QNH to QFE? (need help with chart)

[WhySoTough]

So I can't seem to understand this chart.
http://a5.sphotos.ak.fbcdn.net/hphotos-ak-snc7/s720x720/375983_294938033891857_100001269892528_928886_2143973570_n.j pg

On the bottom it has two little examples.
For number 1) I understand the correction of -200, but I don't get how the 2300ft QFE = 933 HPa.

And I don't understand any of question number 2.

Anybody?
Thanks!

[ATCast]

Example 1:
Lookup QNH = 1020 in the left column of the table.
The corresponding correction in the middle column is -200 ft.
The elevation is 2500ft, after correction it is 2500-200 = 2300ft.
Look this value up on the right side of the scale on the left side of the picture and read the corresponding QFE on the left side of the scale. QFE = 933 hPA

Example 2:
Locate QFE = 980 hPA on the left side scale and find that the corresponding pressure altitude reads 920 ft. The actual elevation is 1500 ft, so the correction is (920-1500) = -580 ft.
Looking up -580 in the middle column of the table yields the corresponding QNH of 1032 hPA in the left column.

ATCast

[WhySoTough]

ATCast,

Thanks very much for your quick response.
Can you/ or someone else, explain to me this.
http://a3.sphotos.ak.fbcdn.net/hphotos-ak-ash4/s720x720/387673_295009730551354_100001269892528_929043_1729267774_n.j pg

Is this basically just showing us for a certain TAT, at a certain pressure altitude, this is what our EPR will be?
Not sure what EPR is. :O

[ATCast]

Not 100% sure, but if I understand it correctly it shows you the Maximum Continuous EPR for given TAT and pressure altitude for a IAE 2527(E) engine.
EPR is Engine Pressure Ratio, the ratio of the core nozzle outlet pressure and the inlet pressure. It has it's own thread (http://www.pprune.org/tech-log/473659-alternative-epr.html)running.

[WhySoTough]

Thanks a lot for the help ATCas,

One last one for you.
In the first chart I sent you, calculating the QNH or QFE.
Is it always correct if for such questions I only calculated 30 feet per HP and do it myself, rather than confuse myself with the chart?

1) 2500/30 = 83
1020-83 = 937. Comes pretty close if it were a multiple question.

Thanks again.

[ATCast]

It comes pretty close, especially for the lower elevations, so you could use it as a check to verify if your result obtained from the chart is correct. See for yourself if it works for Cochabamba airport in Bolivia (elevation 8360) and QNH 970.

On the other hand you are 4 hPa off in your example, and according to your 30 ft / hPa rule that is 120 ft too low. I am not sure if you'd be comfortable with those kind of errors when it comes to say... decision height?

Good luck on the tests!

[BOAC]

30'/mb(HP) only 'works' at sea level in a Standard Atmosphere, going from 30ft to many Km near the edge of the atmosphere, to, in theory, infinity:)..