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Critical point question
Unusualattitude
Topic Author
Unusualattitude created the topic: Critical point question
An airplane cruising at a true airspeed of 250 knots is flying from A to B track of 095 degrees, a distance of 1800 NM. The average wind over the route is estimated to be from 030 degrees at 30knots. The airspeed is assumed to be 60% of the normal cruise speed or 150knots
1. Find the critical point
a.973nm b.995nm c.796nm d.866nm
I use the formula: DH/O+H = 1800*163/137+163 = 978 which is close to A.
2.Find the time of critical point
a.3 hours 47 minutes
b.4 hours 7 minutes
c.4 hours 12 minutes
d.4 hours 18 minutes
So I use this formular: X/H or D-X/O either one should give me the same answer?
973/163 = 5 hours something which is not one of the answer. What am I missing here?
3.Find the reduce ground speed home
a.150K b.159K c.236K d.250K
X/H = 973/163 = 300 something knots. This can't be right.
Any helps appreciated
Cheers
Richard replied the topic: Re: Critical point question
This question would appear to be referring to a single-engine CP. The formula for Critical Point (or ETP) is:
CP = (Distance x GS Home) / (GS Out + GS Home)
The GS out and the GS home are based on the TAS after we've had the engine failure. Therefore we work with the reduced TAS of 150kt. Using the flight computer we get:
GS Out = 138 kt
GS Home = 162 kt
Distance = 1800nm
CP = (1800 x 162) / (162 + 138) = 972 nm
To find the time to the CP remember you will be cruising at 250 kt until you hit the engine out CP so you base your calculation on a TAS of 250 kt not 150kt. The GS out with a TAS of 250kt is about 238 kt.
973 nm @ 238 kt = 4.088 hours or close enough to 4 hrs 7 minutes.
The reduced ground speed home would have to be your GS home with an engine out and you calculated that in the first step: closest answer is 159kt. Your calculation of X/H = 973/163 will give you the time it takes to fly back from the CP not the speed.
That's my take on it based on the information you have provided in the question anyway.
Nikolaou replied the topic: Re: Critical point question
Hi Richard / Bob
Just been going through the topics and came across this one "critical point". Would this be under the CPL Nav theory casa exam or is it Performance? Reason I ask is that I have not come across this in Bob Taits CPL Nav work book & will be sitting the exam soon.
bobtait replied the topic: Re: Critical point question
John
You won't find mention of Critical Point [CP] in the NAV book nor in the CASA NAV exam. It is covered in the Performance course. Critical Point is a term that was once widely used in flight planning but it has now been largely replaced by the term Equi-time Point [ETP].
