Cockpit mental arithmetic

[L'aviateur]

Recently i've been introduced to some good and quick 'rough' calculations that can be made whilst flying in addition to some good rules of the thumb. Please would anyone be so kind as to post a few of the favourites; i.e simple ways to calculate a crosswind component on a final approach, diversions, off track errors.
I think these could be really useful to see the different techniques and see which is the easiest and works best.

Thanks

[PompeyPaul]

I was told that if you take the difference between wind & runway, and imagine it on a watch face, then it will tell you how much of the wind is crosswind, i.e.

runway 20
Wind 230\10

230-200 = 30
30mins on the watch face is half, so that's 10* 1/2 = 5kts crosswind

Let's say,

runway 18
Wind 195\05

195-180 = 15
15mins on the watch face is a quarter, so that's just over 1 kt crosswind.

Although, the floor in this, as far as I can see, is for significant crosswind. I.e.

runway 09
Wind 180/10

180-090 = 90 degrees (therefore 100% crosswind)

But you can't put 90 degrees on the watch face. I guess you work on the assumption > 60 degrees is 100% crosswind

[Fg Off Max Stout]

The RAF method for mental dead reckoning:

Max Drift Angle:

MDA=(60/TAS)*Wind Speed

eg If you cruise at 120kts and there's a 20kt wind, the maximum amount of drift you'll experience (ie when flying at 90 deg to the wind) is 10 deg.

Clock face method:

If the wind is 15 deg off the nose or tail your actual drift will be quarter of the MDA (think 15 minutes = quarter of an hour).
30 deg off = 1/2 MDA.
45 deg off = 3/4 MDA.
60 deg off or more = MDA.

Similarly your head or tailwind component. If the wind is 15 degrees off the lateral axis, the head/tail wind component will be 1/4 of the wind speed, and so on.

Glideslopes:

An x degree glideslope is 100x feet per mile. eg 3 degree = 300 ft/nm
For a 3 degree glideslope your rate of descent should be 5 times the groundspeed. eg 90kts 450fpm.



These are just a few little tips. Of course they are approximations but a good enough for real world use and with a bit of practise can be calculated mentally in no time at all. Using a whizzwheel for such calculations suddenly seems ridiculous!

[FullyFlapped]

I use "sixths" as taught by my class act of an instructor (who would bellow "try not to rip the ***** nosewheel off" as we turned finals by way of encouragement - top bloke:p)

Ie, wind up to 10 degs of runway heading = XW is 1/6th of wind
20 degs = 2/6ths (1/3 for the hard of riffmaticking)
30 degs = 3/6ths (1/2)
and so on, up to
60 degs and above = it's all crosswind.

Common sense should tell you that these are just broadbrush guidelines, not accurate but good enough to give you a feel for the situation.

[Fg Off Max Stout]

Here's another:

Angle of bank for a rate 1 turn = (Airspeed/10)+7
eg 120kts, 19degrees AoB

Works well enough up to about 150kts

[BackPacker]

Distance to a VOR/NDB station for adequate reception: 1.2 times the square root of your altitude (in feet) gives the distance (in nm).

[PompeyPaul]

Here's another:

Angle of bank for a rate 1 turn = (Airspeed/10)+7
eg 120kts, 19degrees AoB

Works well enough up to about 150ktsI don't get why you would need to do that ? Given the turn indicator has the rate 1 turn labeled on there. IS that for aircraft that don't have turn indicators ?

Also how long is to turn 180 degrees in a rate 1 bank turn ? I remember being told this on my PPL and have forgotton. I always thought if I, inadvertently, went into cloud I'd look at the DI and wait for it to turn 180 degrees. Although in the heat of battle I'm not sure if I would remember to do that. Isn't it 2mins for a 180 degree turn ?

[PoppyPilot]

The digits of the heading for each leg (crosswind, downwind, base and final) all add up to same number. For example if runway 35 is in use (3+5=8), then crosswind heading (080) downwind (170), base leg (260) and of course final (350) all add up to eight. Now try it for any other rundway! (Okay, if you picked a 290 heading, you will have to add the digits of the '11' you get from 2+9 once again and make 1+1=2, because the otherheadings (020, 110 and 200) all add up to 2).

Another good rule of thumb :-

Kilograms to pounds rule is simply to double kilos and 10% (This accurate to within five pounds in a ton).

Both these were in an article in the Pilot magazine about a year and a half ago, if you can dig out a copy there were about 16 really useful 'Rules of Thumb'. It would be worth typing out the ones you would use most and getting a copy laminated to keep in the cockpit!!

Good luck!

[Fg Off Max Stout]

Pompey,

Yes, there are some quite well equipped aircraft out there that lack a turn needle or do not have rates marked on it. A rate 1 turn is 3deg/sec or 180deg per minute.

