Four Finger Rule

[Not the Usual]

Can anyone explain this concept to me?
I keep messing up the questions regarding the affect of TAS/CAS/EAS/MACH in a climb/decent/SaL with constant/increasing/decreasing deperature, etc.
I'm sure you guys know which questions i'm talking about. I've been trying to think about it logically but it always seems as though i pick the answer that is the complete opposite to the correct (as in i would choose increase instead of decrease and vica verca).

[LVL_CHG]

I've not heard of the 4 finger rule. I can probably explain to you the 3 finger rule though or "Chicken Tikka Masala"

C- CAS
T-TAS
M- MACH

Lets try an example.Using your thumb and next two fingers on your left hand with your palm facing towards you. Your thumb is CAS, middle finger TAS and other MACH.

Now, imagine you were to climb at a constant CAS. What you want to figure out is what is happening to the TAS and the MACH. So CAS is a bench mark for this example and therefore we need to do something with our thumb.

Rotate your hand so that your thumb is pointing straight up in the air. Whats happened to your other fingers? They're both pointing to the right, right? Or if you imagine that is a graph format, they are increasing. So TAS and MACH increase

Look at it the other way, if you were decending at the CAS, so starting at the top of your thumb and coming vertically downwards, so coming back down the fingers they are now decreasing also. So decending at a CONSTANT CAS the TAS and the MACH decrease.

Now rotate your hand so that the middle finger is pointing upwards.

So a climb at constant TAS, the CAS decreases and the MACH increases.

I'm not the greatest at explaining things but hope this has helped you.

[mad_jock]

I would suggest you don't ask anyone in the engineering fraternity what the four finger rule is.

Its likely to be very graphic and might put you off your lunch.

[pudoc]

4 finger rule works but I prefer my method.

Basically, write down (or visualise it in your head) ECTM.

Next, ask yourself are you climbing or descending? If you are climbing you are going from low altitude (-) to high altitude (+)

-ECTM+

So as you climb at constant TAS, for example, to the right of the T there is a + so that means mach increases. To the left is a - so that means EAS and CAS decrease.

Switch the - and + around for when you are descending.

Never let me down.

[Genghis the Engineer]

To be honest I just learned and understood it. Airspeed is such a fundamental concept I'm uncomfortable applying rules of thumb and mnemonics to it.

G

[pudoc]

Try doing that in an exam with stupidly written questions and little time.

Yes learn it all, but in the exam you have to save as much time as possible to try and work out what the poorly written questions are asking you. Mnemonics and rules are crucial time savers.

Gotta play the exam game I'm afraid.

[Genghis the Engineer]

I know, I do have a CPL, and on things like heading correction and the 10001 ICAO Annexes I did exactly the same. But airspeed I still think is just too fundamental and you need to properly understand it.

G

[what next]

...but in the exam you have to save as much time as possible...

If you know your stuff, you have plenty of time at the exam. I don't want to sound arrogant, but you don't study for the exam, you study for your whole profesional life, if "pilot" is the only profession you ever learn. Invest a little more time and try to understand what you are learning, then you won't need rules of thumb and mnemonics. (As an instructor, I am really fed up with those students who can tell you for every question if the answer is a, b, c or d, but have no idea at all what the question is about.). Sorry.

[pudoc]

Just because you know something it doesn't mean you can work it out quickly. I know the relationship but I can't just pop out with the answer. I'm not fluent enough with it. Remember when you first started flying and your RT was far from perfect, but once flew more and more it became better? Maybe on your QXC you could spill out the RT for a basic service but you'd have to think about what to say before hand? Perhaps you have a mnemonic written down that reminds you what to say in the right order? Well it's the same with theory. I have to think exactly what's happening and I will be doing everything I can to play the exam game.

If it you are going to categorise me as a rote learner (where did you get A b c d from?? who said I use the question bank?), then you shouldn't worry about me being a pilot because I'll get caught out at the CPL skills test or in an interview. :ok:

[mad_jock]

They have banned the four finger rule anyway in the uk because it descriminates against folk from the west country who have 3 or 6 depending which family they come from.

[mad_jock]

Nah the vikings did us a favour up North.

[mad_jock]

Nope thats the mark of the devil.

You would never eat a red frog would you?

[Not the Usual]

Thank's for the replies, especially LVL_CHG. I think it actually is the 3 finger rule, but a friend of mine must have just added IAS in there somewhere too. I think i actually finally understand it now.

And to the two or three people telling me to just "simply learn the content".
To be honest, with the small amount of time i've had for the 7 exams i'm taking in June, reading and understanding 7 enormous books is the last thing on my mind, especially when english is not my first language and i've always found myself reading longer on an english book. It would have take me more then 4 months just to get through the books.
I understand that i am not just studying for an exam, but also for a career (which is quite a silly statement, because in every profession this also applies), but for now, whilst i have to deal with the CAA and their total incompetence when it comes to setting up/creating a decent (and i do not mean easy, i mean understandable) exam, "understanding" the work is the last thing on my mind, because the last time i tried that... it did not end well.

[mad_jock]

just straight hair all the way down her back

See west country and its normal for the women to have hairy backs. :eek:

did the original question ever get answered?

Nah because most haven't bothered to learn it because the relationships are as G said are that easy and to be honest its not something you need to think about every day at work.

[truckflyer]

Yes the C T M - I agree, having the Mnemonic is nice, but you might as well use that as a reminder only, and understand the concept.

Why it happens like it does.

The formula for Dynamic Pressure, Mach and LSS are the ones that form the whole concept of the Chicked Tikka Masala rule!

CAS = Dynamic pressure = 1/2 r V2 (Vsquared) the V is TAS, r is density based on ISA conditions.
You go up, density decreases, so to keep the Dynamic pressure constant, TAS must increase.
This because you climb at a constant CAS.
So if you now climb at a constant TAS, because your density decreases, CAS must decrease.

However the reason Mach increases, is another formula and concept you should know.

LSS depends on temperature only, decreasing temperature, lower LSS. With altitude unless an inversion, LSS will get lower and lower until you reach the Tropopause.

Mach = TAS / LSS, so if TAS is constant, and your LSS is getting smaller the higher you fly, then Mach will increase.

It's all based on simple maths and common sense, if you apply that thought process, than it will make it much easier to explain if you get the question.
One thing is getting the question on a multiple choice, and you have all the time in the world, different the day somebody asks you, what happens to TAS when you climb at constant Mach? You will know the initial answer, however the follow up will be, ok, now explain me why?

Understanding the concept, makes it much easier to understand!

Ok, last one, I did not explain, but in brief, climb at constant Mach, your LSS will decrease with altitude, so to keep Mach constant, TAS also have to decrease with altitude, as you can see from the equation. (LSS = Local Speed of Sound)

[mad_jock]

No mate

And to stop the velco effect try turning her over. Thats also considered normal elsewhere.

[Genghis the Engineer]

Reminds me of interviewing somebody for a job in an airworthiness office who claimed a degree in aeronautical engineering, who couldn't give even a beginning of an explanation of how to derive the main V-speeds.

I wonder where he is now?

G