Definition of 'critical point'?

[r44flyer]

Hi all,

Can anyone provide a good definition and description of 'critical point', ie. the point which denotes whether to carry on to destination B or return to A when looking for the quickest option.

Perhaps there is a good resource online? I have googled but I think it might be known by something other than critical point as I haven't found anthing.

Thanks
Jim

[Luke SkyToddler]

It's also defined as "equitime point", i.e. the point at which it will take the same time to either return to origin or continue to destination.

Is that what you meant or did you want something more in depth?

[r44flyer]

It's also defined as "equitime point", i.e. the point at which it will take the same time to either return to origin or continue to destination.
Is that what you meant or did you want something more in depth?

'Equitime' helped in the search, thanks!

I found these threads which explain everything quite well...

http://www.pprune.org/forums/showthread.php?t=82740
http://www.pprune.org/forums/showthread.php?p=306640

Thanks again :)

[class a]

Equal time point (say time to go on to destination as it is to return to dep airport)

Critical point moves into wind

[vee en]

For calculating the Critical ( Equitime) Point between the Destination and an Alternate airfield, look up the Vir Narain Formula on Wikipedia (Flight Planning: Fuel)

[r44flyer]

Holy thread resurrection, Batman!

Thanks for the tip, though :ok:

[Exaviator]

Hi Jim,

CP is defined as the equal time point and usually based on an engine failure taking into consideration the resulting reduced airspeed. The formula to calculate a C.P. is:

Dist x H Where H = G.S Home all engines
O + H Where O = G.S. Out & H = G.S. H at reduced TAS

If you use the same TAS above and below the line it becomes a simple equal time point for a single engine aircraft.

On long haul four engine aircraft we calculate a C.P. for both one engine out and two engines out. :hmm:

[vee en]

Exaviator. A more interesting - and perhaps practically useful - case is that of the Critical Point between the destination and a diversionary airfield. Older aircrew will recall the "Press On Divert Or Turn Back?" controversy of the 'fifties.

In case of an in-flight emergency it may be necessary to determine whether it is quicker to divert to the alternate airfield or continue to the destination. This can be calculated according to the formula (known as the Vir Narain Formula) as follows:-
C= D.O Sec θ/ 2A, where C is the distance from the Critical Point (equitime point)to the destination, D the distance between the destination and the alternate airfield, O the Groundspeed, A the airspeed and θ = Φ +/- d, where Φ is the angle between the track to the destination and the track from the destination to the alternate airfield and d the Drift (plus when the drift and the alternate airfield are on the opposite sides of the track, and minus when they are on the same side). Published reference 'Air Clues' UK July 1952.
This has many applications in tactical navigation

Incidentally, the "equitime" (defined as the time taken from the Critical Point to either the destination or the alternate airfield) thus turns out to be solely dependent on the drift!

[Exaviator]

Yes you are quite correct Vee en most long haul operators utilise an off-route diversion C.P. and also a three way PNR taking into consideration flight at lower altitude following a decompression. All good stuff but today the lads have a computer to do the sums. No more slide stick and basic navigation.:eek:

[kwachon]

We used to call it PNR during flight planning.

Point of No Return, self explanatory I would say.

KW

[Exaviator]

Not quite Kwachon, there is a difference:

PNR calculations are based on Fuel Endurance and is the point at which you have enough fuel to proceed to destination, or return to point of departure, or off track diversion airfield.

Whereas C.P. is based on Distance and gives equal flying time.:ok:

[kwachon]

A Clarification,

The CP occurs at the moment when flight time to destination and the flight time back to base are the same; the PNR occurs when we will have just sufficient fuel to return to base.

KW

[Mikehotel152]

Point of No Return, self explanatory I would say.


I wouldn't.

The PNR as correctly defined by KW is the last point at which you can still make a choice to continue or go back. Beyond the PNR you cannot return.

IMHO a pedant might say the term should be Last Point of Return! :)

As currently labelled it's therefore not 'self-explanatory' and a bit misleading.

[kwachon]

Mikehotel152

Interesting comment but pure semantics......