A quick trick for reciprocal headings is to add 2 to the first digit and subtract 2 from the second digit (or vice-versa). eg 315 135 or 253 073.

[BackPacker]

I don't get why you would need to do that ? Given the turn indicator has the rate 1 turn labeled on there. IS that for aircraft that don't have turn indicators ?

Also how long is to turn 180 degrees in a rate 1 bank turn ? I remember being told this on my PPL and have forgotton. I always thought if I, inadvertently, went into cloud I'd look at the DI and wait for it to turn 180 degrees. Although in the heat of battle I'm not sure if I would remember to do that. Isn't it 2mins for a 180 degree turn ?

A standard turn (or rate one) is 360 degrees in two minutes, or 180 degrees in one minute.

Knowing the required bank angle is useful because the AI gives you immediate feedback on the bank angle, while the CDI takes a few seconds to stabilize (because your turn will take a few seconds to roll into and stabilize).

[PompeyPaul]

Makes sense.

[B2N2]

Up to 150 kts WCA (Wind Correction Angle)= 1/2 x windspeed.
Your max drift or max correction angle is half the windspeed.
Above 150 kts it's 1/3.

Check it with your CRP, it's surprisingly accurate.


Descent angle of appr. 3 degrees GS/2 + 0
eg GS= 90 kts.......divide by two= 45 +0 = 450 fpm rate of descend
If you have no GS indication use TAS and approximate the wind.
No TAS indication use IAS for quick reference.
You can fly surprisingly accurate non-precision approaches that way.

Quick way to get from IAS to TAS:

TAS increases over IAS with 2 kts/1000 feet.
Easy to remember, standard lapse rate is 2 degr./1000 feet.
eg altitude is 8000 feet IAS 100kts, TAS is 8 x 2= 16 kts higher then IAS, 116 KTAS.

[IO540]

The only one I use is

VS (feet/min) = 200 x height to lose in 1000s of feet (at 10nm to run, 120kt)

e.g. if at 5000ft and need to descend to 2000ft, VS= 200x3 = -600fpm

OTOH if one has 20nm to run, one needs only -300fpm.

Everything else comes from the GPS.

Up to 150 kts WCA (Wind Correction Angle)= 1/2 x windspeed.
Your max drift or max correction angle is half the windspeed.
Above 150 kts it's 1/3.
Check it with your CRP, it's surprisingly accurate.

The above is very good - one of the best ever. One of the great secrets (never mentioned in PPL training) is that the winds aloft forecasts are usually so far out that there is little point in using the circular slide rule - these simple rules of thumb are just as good.

[L'aviateur]

These are excellent, thank you very much.

[India Four Two]

Knowing the required bank angle is useful because the AI gives you immediate feedback on the bank angle

I had a surprise in my one and only fast jet ride (well, it seemed fast to me - a Vampire), when I was reminded that I needed a lot more bank to do a rate one turn at 250 kts - about 35 degrees. A lot more than the 15 degrees I was used to in a Chipmunk.

[stickandrudderman]

I really like this:

A quick trick for reciprocal headings is to add 2 to the first digit and subtract 2 from the second digit (or vice-versa). eg 315 135 or 253 073.

I've been sailing for 20 years and flying for four and have never heard of that one!:D

[IO540]

That great trick fails for headings just over 180 and 360.

e.g. 190 / 370!! but 370=010

You have to memorise those :)

[India Four Two]

A quick trick for reciprocal headings is to add 2 to the first digit and subtract 2 from the second digit (or vice-versa). eg 315 135 or 253 073.

I use the same trick, but I find "Add 200 and subtract 20" or "Subtract 200 and add 20" is easier for me to figure out.

As IO540 says, it doesn't strictly work for 181 to 199, but it easy to convert a result like 375 to 015.

[FullyFlapped]

Simple one for two-digit reciprocals is the "opposite compass section" method. Think of the compass as having four sections, containing headings beginning with zero, 1, 2 and 3. Whatever your heading is, the digits in the opposite quadrant will add up to the same number. You just need to remember the two opposing pairs 0 and 2, 1 and 3 ...

Eg Heading 24, recip 06, heading 13, recip 31, and so on.

Works for all combinations apart from 01/19, 10/28 and 11/29.

[Ringway Flyer]

The average thumb, between the first joint and the end of the thumb, = 10 miles on the 1/2 mil chart. At 100 knots GS, this equals 6 minutes. So 3 'thumbs' = 18 minutes, two 'thumbs' 12 minutes, etc.....:ok:

[Captain_djaffar]

one really instructive thread indeed it is.thanks to you all.

[Henry Hallam]

Could this thread be made "sticky"? It's a lot of useful information in one place.

[Robbo0885]

This one might sound simple to everyone, but I am the worlds worst at mental math.

What was killing me was diversions, where my instructor would ask how long it will take. Speed for the leg, 87kts distance 18nm. I would tie my head in knots trying to figure it. Somebody told me a much easier way to think of it.

