709), or NavCom products, etc, all of which are typically found in light aircraft (e.g. I used to have a KN64 in my own spam-can).... then you might well indeed have a combined (or even single) distance and (or) G/S display.

In any event, and whilst ball park figures are a great aid memoir, I'd caution against trying to do math in the kind of weather that I had today, during an approach through the mountains situated north of Malaga, i.e. it was embedded Cb's / Heavy Rain / Turbulence / cloud base only 100' above the Cat I minima / plus a steeper than normal glideslope.

Accordingly, imho, I still reckon that you should read your groundspeed from the electronics, and then compare that with the V/S required to maintain the glideslope from the relevant section of approach plate which is (should be) right in front of you - it then all becomes a veritable 'no brainer', so leaving the old grey matter to cope with more secular activities, like, flying the aeroplane !

Also I'd still suggest that you should be cautious (particularly in rough weather; see above) about following a barometrically derived VSI, i.e. instead of one that's derived via electronic means, e.g. from the aircrafts INS platform(s) and which therefore is an instantaneous measure of the aircrafts up'in and down'in.

Ps. (and hence the edit) I must correct what I said above, in that 'Hystersis' is actually of Greek origin, e.g. husteresis from hustereo (be behind), from husteros (coming after) - uhm, I never really did get to grip with the classics !

[This message has been edited by Devils Advocate (edited 03 May 2001).]