The term PNR—"point of no return," more often referred to by pilots as the "Radius of Action formula"—originated, according to the Oxford English Dictionary (http://en.wikipedia.org/wiki/Oxford_English_Dictionary), as a technical term in air navigation (http://en.wikipedia.org/wiki/Air_navigation) to refer to the point on a flight at which, due to fuel consumption, a plane is no longer capable of returning to its airfield of origin. After passing the point of no return, the plane has no option but to continue to some other destination. In this sense, the phrase implies an irrevocable commitment.

For nonstop flights between two definite locations, the PNR is actually beyond the halfway (more exactly, the "equitime") point, since aircraft usually carry more fuel than is necessary to reach the destination. For example, on a 2000-mile flight, should the tanks have enough fuel for a 3000-mile flight, the halfway point would be at 1000 miles, but the PNR would be at more than 1500 miles

Have a nice day

KW

[igarratt]

I guess these cant be simple definitions, I would expect them to be amazingly complex and in some instances unable to actually define due to unknowns.

The path followed from a to b is never going to be the same as b to a, differences in flight level, atc route, wind directing differences from opposite heading, lighter aircraft as you dont have as much fuel, change of c of g.
I assume that given that optimum flight level changes with weight then as well as heading FL rule change optimum level will change too.

so it may have taken 2 hrs and X amount of fuel to get to point b but to go back to point a may take more or less time or fuel.

does this mean it cant be calculated until actual is known ? (ie dynamic point of no return) or is it kept simple ? or is it planned (as in this is how we would get back) ?

not trying to be clever.. just learning !

[Mikehotel152]

Interesting comment but pure semantics......

Ha ha ha. No, it's not semantics. It's grammatically and technically correct. Anyway, you understand my point and I've agreed that your explanation of PNR is correct, so stop being a silly sausage! :p

Flap limiting speed is X; fly at X+1 knot with flaps down and your Captain might wallop you with a smelly old piece of cod even though no damage was done...It may be semantics, but correct definitions matter. :)

Sorry, as Manuel might say: "I speak good English - I learned it from a book"...I can't help but point out these things. Such incorrect use of the English language has the potential to confuse people.

[kwachon]

Flap limiting speed is X; fly at X+1 knot with flaps down and your Captain might wallop you with a smelly old piece of cod even though no damage was done...It may be semantics, but correct definitions matter.

Now to your points!
pedant
Noun
a person who is concerned chiefly with insignificant detail or who relies too much on academic learning

I have never understood why folks drift off thread and resort to personal attacks either with humour or sarcasm when they get a response they do not like.

Not sure what Captains you fly with but in my 19 years as a Captain I have never carried a "smelly old piece of cod" nor attacked my First Officer.

Perhaps you could enlighten us all to the airline you fly for, assuming you do, so we might all avoid it lest we get attacked by a fish weilding Captain. :hmm:

semantics
Noun
the branch of linguistics that deals with the study of meaning
Collins Essential English Dictionary (http://www.thefreedictionary.com/_/misc/HarperCollinsProducts.aspx?English) 2nd Edition 2006 © HarperCollins Publishers 2004, 2006

The comment was made in response to your statement not the original question.

KW

[vee en]

Exaviator. Whether you use a slide-rule or a computer, the formula still holds.
The reference to PNR (underline Return) was perhaps a needless (and surprisingly acrimonious) digression.

[Mikehotel152]

:confused:

I simply made a valid comment on a grammatically incorrect definition, because the words used to describe the PNR are slightly misleading and can confuse those learning about these subjects for the first time. Anyone who has recently sat the ATPL theory exams will know that JAA/CAA regularly exploit semantics by asking pedantic questions to catch-out students.

In any event, I said that we all agreed about what it actually meant and insofar as I made a 'personal attack', I did it with a :) in the first instance and in a very light-hearted jokey way in the second. I accept that some might not have watched Fawlty Towers and simply regard this digression as codswallop...

Happy flying :}

[Keygrip]

Enough now.

If you can't discuss it without "willy waving" then leave it alone.

Question has been asked and answered - now grow up.