Divide the speed by 10, so this gives the distance covered every 6 mins, i.e. 8.7 nm. I find this way I can then put this into distance, i.e. approx 12mins.

I'm sure many of you are looking at this and going "yeah, thats obvious!" but I found working out the times and speed in my head tricky when asked and also flying and navigating a diversion. If one person says "thats quite helpful" then I will have fulfilled the purpose of this post!

[jonkil]

Very Simple one, I use for WB on the microlight:

Mogas Weight:

Weight of Mogas = 3/4 the volume.
Example
40 Litres Mogas = 30 KGS in weight
60 Litres Mogas = 45 KGS in weight

[whitehorse]

If this is your thing a "rule of thumb book" is produced by Flight Time Publications www.flight-time.com (http://www.flight-time.com) It´s a pocket sized book of about 20 pages if I remember correctly. The latest price on the site is $7.95. Gives all the usual rules and some which are quite exotic, how you are supposed to work these out in your head I don´t know. :eek:

The rule of thumb methods are good, just to confirm that what you are doing is reasonable, it´s a question of finding out which ones suit you.

For those of you who only use the GPS, the new 1 in 60 rule is -
1 time in 60 the GPS will fail, due to batteries, not updated software, incorrect data put in etc etc etc.
PS I love my GPS:ok:

WH

[Blues&twos]

Robbo0885 - that's quite helpful.

I'm a newby to flying - all of these are great.

Provided I remember them when the chips are down......hmmm.

[IFollowRailways]

For diversions - Finding a rough heading.

Think of your present position as the centre of a clock face. Draw a mental line to where you want to go and think of it as a clock hand - Mentally give that hand a 'time' (Where it would be pointing on the clock face)

Multiply the 'time' x 3 and stick a 0 on the result and you get the heading. For example -

Imaginary line is in the '8' position, 8 x 3 = 24. Add 0 = 240 degrees

Line in '5' position. 5 x 3 = 15. Add 0 = 150 degrees

[wbryce]

Another one for working out time taken when doing diversions...

GS - 140kts per hour so:
140/60, take it down to basic fractions - 7/3.

35nm leg, divide the top number into the distance to fly (7 into 35 = 5 then times it by the bottom number 3 * 5 = 15 mins). Its easy once you have some yardsticks stored in your head if your not good at working the fractions out...once you realise estimates are good enough then you end up just using simple rules like 120 = 2 miles a minute, if your slightly over or under then it doesnt matter much, the estimate will still be good enough, just use common sense and add/subtract a minute if necessary. 90kts is 1.5 miles a min or 3/2 in fractions, 100kts - 5/3.

[Final 3 Greens]

A useful approximation is ground speed divided by 2 x 10 = descent rate in fpm for 3 degree glidepath.

e.g. 90 divided by 2 = 45 * 10 = 450 fpm

[Fg Off Max Stout]

...as stated in post #3!

[MIKECR]

Descent planning:-

height to lose x 3 = distance to top of descent, then add 5nm to slow a/c and configure.
Rate of descent = 5 x groundspeed

Example - 20,000ft descent to 3000ft at 200knots =

20 - 3 = 17, x3 = 51nm, add 5nm to slow a/c = 56nm

5 x 200 = 1000ft per minute

Oh and for Final 3 greens post - perhaps easier just to work out 5 x groundspeed = rate of descent for 3 degree ILS glidepath

[al446]

I'm with Henry Hallam and G-EMMA on this. The learning curve for us aspiring PPLs can be quite steep and having all this in one place is VERY useful. I know there are archaic things knocking around called "books", I have been known to use one or two, and I dont for a minute think this is any kind of replacement but, when experiencing severe brain fade, a thread such as this is useful, it also informs us as to what peeps actually use in real world flying.

Cheers

[Rattus]

If you're planning a sea crossing, the Coastguard can be a useful source of met information, though wind speeds in maritime forecasts are always given with reference to the Beaufort Scale. These can easily be converted to knots, though not being round numbers, some of these are not easy to remember. The Corlett Scale simply involves rounding up or down by 1 or 2 knots, so that with the exception of Force 1, which is unlikely to bother most aviators anyway, all are multiples of 5:
Force 1 = 02 kts call it CALM
Force 2 = 05 kts call it 05
Force 3 = 09 kts call it 10
Force 4 = 13 kts call it 15
Force 5 = 19 kts call it 20
Force 6 = 24 kts call it 25

Very few recreational pilots are likely to be flying in much over force 6, but the scale can be extended right up to Hurricane Force. For completeness, here's the rest:
Force 7 = 30 kts call it 30
Force 8 = 37 kts call it 35
Force 9 = 44 kts call it 45
Force 10 = 52 kts call it 50
Force 11 = 60 kts call it 60
Force 12 = 64 kts call it 65

Regards
Rattus