Hi Bob,

I was doing one of the online exam prep tests and had the below question.

Your working in the answer used density height to enter the chart whereas I used pressure height and got the wrong answer. For such questions, I assume both methods should produce the same result and can use either? Thanks you.

Refer to page 22 of the work booklet.

Given that the following information is extracted from the ERSA.
RWY: 07/25
TORA : 750
TODA : 810 [2.5 percent]
ASDA : 750
LDA : 750

Slope: 0.4 percent down to the NE
RWY WID 30 RWS WID 90
GRAVEL : UNRATED

Ambient conditions for take-off are -
Wind.................[M/Kt] 070/05
Temperature..........+ 31 degrees C
Pressure height..... +100 ft

The heaviest take-off weight permitted at this aerodrome is -

2650 kg
2840 kg ---> Correct Answer
2740 kg
2950 kg ---> My answer using pressure height.

This is a problem with the VFR (Day) Booklet. Richard has recently answered a post dealing with the same issue. For example if you go into the graph at 2000 feet pressure height and 11°C (the ISA temperature at 2000 feet), you should be at exactly the same point as someone who entered at 2000 feet density height. However you will find that the chart is somehow skewed and those two points do not coincide.



This is a problem with the chart itself and I don't know what you can do about it. I suggest that you enter at pressure height and go to the temperature as given in the question because I assume that would be what the examiner has done. However it seems that anyone who enters this chart at at pressure height and temperature will get a different answer than someone who calculates the density height before entering the chart.

I'll send this post to the examination section in Canberra and see what they suggest.

Bob

Here is the reply I got from Flight Crew Licensing (Exam section) in Canberra....

Hi Bob,

Thank you for your comment on the Day VFR Workbook. Candidate's should enter the graph using the pressure height, then move horizontally across to the temperature line prior to moving vertically up the page for the take-off distance available.

I do realise that a combination of pressure height and temperature should result in the same point as the density height - but clearly with this graph there is a difference. As numerous schools have been using this graph in its present format for many, many years, to re-master it would also require those schools to alter their questions they us when teaching candidates (and of course CASA and the exams too). I'm not certain whether there is much to be gained from making the change. I will take note of it and pass your comment onto the FCL (Exam) Review Panel at their next meeting.

Once again, thank you for bringing this to my attention.


Regards,

Hi Bob,

Thank you for your reply and also your time for writing to CASA FCL examination team about the matter.

Interesting to note that while the Echo Take-Off Weight chart has provision to enter at both Pressure Height and Density Height, we should just enter at Pressure Height, not Density Heigh (for the sake of the examination).